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Proc. R. Soc. A (2010) 466, 927–963
doi:10.1098/rspa.2009.0558
Published online 25 January 2010
REVIEW
Heterofunctionalization catalysis with
organometallic complexes of calcium,
strontium and barium
BY ANTHONY G. M. BARRETT1 , MARK R. CRIMMIN2, *, MICHAEL S. HILL2
AND PANAYIOTIS A. PROCOPIOU3
1 Department
of Chemistry, Imperial College London, Exhibition Road,
South Kensington, London SW7 2AZ, UK
2 Department of Chemistry, University of Bath, Claverton Down,
Bath BA2 7AY, UK
3 GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage,
Hertfordshire SG1 2NY, UK
Despite the routine employment of Grignard reagents and Hauser bases as stoichiometric
carbanion reagents in organic and inorganic synthesis, a defined reaction chemistry
encompassing the heavier elements of Group II (M = Ca, Sr and Ba) has, until recently,
remained unreported. This article provides details of the recent progress in heavier Group
II catalysed small molecule transformations mediated by well-defined heteroleptic and
homoleptic complexes of the form LMX or MX2 ; where L is a mono-anionic ligand and X
is a reactive s-bonded substituent. The intra- and intermolecular heterofunctionalization
(hydroamination, hydrophosphination, hydrosilylation and hydrogenation) of alkenes,
alkynes, dienes, carbodiimides, isocyanates and ketones is discussed.
Keywords: calcium; strontium; barium; heavier alkaline earth; heterofunctionalization; catalysis
1. Introduction
Over the past few decades, the understanding of the coordination chemistry of
the elements of the heavy alkaline earth metals (M = Ca, Sr, Ba) has advanced
dramatically, owing primarily to an interest in their application as chemical
vapour deposition precursors (Hanusa 1990, 1993, 2000, 2002; Westerhausen
1998, 2001, 2006, 2008; Alexander & Ruhlandt-Senge 2002; Westerhausen
et al. 2007). Reaction studies upon organometallic compounds of calcium,
strontium and barium have, however, been largely overshadowed by those of
the lighter congener, magnesium. Despite the routine employment of Grignard
reagents and Hauser bases as stoichiometric carbanion reagents in organic and
*Author for correspondence ([email protected]).
One contribution to the 2010 Anniversary Series: a collection of reviews celebrating the Royal
Society’s 350th Anniversary.
Received 21 October 2009
Accepted 15 December 2009
927
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928
A. G. M. Barrett et al.
inorganic synthesis, a well-defined reaction chemistry encompassing the heavier
elements of Group II has remained unreported. It is revealing that, although
syntheses of ‘heavy Grignards’ (ArMX and RMX, X = halide) were documented
contemporaneously with Grignard’s original reports of his eponymous reagents
(Grignard 1900; Beckmann 1905; Gilman & Schulze 1926; Gilman et al. 1943),
only in the last 3 years have arylcalcium halides been isolated and structurally
characterized (Gartner et al. 2007b; Westerhausen 2008). Furthermore, the solidstate structures of the analogous strontium and barium complexes are, as yet,
unknown (Langer et al. 2007).
With some recent remarkable exceptions (Green et al. 2007, 2008; Bonyhady
et al. 2009; Krieck et al. 2009), the organometallic compounds of the heavy
alkaline earths are overwhelmingly redox inactive species, which demonstrate
a +2 oxidation state. The M2+ cations achieve noble gas configurations, and
compounds of the heavy metals possess a d0 electron configuration. The ionic
radii of the dications increase as the group is descended (Shannon 1976),
whereas the Pauling electronegativity of the elements decreases as the group is
descended. The combined result of these factors is that, while organometallic
compounds of magnesium may show a degree of covalency in metal–ligand
interactions, bonding in organometallic complexes of the heavier elements are
dictated almost exclusively by ionic and non-directional interactions between
the metal and auxiliary ligands. In this regard, parallels have often been drawn
between the chemistry of the heavier alkaline earths and that of the trivalent
organolanthanides. The latter compounds are also defined by highly ionic metal–
ligand interactions and have begotten a versatile reaction chemistry. Since the
pioneering work of Marks (Hong & Marks 2004), it has been shown that trivalent
organolanthanide compounds of the form L2 MX1 , where L is a mono-anionic
spectator ligand and X1 is a mono-anionic s-bonded substituent, demonstrate
two fundamental types of reactivity: (i) s-bond metathesis and (ii) insertion of
unsaturated carbon–carbon or carbon–heteroatom bonds into Ln–X1 s-bonds
(figure 1a,b).
By incorporating these two reaction steps into catalytic cycles, a vast
number of lanthanide-mediated synthetic reactions have been developed, many
of which have no direct parallels in conventional organic synthesis. Initial
research in this area centred upon the application of sterically demanding
pentamethylcyclopentadienyl (L = Cp∗ ) or ansa-bridged cyclopentadienyl [L2 =
(C5 Me4 )2 SiMe2 , (C5 Me4 )SiMe2 (N-t-Bu)] ligand sets to stabilize low-coordinate,
highly reactive, organolanthanide intermediates. Work by Marks has
demonstrated the application of compounds of the form L2 MX1 [X1 = H,
CH(SiMe3 )2 , N(SiMe3 )2 ] to the hydroamination, hydrophosphination,
hydrosilylation, hydrogenation and hydroboration of unsaturated carbon–
carbon bonds (Togni & Grützmacher 2001; Molander & Romero 2002;
Hong & Marks 2004).
Given the parallels between the bonding within heavier Group II and trivalent
lanthanide compounds, it has been suggested that these complexes may be
employed as homogeneous catalytic reagents using s-bond metathesis and
insertion reaction chemistries to construct catalytic cycles. Within the last 5
years, this hypothesis has been realized; herein, we present a review article
on the emerging area of catalysis by organometallic complexes of the heavier
alkaline earths.
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Review. Group II catalysis
(a) (i)
-
d
d
+
L2MX1 + X2 – Y
L
L M X1
L2MX2 + X1 – Y
2
X –Y
+
d
d
-
L2MX1 + X2 – Y
(ii)
2
1 2
L2MX + R RC = Y
L
L M X1
Y X2
L2MY + X1 – X2
L
L M X1
Y CR1R2
L2MX2 + X1 – Y
YH
R1
(b)
X2
R2
L2M X2
1R2RC
=Y
HX2
insertion
s-bond
metathesis
L
L M X2
Y CR1R2
L
L M X2
Y CR1R2
‡
Figure 1. (a) Fundamental reactions of trivalent lanthanide organometallics: (i) s-bond
metathesis and (ii) insertion of unsaturated substrates into M–X2 s-bonds. (b) Catalytic
heterofunctionalization with organolanthanides.
Although striking progress has been made in the application of calcium,
strontium and barium complexes for the polymerization of activated alkenes,
such as styrene (Feil & Harder 2000, 2001, 2003; Weeber et al. 2000; Harder et al.
2001a,b; Harder & Feil 2002; Feil et al. 2003; Harder 2004; Piesik et al. 2007),
the ring-opening polymerization of cyclic esters (Chisholm et al. 2003; Piao et al.
2003a,b; Westerhausen et al. 2003; Chisholm et al. 2004; Sarazin et al. 2006;
Davidson et al. 2007), the dimerization of aldehydes (Crimmin et al. 2007b)
and the trimerization of isocyanates (Orzechowski & Harder 2007), this
review focuses on catalytic methods for carbon–heteroatom bond formation
via the heterofunctionalization of unsaturated substrates. Furthermore, owing
to uncertainties relating to the chemical composition of the active species,
applications of complexes of Group II alkoxides as bases for asymmetric
carbon–carbon bond-forming reactions are not covered by this review article
(Yamada & Shibasaki 1998; Kumaraswamy et al. 2001, 2003; Suzuki et al. 2001;
Saito et al. 2007).
While the coordination chemistry of heavier Group II metals has been
reviewed numerous times (see the introductory paragraph for references) and
several recent papers have highlighted results in heavier Group II catalysis
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A. G. M. Barrett et al.
(Coles 2008; Smith 2009; Westerhausen 2009), there are no extensive review
articles that discuss the catalytic reactivity of calcium, strontium and barium
reagents.
2. Heterofunctionalization catalysis
(a) s-bond metathesis and insertion reactivity at heavier Group II centres
Several precedents exist in stoichiometric heavier Group II chemistry that are
consistent with the fundamental reaction types observed for trivalent organolanthanide complexes. Westerhausen, for example, has observed the insertion of
both 1,4-diphenylbutadiyne and benzonitrile into the metal–phosphorus bond of
a series of homoleptic heavier alkaline earth phosphides, [M{P(SiMe3 )2 }2 (THF)4 ]
(M = Ca, Sr and Ba; THF = tetrahydrofuran; Westerhausen et al. 1997, 1998c,
1999, 2000b). In a related study, it has been demonstrated that the analogous
amide complexes [M{N(SiMe3 )2 }2 (THF)2 ] also undergo insertion reactions with
benzonitrile (Westerhausen & Schwarz 1992). Although in all cases, the initial
reaction products underwent decomposition with silyl group migration, the
isolated products can be rationalized in terms of the insertion step. This work
parallels early studies by Gilman and Coles, which showed that ill-defined heavier
alkaline earth complexes, proposed to contain metal–carbon s-bonds, react
with unsaturated substrates such as CO2 , benzonitrile and 1,1-diphenylethene
(Gilman et al. 1943, 1945; Gilman & Woods 1945; Coles & Hart 1971). Mingos
and coworkers have demonstrated the insertion of carbonyl sulphide, carbon
disulphide and sulphur dioxide into Group II metal–alkoxide bonds (Arunasalam
et al. 1994, 1995, 1998; Bezougli et al. 1997, 1998a,b). Feil et al. reported
the insertion of 1,3-dicyclohexyl carbodiimide into the calcium amide bonds of
[Ca{N(SiMe3 )2 }2 ] (Feil & Harder 2005). Perhaps more importantly, the work of
Harder has shown that highly reactive heavier Group II benzyl complexes are
suitable initiators for the polymerization of styrene. These reactions have been
shown to occur through multiple insertions of the alkene into the metal–carbon
s-bond of intermediate organometallic species (Weeber et al. 2000).
Further to these observations, s-bond metathesis (or protonolysis) has been
frequently employed in stoichiometric heavier Group II chemistry to synthesize
new organometallic complexes. Examples include the reaction of heavier Group
II metal amides [M{N(SiMe3 )2 }2 (THF)n ] (M = Ca, Sr and Ba; n = 0 or 2)
with alcohols, thiols, selenols and tellurols, pyrroles and pyrazoles, terminal
alkynes, cyclopentadiene and derivatives, phosphines or arsines to yield the
corresponding metal alkoxide (Westerhausen et al. 2003; Sarazin et al. 2006;
Davidson et al. 2007), thiolate (Chadwick et al. 1998), selenolate and tellurate
(Gindelberger & Arnold 1992, 1994), pyrrolide and pyrazolide (Vargas et al.
2002; Hitzbleck et al. 2004), acetylide (Burkey & Hanusa 1996; Green et al.
1999; Avent et al. 2005a; Barrett et al. 2008b; Schumann et al. 2009),
cyclopentadienide (Tanner & Hanusa 1994; Tanner et al. 1995; Westerhausen
et al. 1995; Hays et al. 1996; Avent et al. 2006), phosphide (Westerhausen &
Schwarz 1993; Westerhausen 1994; Westerhausen et al. 1996a,b, 1998b, 2000a,c)
or arsenide (Westerhausen & Schwarz 1995; Westerhausen et al. 1998a, 2001)
species, MX2 , along with the reaction by-product HN(SiMe3 )2 . In many cases,
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Review. Group II catalysis
L2M + MX12, M = Mg, Ca, Sr, Ba
2 LMX1
Figure 2. The Schlenk equilibrium of heavier Group II complexes.
Me3Si
Me3Si
i-Pr
i-Pr
SiMe3
M
THFn
X1
i-Pr
i-Pr
M
THF
X1
Me
i-Pr
R
Me
i-Pr
N
N
Ca 1
X
THF
i-Pr i-Pr
R N N
X Ca N N BH
N N
1
R
1
M = Ca; X = I; n = 1
1
M = Sr, Ba; X = I; n = 2
M = Ca; X1 = HBEt3; n = 2
M = Ca; X1 = N(SiMe3)2; n = 1
1
M = Ca; X = CCR
1, X1 = N(SiMe3)2
1
M = Ca; X = I
1
M = Ca; X = N(SiMe3)2
1
R = i-Pr; X = N(SiMe3)2
R = t-Bu; X1 = O(2,6-i-PrC6H3)
Figure 3. Selected examples of well-defined, kinetically stabilized, heteroleptic heavier Group II
complexes. Empirical formulae only (many compounds are of higher nuclearity in the solid state).
homoleptic organometallic complexes commonly demonstrate low solubility in
hydrocarbon solvents and are often isolated with the addition of a charge neutral,
chelating ligand.
(b) The Schlenk equilibrium
An underlying challenge in the development of catalytic reagents based upon
the heavier alkaline earths is the propensity of heteroleptic compounds of the form
LMX1 to undergo Schlenk-like solution redistribution reactions to the homoleptic
compounds L2 M and MX12 (figure 2). The latter species are often polymeric and of
low solubility in non-coordinating solvents, and the former species are kinetically
stabilized and unreactive. This solution redistribution reaction results in the
formation of a mixture from a potentially catalytically active species and may
result in the loss of any ligand control over reactivity.
This problem becomes increasingly important for the heavier alkaline earth
congeners, as ionic radius increases and Lewis acidity decreases on descending
the group and, for a given ancillary ligand set, the tendency towards solution
redistribution of heteroleptic Group II organometallics, LMX1 , to MX1 and L2 M
increases across the series M = Mg < Sr < Ca < Ba.
To overcome this problem, we, and others, have proposed that the application
of sterically demanding monoanionic ligand sets to the kinetic stabilization of
heteroleptic alkaline earth complexes should slow the rate of the redistribution
reaction, allowing subsequent study of the s-bonded substituent in the
heteroleptic complexes. Work by Hanusa has shown that a number of heteroleptic
heavier alkaline earth complexes including halides (McCormick et al. 1989;
Burkey et al. 1994; Harvey & Hanusa 2000), acetylides (Burkey & Hanusa
1996), amides (Sockwell et al. 1992) and a calcium borohydride (Harvey et al.
2000; figure 3; X1 = I, C ≡ CR, NR2 , HBR3 ) may be kinetically stabilized by
the application of bulky cyclopentadienyl ligands allowing their synthesis and
characterization in the solid state. Many of these compounds, however, have
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932
A. G. M. Barrett et al.
t-Bu
O
n
O
O
Cat.
THF
O
O
O
O
O
O
t-Bu
O
O
O
Cat. =
n
N N
1
X – Ca N N BH
N N
t-Bu
1
X = N(SiMe3)2, O(2,6-i-PrC6H3)
Figure 4. Calcium-mediated stereocontrolled polymerization of rac-lactide. Cat. is catalyst.
proven unstable in solution and readily undergo Schlenk-like equilibria, and it
is notable that little or no further reaction chemistry has been forthcoming from
this approach.
Chisholm et al. (2003, 2004) reported the isolation and characterization of the
b-diketiminato stabilized calcium amide 1 and a tris(pyrazolylborate) stabilized
analogue (figures 3 and 4), along with their application to the ring-opening
polymerization of rac-lactide to form heterotactic polylactide.
This paper represented a breakthrough in heteroleptic heavier Group II
chemistry; demonstrating controlled reactivity at an alkaline earth centre
supported by a mono-anionic spectator ligand, it paved the way for the
application of heavier Group II catalysts in small molecule transformations. To
date, the majority of catalytic applications are based upon heteroleptic complexes
of the form LMX1 containing kinetically stabilizing b-diketiminato, triazenide,
aminotropiniminato, bis(imidazolin-2-ylidene-1-yl)borate and tris(imidazolin-2ylidene-1-yl)borate spectator ligands, and reactive s-bonded amide, hydride
or alkyl ligands (X1 = N(SiMe3 )2 , H or benzyl). A number of homoleptic
complexes of the form MX12 , however, have also been reported as pre-catalysts
for heterofunctionalization of unsaturated substrates.
(c) Hydroamination (C–N) bond formation
(i) Intramolecular hydroamination of aminoalkenes
In 2005, the intramolecular hydroamination of a number of aminoalkenes
catalysed by 1 was reported (Crimmin et al. 2005, 2009). Reactions were
shown to proceed in high yield under mild conditions (25–80◦ C, 0.25–132 h),
allowing the synthesis of pyrrolidines, piperidines and hexahydroazepines
from the n-exo-trig (n = 5, 6, 7) cyclization of 1-amino-4-pentenes, 1amino-5-hexenes and 1-amino-6-heptenes, respectively (figure 5). Reactions at
higher temperatures were found to proceed with the Schlenk-like solution
redistribution of 1 to the homoleptic complexes [Ca{N(SiMe3 )2 }2 (THF)2 ] and
[{ArNC(Me)CHC(Me)NAr}2 Ca] (Ar = 2,6-di-iso-propylphenyl). This work set
a precedent for heterofunctionalization catalysis at heavier Group II metal
centres.
As a point of comparison, the authors also synthesized the magnesium precatalyst [{ArNC(Me)CHC(Me)NAr}MgMe(THF)] (2) and studied its reaction
with aminoalkenes. While side-by-side kinetic analysis showed that the calcium
pre-catalyst was more active than the magnesium analogue for the cyclization of
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Review. Group II catalysis
R1
R2
R2 R2
4
2.5–20 mol
1 or 2
n
NHR
n
3
R
C6D6, 25–80°C
0.25–132 h
R1 = H, Me
2
R = H, Me, H/allyl, –(CH2)5–
3
R = H, Me, Ph
n = 1–3
3
Ar
R
NR4
1
R
Me
R2
Me
N
Ca
yield
Me
Ar
Ar
THF
(Me3Si)2N
1
12 examples 60–90
N
Me
Me
N
N
Mg Ar
Me
THF
2
Ar = 2,6-di-iso-propylphenyl
Figure 5. Reaction scope of calcium and magnesium mediated intramolecular hydroamination
of aminoalkenes.
(1-allylcyclohexyl)methylamine (for reactions conducted at an initial aminoalkene
concentration of 0.44 M, turn over frequency (TOF) 1, 146 h−1 ; 2, 48.5 h−1 ),
the synthetic utility of the calcium complex proved more limited. Not only
were reactions of 2,2-disubstituted 1-amino-5-hexene substrates accompanied
by alkene isomerization products, but also the cyclization of 1-amino-2,2diphenyl-6-heptene could not be achieved with this catalyst. Nevertheless,
both 1 and 2 demonstrated activities commensurate with those reported
for the organo(III)lanthanides (Hong & Marks 2004) and in vast excess of
those reported for organozinc complexes (Dochnahl et al. 2006, 2007; Meyer
et al. 2006).
The reaction scope is currently limited to the hydroamination cyclization
of 1◦ and 2◦ aminoalkenes incorporating the olefin at the terminal position.
The reaction is reported to be influenced greatly by the substitution pattern
of the aminoalkene. Typically, geminal disubstituted alkenes gave the fastest
reaction times and could be cyclized using just 2 mol% of 1 or 2–5 mol%
of 2. These substrates benefit from a favourable kinetic effect owing to the
geminal groups decreasing the conformational freedom of the aminoalkene and
favouring reactive conformations (Jung & Piizzi 2005). For a given catalyst,
both loadings and reaction times decreased with the increasing steric demands of
the geminal substituents. Substitution about the C=C bond lengthened reaction
times, and the intramolecular hydroamination of aminoalkenes possessing internal
olefins catalysed by either 1 or 2 is yet to be reported. The inclusion of alkyl
groups on the terminal position of the alkene can be expected to disfavour the
hydroamination reaction owing to the need to form an insertion transition state
with partial tertiary Group II alkyl character. Similar observations have been
made in organolanthanide chemistry, and a number of coordinatively unsaturated
ansa-bridged metallocene and half-sandwich organolanthanide catalysts have
been designed to effect the hydroamination/cyclization of highly substituted
aminoalkenes (Molander & Dowdy 1999). Consistent with Baldwin’s (1976)
guidelines for ring formation, the ease of the catalytic reactions increases
with decreasing ring size (5 > 6 > 7-membered ring closures). In all cases,
more forcing reaction conditions (higher reaction temperatures, higher catalyst
loadings) were reported for the hydroamination of aminoalkenes to 6- and
7-membered rings.
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A. G. M. Barrett et al.
5–20 mol
1 or 2
R
NH2
C6D6, 25°C
R
R = Me
R = Ph
R
A
H
N
H
Me
H
R
Me
Me
C
N
H
H
R
M N
HH R
Me
N
H
Me
N
H
‡
‡
N
M H
‡
H
H
B
N
H
1 (20 mol ), 48 h, 99 , 89 : 11
2 (20 mol ), 48 h, 81 , 92 : 8
1 (5 mol ), 5 h, 87 , 95 : 5
2 (10 mol ), 96 h, 72 , 99 : 1
‡
M N
HH H
R
+
R
R
H
Me
N
H
D
R
N
M H
R
Figure 6. Diastereoselectivity in the intramolecular hydroamination of pro-chiral aminoalkenes.
The cyclization of aminoalkenes possessing two pro-chiral centres potentially
results in the formation of a mixture of diastereoisomeric products. Although
substitution on the b-position of the aminoalkene had little effect upon the
diastereoselectivity of the reaction, a-substituted aminoalkenes underwent a
diastereoselective intramolecular hydroamination cyclization reaction. Thus, the
catalytic reaction of 1-amino-1-phenylpent-4-ene with both 1 and 2 yielded
the corresponding trans-pyrrolidine in a diastereoisomeric excess of 90 and
98 per cent, respectively. A similar reaction of 2-aminohex-5-ene catalysed
by 20 mol% 1 was reported to proceed to give the trans-pyrrolidine in
78 per cent diastereoisomeric excess (figure 6). Similar selectivies have been
observed in the organolanthanide series, and reaction of 2-aminohex-5-ene with
[{(C5 Me4 )2 SiMe2 }La{CH(SiMe3 )2 }] has been reported to yield a 8 : 1 mixture of
trans : cis heterocyclic products at 0◦ C (Gagne et al. 1992).
The observed diastereoselectivity is most readily explained by consideration of
the energetically dissimilar diastereoisomeric transition states to carbon–nitrogen
bond formation in the insertion step. Of the four 7-membered diastereoisomeric
transition states that can be envisaged for the carbon–nitrogen-bonding forming
step, transition states A and D (figure 6), leading to the cis-pyrrolidine, possess
a potentially destabilising 1,3-diaxial steric interaction. This latter conformation
may raise the activation energy to carbon–nitrogen bond formation and, hence,
favour the trans-diastereoisomer via transition states B and C. In this regard,
it appears that substitution at the b-position of the aminoalkene does not
suffer from similar unfavourable non-bonding interactions and does not affect
the diastereoselectivity of the reaction. By analogy to the organo(III)lanthanide
system, the improved diastereoselectivity in the case of the magnesium catalyst
can be attributed to the shorter M–N and M–C bond lengths in magnesiumcontaining relative to calcium-containing complexes tightening the transition
state of the insertion reaction and therefore increasing any effects exerted by
non-bonding interactions.
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Review. Group II catalysis
(a)
(b)
Me
Ar
Me
N
N
C 6 D6
Me
Ar + R 2 NH
Ca
(Me 3 Si) 2 N THF
Me
THF Ar
R2
N
N
Ca
N
N
R2
Ar THF Ar
Ar
N
Ca
N
Me
+ HN(SiMe 3 )2
N1
Me
Ca
N2
Me
Ar
Me
N
N
C 6 D6
Me
Ar + R 2 NH
30 min, 25° C
Me
n/s Bu
Mg
Ar
N
Mg
N
Ar
Ar
R2
N
N
Mg
N
N
R2
Ar
Me
N3
O
+
n/s BuH
Me
Figure 7. (a) Reaction of pre-catalysts 1 and 2 with primary amines. (b) Oak Ridge Thermal
Ellipsoid Plot (ORTEP) representation of [{ArNC(Me)CHC(Me)NAr}Ca(h2 -NH(CH2 )2 OMe)]2
(Ar = 2,6-di-iso-propylphenyl). H atoms are omitted for clarity; thermal ellipsoids at 20%
probability.
While the stoichiometric protonolysis reaction of an analogue of 2,
[{ArNC(Me)CHC(Me)NAr}Mgn/s Bu], with benzylamine, 2-methoxyethylamine
and pyrrolidine to yield the corresponding dimeric magnesium amides and
methane has been reported (Barrett et al. 2009b) to proceed rapidly and nonreversibly at room temperature, the reaction of 1 with benzylamine forms a
quantifiable equilibrium between monomeric bis(trimethylsilyl)amide and dimeric
benzylamide reaction products (figure 7). A van’t Hoff analysis of this equilbrium
mixture allowed the derivation of DG◦ (298 K) as 11.3 kJ mol−1 , consistent with
facile pre-catalyst activation via protonolysis with a primary amine (Barrett
et al. 2008d). Further reactions of 1 with 2-methoxyethylamine and 2,6-di-isopropylaniline demonstrated that the equilibrium could be effectively perturbed
to the reaction products in the presence of chelating or more acidic substrates.
(Avent et al. 2004, 2005b; figure 7). These studies, along with a report by Harder
and coworkers upon the addition of ammonia to 1, have demonstrated that
b-diketiminato calcium 1◦ amide complexes undergo facile external amine/amide
exchange along with intramolecular site exchange between amide and amine
ligands (Ruspic & Harder 2007; Crimmin et al. 2009).
Although products of the intramolecular insertion of alkenes into M–N bonds
have not been directly observed, a deuterium labelling experiment has implied
their formation. Thus, the cyclization of (1-allylcyclohexyl)methylamine-d2 with
either 1 or 2 has been reported to yield the corresponding hydroamination product
with deuterium incorporation upon the carbon framework solely at the exo-cyclic
methyl group. Crimmin et al. proposed that the two pre-catalysts 1 and 2 give
rise to two distinct catalytic systems proceeding via non-reversible and reversible
catalyst initiation, respectively (figure 8). Data were reported that are consistent
with rate-determining insertion of the alkene into the metal–nitrogen bond of
a coordinatively unsaturated monomeric metal amido reaction intermediate.
Kinetic studies upon the magnesium pre-catalyst allowed the derivation of the
rate law y ≈ kd k3 [Cat]o /ka [Sub]o + k3 , where kd is a dissociation rate constant of
ligand dissociation from a coordinatively saturated catalyst resting state, ka is an
association rate constant of a ligand with the coordinatively unsaturated resting
state (i.e. the reverse of kd ), k3 is the rate constant of alkene insertion into a
coordinatively unsaturated resting state, Cat is catalyst and Sub is substrate.
In line with the findings of Hultzsch and coworkers for lanthanide(III) systems
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936
A. G. M. Barrett et al.
(a)
(b)
s-bond
metathesis
N
H
H2N
Ar
N
M
1
X
Ar
THF
H2N
HX1
N
Ar N M Ar
Sn NH
Ar
N Sn–1
M
N HN
Ar
N
H
N
HX1
s-bond
metathesis
ligand
dissociation
‡
rate determining insertion of
M = Mg; X1 = Me
alkene into M–N bond
S = THF, substrate, product
Ar = 2,6-di-iso-propylphenyl
N
X
Ar
M = Ca; X1 = N(SiMe3)2
S = THF, substrate, product
Ar = 2,6-di-iso-propylphenyl
N
Ar
THF
H2N
HX1
NM N
Sn
Ar
N Sn–1
M
N HN
Ar
M
1
N
H
H2N
Ar
N Sn–1
M
N HN
Ar
Ar
Ar
NH
ligand
dissociation
Ar
N Sn–1
M
N HN
Ar
rate determining insertion of
alkene into M–N bond
Figure 8. Proposed mechanism of Group II mediated hydroamination of aminoalkenes with
(a) non-reversible and (b) reversible pre-catalyst generation.
(Gribkov et al. 2006), it was suggested that both substrate and product inhibit
the reaction to similar extents; hence, reaction-rate constants are dependent upon
the substrate concentration at to .
While Crimmin et al. refrained from application of heavier analogues of 1 and
2 (M = Sr, Ba; Avent et al. 2005b) in hydroamination catalysis owing to their
propensity to undergo Schlenk-like redistribution under the reaction conditions,
Datta et al. (2007, 2008a,b) reported aminotroponoate and aminotroponiminate
supported calcium and strontium amide complexes 3–5 as competent catalysts
for the intramolecular hydroamination of aminoalkenes (figure 9). Although, in
most instances, selectivities and catalyst activities were commensurate with those
reported for the b-diketiminato-stabilized calcium amide 1, it is noteworthy that
Datta et al. reported not only that 4 mol% 4 was effective for the intramolecular
hydroamination of an aminoalkyne to the corresponding imine in more than
90 per cent after 22 h at room temperature, but also that both 4 and 5 affect
the cyclization of the internal (activated) alkene trans-1-amino-2,2-dimethyl-5phenyl-4-pentene to the corresponding pyrrolidine in 80 per cent yield after 10 min
at room temperature (figure 10). Despite side-by-side experiments suggesting that
the strontium compound 5 is less active than the calcium analogue 4, the kinetic
stability of the Group II catalytic intermediates with respect to Schlenk-like
solution redistribution was not discussed.
Based upon initial studies upon the coordination chemistry of N-heterocyclic
carbenes to Group II metal centres (Barrett et al. 2008e; Arrowsmith et al. 2009c)
and inspired by Chisholm’s et al. (2003, 2004) application of tripodal ligand
sets in Group II lactide polymerization catalysis, Arrowsmith et al. (2009a,b)
have reported the application of calcium and strontium-based bis(imidazolin2-ylidene-1-yl)borate complexes 6–7 and calcium, strontium and barium
tris(imidazolin-2-ylidene-1-yl)borate complexes 8–10 to the intramolecular
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937
Review. Group II catalysis
i-Pr
THF
N
Ca
O
THF
i-Pr
N(SiMe3)2
N
N(SiMe3)2
M
(Me3Si)2N Ca
i-Pr
N
THF
i-Pr
N
N
H
N
N(SiMe3)2
H
B
HB
THFn
N
t-Bu
N
N(SiMe3)2
M
N
N
N
N
N
N
t-Bu
t-Bu
N
i-Pr
M
THF
(Me3Si)2N
n
t-Bu
M = Ca, n = 1, 6
M = Sr, n = 2, 7
i-Pr
i-Pr
t-Bu
N
M
N
M = Ca, 4
M = Sr, 5
3
t-Bu
N
THF
N
O
M = Ca, n = 0, 8
M = Sr, n = 1, 9
M = Ba, n = 1.5, 10
i-Pr
M = Ca, n = 2, 11
M = Sr, n = 3, 12
Figure 9. Aminotroponate, aminotropiniminate, bis(imidazolin-2-ylidene-1-yl)borate, tris (imidazolin-2-ylidene-1-yl)borate and triazenide supported Group II pre-catalysts for intramolecular
hydroamination.
R1
R3
R3 R3
NH2
n
2.5–20 mol
n
C6D6, 25–80°C
2
R
R3
3–9 or 11–12
R
0.1–144 h
1
R2
N
Me H
R1 = H, Me; R2 = Me, H, Ph; R2 = H;
R3 = H, Me, Ph, –(CH2)5–; n = 1–3
Me
Me
Me
Me
NH2
Ph
10 mol
4 or 5
Bn
C6D6, 25°C
N
H
90
yield
0.15 h
Me
Ph
Me
Me
Me
NH2
10 mol
4
Bn
C6D6, 25°C
Bn = benzyl
22 h
N
90
yield
Figure 10. Scope of hydroamination catalysis with complexes 3–9 and 11–12.
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938
A. G. M. Barrett et al.
hydroamination of aminoalkenes. Although in all cases, reactions proceeded with
no evidence for the Schlenk-like redistribution of the pre-catalysts or reaction
intermediates, these complexes have been reported to be rather thermally labile
and susceptible to B–N bond cleavage. Indeed, while bis(imidazolin-2-ylidene1-yl)borate complexes proved robust under catalytic conditions, their tripodal
analogues proved fragile with ligand cleavage reactions occurring not only during
their synthesis, resulting in their isolation as imidazole solvates, but also under
protic reaction conditions at temperatures above 50◦ C. The current scope of
hydroamination catalysis with these complexes is limited to 1◦ aminoalkenes
incorporating terminal olefins (figure 10). Although the poor performance of
tris(imidazolin-2-ylidene-1-yl)borate Group II bis(trimethylsilyl)amide complexes
8–10 in hydroamination catalysis limited the reaction scope to the cyclization
of 1-amino-2,2-diphenyl-4-pentene and (1-allylcyclohexyl)methylamine, it has
been reported that bis(imidazolin-2-ylidene-1-yl)borate Group II complexes 6–7
effect the cyclization of a number of substrates, allowing the formation of
5-, 6- and 7-membered heterocycles. It is noteworthy that the b-diketiminato
calcium amide 1 was reported to be ineffective for the cyclization of 1-amino-2,2diphenyl-6-heptene to the corresponding 7-membered heterocycle. In all cases,
strontium-based catalysts were reported to be more active than the lighter
calcium analogues. The barium complex 10 proved ineffective for hydroamination
catalysis of aminoalkenes.
Barrett et al. (2008a) have also reported the application of calcium and
strontium amide catalysts supported by sterically demanding triazenide ligand
sets 11–12 to the intramolecular hydroamination of 1-amino-2,2-diphenyl-4pentene. While the pre-catalysts were shown to be highly sensitive to Schlenk
equilibrium in solution, in this instance, kinetic experiments were conducted and,
in contrast to the findings of Arrowsmith et al. but consistent with Datta et al.,
the calcium species 11 provided higher turnover frequencies than the strontium
analogue 12 (for reactions conducted at an initial aminoalkene concentration of
0.4 M, TOF 11, 500 h−1 ; 12, 75 h−1 ).
The discrepancies in the reported activities of calcium and strontium-based
catalysts are indicative of the difficulties in studying heavier Group II reaction
chemistry resulting from facile exchange of both neutral and mono-anionic
ligand sets (Schlenk equilibration). To further complicate this issue, current
reports of Group II hydroamination catalysis are plagued by discussion of
‘rates’ of reaction based upon reaction times and conditions; unless discussion is
supported by reaction-rate constants, turnover frequencies derived from kinetic
data or reaction half-lives at known concentrations of the reactants, it is
likely to be more misleading than informative. The current observed decrease
in reactivity across the series Ca/Sr > Mg/Ba, however, is consistent with
density functional theory studies upon intermolecular hydroamination catalysis
(vide infra).
While the different coordination environment at the metal centres of
1–12 undoubtedly influence reactivity, the development of an ‘intrinsic’
reaction scale through interpretation of experimental data has, to date, been
complicated by the facile exchange processes at Group II metal centres
(figure 11). In this regard, it is noteworthy that initial attempts to achieve
the stereocontrolled intramolecular hydroamination of 1-amino-2,2-diphenyl4-pentene by use of a chiral calcium amide catalyst 13, based upon the
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Review. Group II catalysis
Ph
Ph
Ph Ph
NH2
mol
1–12
C6D6, 25°C
time
Me
time (h)
N
H
catalyst
mol ( )
yield ( )
1
2
0.25
99
2
2
2
99
3
5
1
99
4
2
1
99
5
3
1.5
99
6
10
2
>95
7
5
1
>95
8
5
96 (45° C)
85
9
5
1.8
97
10
5
168 (50° C)
5
11
2
0.25
99
12
2
1
99
Figure 11. Side-by-side comparison of reaction conditions for the intramolecular cyclization of
1-amino-2,2-diphenyl-4-pentene.
(S)-Ph-pybox ligand, have provided non-racemic products, but in extremely
low enantioselectivity (less than 10% enantiomeric excess (e.e.)). In this
study, it was proposed that the pre-catalyst L∗ MX1 (X1 = N(SiMe3 )2 ) is
unstable with respect to Schlenk-like solution equilibria with the formation
of homoleptic calcium complexes (L∗ )2 M and M(X1 )2 . Furthermore, the
latter bis(trimethylsilyl) amide complex [Ca{N(SiMe3 )2 }2 (THF)2 ] is proposed
to be catalytically active, giving rise to an achiral background reaction
(Buch & Harder 2008).
(ii) Intermolecular hydroamination of alkenes
Barrett et al. (2009a) reported a combined computational and experimental
study upon the addition of amines to alkenes catalysed by coordination complexes
of the heavier alkaline earths. The reaction of benzylamine, pyrrolidine and
piperidene with styrene, 4-methylstyrene, 4-methoxystyrene and 4-chlorostyrene
catalysed by 5 mol% [M{N(SiMe3 )2 }2 ]2 (M = Ca, Sr) was reported to proceed
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940
A. G. M. Barrett et al.
5 mol
[M{N(Sime3)2}2]2
+
NR1R2
R1R2NH
X
60°C, neat, 1–144 h
M = Ca, Sr
X
5 examples 63–92
X = Cl, H, Me, OMe
R1 = R2 = –(CH2)4–, –(CH2)5–; R = H1, R2 = Bn
conversion
Figure 12. Intermolecular hydroamination mediated by Group II complexes.
Me
‡
R2
Ar
R1
N
N
2,1-insertion
N
Me
Me
Ph
Ar
R2 R1
N
N
Ca
N
Me
Ar
Me
CH
Ar
Ph
‡
1,2-insertion
R2
Ar
Me
N
CHPh
Ca
N
Me
R1
N
CHPh
C
H2
CH2
Ca
N
CH
Ar
R1
N
N
CH2
Ca
R2
Ar
Me
Ar
C
H2
not observed
Figure 13. Proposed transition states to C–N bond formation in the anti-Markovnikov
hydroamination of styrenes.
under mild solvent-free reaction conditions at 60◦ C (figure 12). While the
products were readily isolable following the reaction, current data upon reaction
yields are reported for in situ conversions only.
In all cases, reactions are reported to proceed with an anti-Markovnikov or
2,1-addition of the amine to the alkene. Similar observations have been made
in organo(III)lanthanide catalysis and the preferential anti-Markovnikov or 2,1addition to styrene can be attributed to the organization of the transition state
to N–C bond formation. Factors that stabilize the developing anionic charge
upon the atom adjacent to the metal centre in the transition state to N–C
bond formation will be expected to lower the activation energy of the insertion
step. In the case of the 2,1-insertion of styrene into the Ca–N bond, the phenyl
group may stabilize the adjacent anionic centre. In the case of a 1,2-insertion,
no such stabilization exists (figure 13). Despite the strontium-based catalyst
demonstrating superior reactivity (lower reaction times and higher reaction
yields) to the calcium analogue, the magnesium and barium analogues of these
catalysts (M = Mg, Ba) effected the hydroamination of styrene in very low yields.
Density functional theory calculations of the model reaction of ethylene with
ammonia catalysed by the model complexes [{HNC(Me)CHC(Me)NH}M(NH2 )]
in the gas phase using B3LYP theory employing the LANL2DZ basis set provided
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Review. Group II catalysis
N
M
N
Gibbs free energy (kJ mol–1)
100
‡
H2
C
H
CH2
N
N
N
H2
90
alkene insertion TS
60
N
M
H2N
80
70
H
‡
H
CH2
CH2
protonolysis TS
atom numbering
C1
N M
C2
N
N
50
40
30
N M
N
20
N
H2
10
N M
NH2
N
0
+NH3 + C2H4
–10
–20
H2
C
N
N M
CH2
NH3
N M
CH2
N
N
H
H2N
H
reaction coordinate
CH2
N M
NH2
N
+CH3CH2NH2
Figure 14. Free energy profiles for the reaction of ethylene with ammonia catalysed by
[{HNC(Me)CHC(Me)NH}M(NH2 )]. Dot, Mg; filled circle, Ca; filled triangle, Sr; filled square, Ba.
TS is transition state.
(a)
(b)
Figure 15. Electron density difference maps for the alkene insertion (a) transition state and (b)
intermediate for the Mg-mediated cycle. Density difference is relative to a pro-molecule composed
of spherical atoms, solid lines indicate areas of build up in electron density, and dotted lines indicate
areas of depletion. Slice taken in the plane of atoms C1 –C2 –N3 and contours at intervals of 0.003.
considerable insight into the observed reactivity (figures 14 and 15). In line with
experimental findings upon the intramolecular hydroamination of aminoalkenes,
results were consistent with rate-determining alkene insertion within a catalytic
cycle dominated by Coulomb interactions. Alkene insertion into the M–N bond
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942
A. G. M. Barrett et al.
was reported to occur via a four-centre transition state that is highly polarized;
electron density equivalent to almost an entire electron is polarized onto the
carbon adjacent to the metal and occurs with simultaneous depletion of electron
density in the nitrogen-based lobe, with electron density being directed to
the newly forming carbon–nitrogen bond. Examination of the barrier heights
of insertion (M = Mg, 87.9 kJ mol−1 ; Ca, 69.9 kJ mol−1 ; Sr, 64.9 kJ mol−1 ; Ba,
77.8 kJ mol−1 ) and protonolysis (M = Mg, 82.8 kJ mol−1 ; Ca, 36.0 kJ mol−1 ) steps
revealed a series of data consistent with the experimental observations. Barrett
et al. suggest that the relative barrier heights for alkene insertion into a M–N
bonds may be viewed as a result of a balance of the polarity of the M–N bond
(i.e. the ability of the M–N bond to induce a dipole in the non-polarized alkene)
and the polarizability of the M2+ cation.
Eyring analysis of kinetic reaction data upon the addition of piperidene
to styrene catalysed by [M{N(SiMe3 )2 }2 ]2 (M = Ca, Sr) provided activation
energies of DG‡ = 100.8 and 97.9 kJ mol−1 for calcium and strontium, respectively.
Although these data are overestimated somewhat compared with those calculated
on the model system, it was suggested that the strontium and calcium reagents
provide the ideal balance of polarization and polarizability that allow facile
insertion reactions with alkene substrates. Furthermore, the increased activity
of the strontium catalyst was attributed to an influential entropic advantage,
with the calcium catalyst providing data consistent with a much tighter and
more ordered transition state in the insertion step than the strontium system
(DS‡ = −167.8 J mol−1 K−1 and −22.0 J mol−1 K−1 for calcium and strontium,
respectively).
With regard to the free-energy diagram presented in figure 14, it is noteworthy
that, while the metal alkyl intermediates have been implied by deuterium labelling
studies upon the intramolecular hydroamination of alkenes, recent studies by
Wiecko et al. (2008) and Schumann et al. (2004) have provided experimental
evidence for the formation of Group II alkene adducts. In both these studies,
metallocene complexes of the heavier alkaline earth metals bearing pendent
alkenyl groups (tethered to the cyclopentadienyl ring) were shown to demonstrate
metal coordination to the p-system of unsaturated carbon–carbon bonds in
solid state. The importance of crystal-packing effects in these latter interactions,
however, cannot be underestimated.
(iii) Intermolecular hydroamination of carbodiimides
In contrast to the unstable reaction intermediates implied in the intraand intermolecular hydroamination of alkenes (vide supra), products derived
from the insertion of carbodiimides into heavier alkaline earth metal–nitrogen
bonds, i.e. Group II guanidinates, are kinetically and thermodynamically stable
and readily isolable. Several stoichiometric reaction studies have documented
these insertion products. Feil & Harder (2005) reported the insertion of 1,3dicyclohexyl carbodiimide into the calcium amide bonds of [Ca{N(SiMe3 )2 }2 ].
Based upon this finding, Barrett et al. (2008c) reported that a number of
analogues of 1, in which the bis(trimethylsilylamide) ligand had been replaced
by a less basic amide ligand, readily react with dialkyl carbodiimides to
yield calcium complexes containing both b-diketiminate and guanidinate ligands
(figure 16).
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Review. Group II catalysis
R2NH
Ar
N
(Me3Si)2N
N
Ca
Ar
–HN(SiMe3)2
R1N=C=NR1
Ar
THF
N
N
Ca
R2N
Ar
C6D6
25°C, 0.1–14 h
THF
Ar = 2,6-di-iso-propylphenyl
R2N = NH(CH2)2OMe, NPh2, NHAr
>95
yield by NMR
Ar
N
N
Ar
THF
NR1
Ca
R1–N
R2N
R1 = Cy, i-Pr
Figure 16. Insertion of 1,3-dialkyl carbodiimides into the Ca–N bonds of derivatives of 1. NMR is
nuclear magnetic resonance.
ArNH2 + R1N=C=NR2
2–4 mol
1 or [M{N(SiMe3)2}2(THF)2]
0.1–72 h, hexane
H
R1HN
N
Ar
1,3-proton
shift
NR2
25–80°C
N
R1HN
Ar
NHR2
12 examples 37–81 yield
Ar = 2-FC6H4, 4-MeC6H4, Ph, 2-MeOC6H4, 1-napthyl, 2,6-i-PrC6H3
R1 = R2 = i-Pr, Cy, t-Bu; R1 = Et, R2 = t-Bu
M = Ca, Sr, Ba
Figure 17. Scope of Group II mediated hydroamination of carbodiimides.
While in all cases, the insertion reaction products in the aforementioned studies
were characterized by multi-nuclear nuclear magnetic resonance (NMR) and
single-crystal X-ray diffraction, application of this stoichiometric reactivity in the
Group II catalysed hydroamination of carbodiimides was reported by Lachs et al.
(2008). The reaction of electron-deficient and electron-rich primary arylamines
with 1,3-dialkyl carbodiimides was reported to proceed rapidly at room
temperature catalysed by 2–4 mol% [M{N(SiMe3 )2 }2 (THF)2 ] (M = Ca, Sr, Ba)
to yield the corresponding guanidines, [ArNC(NHR)2 ] (figure 17). The reaction
scope included sterically encumbered amines (e.g. 2,6-di-iso-propylaniline) and
carbodiimides (e.g. 1,3-di-tert-butyl carbodiimide) and, in most cases, the
reaction products were reported to crystallize directly from preparations in
hexane solvent (0.15 M concentration of substrates), allowing their isolation
by simple filtration. Characterization of the hydroamination products by
multinuclear NMR spectroscopy and, in one instance, single-crystal X-ray
diffraction revealed that the guanidines form with a concomitant 1,3-proton shift
(figure 17). Lachs et al. explored the limit of this catalysis and, by conducting
the reaction of aniline with 1,3-dicyclohexyl carbodiimide in hexane on a
60 mmol scale, demonstrated that catalysis shutdown below a 0.5 mol% loading
of [Ca{N(SiMe3 )2 }2 (THF)2 ]. In this instance, the product was isolated in 86
per cent yield.
Through a series of stoichiometric reactions, Lachs et al. suggested that the
calcium-mediated hydroamination of carbodiimides proceeds via a catalytic cycle
incorporating a dimeric calcium guanidinate intermediate. It was reported that
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944
(a)
M(X1)2
A. G. M. Barrett et al.
R1N=C=NR1
(b)
insertion
R2N
initiation
N
R1
[M(NHAr)2]n
protonolysis
N Ar
R1HN
R1
N
Ar
N
M
NR2
N
Ar observed
catalyst
resting state
ArNH2
NHR2
C8
C11
N3 H20
N2
Ca
N1
C1
N4 N6
C20
N5 C21
C24 H50
13
Figure 18. (a) Proposed mechanism of the Group II mediated hydroamination of carbodiimides
and (b) ORTEP representation of 13 thermal ellipsoids at 40% probability; H atoms are omitted
for clarity.
the reaction of [Ca{N(SiMe3 )2 }2 (THF)2 ] with either two equivalents of aniline
and two equivalents of 1,3-di-iso-propyl carbodiimide or two equivalents of
[PhNC(NHi Pr)2 ] yielded a compound that gave identical spectroscopic data in
benzene-d6 solution to that formed from the addition of four equivalents of
[PhNC(NHi Pr)2 ] to [Ca{N(SiMe3 )}2 ]2 . Complex 13 was isolated from this latter
reaction following crystallization from hexane solution and was characterized
by variable temperature NMR spectroscopy, along with single-crystal X-ray
diffraction (figure 18). While site exchange between the terminal and bridging
ligands of 13 (DG‡ = 67.8 kJ mol−1 ) was observed in d8 -toluene solution, this latter
dimeric complex proved kinetically competent for the catalytic hydroamination
of 1,3-di-iso-propyl carbodiimide with aniline. Based upon this reactivity, the
stoichiometric insertion studies and by analogy to the hydroamination of
aminoalkenes, the authors proposed that the reaction proceeds via a catalytic
cycle constructed with insertion and protonolysis steps (figure 18).
(iv) Intermolecular hydroamination of isocyanates
In 2008, Barrett et al. reported that the hydroamination of 2,6-di-isopropylphenyl isocyante and 1-adamantyl isocyanate with diphenylamine could be
catalysed by 5–6 mol% 1 or [M{N(SiMe3 )2 }2 ]2 (M = Ca, Sr and Ba) in benzene
or toluene solution. Although the aryl-substituted isocyanate was reported to
undergo hydroamination at room temperature, the alkyl-substituted analogue
required slightly more forcing conditions (60◦ C). In both cases, the urea products
were readily isolable by crystallization from hydrocarbon solution and isolated
in 48–93% yield. The reported reaction scope is currently very restricted, being
limited to these two preparations (figure 19). The catalytic reaction, however,
does offer distinct advantages over the uncatalysed variant, and a background
experiment demonstrated no reaction between diphenylamine and 1-adamantyl
isocyanate over a period of four weeks at room temperature or 16 h at 80◦ C.
Kinetic analysis upon this latter reaction catalysed by 5 mol% [Ca{N(SiMe3 )2 }2 ]2
at room temperature demonstrated that both starting materials were consumed
at the same rate, indicative of a clean heterofunctionalization of the isocyanate
without deleterious (polymerization) side reactions.
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Review. Group II catalysis
5 mol 1
or [M{N(SiMe3)2}2]2
Ph2NH + AdN=C=O
O
Ad
C6D6, 2 h, 25°C
5 mol 1
or [M{N(SiMe3)2}2]2
Ph2NH + ArN=C=O
NPh2
N
H
O
Ad
C6D6, 2 h, 25°C
NPh2
N
H
Ar = 2,6-di-iso-propylphenyl; Ad = 1-adamantyl
M = Ca, Sr, Ba
Figure 19. Group II catalysed addition of diphenylamine to isocyanates.
(a)
(b)
R1N=C=O
insertion
O
L M
1
LM(X )
initiation
[LM(NPh2)]
protonolysis
N1
R
NPh2
O2
O1
N3
observed
catalyst
resting state
N1
C1
Ca
N2
N4
Ph2NH
O
R1HN
NPh2
14
Figure 20. (a) Proposed mechanism of the Group II catalysed addition of diphenylamine to
isocyanates. (b) ORTEP representation of 14 thermal ellipsoids at 20% probability; H atoms are
omitted for clarity.
In line with this observation, Barrett et al. demonstrated that the
controlled insertion of 1-adamantyl isocyanate into the metal–nitrogen bond
of [{ArNC(Me)CHC(Me)NAr}Ca(NPh2 )(THF)] occurs upon reaction in a 1 : 1
stoichiometry in hydrocarbon solution at room temperature. The resulting
ureido complex 14 was isolated in an unoptimized yield of 36 per cent,
crystallographically charaterized, and proven to be catalytically competent for
the hydroamination of 2,6-di-iso-propylphenyl isocyanate with diphenylamine
(figure 20). In contrast to a report by Crimmin et al. (2009) documenting the
intramolecular hydroamination of aminoalkenes, hydroamination reactions of
isocyanates with diphenylamine were reported to yield kinetic data consistent
with significant product inhibition of catalysis. Barrett et al. ascribe the observed
product inhibition to the urea product effectively binding Group II metal centres
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A. G. M. Barrett et al.
+ HPPh2
Ph
10 mol
1 or 15
Ph
PPh2
C6D6, 20 h, 75°C
95
Me
10 mol
1
PPh2
+ HPPh2
C6D6, 24 h, 25°C
95
10 mol
PPh2
Me
+
Me
Me
79 : 21
1
PPh2
+ HPPh2
C6D6, 24 h, 75°C
78
Ph
Ph
20 mol
PPh2
1
+ HPPh2
Ph
PPh2
+
C6D6, 13 h, 75°C
94
Ph
Ph
98 : 2
Ph
Figure 21. Scope of alkene and alkyne hydrophosphination mediated by 1.
and preventing substrate coordination and activation. Initial attempts to quantify
the effect of the metal centre upon the rate of reaction were reported to
be complicated by the insolubility of intermediate barium ureido species in
hydrocarbon solvents. A cross-over reaction conducted in C6 D6 between one
equivalent of the urea derived from 1-adamantyl isocyanate and one equivalent
of 2,6-di-iso-propylphenyl isocyanate catalysed by 5 mol% [Ca{N(SiMe3 )2 }2 ]2
demonstrated the potential reversibility in the insertion step. Thus, it was
proposed that catalytic turnover occurs via reversible s-bond metathesis and
insertion reaction steps (figure 20a).
(d) Hydrophosphination (C–P) bond formation
(i) Intermolecular hydrophosphination of alkenes, alkynes and dienes
In 2007, Hill and coworkers reported the catalytic hydrophosphination of a
series of unhindered activated alkenes and an alkyne with diphenylphosphine
mediated by 10–20 mol% of 1 (figure 21). The structures of the isolated products
were consistent with an anti-Markovnikov, syn-addition of the P–H bond across
the least hindered, unsaturated C–C bond of the substrate. The reaction proved
to be highly dependent upon the steric demands of the alkene and more
hindered substrates, such as a-methylstyrene, 1,2-diphenylethene and transstilbene, did not readily undergo hydrophosphination under these reaction
conditions. Similar observations have been made in the hydrosilylation of
styrenes with calcium benzyl complexes, with a-methylstyrene being less reactive
towards hydrosilylation than styrene itself (vide infra). Despite the precedent in
lanthanide(III) chemistry for the intermediate species to initiate polymerization
reactions (Kawaoka & Marks 2005), oligomeric or polymeric reaction by-products
were not reported.
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947
Review. Group II catalysis
Me
‡
R2
Ar
R1
2,1-insertion
P
N
N
Me
Me
R2 R1
P
N
Ca
N
Me
Ar
Me
CH
Ar
Ph
Ar
Ph
‡
1,2-insertion
R2
Ar
Me
N
CHPh
Ca
N
Me
R1
P
CHPh
C
H2
CH2
Ca
N
CH
Ar
R1
P
N
CH2
Ca
R2
Ar
Me
Ar
C
H2
not observed
Figure 22. Explanation for the observed regiochemistry in the Group II mediated hydrophosphination of styrenes.
The observed regio- and stereo-selectivity were consistent with the precedent
set by not only organo(III)lanthanide (Hong & Marks 2004), but also
calcium-mediated intermolecular hydroamination (§2c). As in the case of the
hydroamination of styrenes, the preferential anti-Markovnikov or 2,1-addition
of diphenylphosphine to styrene can be attributed to the organization of the
transition state to P–C bond formation. Factors that stabilize the developing
anionic charge upon the atom adjacent to the metal centre in the transition state
to P–C bond formation will be expected to lower the activation energy of the
insertion step. In the case of the 2,1-insertion of styrene into the Ca–P bond, the
phenyl group may stabilize the adjacent anionic centre (figure 22). In the case of
a 1,2-insertion, no such stabilization exists. Furthermore, the almost exclusive
syn-addition of diphenylphosphine to diphenylacetylene can be attributed to
a concerted insertion of the alkyne into the Ca–P s-bond of an intermediate
phosphide complex.
The stoichiometric reaction between 1 and diphenylphosphine was conducted
on an NMR scale, and monitoring of the reaction by 1 H and 31 P NMR
spectroscopy revealed the consumption of diphenylphosphine with concurrent
production of HN(SiMe3 )2 and a single new product peak at −21.3 ppm in the 31 P
NMR spectrum owing to the formation of heteroleptic b-diketiminate-stabilized
phosphide 15. The half-life of this reaction under pseudo-first-order conditions
(15 equivalent HPPh2 ) was reported as approximately 200 min. To eliminate the
possibility that the slow s-bond metathesis step was due to steric factors, the
authors conducted a competition experiment between 1 and a 1 : 1 mixture of
HNPh2 and HPPh2 . After 30 min at room temperature, exclusive formation of the
calcium diphenylamide in preference to 15 was observed, despite the fact diphenylphosphine has a lower pKa than diphenylamine (Li et al. 2006). This disparity
in the rate of the reaction of 1 with diphenylphosphine compared with diphenylamine was attributed to the fact that a coordination of the substrate to the metal
is required for the s-bond metathesis (protonolysis) step to occur, and that the
soft phosphine is a poorer ligand than the amine for the hard calcium centre.
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A. G. M. Barrett et al.
protonolysis
(a)
Ar
THF
N
Ca
N
N(SiMe3)2
Ar
1
Ph
HN(SiMe3)2
P1
Ar
H
Ph PPh2
d31P = 8.9
(b)
31
HPPh2 d P = –40.1
Ar = 2,6-di-iso-propylphenyl
C36
C30
THF
N
Ca
d31P = –21.3
PPh2
N
Ar 15
Ca1
N2
O1
Ph
N1
Ph
HN(SiMe3)2
Ar Ph Ph
P
N
Ca C Ph
N
C
Ar
Ph
insertion
15
Ph Ph
P
Ph
C
Ca
N Ar C
Ph
Ar
N
via:
Figure 23. (a) Proposed mechanism of the hydrophosphination of diphenylacetylene catalysed by
1 and (b) ORTEP representation of 15 thermal ellipsoids at 20% probability; H atoms are omitted
for clarity.
Support for the postulated mechanism of intermolecular hydrophosphination
was provided by the reactivity of 15. The reaction of diphenylphosphine with
styrene could be catalysed by 10 mol% 15, demonstrating that the latter is
kinetically active in the proposed catalytic cycle. More importantly, the reaction
of 15 with an excess of diphenylacetylene and a single equivalent of HN(SiMe3 )2
in C6 D6 at 75◦ C for 45 min gave a 1 : 1 mixture of (E)-PhC(H) = C(PPh2 )Ph,
observed at 8.9 ppm in the 31 P NMR, and 1 in near quantitative yield. This latter
experiment can be explained by considering the concerted insertion of the alkyne
into the Ca–P bond of 15 to generate a highly unstable calcium vinyl intermediate
that then undergoes a subsequent s-bond metathesis with hexamethyldisilazane
to liberate the hydrophosphinated product and regenerate 1 (figure 23). Thus, it
was proposed that the reaction proceeds via (i) catalyst initiation by the reaction
of 1 with diphenylphosphine to form the analogous phosphide, (ii) the concerted
insertion of the unsaturated carbon–carbon bond into the Ca–P sigma bond,
and (iii) a facile s-bond metathesis (protonolysis) reaction of the calcium alkyl
intermediate.
Hill and coworkers attempts to catalyse the hydrophosphination of alkenes with
[Ca{N(SiMe3 )2 }2 (THF)2 ] in benzene solution met with limited success, and the
reaction was reported to be accompanied by the precipitation of a small amount
of a yellow insoluble solid, which was later confirmed to be [Ca(PPh2 )2 (THF)4 ] by
independent synthesis (Crimmin et al. 2007a; Gartner et al. 2007a). Despite this
observation, Westerhausen and coworkers have reported that [Ca(PPh2 )2 (THF)4 ]
in THF is an excellent catalyst for the hydrophosphination of diphenylacetylene
and 1,4-diphenylbutadiene with diphenylphosphine (Al-Shboul et al. 2008).
Reactions proceeded to high yield in less than 2 h at room temperature
with 5–6 mol% catalyst loading and, in the instance of the latter substrate,
the product was trans,trans-1,4-diphenyl-1,4-bis(diphenylphosphanyl)buta-1,3diene (formed from addition of two molecules of phosphine to the diyne;
figure 24). While the discrepancy between the two catalyst systems is
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Review. Group II catalysis
5 mol
Ph
Ph
[Ca(PPh2)2(THF)4]
+ HPPh2
THF, 2 h, 25°C
100
Ph
Ph
+ HPPh2
5 mol
[Ca(PPh2)2(THF)4]
PPh2
H
Ph
Ph
Ph2P
H
Ph
Ph
THF, 2 h, 25°C
100
H
PPh2
Figure 24. Hydrophosphination of diphenylacetylene and 1,4-diphenylbutadiyne catalysed by
[Ca{PPh2 }2 (THF)4 ].
Ar2PH + R1N=C=NR2
1.5–5 mol
1 or [M{N(SiMe3)2}2(THF)2]
0.25–28 h, hexane
25°C
PAr2
R1HN
NR2
5 examples 68–99
yield
Ar = Ph, 4-MeC6H4
R1 = R2 = i-Pr, Cy, p-tol; R1 = Et, R2 = t-Bu
M = Ca, Sr, Ba
Figure 25. Group II mediated hydrophosphination of carbodiimides.
likely due to the solubility properties of [Ca(PPh2 )2 (THF)4 ], further studies
employing the analogous strontium and barium phosphides have yet to
be reported.
(ii) Intermolecular hydrophosphination of carbodiimides
In 2008, Hill and coworkers demonstrated that phosphaguanidines could
be synthesized in high yield by the Group II catalysed hydrophosphination
of carbodiimides. A series of heavier alkaline earth-based catalysts including
the heteroleptic calcium amide 1 and the homoleptic alkaline earth
amides [Ca{N(SiMe3 )2 }2 ]2 , [Ca{N(SiMe3 )2 }2 (THF)2 ], [Sr{N(SiMe3 )2 }2 (THF)2 ]
and [Ba{N(SiMe3 )2 }2 (THF)2 ] were applied to the hydrophosphination of carbodiimides with diphenylphosphine, di-p-tolylphosphine and dicyclohexylphosphine
(figure 25).
The hydrophosphination of a number of symmetric and unsymmetric
carbodiimides with secondary arylphosphines was reported to proceed at room
temperature using catalyst loadings as low as 1.5 mol%. While 1 was found to
catalyse this reaction, the simpler homoleptic alkaline earth amides proved more
active. The reaction products were isolated as colourless solids by crystallization
from hexane solution at low temperature. Under these reaction conditions, the
hydrophosphination of carbodiimides with dicyclohexylphosphine could not be
achieved with either the homoleptic or heteroleptic alkaline earth-based catalysts,
and it was postulated that, in these cases, catalyst activation did not occur.
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A. G. M. Barrett et al.
Additionally, the hydrophosphination of sterically demanding carbodiimides such
as 1,3-di-tert-butyl carbodiimide with Group II amides was not catalysed under
the reported reaction conditions.
Based upon a series of stoichiometric reactions, and in contrast to the
previously discussed hydrophosphination reactions (vide supra), it was postulated
that unsaturated homoleptic heavier alkaline earth phosphides are unlikely to
be long-lived intermediates in the catalytic hydrophosphination of carbodiimides
conducted in C6 D6 solutions. Rather, it was proposed that the phosphaguanidine
product was acting as a ligand for the Lewis acidic metal centre. The individual
reaction steps for the reaction of diphenylphosphine with 1,3-di-iso-propyl carbodiimide catalysed by the model complex 1 were followed by 31 P NMR spectroscopy. Following the reaction of diphenylphosphine (d31 P = −40.1 ppm) with 1 to
form the calcium phosphide 15 (d31 P = −21.3 ppm), an insertion reaction with
the carbodiimide to form the phosphaguanidinate 16 (d31 P = −21.1 ppm) was
observed. The reaction product was characterized by single-crystal X-ray diffraction and shown to exist in the solid state as a mononuclear fivecoordinate calcium complex in which the phosphaguanidinate ligand binds via a
symmetric−NCN−chelate with auxiliary coordination at calcium provided by the
b-diketiminate spectator ligand and a single molecule of THF. Consistent with
previous studies upon the coordination chemistry of phosphaguanidine ligands
(Mansfield et al. 2006), the phosphorus lone pair is not delocalized across the amidinate moiety, and there is a significant degree of pyramidalization at phosphorus.
Further studies demonstrated that, while 16 was kinetically competent for
hydrophosphination catalysis, the addition of diphenylphosphine to this isolated
complex did not result in the formation of the phosphide complex 15 as
monitored by 1 H and 31 P NMR spectroscopy. Catalytic turnover was observed,
however, upon addition of a mixture of diphenylphosphine and carbodiimide to
16. The authors explained these observations in terms of the Curtin–Hammett
principle, and suggest that an equilibrium between 16/HPPh2 and the phosphide
15/phosphaguanidine product exists in solution. Thus, complex 15 is formed in
low concentration, but readily reacts with the carbodiimide to reform 16 achieving
catalytic turnover. While these experiments suggested a degree of reversibility in
the s-bond metathesis step, the potential for reversibility in the insertion step
was demonstrated by a cross-over experiment. The reaction of one equivalent
of [Ph2 PC{NHCy}{NCy}] (d31 P = −18.1 ppm) and one equivalent of 1,3-di-isopropyl carbodiimide catalysed by either 10 mol% 1 or 5 mol% [Ca{N(SiMe3 )2 }2 ]2
in C6 D6 resulted in the formation of the cross-over products [Ph2 PC{NH-i-Pr}{Ni-Pr}] (d31 P = −18.5 ppm) and 1,3-dicyclohexyl carbodiimide, as observed by
NMR spectroscopy. Based upon these experiments, Hill and coworkers suggested
that the catalytic reaction proceeds via fast catalyst initiation, with turnover
proceeding via s-bond metathesis and insertion reaction steps, with both steps
being reversible under the catalytic conditions (figure 26).
(e) Hydrosilylation (C–Si) bond formation
(i) Intermolecular hydrosilylation of alkenes and dienes
In 2006, Harder and coworkers described the application of the heavier alkaline
earth benzyl complexes, [{{2-(Me2 N)C6 H4 }CHSiMe3 }2 M(THF)2 ] (M=Ca, Sr), to
the intermolecular hydrosilylation of vinylarenes and dienes with phenylsilane
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951
Review. Group II catalysis
(a)
2
R1N=C=NR
(b)
insertion
R2
N
L M
initiation
[LM(PAr2)]
LM(X1)
PAr2
O
N1
R
observed
catalyst
resting state
protonolysis
N4
Ca
N1
N2
N3
PP
Ar2PH
PAr2
R1HN
NR2
Figure 26. (a) Proposed mechanism of Group II mediated hydrophosphination of carbodiimides.
(b) ORTEP representation of 16; thermal ellipsoids at 20% probability; H atoms are omitted
for clarity.
R1
20–50°C, C6H6
+ PhSiH2R2
Ph
2.5–10 mol Cat.
0.1–20 h
R1
SiHPhR2
Ph
R1 = H, Me, Ph; R2 = H, Me
25°C, C6H6
+ PhSiH3
Ph
Ph
+ PhSiH2R2
SiH2Ph
2.5 mol Cat.
0.1 h
Cat.
Me3Si
M
SiMe3
Me2N
Ph
25°C, THF
2.5–10 mol
NMe2
Cat. =
Ph
SiH2Ph
9 examples 20–98
yield
Figure 27. Scope of Group II mediated hydrosilylation of alkenes and dienes.
and phenylmethylsilane (figure 26; Buch et al. 2006). Reactions proceeded with
low catalyst loadings (0.5–10 mol%) under relatively mild conditions (50◦ C,
0.5–16 h), with the strontium catalyst providing shorter reaction times than
the calcium analogue (figure 27). The reaction scope is limited to activated
alkenes, and norbornene and allylbenzene were reported to not undergo
hydrosilylation with Group II catalysts. Despite the possibility that [{{2(Me2 N)C6 H4 }CHSiMe3 }2 M(THF)2 ] may initiate the anionic polymerization of
styrene, clean hydrosilylation reactivity was observed.
The regiochemistry of the isolated products proved solvent dependent for the
hydrosilylation of diphenylethene; while preparations in benzene gave products
deriving from 2,1-insertion of the alkene into the M−H s-bond, preparations in
THF gave the opposite regioisomer proposed to derive from a similar 2,1-insertion
into an M−Si s-bond. Despite the possible formation of CaH2 upon catalyst
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952
A. G. M. Barrett et al.
Me
Me
N
Ar
Ca
N
Ar
THF
(Me3Si)2N
Ar
Me
C6D6
N
Ca
60°C, PhSiH3
1
H
Ar
Ar THF
Ar = 2,6-di-iso-propylphenyl
Me
N
Ca
H
N
Me
THF
N
Ar
Me
17
Figure 28. Reaction of 1 with PhSiH3 to yield the molecular hydride 17.
R1
R1
R2
Cat.
SiH2Ph
H
R1
R2
cycle B
SiH2Ph
R
ML
SiH2Ph
PhSiH3
2
R2
LMH
+
R1
Ph
cycle A
PhSiH3
R1
R2
H
ML
ML
H
H Si
H
H
PhSiH3
–
– H2
Ph
LM Si H
H
M = Ca, Sr
R1
R2
Figure 29. Proposed mechanisms of the Group II mediated hydrosilylation of alkenes.
initiation via reaction of [{{2 − (Me2 N)C6 H4 }CHSiMe3 }2 M(THF)2 ] with Rn SiH4−n
(vide infra), commercially available CaH2 proved inactive for the hydrosilylation
of alkenes under these reaction conditions.
While not directly observed, Buch et al. suggest that the catalyst
resting state may be by hydride-rich clusters of the general formulae
[{{2-(Me2 N)C6 H4 }CHSiMe3 }<1 MH1> ]n . Detailed mechanistic studies have not
been conducted, but the stoichiometric reaction of phenylsilane with 1
has been shown to yield the corresponding molecular calcium hydride
complex [{ArNC(Me)CHC(Me)NAr}CaH(THF)]2 (Ar = 2,6-di-iso-propylphenyl,
17; Harder & Brettar 2006; figure 28).
Based upon this evidence and the observed reaction products, Harder has
proposed two distinct ‘lanthanide-mimetic’ catalytic cycles, proceeding via either
a metal hydride (figure 29, cycle A) or silanide intermediate (figure 9, cycle B).
Furthermore, it has been suggested that more polar solvents favour the formation
of charge-separated species of the form [LM]+ [H4 SiPh]− that, in turn, decompose
to yield the metal silanide [LM(SiPhH2 )] with liberation of H2 . Although this
rationale neatly accounts for the observed solvent-dependent regiochemistry, it
remains a possibility that the two regio-isomeric transition states of the s-bond
metathesis step between LMX1 (X1 = alkyl, M=Ca, Sr) and Rn SiH4−n have
similar activation energies that may be perturbed by solvent polarity.
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Review. Group II catalysis
O
1–2.5 mol
2
2
R1
17
O
R
R1
O
Si
50°C, C6H6
R1 = R2 = –(CH2)5–, Ph, CH2Ph, C9H14
1
R1
+ H3SiPh
R2
H
Ph R2
5 examples 91–96
yield
2
R = Ph, R = Me
Figure 30. Group II catalysed hydrosilylation of ketones.
As with intramolecular hydroamination catalysis (§2c), initial attempts
to achieve a catalytic asymmetric hydrosilylation of substrates containing
unsaturated carbon–carbon bonds has met with limited success (Buch & Harder
2008). While a number of chiral calcium catalysts based upon non-racemic
b-diketiminate (derived from acetylacetone and (S )-a-Me-benzylamine) and (S )Ph-pybox ligands have proven catalytically active for the hydrosilylation of
styrene, in all instances reaction products were isolated with low enantioselectivies
(5–10% e.e.). These results have been explained in terms of the loss of ligand
control over reactivity, and Buch et al. suggest that, as with intramolecular
hydroamination catalysis, under the catalytic reaction conditions, the Schlenk
equilibrium is operative with non-racemic catalysts L∗ MX forming inactive chiral
species L∗2 M and racemic active catalysts MX2 .
More recently, the grafting of calcium reagents onto silica supports, as
a means to control this deleterious solution equilibrium, has been achieved.
The hydrosilylation catalysts [{{2-(Me2 N)C6 H4 }CHSiMe3 }2 Ca(THF)2 ] and
[Ca{N(SiMe3 )2 }2 (THF)2 ] have been reported to react with silica, prepared
by dehydroxylation at 700◦ C, to afford materials that bear solid supported
(bound to the solid surface via a silanol group) calcium complexes which
still contain a single reactive s-bonded substituent (i.e. [≡SiO–Ca{N(SiMe3 )2 }]
and [≡SiO-Ca{{2-(Me2 N)C6 H4 }CHSiMe3 }]). Characterized by IR spectroscopy
and one- and two-dimensional solid-state NMR spectroscopy, these materials
proved catalytically active for the hydrosilylation of styrene, 1,3-cyclohexadiene
and 1,1-diphenylethylene with PhSiH3 , albeit with lower activities than those
reported for homogeneous catalysis, particularly for the hydrosilylation of
1,1-diphenylethylene. While it is unclear whether the active catalyst is
bound to the silica surface, the authors suggested that the sensitivity of
the reaction to substrate size is an indication of a heterogeneous process
(Gauvin et al. 2009).
(ii) Intermolecular hydrosilylation of ketones
The scope of the hydrosilylation reactivity has been extended to incorporate
ketones and the intermolecular hydrosilylation of cyclohexanone, benzophenone,
adamantone and acetophenone with phenylsilane catalysed by the molecular
calcium hydride 17 to afford the corresponding silyl-ethers was reported in 2008
(figure 30; Spielmann & Harder 2008). Reactions were reported to proceed under
mild conditions (50◦ C, 0.2–34 h), and by-products originating from competitive
carbonyl enolization reactions are only observed in low yield (less than 5%).
It is noteworthy that, irrespective of the initial silane : ketone ratio, the major
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954
A. G. M. Barrett et al.
20–60°C
C6D6 or THF/HMPA
R1
Ph
R1
+ H2
2.5–10 mol Cat.
0.1–20 h, 20 bar H2
Ph
Me3Si
R1 = H, Me, Ph; R2 = H, Me
+ H2
NMe2 or 17
Cat. =
M
SiMe3
Me2N
20°C, C6D6
2.5 mol
M = Ca, Sr
Cat.
22 h, 20 bar H2
4 examples 41–98
Figure 31. Reaction scope of the calcium-mediated hydrogenation of activated alkenes. HMPA is
hexamethylphosphoramide.
reaction products are those derived from a 1 : 2 reaction stoichiometry, i.e.
[PhHSi(OCHR1 R2 )2 ]. At low silane : ketone ratios, small amounts of the trialkoxy
product could also be obtained.
The pre-catalyst 17 has been shown to react with ketones to yield the
corresponding dimeric b-diketiminato calcium alkoxide products derived from
the insertion of the ketone into the Ca–H bond of the molecular hydride.
These latter species have been shown to be kinetically competent for the
hydrosilylation of ketones. Stoichiometric preparations included the reaction of
17 with benzophenone, acetophenone, cyclohexanone, 1,3-diphenylacetone and
2-adamantone to yield the corresponding alkoxides in 9–73% yield following
crystallization from hydrocarbon solutions. Quenching the reaction mixtures with
Me3 SiCl showed, in addition to the expected reduction products, moderate to
significant amounts of substrate enolization and aldol condensation products.
Despite this observation, under the catalytic reaction conditions, these side
reactions are apparently largely inhibited.
Despite a report by Spielmann & Harder (2007) documenting that 17 readily
reacts with alternative substrates, including benzonitrile, cyclohexene oxide,
2-methylpropene oxide, 1,1,3,3-tetramethylbutyl isonitrile and diphenylmethylN -phenylimine, via insertion of unsaturated carbon–nitrogen and carbon–oxygen
bonds into the calcium–hydride bond of the organometallic reagent, catalytic
preparations for the hydrosilylation (or hydrogenation) of these substrates with
Group II reagents have yet to be reported.
(f ) Hydrogenation (C–H) bond formation
Based upon their studies of the calcium-mediated hydrosilylation of
alkenes, Spielmann et al. (2008) reported the calcium-catalysed hydrogenation
of styrene, 1,1-diphenylethylene, 1,3-cyclohexadiene, a-methylstyrene and
1-phenylcyclohexene. Reactions were reported to proceed under mild conditions
(20◦ C, 20 bar H2 ) and while both 17 and [{{2-(Me2 N)C6 H4 }CHSiMe3 }2 M(THF)2 ]
proved catalytically active, finely ground commercially available CaH2 was an
ineffective catalyst under these reaction conditions. As with the previously
reported hydrosilylation catalysis (§2d(i)), reactions proved to be sensitive to
solvent polarity. Although preparations in THF or a THF/HMPA mixture
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Review. Group II catalysis
(a)
(b)
insertion
R1
Ar
Me
N
H
Ca
LM
R1
LMX1
H2
[LMH]n
Me
N
H
Ca
H
N
Me
Ar
THF
N
Ar THF
Me
Ar
17
Ph
C6D6, 60°C
s-bond
metathesis
Me
Ar
N
Ca
2
R1
–Ph2CHMe
Ph
H2
Me
H2
20 bar
Ph
CH3
Ph
N
THF
Ar
Figure 32. (a) Proposed mechanism for the Group II mediated hydrogenation of alkenes.
(b) Observed reactivity of 17 under catalytically relevant conditions.
proceeded rapidly, in most instances, the reaction products were accompanied
by side products derived from the dimerization/hydrogenation of the alkene
(figure 31). In the case of styrene, while a reaction in THF/HMPA led to
a calcium-mediated polymerization of this substrate, changing the solvent to
benzene gave the hydrogenation product in 80 per cent yield. Spielmann et al.
suggested that the side products derive from single or multiple insertions of the
alkene substrate into the Group II alkyl intermediate. As with hydrosilylation,
hydrophosphination and hydroamination catalysis, the reaction scope is currently
limited to activated alkenes.
Stoichiometric studies demonstrated that 17 readily undergoes insertion
reactions with 1,1-diphenylacetylene and myrcene (7-methyl-3-methylene-1,6octadiene), a terpene that contains three double bonds. In both reactions, the
insertion products were isolated and fully characterized, including single-crystal
X-ray diffraction studies. In the case of myrcene, the reaction proved highly
selective, with 17 adding to the least hindered end of the diene. Experimental
evidence for the metathesis step was provided by not only the reaction of the
calcium complexes isolated from these latter reactions, but also the exchange
reaction of 17 with D2 . Treatment of the calcium deuteride 17-d2 with 1 bar
H2 at 20◦ C led to complete H/D exchange after 20 min at room temperature, as
evidenced by NMR spectroscopy. The reaction of H2 with Ca−C bonds required
slightly more forcing reactions conditions and, consistent with those reported for
the catalytic reaction, could be achieved at 20 bar H2 at 20◦ C in either benzene or
THF solution. Based upon the reactivity of the observed reaction intermediates,
it was suggested that alkene hydrogenation occurs via the catalytic cycle outlined
in figure 32.
3. Concluding remarks
In the past few years, well-defined coordination complexes of the heavier
Group II metals have been applied to the heterofunctionalization of unsaturated
substrates, including the hydroamination, hydrophosphination, hydrosilylation
Proc. R. Soc. A (2010)
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and hydrogenation of substrates containing unsaturated carbon–carbon, carbon–
nitrogen and carbon–oxygen bonds. Organometallic reagents derived from these
elements have shown a versatile reaction chemistry and are beginning to emerge
from merely lanthanide-mimetic applications. Noteworthy recent studies have
demonstrated not only C–F bond activation at calcium (Barrett et al. 2007), but
also the dehydrocoupling of silanes with amines and terminal alkynes mediated
by a calcium catalyst (Buch & Harder 2007). These studies, in addition to a
recent report upon the facile synthesis of Group II alkyl complexes through saltmetathesis reactions (Crimmin et al. 2008), are likely to provide a key starting
point to extend the rapidly growing field of Group II catalysis beyond the
heterofunctionalization of unsaturated substrates.
We thank GlaxoSmithKline for the generous endowment (to A.G.M.B.), the Royal Society for
a University Research Fellowship (M.S.H.) and Royal Society Wolfson Research Merit Award
(A.G.M.B.), and the Engineering and Physical Sciences Research Council and GlaxoSmithKline
for generous support of our studies.
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