The Chemistry of Frustrated Lewis Pairs
MacMillan Group Meeting
Tracy Liu
4 December 2013
RR
B
R
R
N
R
R
RR
B
R
R
N
R
R
RR
B
R
R
N
R
R
R
R
R
B
R
N
R
R
RR
B
R
N
R
R
R
RR
R
B
R
N
R
H2
R
R
R
R
B
N
R
R
R
R
R
R
B
R
H
H
N
R
R
RR
B
R
N
R
R
R
RR
R
B
R
N
R
H2
R
R
R
R
heat
B
N
R
R
R
R
R
R
B
R
H
H
N
R
R
Number of Publications
Rapid Emergence of FLP Chemistry
100
80
60
40
20
0
2000
2006
2009
2012
■ 289 total publications in FLP chemistry
■ Leading Academics:
Douglas W. Stephan, University of Toronto, Canada
Gerhard Erker, Westfälische Wilhelms-Universität Münster, Germany
Imre Pápai, Chemical Research Cent. of the Hungarian Acad. of Sciences, Hungary
The Advent of FLP Chemistry
1942 - Brown makes an
interesting observation
Me
N
Me
Me
BF 3
BF 3
N
BMe 3
No Reaction
Me
Brown, H. C.; Schlesinger, H. I. JACS, 1942, 64, 325-329.
The Advent of FLP Chemistry
1942 - Brown makes an
interesting observation
Me
N
Me
Me
BF 3
BF 3
N
BMe 3
No Reaction
Me
Brown, H. C.; Schlesinger, H. I. JACS, 1942, 64, 325-329.
The Advent of FLP Chemistry
1942 - Brown makes an
interesting observation
1959 - Wittig finds PPh 3 and BPh 3
does not form an adduct
F
Mg
PPh 3
Br
PPh 3
BPh 3
BPh 3
No formation of classical Lewis Acid/Base adduct
Wittig, G.; Benz, E. Chem. Ber., 1959, 92, 1999-2013.
The Advent of FLP Chemistry
1942 - Brown makes an
interesting observation
1966 - Tochtermann reports Ph 3Cand BPh 3 trap across olefins;
coining of the term "FLP"
1959 - Wittig finds PPh 3 and BPh 3
does not form an adduct
BPh 3
[Ph 3C-]Na+
Ph 3C
and
BPh 3
Ph 3C
BPh 3
no observation of polybutadiene
Tochtermann, W. ACIE, 1966, 5, 351-371.
The Advent of FLP Chemistry
2006 - Stephan finds FLPs
can reversibly split H 2
1942 - Brown makes an
interesting observation
1966 - Tochtermann reports Ph 3Cand BPh 3 trap across olefins;
coining of the term "FLP"
1959 - Wittig finds PPh 3 and BPh 3
does not form an adduct
Me
F
F
F
Me
PH
+
B(C 6F 5)3
(Me 3C6H 2)2P
B(C 6F 5)2
H
Me 2
F
F
F
Me 2SiHCl
F
H
(Me 3C6H 2)2P
B(C 6F 5)2
H
F
F
Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Science, 2006, 314, 1124-1126.
The Advent of FLP Chemistry
2006 - Stephan finds FLPs
can reversibly split H 2
1942 - Brown makes an
interesting observation
1966 - Tochtermann reports Ph 3Cand BPh 3 trap across olefins;
coining of the term "FLP"
1959 - Wittig finds PPh 3 and BPh 3
does not form an adduct
F
F
F
H
(Me 3C6H 2)2P
heat
B(C 6F 5)2
F
(Me 3C6H 2)2P
B(C 6F 5)2
H2
H
F
F
F
F
Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Science, 2006, 314, 1124-1126.
The Advent of FLP Chemistry
2006 - Stephan finds FLPs
can reversibly split H 2
1942 - Brown makes an
interesting observation
1966 - Tochtermann reports Ph 3Cand BPh 3 trap across olefins;
coining of the term "FLP"
1959 - Wittig finds PPh 3 and BPh 3
does not form an adduct
F
F
F
H
(Me 3C6H 2)2P
heat
B(C 6F 5)2
F
(Me 3C6H 2)2P
B(C 6F 5)2
H2
H
F
F
F
F
Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Science, 2006, 314, 1124-1126.
The Advent of FLP Chemistry
2006 - Stephan finds FLPs
can reversibly split H 2
1942 - Brown makes an
interesting observation
1966 - Tochtermann reports Ph 3Cand BPh 3 trap across olefins;
coining of the term "FLP"
1959 - Wittig finds PPh 3 and BPh 3
does not form an adduct
2007 - FLPs catalytically
perform hydrogenations
B(C 6F 5)2
(tBu) 2P
N
tBu
(5 mol %)
1 atm H 2
Ph
HN
tBu
Ph
98%
(C 6F 5)Ph2P (20 mol %)
B(C 6F 5)3 (20 mol %)
Ph
Ph
5 bar H 2
Me
Ph
Ph
99%
Chase, P.A.; Welch, G. C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
Greb, L.; Ona-Burgos, P.; Schirmer, B.; Grimme, S.; Stephan, D. W.; Paradies, J. ACIE, 2012, 51, 10164-10168.
The Advent of FLP Chemistry
2006 - Stephan finds FLPs
can reversibly split H 2
1942 - Brown makes an
interesting observation
2008 - FLPs reversibly
bind CO2
1966 - Tochtermann reports Ph 3Cand BPh 3 trap across olefins;
coining of the term "FLP"
1959 - Wittig finds PPh 3 and BPh 3
does not form an adduct
tBu 3P
+
B(C 6F 5)3
2007 - FLPs catalytically
perform hydrogenations
CO2, 25 ºC
(tBu) 3P
B(C 6F 5)3
O
-CO2, 80 ºC
vacuum, 5 hrs
O
Mömming, C. M.; Otten, E.; Kehr, G.; Frölich, R.; Grimme, S.; Stephan, D. W.; Erker, G. ACIE, 2009, 48, 6643-6646.
The Advent of FLP Chemistry
2006 - Stephan finds FLPs
can reversibly split H 2
1942 - Brown makes an
interesting observation
2008 - FLPs reversibly
bind CO2
1966 - Tochtermann reports Ph 3Cand BPh 3 trap across olefins;
coining of the term "FLP"
1959 - Wittig finds PPh 3 and BPh 3
does not form an adduct
2007 - FLPs catalytically
perform hydrogenations
Me
Me
B(C 6F 5)2
OMe
Me
N
2010 - Asymmetric
hydrogenation
of imines with FLPs
Ph
OMe
HN
(5 mol %)
Me
tBu 3P (5 mol %)
25 bar H 2
Me
>99%
81% ee
Chen, D.; Wang, Y.; Klankermeyer, J. ACIE, 2010, 49, 9475-9578.
The Advent of FLP Chemistry
2006 - Stephan finds FLPs
can reversibly split H 2
1942 - Brown makes an
interesting observation
1966 - Tochtermann reports Ph 3Cand BPh 3 trap across olefins;
coining of the term "FLP"
1959 - Wittig finds PPh 3 and BPh 3
does not form an adduct
2008 - FLPs reversibly
bind CO2
2010 - Asymmetric
hydrogenation
of imines with FLPs
2007 - FLPs catalytically
perform hydrogenations
■ I. Theories on the Mechanism of H 2 Activation
■ II. Applications of FLPs in Hydrogenation Reactions
and Storage of Small Molecules
The Advent of FLP Chemistry
2006 - Stephan finds FLPs
can reversibly split H 2
1942 - Brown makes an
interesting observation
1966 - Tochtermann reports Ph 3Cand BPh 3 trap across olefins;
coining of the term "FLP"
1959 - Wittig finds PPh 3 and BPh 3
does not form an adduct
2008 - FLPs reversibly
bind CO2
2010 - Asymmetric
hydrogenation
of imines with FLPs
2007 - FLPs catalytically
perform hydrogenations
■ I. Theories on the Mechanism of H 2 Activation
■ II. Applications of FLPs in Hydrogenation Reactions
and Storage of Small Molecules
Two Competing Models on the Mechanism of H 2 Activation
R
R
R
H
R
H
A
R
R
D → σ* donation
R
R
H
H
D
R
R
R
σ → A donation
Electron Transfer Model
R 3P
R
BR 3
Electric Field Model
Initial Hypotheses on the Mechanism of H 2 Activation
tBu 3P
+
B(C 6F 5)3
Mes 3P
+
B(C 6F 5)3
H2
1 atm
H2
1 atm
[tBu 3PH][HB(C 6F 5)3]
[Mes 3PH][HB(C 6F 5)3]
Welch, C.; Stephan, D. W. JACS, 2007, 129 , 1880-1881.
Initial Hypotheses on the Mechanism of H 2 Activation
tBu 3P
+
B(C 6F 5)3
Mes 3P
+
B(C 6F 5)3
H2
[tBu 3PH][HB(C 6F 5)3]
1 atm
H2
[Mes 3PH][HB(C 6F 5)3]
1 atm
R 3P
H
BR 3
H
σ (H 2) → p (B)
PR 3 + BR 3
R 3P
H
[R 3PH][HBR 3]
H
BR 3
LP (P) → σ* (H 2)
Welch, C.; Stephan, D. W. JACS, 2007, 129 , 1880-1881.
First DFT Study on the Mechanism of H 2 Activation
tBu 3P ------ B(C 6F 5)3
■ Multiple H-bonds between C–H---F groups give rise to a preorganized complex
■ Non-directional dispersion forces between tBu and C6F 5 groups render the complex flexible
Rokob, T. A.; Hamza, A.; Stirling, A.; Soós, T.; Pápai, I. ACIE, 2008, 120 , 2469-2472.
First DFT Study on the Mechanism of H 2 Activation
tBu 3P --- H–H --- B(C 6F 5)3
■ Located T.S. features a nearly linear P–H–H–B axis
■ H 2 bond elongated from 0.74 A to 0.79 A indicative of an early T.S.
■ 10.4 kcal/mol higher than tBu 3P---B(C 6F 5)3 + H 2
Rokob, T. A.; Hamza, A.; Stirling, A.; Soós, T.; Pápai, I. ACIE, 2008, 120 , 2469-2472.
First DFT Study on the Mechanism of H 2 Activation
tBu 3P --- H–H --- B(C 6F 5)3
■ Discovered significant H 2 polarization
■ Electron transfer proceeds via simultaneous tBu 3P → σ∗(H 2) and σ(H 2) → B(C 6F 5)3 donation
Rokob, T. A.; Hamza, A.; Stirling, A.; Soós, T.; Pápai, I. ACIE, 2008, 120 , 2469-2472.
First DFT Study on the Mechanism of H 2 Activation
The Electron Transfer Model
■ Non-bonding interactions between bulky substituents lead to a higher E frustrated complex
Rokob, T. A.; Hamza, A.; Stirling, A.; Soós, T.; Pápai, I. ACIE, 2008, 120 , 2469-2472.
First DFT Study on the Mechanism of H 2 Activation
The Electron Transfer Model
■ Non-bonding interactions between bulky substituents lead to a higher E frustrated complex
■ Frustration energy, ΔEt, decreases the activation energy and renders hydrogen splitting
facile and highly exothermic via reactant state destabilization
Rokob, T. A.; Hamza, A.; Stirling, A.; Soós, T.; Pápai, I. ACIE, 2008, 120 , 2469-2472.
First DFT Study on the Mechanism of H 2 Activation
The Electron Transfer Model
■ Non-bonding interactions between bulky substituents lead to a higher E frustrated complex
■ Frustration energy, ΔEt, decreases the activation energy and renders hydrogen splitting
facile and highly exothermic via reactant state destabilization
■ Non-bonding interactions between bulky substituents stabilizes both the T.S. and product
Rokob, T. A.; Hamza, A.; Stirling, A.; Soós, T.; Pápai, I. ACIE, 2008, 120 , 2469-2472.
An Alternative Mechanism of H 2 Activation
Mes
C6F 5
C6F 5
B
C6F 5
P
Mes
H2
P
Mes
H
C6F 5
H
Mes
Ph
Ph
Mes
Mes
B
P
Mes
H2
B
Ph
Mes
P
B
H
Ph
H
Linear T.S. not possible for certain tethered FLPs
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
An Alternative Mechanism of H 2 Activation
Calculated T.S. employing a dispersion corrected DFT model:
(C 6F 5)3B ---- PtBu 3 + H 2
(C 6F 5)2B
PMes 2
+
H2
Calculated T.S. do not have a linear relationship along P–H–H–B axis
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
An Alternative Mechanism of H 2 Activation
incr. E
2 kcal/mol between
each line
2-D potential E surface for tBu 3P + B(C 6F 5)3 system
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
An Alternative Mechanism of H 2 Activation
incr. E
peak
2 kcal/mol between
each line
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
An Alternative Mechanism of H 2 Activation
artificial T.S.
peak
Pápai's non-dispersion corrected DFT model overestimated the P–B bond distance
resulting in a T.S. that is otherwise not present
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
An Alternative Mechanism of H 2 Activation
The Electric Field Model
R 3P
BR 3
■ Neglect the FLP as a molecular species
and replace it by an electric field
■ Polarization from electric field generated in
the interior of the FLP cavity allows for H 2 splitting
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
An Alternative Mechanism of H 2 Activation
The Electric Field Model
R 3P
BR 3
■ Neglect the FLP as a molecular species
and replace it by an electric field
■ Polarization from electric field generated in
the interior of the FLP cavity allows for H 2 splitting
F = Electric field strength (a.u.)
(1 a.u. = 5.1422 x 1011 Vm-1)
Critical field strength needed for H 2 splitting is 0.05 – 0.06 a.u.
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
An Alternative Mechanism of H 2 Activation
The Electric Field Model
F = Electric field strength (a.u.)
(1 a.u. = 5.1422 x 1011 Vm-1)
Critical field strength needed for H 2 splitting is 0.05 – 0.06 a.u.
tBu
C6F 5
P
B
tBu C6F 5
C6F 5
tBu
0.04 - 0.06 a.u.
C6F 5
Mes
P
B
C6F 5
Mes
0.04 - 0.06 a.u.
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
An Alternative Mechanism of H 2 Activation
The Electric Field Model
■ H-bonding interactions form the FLP while non-directional dispersion forces instill
flexibility allowing H 2 entry – termed the "preparation step"
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
An Alternative Mechanism of H 2 Activation
The Electric Field Model
■ H-bonding interactions form the FLP while non-directional dispersion forces instill
flexibility allowing H 2 entry – termed the "preparation step"
■ Activation energy is the "preparation step"; after entrance H 2 splitting is barrierless
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
An Alternative Mechanism of H 2 Activation
The Electric Field Model
■ H-bonding interactions form the FLP while non-directional dispersion forces instill
flexibility allowing H 2 entry – termed the "preparation step"
■ Activation energy is the "preparation step"; after entrance H 2 splitting is barrierless
■ No molecular orbitals arguments invoked; believe this accounts for the similar reactivity
of chemically different FLPs
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
The Electron Transfer Model Revisited
C6F 5
tBu
P
tBu
tBu
+
tBu
B
C6F 5
C
C6F 5
C6F 5
N
N
tBu
+
C6F 4H
+
B
C6F 5
C6F 5
N
Mes
B
C6F 4H
Rokob, T. A.; Bakó, I.; Stirling, A.; Hamza, A.; Pápai, I. JACS, 2013, 135 , 4425-4437.
The Electron Transfer Model Revisited
C6F 5
tBu
P
tBu
tBu
+
tBu
B
C6F 5
C
C6F 5
C6F 5
N
N
tBu
+
C6F 4H
+
B
C6F 5
C6F 5
N
Mes
B
C6F 4H
Rokob, T. A.; Bakó, I.; Stirling, A.; Hamza, A.; Pápai, I. JACS, 2013, 135 , 4425-4437.
The Electron Transfer Model Revisited
F = Electric field strength (a.u.)
(1 a.u. = 5.1422 x 1011 Vm-1)
■ At the "critical field" of 0.06 a.u., the activation energy is 75 kcal/mol
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
Rokob, T. A.; Bakó, I.; Stirling, A.; Hamza, A.; Pápai, I. JACS, 2013, 135 , 4425-4437.
The Electron Transfer Model Revisited
F = Electric field strength (a.u.)
(1 a.u. = 5.1422 x 1011 Vm-1)
■ At the "critical field" of 0.06 a.u., the activation energy is 75 kcal/mol
■ Only at higher electric fields, 0.09 a.u. and above, does the activation energy lower
to a reasonable range (20 kcal/mol or lower)
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
Rokob, T. A.; Bakó, I.; Stirling, A.; Hamza, A.; Pápai, I. JACS, 2013, 135 , 4425-4437.
The Electron Transfer Model Revisited
F = Electric field strength (a.u.)
(1 a.u. = 5.1422 x 1011 Vm-1)
■ At the "critical field" of 0.06 a.u., the activation energy is 75 kcal/mol
■ Only at higher electric fields, 0.09 a.u. and above, does the activation energy lower
to a reasonable range (20 kcal/mol or lower)
■ Only at field strengths above 0.1 a.u., does H 2 splitting become "barrierless"
Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. ACIE, 2010, 49, 1402-1405.
Rokob, T. A.; Bakó, I.; Stirling, A.; Hamza, A.; Pápai, I. JACS, 2013, 135 , 4425-4437.
The Electron Transfer Model Revisited
C6F 5
tBu
P
tBu
tBu
+
tBu
B
C6F 5
C
C6F 5
C6F 5
N
N
tBu
+
C6F 4H
+
B
C6F 5
C6F 5
N
Mes
B
C6F 4H
Rokob, T. A.; Bakó, I.; Stirling, A.; Hamza, A.; Pápai, I. JACS, 2013, 135 , 4425-4437.
The Electron Transfer Model Revisited
C6F 5
tBu
P
tBu
tBu
+
tBu
B
C6F 5
C
C6F 5
C6F 5
N
N
tBu
+
C6F 4H
+
B
C6F 5
C6F 5
N
Mes
B
C6F 4H
Rokob, T. A.; Bakó, I.; Stirling, A.; Hamza, A.; Pápai, I. JACS, 2013, 135 , 4425-4437.
The Electron Transfer Model Revisited
C6F 5
tBu
P
tBu
tBu
+
tBu
B
C6F 5
C
C6F 5
C6F 5
N
N
tBu
+
C6F 4H
+
B
C6F 5
C6F 5
N
Mes
B
C6F 4H
EFs within FLP cavities are not homogeneous and are not always strong enough to effect H 2 splitting
Rokob, T. A.; Bakó, I.; Stirling, A.; Hamza, A.; Pápai, I. JACS, 2013, 135 , 4425-4437.
The Electron Transfer Model Revisited
Me
tBu
P
tBu
tBu
tBu
C
N
N
tBu
Mes
N
+ B(C 6F 5)3
+ B(C 6F 5)3
+ B(C 6F 5)3
1
2
3
Mes
B
B
C6F 5
tBu
C6F 5
Me
4
C6F 5
N
C6F 5
P
Me
Me
tBu
5
Ph
P
B
Ph
6
Rokob, T. A.; Bakó, I.; Stirling, A.; Hamza, A.; Pápai, I. JACS, 2013, 135 , 4425-4437.
The Electron Transfer Model Revisited
0.09 a.u.
For none of the FLPs studied is the EF along the H 2 axis high enough to permit H 2 splitting
Me
tBu
P
tBu
tBu
tBu
C
N
N
tBu
Mes
N
+ B(C 6F 5)3
+ B(C 6F 5)3
+ B(C 6F 5)3
1
2
3
Mes
B
B
C6F 5
tBu
C6F 5
Me
4
C6F 5
N
C6F 5
P
Me
Me
tBu
5
Ph
P
B
Ph
6
Rokob, T. A.; Bakó, I.; Stirling, A.; Hamza, A.; Pápai, I. JACS, 2013, 135 , 4425-4437.
The Electron Transfer Model Revisited
H
H
H
R
R
H
A
R
R
R
R
D → σ* donation
H
H
R
R
H
H
D
R
R
R
R
σ → A donation
■ Use of dispersion corrected DFT basis set results in a generally bent D–H–H–A geometry
explained by frontier orbitals aligning themselves for optimal orbital overlap
■ Deviation from ideal 180º D---H 2 angle and 90º H 2---A angle due to polarization of the
σ / σ* orbitals of H 2
Rokob, T. A.; Bakó, I.; Stirling, A.; Hamza, A.; Pápai, I. JACS, 2013, 135 , 4425-4437.
The Advent of FLP Chemistry
2006 - Stephan finds FLPs
can reversibly split H 2
1942 - Brown makes an
interesting observation
1966 - Tochtermann reports Ph 3Cand BPh 3 trap across olefins;
coining of the term "FLP"
1959 - Wittig finds PPh 3 and BPh 3
does not form an adduct
2008 - FLPs reversibly
bind CO2
2010 - Asymmetric
hydrogenation
of imines with FLPs
2007 - FLPs catalytically
perform hydrogenations
■ I. Theories on the Mechanism of H 2 Activation
■ II. Applications of FLPs in Hydrogenation Reactions
and Storage of Small Molecules
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
F
F
PR 2
(C 6F 5)2B
F
F
1 R = Mes
2 R = tBu
N
Ph
tBu
5 mol % cat.
1 atm H 2
80 ºC
HN
tBu
Ph
79% (1)
98% (2)
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
F
F
PR 2
(C 6F 5)2B
F
N
1 R = Mes
2 R = tBu
tBu
5 mol % cat.
1 atm H 2
80 ºC
Ph
F
HN
tBu
Ph
79% (1)
98% (2)
Ph
N
Ph
SO2Ph
Ph
97% (1)
87% (2)
N
Ph
Ph
N
Ph
88% (1)
Ph
98% (1)
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
■ Catalytic hydrogenation only possible for sterically hindered imines
F
F
Mes 2P
B(C 6F 5)2
F
N
Ph
Ph
F
HN
5 mol %
5 atm H 2
120 ºC
Ph
Ph
5%
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
■ Catalytic hydrogenation only possible for sterically hindered imines
F
F
Mes 2P
B(C 6F 5)2
F
N
Ph
Ph
F
HN
5 mol %
5 atm H 2
120 ºC
Ph
Ph
5%
F
F Bn
H
N
Mes 2P
Bn
B(C 6F 5)2
F
F
adduct formation between Lewis Basic product
and FLP shuts down catalytic activity
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
■ Catalytic hydrogenation only possible for sterically hindered imines
F
F
Mes 2P
B(C 6F 5)2
F
N
Ph
F
HN
5 mol %
5 atm H 2
120 ºC
Ph
Ph
Ph
5%
■ Protecting Imine with B(C 6F 5)3 recovers catalytic activity
F
F
Mes 2P
(C 6F 5)3B
Ph
B(C 6F 5)2
F
N
Ph
F
5 mol %
5 atm H 2
120 ºC
(C 6F 5)3B
N
Ph
Ph
57%
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
■ Catalytic hydrogenation only possible for B(C 6F 5)3 protected nitriles
F
F
Mes 2P
B(C 6F 5)2
B(C 6F 5)3
F
N
5 mol %
R
5 atm H 2
120 ºC
F
HN
B(C 6F 5)3
R
R = Me, 75%
R = Ph, 84%
Only the amine is isolated (cannot isolate imine)
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
F
F
Mes 2P
B(C 6F 5)2
F
F
H2
F
F
H
Mes 2P
B(C 6F 5)2
H
F
F
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
F
F
Mes 2P
B(C 6F 5)2
F
F
H
N
F
H2
F
F
R
F
H
Mes 2P
H
Mes 2P
B(C 6F 5)2
B(C 6F 5)2
H
Ph
F
F
F
F
proton transfer
N
R
Ph
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
F
HN
Mes 2P
R
F
N
B(C 6F 5)2
F
Ph
H
F
F
H2
F
F
R
F
H
Mes 2P
H
Mes 2P
B(C 6F 5)2
B(C 6F 5)2
H
Ph
F
F
F
F
proton transfer
N
R
Ph
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
F
HN
Mes 2P
R
F
N
B(C 6F 5)2
F
Ph
H
F
F
H2
F
F
R
F
H
Mes 2P
H
Mes 2P
B(C 6F 5)2
B(C 6F 5)2
H
Ph
F
F
F
F
proton transfer
N
R
Ph
Proton transfer precedes hydride delivery
Increased rates with electron rich imines (R = tBu, 1 hr vs. R = SO2Ph, >10 hrs)
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
F
F
Mes 2P
B(C 6F 5)2
F
F
H2
F
F
H
Mes 2P
B(C 6F 5)2
H
F
F
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
F
F
Mes 2P
B(C 6F 5)2
F
F
(C 6F 5)3B
Ph
N
F
H2
F
F
F
R
H
Mes 2P
H
Mes 2P
B(C 6F 5)2
H
B(C 6F 5)2
H
F
F
F
F
hydride delivery
(C 6F 5)3B
N
R
Ph
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
F
(C 6F 5)3B
H
N
Mes 2P
R
F
Ph
N
B(C 6F 5)2
F
Ph
(C 6F 5)3B
F
F
H2
F
F
F
R
H
Mes 2P
H
Mes 2P
B(C 6F 5)2
H
B(C 6F 5)2
H
F
F
F
F
hydride delivery
(C 6F 5)3B
N
R
Ph
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
The First Example of Catalytic Hydrogenations with FLPs
Hydrogenation of Imines, Nitriles, and Aziridines
F
(C 6F 5)3B
H
N
Mes 2P
R
F
Ph
N
B(C 6F 5)2
F
Ph
(C 6F 5)3B
F
F
H2
F
F
F
R
H
Mes 2P
H
Mes 2P
B(C 6F 5)2
H
B(C 6F 5)2
H
F
F
F
F
hydride delivery
(C 6F 5)3B
N
R
Ph
Hydride delivery precedes proton transfer
Chase, P. A.; Welch, C.; Jurca, T.; Stephan, D. W. ACIE, 2007, 46, 8050-8053.
An Improved FLP
Hydrogenation of Ketimines and Enamines
H
H2
Mes 2P
B(C 6F 5)2
B(C 6F 5)3
Mes 2P
H
N
Ph
tBu
Mes 2P
B(C 6F 5)2
HN
20 mol %
2.5 bar H 2
r.t.
tBu
Ph
87%
Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Fröhlich, R.; Erker, G. ACIE, 2008, 47, 7543-7546.
An Improved FLP
Hydrogenation of Ketimines and Enamines
H
H2
Mes 2P
B(C 6F 5)2
B(C 6F 5)3
Mes 2P
H
N
Mes 2P
tBu
B(C 6F 5)2
HN
20 mol %
2.5 bar H 2
r.t.
Ph
tBu
Ph
87%
O
N
N
Ph
tBu
N
N
70%
(5 mol %)
70%
(3 mol %)
Me
70%
(5 mol %)
70%
(10 mol %)
Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Fröhlich, R.; Erker, G. ACIE, 2008, 47, 7543-7546.
Catalytic Hydrogenations Using only B(C6F 5 )3
Hydrogenation of Imines
N
Ph
tBu
B(C 6F 5)3 (cat.)
4 atm H 2
toluene, 25 ºC
(C 6F 5)3B
Ph
N
tBu
H 2N
4 atm H 2
toluene, 80 ºC
tBu
HB(C 6F 5)3
Ph
Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Comm., 2008, 1701-1703.
Catalytic Hydrogenations Using only B(C6F 5 )3
Hydrogenation of Imines
N
Ph
tBu
B(C 6F 5)3 (cat.)
4 atm H 2
toluene, 25 ºC
(C 6F 5)3B
Ph
N
tBu
H 2N
4 atm H 2
toluene, 80 ºC
tBu
HB(C 6F 5)3
Ph
Isolation of zwitterion suggests adduct formation with the product is reversible
and that the B(C 6F 5)3 catalyst can be recycled
Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Comm., 2008, 1701-1703.
Catalytic Hydrogenations Using only B(C6F 5 )3
Hydrogenation of Imines
N
Ph
tBu
B(C 6F 5)3 (cat.)
4 atm H 2
toluene, 25 ºC
(C 6F 5)3B
N
tBu
H 2N
4 atm H 2
toluene, 80 ºC
Ph
tBu
HB(C 6F 5)3
Ph
Isolation of zwitterion suggests adduct formation with the product is reversible
and that the B(C 6F 5)3 catalyst can be recycled
N
Ph
tBu
5 mol % B(C 6F 5)3
5 atm H 2
120 ºC
HN
tBu
Ph
89%
Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Comm., 2008, 1701-1703.
Catalytic Hydrogenations Using only B(C6F 5 )3
Hydrogenation of Imines
N
Ph
tBu
B(C 6F 5)3 (cat.)
4 atm H 2
toluene, 25 ºC
(C 6F 5)3B
N
tBu
H 2N
4 atm H 2
toluene, 80 ºC
Ph
tBu
HB(C 6F 5)3
Ph
Isolation of zwitterion suggests adduct formation with the product is reversible
and that the B(C 6F 5)3 catalyst can be recycled
B(C 6F 5)3
R
H2 +
N
R'
R
N
H
HB(C 6F 5)3
R'
Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Comm., 2008, 1701-1703.
Catalytic Hydrogenations Using only B(C6F 5 )3
Hydrogenation of Imines
N
Ph
tBu
B(C 6F 5)3 (cat.)
(C 6F 5)3B
4 atm H 2
toluene, 25 ºC
N
tBu
H 2N
4 atm H 2
toluene, 80 ºC
Ph
tBu
HB(C 6F 5)3
Ph
Isolation of zwitterion suggests adduct formation with the product is reversible
and that the B(C 6F 5)3 catalyst can be recycled
B(C 6F 5)3
R
H2 +
N
R'
H
R N B(C 6F 5)3
H
R'
R
N
H
HB(C 6F 5)3
R'
Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Comm., 2008, 1701-1703.
Catalytic Hydrogenations Using only B(C6F 5 )3
Hydrogenation of Imines
N
Ph
tBu
B(C 6F 5)3 (cat.)
(C 6F 5)3B
4 atm H 2
toluene, 25 ºC
N
tBu
H 2N
4 atm H 2
toluene, 80 ºC
Ph
tBu
HB(C 6F 5)3
Ph
Isolation of zwitterion suggests adduct formation with the product is reversible
and that the B(C 6F 5)3 catalyst can be recycled
R
NH
B(C 6F 5)3
R
H2 +
R'
N
R'
H
R N B(C 6F 5)3
H
R'
R
N
H
HB(C 6F 5)3
R'
Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Comm., 2008, 1701-1703.
Expanding the Scope to Oxygenated Substrates
Hydrogenation of Silyl Enol Ethers
PPh 2 PPh 2
H
H 2, r.t.
PPh 2 PPh 2
HB(C 6F 5)3
–H 2, 60 ºC
Wang, H.; Fröhlich, R.; Kehr, G.; Erker, G. Chem. Comm., 2008, 5966-5968.
Expanding the Scope to Oxygenated Substrates
Hydrogenation of Silyl Enol Ethers
PPh 2 PPh 2
H
H 2, r.t.
PPh 2 PPh 2
HB(C 6F 5)3
–H 2, 60 ºC
OTMS
R
OTMS
20 mol % cat.
2 bar H 2, r.t.
R
R'
Wang, H.; Fröhlich, R.; Kehr, G.; Erker, G. Chem. Comm., 2008, 5966-5968.
Expanding the Scope to Oxygenated Substrates
Hydrogenation of Silyl Enol Ethers
H
B(C 6F 5)3
H 2, r.t.
PPh 2 PPh 2
PPh 2 PPh 2
HB(C 6F 5)3
–H2, 60 ºC
OTMS
R
R
2 bar H 2, r.t.
OTMS
OTMS
OTMS
OTMS
20 mol % cat.
R
OTMS
93%
OTMS
Me
tBu
Ph
R'
89%
86%
85%
>99%
Wang, H.; Fröhlich, R.; Kehr, G.; Erker, G. Chem. Comm., 2008, 5966-5968.
Studies of Hantzsch Ester with B(C6F 5 )3 Unveils New Substrates
Hydrogenation of N-heterocycles
O
O
EtO
O
OEt
Me
N
H
Me
B(C 6F 5)3
-40 ºC
O
O
EtO
OEt
Me
N
H
Me
60%
EtO
HB(C 6F 5)3
-20 ºC
O
OEt
H
Me
B(C 6F 5)3
N
H
Me
55%
(70% at r.t.)
Webb, J. D.; Laberge, V. S.; Geier, S. J.; Stephan, D. W.; Crudden, C. M. Chem. Eur. J., 2010, 16 , 4895-4902.
Geier, S. J.; Chase, P. A.; Stephan, D. W. Chem. Comm., 2010, 46, 4884-4886.
Studies of Hantzsch Ester with B(C6F 5 )3 Unveils New Substrates
Hydrogenation of N-heterocycles
O
O
EtO
O
OEt
Me
N
H
Me
B(C 6F 5)3
-40 ºC
O
O
EtO
OEt
Me
N
H
Me
EtO
HB(C 6F 5)3
-20 ºC
60%
O
OEt
H
Me
B(C 6F 5)3
N
H
Me
55%
(70% at r.t.)
Observation of hydride delivery into pyridinium inspired
experimentation with N-heterocycle substrates
Webb, J. D.; Laberge, V. S.; Geier, S. J.; Stephan, D. W.; Crudden, C. M. Chem. Eur. J., 2010, 16 , 4895-4902.
Geier, S. J.; Chase, P. A.; Stephan, D. W. Chem. Comm., 2010, 46, 4884-4886.
Studies of Hantzsch Ester with B(C6F 5 )3 Unveils New Substrates
Hydrogenation of N-heterocycles
O
O
EtO
O
OEt
Me
N
H
Me
B(C 6F 5)3
-40 ºC
O
O
EtO
OEt
Me
N
H
EtO
HB(C 6F 5)3
-20 ºC
Me
60%
O
OEt
H
Me
B(C 6F 5)3
N
H
Me
55%
(70% at r.t.)
Observation of hydride delivery into pyridinium inspired
experimentation with N-heterocycle substrates
5 mol % B(C 6F 5)3
N
4 atm H 2, 2 hrs, r.t.
80%
N
H
HB(C 6F 5)3
N
H
Webb, J. D.; Laberge, V. S.; Geier, S. J.; Stephan, D. W.; Crudden, C. M. Chem. Eur. J., 2010, 16 , 4895-4902.
Geier, S. J.; Chase, P. A.; Stephan, D. W. Chem. Comm., 2010, 46, 4884-4886.
Studies of Hantzsch Ester with B(C6F 5 )3 Unveils New Substrates
Hydrogenation of N-heterocycles
O
O
O
EtO
OEt
Me
N
H
B(C 6F 5)3
-40 ºC
Me
O
O
EtO
OEt
Me
N
H
EtO
HB(C 6F 5)3
-20 ºC
Me
O
OEt
H
Me
60%
B(C 6F 5)3
N
H
Me
55%
Observation of hydride delivery into pyridinium inspired
experimentation with N-heterocycle substrates
5 mol % B(C 6F 5)3
2 hrs, r.t.
N
N
H
80%
N
H
80%
Ph
N
H
74%
Me
Me
N
H
88%
N
N
H
84%
Webb, J. D.; Laberge, V. S.; Geier, S. J.; Stephan, D. W.; Crudden, C. M. Chem. Eur. J., 2010, 16 , 4895-4902.
Geier, S. J.; Chase, P. A.; Stephan, D. W. Chem. Comm., 2010, 46, 4884-4886.
Aromatic Hydrogenation
Hydrogenation of Anilines to Cyclohexylamines
tBu
tBu
NH
Ph
B(C 6F 5)3
H 2, 25 ºC
tBu
NH 2
Ph
HB(C 6F 5)3
H2
NH 2
HB(C 6F 5)3
110 ºC
93%
Mahdi, T.; Heiden, Z. M.; Grimme, S.; Stephan, D. W. JACS, 2012, 134 , 4088-4091.
Aromatic Hydrogenation
Hydrogenation of Anilines to Cyclohexylamines
tBu
tBu
NH
Ph
B(C 6F 5)3
tBu
H 2, 25 ºC
NH 2
NH 2
H2
HB(C 6F 5)3
HB(C 6F 5)3
110 ºC
Ph
93%
iPr
NH 2
iPr
NH 2
iPr
NH 2
NH 2
Me
OMe
Me
Me
HB(C 6F 5)3
HB(C 6F 5)3
HB(C 6F 5)3
HB(C 6F 5)3
65%, 96 hrs
77%, 36 hrs
61%, 36 hrs
48%, 72 hrs
Mahdi, T.; Heiden, Z. M.; Grimme, S.; Stephan, D. W. JACS, 2012, 134 , 4088-4091.
Aromatic Hydrogenation
Hydrogenation of Anilines to Cyclohexylamines
tBu
tBu
NH
Ph
B(C 6F 5)3
tBu
H 2, 25 ºC
NH 2
NH 2
H2
HB(C 6F 5)3
HB(C 6F 5)3
110 ºC
Ph
93%
iPr
iPr
NH 2
NH 2
iPr
NH 2
NH 2
Me
Me
OMe
Me
HB(C 6F 5)3
HB(C 6F 5)3
HB(C 6F 5)3
HB(C 6F 5)3
65%, 96 hrs
77%, 36 hrs
61%, 36 hrs
48%, 72 hrs
Ph
5 mol % B(C 6F 5)3
N
Ph
Ph
H 2, 110 ºC
H 2N
Ph
HB(C 6F 5)3
Ph
50%, 96 hrs
Mahdi, T.; Heiden, Z. M.; Grimme, S.; Stephan, D. W. JACS, 2012, 134 , 4088-4091.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Olefins
(C 6F 5)Ph2P
+
B(C 6F 5)3
H2
[(C 6F 5)Ph2PH] [HB(C 6F 5)3]
cation posseses much greater bronsted acidity
Greb, L.; Ona-Burgos, P.; Schirmer, B.; Grimme, S.; Stephan, D. W.; Paradies, J. ACIE, 2012, 51, 10164-10168.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Olefins
+
(C 6F 5)Ph2P
B(C 6F 5)3
H2
[(C 6F 5)Ph2PH] [HB(C 6F 5)3]
cation posseses much greater bronsted acidity
Ph
20 mol % (C 6F 5)Ph2P
20 mol % B(C 6F 5)3
Ph
Me
Ph
5 bar H 2, r.t.
Ph
99%
Greb, L.; Ona-Burgos, P.; Schirmer, B.; Grimme, S.; Stephan, D. W.; Paradies, J. ACIE, 2012, 51, 10164-10168.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Olefins
+
(C 6F 5)Ph2P
H2
B(C 6F 5)3
[(C 6F 5)Ph2PH] [HB(C 6F 5)3]
cation posseses much greater bronsted acidity
Ph
20 mol % (C 6F 5)Ph2P
20 mol % B(C 6F 5)3
Ph
Me
5 bar H 2, r.t.
Ph
Ph
99%
Ph
HB(C 6F 5)3
Me
Ph
Ph N
Ph
Me
Ph
hydride delivery
H HB(C 6F 5)3
69%
Ph
Me
Ph
Ph
NMePh
Ph 2NMe
Friedel Crafts
Ph
Ph
Me
31%
Greb, L.; Ona-Burgos, P.; Schirmer, B.; Grimme, S.; Stephan, D. W.; Paradies, J. ACIE, 2012, 51, 10164-10168.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Olefins
R
R
[(C 6F 5)Ph2PH] [HB(C 6F 5)3]
proton transfer
R
R
Me
R
HB(C 6F 5)3
hydride delivery
Me
R
Greb, L.; Ona-Burgos, P.; Schirmer, B.; Grimme, S.; Stephan, D. W.; Paradies, J. ACIE, 2012, 51, 10164-10168.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Olefins
R
[(C 6F 5)Ph2PH] [HB(C 6F 5)3]
proton transfer
R
Me
R
R
Me
R
HB(C 6F 5)3
hydride delivery
Me
R
Me
Me
96%
85%
Greb, L.; Ona-Burgos, P.; Schirmer, B.; Grimme, S.; Stephan, D. W.; Paradies, J. ACIE, 2012, 51, 10164-10168.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Olefins
R
[(C 6F 5)Ph2PH] [HB(C 6F 5)3]
proton transfer
R
Me
R
Me
HB(C 6F 5)3
R
Me
Me
96%
R
hydride delivery
Me
R
Me
MeO
85%
30%
Greb, L.; Ona-Burgos, P.; Schirmer, B.; Grimme, S.; Stephan, D. W.; Paradies, J. ACIE, 2012, 51, 10164-10168.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Olefins
R
[(C 6F 5)Ph2PH] [HB(C 6F 5)3]
proton transfer
R
Me
R
Me
HB(C 6F 5)3
R
Me
Me
96%
R
Me
MeO
85%
hydride delivery
Me
R
Me
Cl
30%
10%
Greb, L.; Ona-Burgos, P.; Schirmer, B.; Grimme, S.; Stephan, D. W.; Paradies, J. ACIE, 2012, 51, 10164-10168.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Olefins
R
[(C 6F 5)Ph2PH] [HB(C 6F 5)3]
proton transfer
R
Me
R
R
Me
HB(C 6F 5)3
R
Me
Me
hydride delivery
Me
R
Me
Me
TMS
Me
96%
MeO
85%
Cl
30%
10%
95%
Greb, L.; Ona-Burgos, P.; Schirmer, B.; Grimme, S.; Stephan, D. W.; Paradies, J. ACIE, 2012, 51, 10164-10168.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Olefins
R
R
[(C 6F 5)Ph2PH] [HB(C 6F 5)3]
Me
proton transfer
R
Me
R
HB(C 6F 5)3
R
Me
Me
hydride delivery
Me
R
Me
Me
TMS
Me
96%
MeO
85%
Cl
30%
10%
95%
99%
Me
Me
Me
Me
Me
99%
Greb, L.; Ona-Burgos, P.; Schirmer, B.; Grimme, S.; Stephan, D. W.; Paradies, J. ACIE, 2012, 51, 10164-10168.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Alkynes to Cis-Alkenes
R
NMe 2
B
H
C6F 5
R'
NMe 2
H
R'
B
C6F 5
R
Chernichenko, K.; Madarász, A.; Pápai, I.; Nieger, M.; Leskela, M.; Repo, T. Nat. Chem., 2013, 5, 718-723.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Alkynes to Cis-Alkenes
R
R'
NMe 2
NMe 2
B
H
H
R'
B
C6F 5
C6F 5
Me
Me
R
H2
N
H H
R'
B
C6F 5 H
R
Chernichenko, K.; Madarász, A.; Pápai, I.; Nieger, M.; Leskela, M.; Repo, T. Nat. Chem., 2013, 5, 718-723.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Alkynes to Cis-Alkenes
R
R'
NMe 2
NMe 2
B
H
C6F 5
Me
Me
H H
R'
R
R
H2
N
H
R'
B
C6F 5
H
H
R'
B
C6F 5 H
R
Chernichenko, K.; Madarász, A.; Pápai, I.; Nieger, M.; Leskela, M.; Repo, T. Nat. Chem., 2013, 5, 718-723.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Alkynes to Cis-Alkenes
NMe 2
C6F 5
B
C6F 5
R
R'
H2
Me
N
Me
H F
B
C6F 5 H
F
NMe 2
NMe 2
80 ºC
F
B
F
H
H
R'
B
C6F 5
C6F 5
R
F
Me
H
Me
H2
N
H
H H
R'
R
R'
B
C6F 5 H
R
Chernichenko, K.; Madarász, A.; Pápai, I.; Nieger, M.; Leskela, M.; Repo, T. Nat. Chem., 2013, 5, 718-723.
Hydrogenation of Non-Polar Substrates
Hydrogenation of Alkynes to Cis-Alkenes
NMe 2
C6F 5
B
C6F 5
R
R'
H2
Me
N
Me
H F
B
C6F 5 H
F
NMe 2
NMe 2
80 ºC
F
B
F
H
R'
B
C6F 5
C6F 5
NMe 2
D2
H
B
D
C6F 5
R
+
F
H
D
Me
H
Me
H2
N
H
R
H H
R'
R
R'
R'
B
C6F 5 H
R
Chernichenko, K.; Madarász, A.; Pápai, I.; Nieger, M.; Leskela, M.; Repo, T. Nat. Chem., 2013, 5, 718-723.
Asymmetric Catalytic Hydrogenation
Hydrogenation of Imines
Me
Me
B(C 6F 5)2
OMe
Me
OMe
Ph
N
HN
(5 mol %)
Me
Me
tBu 3P (5 mol %)
25 bar H 2
>99%
81% ee
Me
Me C6F 5F
B
Me
H F
N
Ph
Me
favored approach
F
Me
F
F
F
Me F
F
F
F
C6F 5
B
OMe
H
Me
Me
N
disfavored approach
Chen, D.; Wang, Y.; Klankermeyer, J. ACIE, 2010, 49, 9475-9578.
Applications in Green Chemistry
Reversible CO 2 Binding
tBu 3P
+
B(C 6F 5)3
CO2, 25 ºC
(tBu) 3P
B(C 6F 5)3
O
80 ºC
vacuum, 5 hrs
-CO2
O
87%
B(C 6F 5)3
Mes 2P
CO2, 25 ºC
Mes 2P
B(C 6F 5)3
O
CH2Cl2, > -20 ºC
-CO2
Mes 2P
B(C 6F 5)3
O
79%
Mömming, C. M.; Otten, E.; Kehr, G.; Fröhlich, R.; Grimme, S.; Stephan, D. W.; Erker, G. ACIE, 2009, 48, 6643-6646.
Applications in Green Chemistry
SO 2 and N 2O Binding
■ SO2 is a major air pollutant, precursor to acid rain
tBu 3P
+
B(C 6F 5)3
SO2, 25 ºC
(tBu) 3P
B(C 6F 5)3
S
O
O
80%
B(C 6F 5)3
Mes 2P
SO2, -75 ºC
Mes 2P
S* O
B(C 6F 5)3
O
Mes 2P
B(C 6F 5)3
83%
Sajid, M. et. al. Chem. Sci., 2013, 4, 213-219.
Applications in Green Chemistry
SO 2 and N 2O Binding
■ SO2 is a major air pollutant, precursor to acid rain
tBu 3P
+
B(C 6F 5)3
SO2, 25 ºC
(tBu) 3P
B(C 6F 5)3
S
O
O
80%
B(C 6F 5)3
Mes 2P
SO2, -75 ºC
Mes 2P
S* O
B(C 6F 5)3
O
Mes 2P
83%
B(C 6F 5)3
■ N 2O is a minor constituent in the atmosphere, but ~300x more potent as a greenhouse gas
tBu 3P
+
B(C 6F 5)3
N 2O, 25 ºC
(tBu) 3P
N
N
O
B(C 6F 5)3
76%
Sajid, M. et. al. Chem. Sci., 2013, 4, 213-219.
Otten, E.; Neu, R. C.; Stephan, D. W. JACS, 2009, 131 , 9918-9919.
Applications in Green Chemistry
Converting CO 2 to Fuels
■ Addition of Et 3SiH to CO2 in the presence of an FLP efficiently reduces CO2 to CH4
Me
Me
NH 2
Me
HB(C 6F 5)3
Me
Berkefeld, A.; Piers, W. E.; Parvez, M. JACS, 2010, 132 , 10660-10661.
Applications in Green Chemistry
Converting CO 2 to Fuels
■ Addition of Et 3SiH to CO2 in the presence of an FLP efficiently reduces CO2 to CH4
O
B(C 6F 5)3
TMPH
O
H
CO2
Me
Me
NH 2
Me
HB(C 6F 5)3
Me
Berkefeld, A.; Piers, W. E.; Parvez, M. JACS, 2010, 132 , 10660-10661.
Applications in Green Chemistry
Converting CO 2 to Fuels
■ Addition of Et 3SiH to CO2 in the presence of an FLP efficiently reduces CO2 to CH4
Et 3SiH + B(C 6F 5)3
O
B(C 6F 5)3
TMPH
O
Et 3Si
H
H
B(C 6F 5)3
CO2
O
O
H
SiEt 3
Me
Me
NH 2
Me
HB(C 6F 5)3
Me
Berkefeld, A.; Piers, W. E.; Parvez, M. JACS, 2010, 132 , 10660-10661.
Applications in Green Chemistry
Converting CO 2 to Fuels
■ Addition of Et 3SiH to CO2 in the presence of an FLP efficiently reduces CO2 to CH4
Et 3SiH + B(C 6F 5)3
O
B(C 6F 5)3
TMPH
O
H
Et 3Si
H
B(C 6F 5)3
CO2
O
SiEt 3
O
H
Me
Me
NH 2
Me
Et 3Si
H
–B(C 6F 5)3
B(C 6F 5)3
O
O
Et 3Si
HB(C 6F 5)3
Me
SiEt 3
H
H
Berkefeld, A.; Piers, W. E.; Parvez, M. JACS, 2010, 132 , 10660-10661.
Applications in Green Chemistry
Converting CO 2 to Fuels
■ Addition of Et 3SiH to CO2 in the presence of an FLP efficiently reduces CO2 to CH4
Et 3SiH + B(C 6F 5)3
O
B(C 6F 5)3
TMPH
O
H
Et 3Si
H
B(C 6F 5)3
CO2
O
SiEt 3
Me
O
Me
H
NH 2
Me
Et 3Si
H
O
O
Et 3Si
Me
–B(C 6F 5)3
B(C 6F 5)3
SiEt 3
HB(C 6F 5)3
Et 3Si
H
H
B(C 6F 5)3
O
H
Et 3Si
H
H
H
(Et 3Si)2O
+ B(C 6F 5)3
Berkefeld, A.; Piers, W. E.; Parvez, M. JACS, 2010, 132 , 10660-10661.
Applications in Green Chemistry
Converting CO 2 to Fuels
■ Addition of Et 3SiH to CO2 in the presence of an FLP efficiently reduces CO2 to CH4
Et 3SiH + B(C 6F 5)3
O
B(C 6F 5)3
TMPH
O
H
Et 3Si
H
B(C 6F 5)3
CO2
O
SiEt 3
Me
O
Me
H
NH 2
Me
Et 3Si
H
O
O
Et 3Si
Me
–B(C 6F 5)3
B(C 6F 5)3
SiEt 3
HB(C 6F 5)3
Et 3Si
H
H
B(C 6F 5)3
O
H
Et 3Si
H
(Et 3Si)2O
+ B(C 6F 5)3
Et 3Si
H
B(C 6F 5)3
H
HCH 3
H
(Et 3Si)2O
+ B(C 6F 5)3
Berkefeld, A.; Piers, W. E.; Parvez, M. JACS, 2010, 132 , 10660-10661.
Limitations and Future Directions
■ Currently cannot hydrogenate aldehydes or ketones catalytically
O
H
Me
Me
N
H2
Me
Me
HB(C 6F 5)3
Me
20 ºC, 1hr
Me
N
H2
O
Me
B(C 6F 5)3
Me
95%
■ Discovery of better FLP catalysts for asymmetric induction; expanding the scope
to olefin hydrogenations
■ Hydrogen gas storage - currently FLPs can achieve 0.25 wt % H 2; whereas to be practical
need 6 - 9 wt % H 2 (i.e. ammonia-borane)
Sumerin, V.; Schulz, F.; Nieger, M.; Leskelä, M.; Repo, T.; Rieger, B. ACIE, 2008, 47, 6001-6003.
Summary
■ I. Theories on the Mechanism of H 2 Activation
■ Electron Transfer Model that Invokes Frontier Molecular Orbitals
■ Electric Field Model that disregards Frontier Molecular Orbitals
■ Secondary non-covalent interactions play key role in lowering H 2 activation barrier
■ II. Applications of FLPs in Hydrogenation Reactions and Storage of Small Molecules
■ Catalytic hydrogenation of imines, enamines, nitriles, aziridines, silyl enol ethers,
cyclic ethers, alkenes, alkynes, ynones
■ Stoichiometric hydrogenations of aldehydes, ketones
■ Asymmetric imine hydrogenations
■ Advances in area of green chemistry