Tetrahedron 72 (2016) 355e369
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Tetrahedron report 1099
[2þ2] Cycloaddition reactions promoted by group 11 metal-based
catalysts
Manuel R. Fructos, Auxiliadora Prieto *
lisis Homog
n en Química Sostenible and Departamento de Química,
Laboratorio de Cata
enea, Unidad Asociada al CSIC CIQSO-Centro de Investigacio
Universidad de Huelva, Campus de El Carmen s/n, Huelva, 21007, Spain
a r t i c l e i n f o
Article history:
Received 25 September 2015
Available online 19 November 2015
Keywords:
[2þ2] Cycloaddition
Copper
Silver
Gold
Catalysis
Contents
1.
2.
3.
4.
5.
6.
7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
[2þ2] Cycloadditions reactions involving allenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356
2.1.
Copper catalyzed reactions of allenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356
2.2.
Gold catalyzed reactions of allenenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
[2þ2] Cycloaddition reactions involving allenamides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359
[2þ2] Cycloaddition reactions involving ketenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362
[2þ2] Cycloaddition reactions involving alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362
5.1.
Ynamides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362
5.2.
Phenylthioacetylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364
5.3.
Siloxy alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365
5.4.
[2þ2] Cycloaddition of non-functionalyzed terminal alkynes and alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366
[2þ2] Cycloaddition reactions between imines and enol ethers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
References and notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
Biographical sketch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
1. Introduction
In modern organic synthesis, cycloaddition reactions are particularly important tools allowing the generation of at least two
* Corresponding author. E-mail address: [email protected] (A. Prieto).
http://dx.doi.org/10.1016/j.tet.2015.11.031
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.
new bonds and one cycle in a single step. Additionally, these
transformations provide the assembly of complex molecular
structures in an easy fashion, with high atom economy and
consequently minimization of waste production.1 Cycloadditions
can be promoted by heat, light, high pressure, sonication or Lewis
acids. Unfortunately, many of these promoters require the presence of polar functional groups in the substrates to carry out
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M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
these transformations. In this regard, metal complexes have
afforded excellent opportunities for uncovering new selective
cycloaddition reactions due to their coordination properties with
non-activated CeC multiple bonds (e.g., alkenes, alkynes, allenes
or 1,3-dienes), even in a catalytic manner.2 In addition, the use of
metal complexes as catalysts provides the opportunity to carry
out enantioselective transformations by the employment of chiral
ligands. Among cycloaddition reactions, [2þ2] metal catalyzed
cycloadditions have been widely studied in the last decades as the
most straightforward approach for the preparation of cyclobutane
and cyclobutene derivatives.3,4 This carbocyclic structure is of
particular relevance in organic synthesis not only for its presence
in bioactive compounds but mainly as useful building blocks due
to its unique reactivity.5 Accordingly, several interesting examples
of metal complexes6 and Lewis acid7 catalyzed transformations
have been reported for the synthesis of these carbocyclic
compounds.4b
In 1967, Friedman published the first example of the use of
a silver salt as promoter in the [2þ2] cycloaddition of benzyne
(Scheme 1).8 However, it was not until 2003 that the progress in the
use of group 11 metals as catalysts in this field began. Since then
a large number of new [2þ2] cycloaddition strategies have been
developed including asymmetric versions. The success obtained
with these metals is due to the high affinity for p-bonds displayed
by group 11 metals. This high p-acidity can be explained in part by
relativistic effects that achieve the highest level in the case of gold.
As a result, group 11 metal complexes exhibit high carbophilicity
being possible the activation of the carbonecarbon multiple bond
systems without the presence of other polar groups.9 On the other
hand, gold(I) complexes showed relatively non-oxophilic properties. Thus, gold(I) complexes exhibit good chemoselectivity and
good functional group compatibility. These unique properties have
made that gold(I) complexes have arised as powerful catalysts
allowing the discover of new and important [2þ2] cycloaddition
reactions involving p-unsaturated systems.
2.1. Copper catalyzed reactions of allenes
The first [2þ2] cycloaddition reaction employing allenes and
catalyzed by group 11 metal complexes was reported by Akiyama’s
group in 2003.13 The authors carried out the enantioselective [2þ2]
cycloaddition between 1-methoxyallenylsilane and a-imino ester
using the catalytic system [Cu(MeCN)4]BF4/(R)-Tol-BINAP in the
presence of 4 A molecular sieves and THF. Under these conditions,
azetidines were obtained in good yields as single diastereomers
and with excellent enantioselectivities of up to 97% ee (Scheme 2).
Moreover, the allenyltriethylgermane also reacted affording the
corresponding cycloadduct with good enantioselectivity. Unfortunately, the cycloaddition worked well only with the a-imino
ester, other less reactive imine derivatives such as PhCH]NPh,
PhCH]NTs or EtOCOCH]Ph did not provide the desired
cycloadducts.
Scheme 2. Copper-catalyzed enantioselective
methoxyallenylsilanes with a-imino ester.
Scheme 1. [2þ2]-Cycloaddition reaction of benzyne and benzene promoted by silver
salts.
So far, reviews focusing on gold, copper and silver catalysis in
organic synthesis have been published,10 in particular, a review
about the development of gold-catalyzed cycloaddition reactions.11 However, in the last decade we have witnessed an exciting progress in the group 11 metal-catalyzed [2þ2]
cycloaddition reactions. This review highlights the recent advances achieved in this area including mechanistic aspects and
further cycloadduct transformations. This contribution is subdivided into five sections, according to the starting substrate
employed in any reaction.
[2þ2]
cycloaddition
of
1-
Additionally, the ring opening of the corresponding azetidines
was accomplished without loss the enantioselectivity and quantitatively affording the acylsilanes by treatment with 1 N HCl in THF
at 0 C (Scheme 3). The absolute stereochemistry of azetidines and
acylsilanes were assigned by analogy after X-ray analysis of the acyl
tert-butyldimethylsilane (R¼Si(t-Bu)Me2).
Scheme 3. Ring opening of azetidines.
2. [2D2] Cycloadditions reactions involving allenes
Allenes represent one of the most powerful and versatile
building blocks in organic synthesis given that they undergo a variety of interesting transformations.12 Notably, allenes have shown
great importance in [2þ2] cycloaddition reactions, and have been
studied under thermal conditions, photochemically and by metal
catalysis.4a Among them, the use of copper and gold complexes, as
catalysts, has provided novel and interesting possibilities to activate allenes in a chemoselective and even in high enantioselective
manner.
1,4-Bisallenes have also been employed as substrates in the
[2þ2] cycloaddition reaction. Thus, Kitagaki et al. managed to
minimize the [3,3] sigmatropic rearrangement reaction (product B)
and to conduct the reaction mainly through [2þ2] cycloaddition
(product A) with the use of a slight excess of CuBr2 in the presence
of iPr2NH at high temperature (Scheme 4).14 According to the authors, the amine features the role of reducing agent to generate the
active Cu(I) specie as well as to stabilize it. Additionally, the amine
makes easier the oxidative addition.
M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
357
2.2. Gold catalyzed reactions of allenenes
€rstner have carried
In the last decade, the groups of Toste and Fu
out an impressive work in the development of gold-catalyzed
enantioselective [2þ2] cycloaddition reactions of allenenes. In
2007, Toste16 predicted that the addition of an alkene to a gold(I)activated allene would generate the stabilize benzylic cation A,
which by intramolecular trapping would afford the cationic intermediate B and consequently would prompt the corresponding
cyclobutane C. Alternatively, C could be obtained from A by direct
reaction with the carbonegold bond (Scheme 6).17
Scheme 6. Intermediates proposed by Toste.
Scheme 4. Cu-mediated [2þ2] cycloaddition reactions of 1,4-bisallenes.
Thus, the authors developed the first gold-catalyzed intramolecular cycloisomerization of g-eneallenes to generate
alkylidene-cyclobutanes using Ph3PAuCl as catalyst. Effective [2þ2]
cycloaddition required the presence of an aryl-substituted alkene
apart from a terminally disubstituted allene. Most interesting, they
carried out the enantioselective version of the reaction using the
catalytic system (R)-DTBM-SEGPHOS(AuCl)2/AgBF4 in a 1:2 mixture
(Scheme 7). The [2þ2] cycloaddition products were obtained in
high yields (up to 92%) and with excellent enantioselectivities (up
In addition, the authors accomplished the tandem Crabb e
homologation, the transformation of alkynes to allenes 15
and [2þ2] cycloaddition reactions using 1,5-hexadiynes
(Scheme 5).
Scheme 5. Direct transformation of 1,5-diynes into [2þ2] cycloaddition products.
Scheme 7. Enantioselective Au(I)-catalyzed intramolecular [2þ2] cycloaddition of
allenenes.
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M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
to 97% ee) in all cases, except for the product wherein the cyclobutane ring is condensed with a pyrrolidine.
Later, in the context of the diastereo- and enantioselective
synthesis of 3,4-substituted pyrrolidines, the group of Toste improved dramatically the enantioselectivity obtained in the reaction
involving N-protecting substrates with the phosphoramidite gold(I)
catalyst shown in the Scheme 8.18 Thus, the authors achieved the
corresponding azabicycles in good yields and with enantioselectivities up to 97%.
metallacyclic intermediate formed after the addition of the alkene to the gold-coordinated allene. The two possible stereochemical cis and trans pathways are competitive. However, in the
presence of a nucleophile the trans-pathway is favored while in the
absence of a nucleophile the cis-pathway is operative (Scheme 9).
€rstner et al. published a family of novel and inAdditionally, Fu
teresting TADDOL-derived phosphoramidite gold complexes to be
employed as chiral catalysts in several reactions.19 Among them,
the [2þ2] cycloaddition reaction of allenenes was achieved
affording the substituted cyclobutenes in excellent yields and
enantioselectivity (Scheme 10).19a The complex, which contains
tert-butyl substituents in the phenyl rings gave the higher ee values,
up to 99% ee.
Scheme 8. Phosphoramidite Au(I)-catalyzed enantioselective synthesis of 3,4substituted pyrrolidines through [2þ2] cycloaddition of allenenes.
In this work, they also performed a computational study (DFT)
in order to elucidate the mechanism of the cycloaddition. They
suggested a stepwise process involving a five-membered
Scheme 10. Phosphoramidite Au(I)-catalyzed enantioselective [2þ2] cycloaddition of
allenenes.
On the other hand, N-heterocyclic carbene ligands (NHCs) have
been also employed in the intramolecular gold(I)-catalyzed [2þ2]€rstner et al. proved
cycloaddition reactions of allenenes.20 Thus, Fu
in an interesting study on the p-acceptor properties of NHCs that
electron-deficient NHCeAu(I) complexes A and B effectively catalyzed the formation of the [2þ2]-cycloadducts instead of the corresponding [3þ2] ones (Scheme 11).
Scheme 9. Toste’s mechanistic proposal based on experimental and computational
studies.
Scheme 11. NHCeAu(I) complexes catalyzed intramolecular [2þ2]-cycloaddition
reaction.
M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
359
3. [2D2] Cycloaddition reactions involving allenamides
In the last four years there has been an impressive advance in
the [2þ2] cycloaddition reaction of allenamides. One of the reasons
for this success stands on their higher electron density compared
with simple allenes, due to the higher p-donating properties of the
nitrogen atom.21 Accordingly, intermolecular and high regioselective transformations have been accomplished by several groups.
Gold has been the metal of choice for the activation of allenamides
to carry out the [2þ2] cycloaddition reactions because, as abovementioned, this metal shows a high carbophilicity and affinity for
p-unsaturated systems.
In 2012, the group of Chen22 encouraged by the excellent results
in the gold catalyzed intramolecular reactions of allenes,16,18,19a,20
disclosed the first [2þ2] intermolecular cycloaddition reaction between allenamides and electron-rich olefins. This methodology
employed a wide range of allenamides with a low Lewis basic
sulfonamide group to avoid the coordination and deactivation of
the gold catalyst. Namely, the authors studied the reaction of vinyl
ethers and vinyl amides with electron-rich styrenes catalyzed by
JohnPhosAuCl to obtained densely functionalized cyclobutane adducts in moderated to high yields (Scheme 12). Unfortunately, a or
g-substituted allenamides did not react even at a high temperature
probably due to steric hindrance.
Scheme 12. Au(I)-catalyzed [2þ2] intermolecular cycloaddition of alleneamides with
alkenes.
Additionally, a series of dimerization products from starting
allenamides were described in the absence of alkenes and using the
mixture PPh3AuCl/AgSbF6 as the catalytic system (Scheme 13).
In the same context, Gonz
alez reported the intermolecular
[2þ2] cycloaddition reaction of N-allenylsulfonamides with enol
ethers (Scheme 14).23 The reactions concluded quickly, within
5e20 min, using low catalyst loadings not only with terminal but
also with internal allenyl sulfonamides. Likewise, the selective
homodimerization of the starting allene was carried out in good to
excellent yields (68e90%) using norbornene as additive and the
same phosphite-base gold catalyst.
Scheme 13. Au(I)-catalyzed dimerization of alleneamides.
Scheme 14. Phosphite-gold(I) catalyzed intermolecular [2þ2] cycloaddition of alleneamides with alkenyl ethers.
~ as published the Au-catalyzed [2þ2]
Simultaneously, Mascaren
cycloaddition reaction of 2-oxazolidinone allene-amide with
enamides and styrene derivatives (Scheme 15).24 The cycloadducts
were obtained in excellent yields and with complete regio-, chemoand stereoselectivity at 15 C in only 1 h. Given that only the trans
stereoisomer of the cycloadducts is obtained with both (E) and (Z)
enamides, the authors suggested a reaction mechanism, which
involved carbocationic intermediates, at least for these substrates.
Thus, the Au-allyl cation species A formed in the first step is
intercepted by the alkene, affording a second cationic intermediate
B.25 The formation of the more stabilized bencylic or imonium
cation would explain the regioselectivity observed.
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M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
~ as’s mechanistic proposal for the [2þ2]-cycloaddition reaction of
Scheme 16. Mascaren
2-oxazolidinone allene-amide and alkenes.
Scheme 15. Phosphite-gold(I) catalyzed intermolecular [2þ2] cycloaddition of alleneamides with alkenes.
Additionally, the rotation around the sigma CeC bond would be
responsible for the loss of the alkene stereochemical information.
Finally, the ring-closing process followed by elimination of the gold
complex would provide the final [2þ2] cycloadduct (Scheme 16).
It must be pointed out that this same year, the first asymmetric
gold-catalyzed intermolecular [2þ2] cycloaddition of N-sulfonylallenamides and vinylarenes was accomplished by Gonzalez’s
group using gold(I)-complexes derived from enantiopure phosphoramidite ligands (Scheme 17).26 Given the high reactivity of the
starting materials the reaction could be carried out at very low
temperatures (70 C), allowing to optimize the enantioselectivity
of the process. So, the [2þ2] cycloadducts were obtained after 1 h in
good yields and with high enantioselectivities (71e94% ee). It is
worth pointing out that synthetically useful cyclobutanes containing challenging quaternary carbon centers were also prepared
in enantioenriched form. Finally, the catalyst loading employed was
as low as 0.5 mol % without reduction in the yield or enantiomeric
excess.
~ as described the use of conjugate N,N-diaAfterward, Mascaren
lkyl hydrazones as substrates in the [2þ2] cycloaddition reaction
with allenamides (Scheme 18).27 The Johnphos-based gold(I)
complex was chosen as the most active and selective catalyst, given
the corresponding [2þ2] cycloadduct as a single product. The asubstituted N,N-diisopropyl hydrazones were the most reactive
substrates providing the substituted cyclobutanes in high yields
even at room temperature. The hydrazone derived from cyclohexene-1-carbaldehyde afforded an interesting bicyclic structure in
excellent yield and complete syn selectivity. The hydrazone analogue derived from cyclopentenecarbaldehyde did not give the
Scheme 17. Enantioselective phosphoramidite-gold(I) catalyzed [2þ2] allenamide-vinylarene cycloaddition.
M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
[2þ2] adduct due to high strain of this bicyclic system. On the other
hand, the N,N-dimethyl hydrazones with aryl or alkyl substituents
at the alkene also afforded the [2þ2] cycloadducts however, higher
reaction times and temperatures were needed. Furthermore, the
SAMP-derived chiral hydrazone afforded the corresponding [2þ2]
cycloadduct in high yield and moderate diastereoselectivity.
361
a nitrile or alcohol by treatment with magnesium monoperoxiphthalate (MMPP) or hydrolysis and subsequent reduction,
respectively.
The authors assumed that the reaction proceed in a stepwise
manner (Scheme 20). Thus, the easy activation of the allenamide by
the gold catalyst to give the zwitterionic intermediate A is proposed
as the initial activation step. This species had been theoretically
identified in the context of intermolecular gold-catalyzed [4þ2]
allenamides and dienes cycloaddition reactions.28 Then, the alkenylhydrazone could attack regioselectively at the g-position of A
affording the intermediate B. Finally, the cycloadduct could be
obtained by direct cyclization with simultaneously elimination of
the catalyst. Alternatively, the formation of the alkyl gold intermediate C could explain the formation of Z/E mixtures
cycloadducts.
Scheme 18. Johnphos-based gold (I) catalyzed [2þ2] allenamide-a,b-unsaturated hydrazone cycloaddition.
Scheme 20. Mechanistic proposal for cycloaddition of allenamides and a,b-unsaturated hydrazones.
In addition, the authors carried out further modifications to
transform the cyclobutane adducts in attractive compounds
(Scheme 19). Thus, the hydrazone moiety was converted into
Scheme 19. Cycloadduct transformations.
Very recently, the synthesis of chiral 2,3-indoline-cyclobutanes
has been achieved through gold catalyzed dearomative [2þ2] cycloaddition between indoles and allenamides.29 As shown in
Scheme 21, this reaction provided direct access to indolines with
two consecutive quaternary stereogenic centers with excellent
stereochemical control (dr>20:1, ee up to 99%) employing the
commercially available (R)-DTBM-Segphos, as chiral ligand, under
mild reaction conditions. Additionally, the authors observed the
key role of the electron withdrawing protecting group (i.e., Boc,
Cbz) in the nitrogen to obtain the cycloadducts in good yields.
From a mechanistically point of view the authors proposed, as
mentioned before, the initial coordination of the allenamide by the
chiral cationic gold complex providing the electrophilic intermediate A. After indole attack at the g-position of A to lead the
gold-alkenyl intermediate B, two possible pathways were proposed. In one hand a two steps mechanism (path a), in which a new
alkyl-gold intermediate C is formed, or alternatively a concerted
mechanism (path b). According to the authors, the concerted
mechanism is the most likely way, given the high stereoselectivity
observed in formation of the exo-C]C double bond (Scheme 22).
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M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
that [Cu(MeCN)4]PF6 in the presence of (R,P)-Walphos-CF3 as chiral
ligand efficiently catalyzed the stereoselective formation of 1,2oxazetidine-3-ones using 2-nitrosopyridine as starting material
(Scheme 23). This transformation was carried out using para- and
meta-aryl substituted ketenes with excellent enantioselectivities.
Nevertheless, poor enantioselectivities were obtained with the ortho-substituted ones. Heteroaryl substituted 3-thienyl ethyl ketene
furnished the corresponding cycloadducts with good enantioselectivities and yields. The potential application of this methodology
was assessed by the transformation of the cycloadducts into the
synthetically useful chiral 1,2-diols through reduction to the corresponding alcohol employing AlLiH4 and subsequent NeO bond
cleavage with CuSO4/MeOH.
Scheme 21. Enantioselective gold(I)-catalyzed dearomative [2þ2] indole-allenamide
cycloaddition.
Scheme 23. Enantioselective
nitrosopyridine and ketenes.
copper
catalyzed
[2þ2]
cycloaddition
of
2-
5. [2D2] Cycloaddition reactions involving alkynes
5.1. Ynamides
Scheme 22. Catalytic cycle proposed by Bandini.
4. [2D2] Cycloaddition reactions involving ketenes
Ketenes are valuable reactive intermediates and starting materials employed in a variety of organic transformations.30 Among
them, the Staudinger [2þ2] cycloaddition reaction to obtain has
been intensively studied in the last century. Nonetheless, only
a single report on group 11 metal catalyzed [2þ2] cycloaddition has
been published to date.31 In 2010, Studer and co-worker reported
The thermal [2þ2] cycloaddition of ynamines with cyclic
enones, described by Ficini in 1969,32 was one of the most useful
carbonecarbon bond-forming reactions involving ynamines.33
However, the synthetic utility of ynamines was only studied during 70’s and 80’s since these substrates are very sensitive toward
hydrolysis so they exhibit difficulties in the preparation and handling. Recently, ynamides34 have been featured as a useful and
broadly available synthetic equivalent of ynamines, mainly due to
the advances in the cross-coupling of amides with alkynyl
bromides.35
The first example of Ficini’s [2þ2] cycloaddition of ynamides,
specifically N-sulfonyl-substituted ynamides, with enones was
M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
reported by Hsung and co-workers employing CuCl2 and AgSbF6 as
catalysts (Scheme 24).36 The reactions were carried out at 0 C
using substrates with different N-sulfonyl groups as well as cyclic
and acyclic enones and different alkyne and nitrogen substitutions.
Unfortunately, the corresponding cyclobutenamides were obtained
in moderate yields using high amounts of catalyst (20 mol % of
CuCl2 and 60% mol % of AgSbF6).
363
Regrettably, it was impossible to obtain the cycloadducts in
which the cyclobutenamide is fused to afford a 5-membered ring.
Possibly, the ring strain made impracticable the cycloaddition
pathway leading to the half-cycloaddition product (Scheme 26).
Scheme 26. Silver-catalyzed half-cycloaddition product.
Subsequently, Mezzetti studied the Ficini reaction of both cyclic
and acyclic unsaturated b-ketoesters using Cu(I) or Cu(II) triflate as
catalysts. Cu(I) triflate showed the best catalytic behavior giving the
expected regioisomers in quantitative yields (Scheme 27).38 Additionally, in this contribution the authors also examined the [2þ2]
cycloaddition of phenylthioacetylene (See section 5.2).
Scheme 24. Copper(II)-catalyzed Ficini [2þ2] cycloaddition of ynamides.
Further studies carried out by the same authors showed that
either Cu(II) or Ag(I) salts could catalyzed the intramolecular Ficini’s cycloaddition being AgNTf2 the most suitable catalyst.37 The
corresponding cycloadducts were obtained in moderate yields using 10 mol % of catalyst at room temperature (Scheme 25).
Scheme 27. Copper(I)-catalyzed Ficini [2þ2] cycloaddition of ynamides.
Scheme 25. Silver(I)-catalyzed intramolecular Ficini’s cycloaddition reaction.
Very recently, remarkable results have been reported by Nakada
and co-workers in the enantioselective [2þ2] cycloaddition reaction of cyclic a-alkylidene b-oxo imides with ynamides employing Cu(OTf)2 and Ishihara’s bisoxazoline ligand as chiral catalyst.39
The reactions were fast (within 1 h) with terminal alkynes using
10 mol % of catalyst at 0 C, affording the cycloadducts in good to
excellent yields and with high ee (Scheme 28). Unfortunately, the
steric effect in the case of internal alkynes reduced the yields and ee
values. No reaction was observed with CO2Me as terminal alkyne
substituent. Higher reactivity was found with cyclohexenone derivatives compare to the corresponding cyclopentenone ones.
Moreover, it was possible to carry out the cycloaddition with hindered substrate even at low temperature. The authors have proposed the complex A, after X-ray crystallographic analysis and NMR
studies, to explain the sense of induction observed in the reaction.
As shown in the Scheme 27, the complex A display a rigid conformation due to an intramolecular hydrogen bond and the coordination of the cyclic a-alkylidene b-oxo imide with Cu(II),
distinguishing the two enantiotopic faces of the reacting double
bond.
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M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
On the other hand, Ishihara reported the use of propiolamides in
the enantioselective copper catalyzed [2þ2]-cycloaddition reactions with silyl enol ethers.40 The corresponding cyclobutenes
were obtained in good yields and with high enantiomeric excess
employing the chiral ligand showed in the Scheme 30. Unfortunately, acyclic dienes and acyclic silyl enol ethers could not be
suitable as reactants. The authors assumed its accomplishment via
a stepwise mechanism, analogous to that proposed previously for
the [2þ2] cycloaddition of alkenes with a-acyloxyacroleins catalyzed by chiral organoammonium salts.41 Thus, the enantioselective Michael aldol addition to the re face of the silyl enol ether
followed by the intramolecular cyclization would explain the observed absolute stereochemistry of the cycloadducts.
Scheme 30. Copper-catalyzed [2þ2] cycloaddition reaction with propiolamides.
Scheme 28. Copper(II)-catalyzed asymmetric [2þ2] cycloaddition of cyclic a-alkylidene b-oxo imides with ynamides.
5.2. Phenylthioacetylene
Finally, further transformations of the products were accomplished to obtained amides, nitriles and esters in moderate to excellent yields. Additionally, it was possible to remove the imide
moiety (Scheme 29).
Scheme 29. Subsequent cycloadduct transformations.
In 2003, Ito and co-workers reported the first copper-catalyzed
[2þ2] cycloaddition reaction of phenylthioacetylene and methyl 2oxocyclopentanecarboxylate as a part of their synthetic strategy for
the preparation of tricycloclavulone (Scheme 31).42 The reaction
was carried out employing 30 mol % of Cu(OTf)2 to afford the bicyclic compound in 64% yield.
Scheme 31. Copper(II)-catalyzed
thioacetylene.
[2þ2]-cycloaddition
reaction
of
phenyl-
M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
Afterward, in order to obtain the (þ)-tricycloclavulone the same
group managed to make this reaction in an enantioselective fashion
(67% yield and 73% ee) through the use of the novel catalyst A
(Scheme 32).43 Additionally, further studies were accomplished to
expand the reaction scope using different thioacetylene derivatives.44 On the other hand, the authors employed this methodology in the synthesis of (þ)-precapnelladiene.45
Scheme 32. Enantioselective
thioacetylene.
[2þ2]-cycloaddition
reaction
of
365
5.3. Siloxy alkynes
The silver-catalyzed [2þ2] cycloaddition of siloxy alkynes with
different a,b-unsaturated carbonyl compounds was reported by
Kozmin in 2004.46 In this contribution, AgNTf2 appeared as the
unique and excellent promoter of this reaction giving the corresponding siloxy cyclobutenes in good yields at room temperature
(Scheme 34). The authors observed that both E- and Z-crotonates
generated single trans-substituted siloxy cyclobutene, suggesting
a stepwise mechanism. In order to support this approach, the authors
carried out an experiment using a deuterated enone (Z:E¼92:8). As
shown in Scheme 33, a mixture of the cis and trans cyclobutenes were
obtained, which is coherent with the stepwise process.
substituted
As mentioned earlier,38 the group of Mezzetti examined the
reaction of phenylthioacetylene with both cyclic and acyclic unsaturated b-ketoesters employing only 1 mol % of Cu(I) or Cu(II)
triflate as catalysts (Scheme 33). Surprisingly, the [2þ2] cycloaddition of phenylthioacetylene onto acyclic unsaturated b-ketoester
showed inversed regioselectivity as usual, affording a 4:1 mixture
of the regioisomers, possibly due to the lower electronic induction
of the sulfane as compared to ynamide and the steric difference
between the doubly substituted a- and the unsubstituted b-position of the ethyl 2-benzoylacrylate. In the case of the cyclic
unsaturated b-ketoester, the reaction was completely regioselective and only the electronically favored regioisomer was
obtained.
So, from a mechanistic point of view, the authors suggested two
plausible pathways (Scheme 35): the classical Lewis acid activation
of enone (path a) and the more likely silver-based activation of
siloxy alkyne (path b). The last one is supported by low-
Scheme 33. Copper-catalyzed [2þ2] phenylthioaceylene-unsaturated b-ketoesters
cycloaddition.
Scheme 35. Mechanistic proposals for silver-catalyzed [2þ2] cycloaddition of siloxy
alkynes and a,b-unsaturated carbonyl compounds.
Scheme 34. [2þ2] Cycloaddition reactions of siloxy alkynes catalyzed by AgNTf2.
366
M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
temperature NMR experiments, given that important changes in
the 1H and 13C chemical shifts of the alkyne were observed in the
presence of AgNTf2.
5.4. [2D2] Cycloaddition of non-functionalyzed terminal alkynes and alkenes
In 2010, Echavarren achieved in a high selective manner the
intermolecular [2þ2] cycloaddition reaction of terminal alkynes
with alkenes. The success of this transformation was accomplished
by using 100% excess of the alkene with respect to the alkyne and
sterically hindered cationic Au(I) complexes. This system allowed
them to obtain interesting substituted cyclobutenes in a regioselective fashion with good yields (up to 81%) (Scheme 36).47 Additionally, the authors employed diethynylbenzene as substrates to
reach the corresponding biscyclobutenes.
Scheme 37. Gold(I)-catalyzed macrocyclization of 1,n-enynes.
Scheme 36. Gold(I)-catalyzed [2þ2] cycloaddition of alkynes and alkenes.
Furthermore, the same group extended this methodology, from
a conceptual standpoint, to the use of large 1, n-enynes (n¼10e16)
to afford macrocycles in moderate to good yields (Scheme 37).48
The mechanism of this reaction has been the subject of research
by several groups.49 These studies have concluded that different
species of gold(I) complexes are formed during the process and
even several s,p-(phenylacetylene)digold(I) complexes have been
isolated and studied by X-ray diffraction (XRD).49a In order to
minimize the formation of digold(I) complexes, Echavarren and coworkers, synthesized a series of gold(I) complexes of the type [tBuXPhosAu(MeCN)]X (X¼anion) with different counterions and
tested them as catalysts in the [2þ2] cycloaddition reactions. The
authors observed that the reaction efficiency of both intermolecular cycloaddition and macrocyclization reactions were
improved by the use of the well-defined [t-BuXPhosAu(MeCN)]
BArF4 complex bearing the bulky and soft anion BArF
4 as counterion
[BArF
4 ¼tetrakis(3,5-bis(trifluoromethyl)phenyl)borate]. In addition, kinetic studies and DFT calculations were carried out to conclude that the rate-determining step of the reaction is the first
ligand exchange to afford the active (h2-phenylacetylene)gold(I)
complex (Scheme 38).
Scheme 38. Echavarren’s mechanism proposal of the [2þ2] cycloaddition between
alkynes and alkenes.
6. [2D2] Cycloaddition reactions between imines and enol
ethers
The synthesis of b-lactams has been one of the most intensively
topic investigated in the last decades.50 Despite the great interest in
these compounds, there are few examples reporting the use of group
11 metal complexes as catalysts in [2þ2] cycloadditions of imines and
ketenes, initially discovered by Staudinger,51 to provide azetidin-2ones (b-lactams). The synthesis of spirocyclopropanated azetidines
M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
367
was reported by Nakamura through the silver-catalyzed cycloaddition reaction of imines to (alkoxymethylene)cyclopropanes.52 The
[Ag(fod)] (fod¼6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato) exhibited the high catalytic activity after an extensive study
carried out with different Lewis acids. Thus, the reaction between
imines and (alkoxymethylene)cyclopropanes was accomplished in
the presence of [Ag(fod)] (10 mol %) at 30 C to obtain the corresponding spirocyclopropanated azetidines in good to excellent yields
and with high cis selectivities (Scheme 39).
Scheme 41. Mechanism of silver catalyzed [2þ2] cycloaddition of imines to (alkoxymethylene)cyclopropanes.
Scheme 39. Silver catalyzed [2þ2]-cycloaddition of imines to (alkoxymethylene)
cyclopropanes.
This methodology proved to be useful in the synthesis of bphenylalanine analogue through a three-step conversion procedure
(Scheme 40).
evidence to predict that the number of examples will increase in
the future. Additionally, in the last few years, there have been remarkable advances in the development of chiral [2þ2] cycloaddition reactions using chiral copper complexes. In fact, copper salts
and chiral copper complexes have proved themselves as excellent
chemoselective catalysts activating allenes, ketenes, phenylthiocetylene and ynamides to afford the corresponding cycloadducts in good yields and enantioselectivities. In addition, there is
no doubt the interest aroused by gold(I) catalysts in this field. The
use of this metal has enabled the development of a series of elegant
and new [2þ2] cycloadditions involving allenenes, allenamides and
the reaction of non-functionalyzed terminal alkynes and alkenes.
Although very interesting chiral transformations have been developed using allenenes and allenamides as substrates, the enantioselective version of the cycloaddition between nonfunctionalyzed terminal alkynes and alkenes has not been reported yet. In conclusion, unique behavior has been observed for
these group 11 metal-based systems in the [2þ2]-cycloaddition
reactions and further developments in the application of these
metal systems will hopefully appear in the near future.
Acknowledgements
We thank MINECO (Project CTQ2011-24502). Thanks are also
rez for helpful comments.
due to Prof. Pedro J. Pe
References and notes
Scheme 40. Synthesis of a-cyclopropane-modified b-phenylalanine.
The authors proposed a two steps mechanism, in which the first
step is the nucleophilic attack of the carbonecarbon double bond
on the silver activated imine leading to the anti-oriented zwitterion. Subsequently, internal rotation of the zwitterion and cyclization would afford the cis or trans azetidine (Scheme 41). According
to the authors the ring closure is reversible; however the cis azetidine is the thermodynamically more stable isomer as proven
experimentally.
7. Conclusions
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powerful tools in the [2þ2] cycloaddition reactions. Although few
examples have been described through the use of silver and none of
them in chiral form, it has been demonstrated the ability of this
metal to activate olefinic species and undoubtedly there is enough
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M.R. Fructos, A. Prieto / Tetrahedron 72 (2016) 355e369
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Biographical sketch
Manuel R. Fructos graduated in Chemistry at the Universidad of Sevilla (2002) obtained his Ph.D. degree in 2007 from the Universidad of Huelva, under the supervision
of Prof. Pedro J. Perez and M. Mar Díaz Requejo. Then, he moved to the Durham University for a postdoctoral stay to work with Prof. Todd B. Marder. In 2009, he returned
to the Universidad de Huelva as Assistant Professor and currently as Junior
Lecturer. His research interests include organometallics chemistry and homogeneous
catalysis.
Auxiliadora Prieto graduated in Chemistry at the Universidad de Sevilla (1998). She
received her PhD (2003) in the Universidad of Sevilla, under the supervision of Dr. Rondez and Dr. Jose
Ma Lassaletta. After a postdoctoral period in Professor Karl
sario Ferna
Anker Jorgensen’s group at University of Aarhus in Denmark, she joined to the group of
rez in the University of Huelva where she is lecture in organic
Professor Pedro J. Pe
chemistry. Her research interests include the development of group 11 metal-based
catalyzed reactions and nickel catalyzed cross-coupling reactions.