10
D. W. Stephan
have proved to be highly effective catalysts for imine and enamine hydrogenation
(Scheme 1.5) [72, 73]. Catalyst optimization found that the borenium cation
[(Cl 2 C 3 (NMe) 2 )(9-BBN)]
+ to be most effective, affording a turnover frequency
of ~ 1000 h
−1 at 102 atm of H 2 and 25 C. Subsequent work by Crudden et al.
[74] has broadened the range of borenium cations to include those derived from
triazole-derived meso-ionic N-heterocyclic carbenes (MIC).
In 2013, we described the high Lewis acidity of electrophilic phosphonium
cations. In contrast to boron-based systems, these derive their Lewis acidity from
a low-lying σ
∗ orbital [75]. Moreover, we showed that [FP(C 6 F 5 ) 3 ][B(C 6 F 5 ) 4 ] in
combination with a bulky amine affected FLP hydrogenation of olefins (Scheme 1.5)
[76]. This proof of principle suggested that other potential catalysts could be derived
from a wider variety of main group Lewis acids.
In 2014, an innovative application of the concept of FLPs was reported by
Ingleson and coworkers [77]. These researchers showed that N-methylacridinium
salts are carbon-based Lewis acids (Scheme 1.5), which in the presence of 2,6lutidine generate a truly organic FLP. Such systems mediate the hydrogenation of
bulky imines.
The concept of FLPs has also been extended to unique pairs. For example,
the complex [((Ph 2 PC 6 H 4 ) 2 B(η
6 -Ph))RuCl][B(C 6 F 5 ) 4 ] is Lewis acidic on the π -
bound arene ring (Scheme 1.5) [78]. This Lewis acid yields an FLP in combination
with Mes 3 P and also acts as Lewis acid catalyst for the catalytic hydrogenation of
aldimines at room temperature. This system is a rather curious example of a catalyst
containing an ancillary metal center.
Extending the concept of FLPs, Wass among others have developed systems in
which transition metal species are used as the Lewis acid component [79]. On the
other hand, metals can also act as the basic component of an FLP. Examples of
this latter situation are a series of Ni species described by Peters and coworkers [80].
The complexes (ArB(C 6 H 4 PPh 2 ) 2 Ni Ar = Ph, Mes) activate H 2 reversibly, whereby a
boronhydride nickel(II) complex is generated. This species affects the hydrogenation
of olefin substrates under mild conditions.
While efforts to develop new FLPs derived from new, strong Lewis acids continue,
an alternative approach exploited a weaker Lewis acid and a stronger base. Perhaps
the most noteworthy examples of such systems are the salts, MPtBu 2 (M = Li, Na,
K), KH and KN(SiMe 3 ) 2 (Scheme 1.5). In a 2018 report, we [81] showed that these
systems, activate H 2 to generate phosphine and MH reversibly and indeed these
systems are capable of mediating imine and olefin hydrogenation.
1.2.5 Heterogeneous Hydrogenation Catalysis
FLPs have also been exploited to advance the development of heterogeneous hydrogenation catalysts. The groups of Guo and Wang [82] demonstrated that while a
clean gold surface is unreactive to H 2 , its combination with an imine or nitrile in
solution prompted the hydrogenation of C–N bonds. In this system, the Au surface
D. W. Stephan
have proved to be highly effective catalysts for imine and enamine hydrogenation
(Scheme 1.5) [72, 73]. Catalyst optimization found that the borenium cation
[(Cl 2 C 3 (NMe) 2 )(9-BBN)]
+ to be most effective, affording a turnover frequency
of ~ 1000 h
−1 at 102 atm of H 2 and 25 C. Subsequent work by Crudden et al.
[74] has broadened the range of borenium cations to include those derived from
triazole-derived meso-ionic N-heterocyclic carbenes (MIC).
In 2013, we described the high Lewis acidity of electrophilic phosphonium
cations. In contrast to boron-based systems, these derive their Lewis acidity from
a low-lying σ
∗ orbital [75]. Moreover, we showed that [FP(C 6 F 5 ) 3 ][B(C 6 F 5 ) 4 ] in
combination with a bulky amine affected FLP hydrogenation of olefins (Scheme 1.5)
[76]. This proof of principle suggested that other potential catalysts could be derived
from a wider variety of main group Lewis acids.
In 2014, an innovative application of the concept of FLPs was reported by
Ingleson and coworkers [77]. These researchers showed that N-methylacridinium
salts are carbon-based Lewis acids (Scheme 1.5), which in the presence of 2,6lutidine generate a truly organic FLP. Such systems mediate the hydrogenation of
bulky imines.
The concept of FLPs has also been extended to unique pairs. For example,
the complex [((Ph 2 PC 6 H 4 ) 2 B(η
6 -Ph))RuCl][B(C 6 F 5 ) 4 ] is Lewis acidic on the π -
bound arene ring (Scheme 1.5) [78]. This Lewis acid yields an FLP in combination
with Mes 3 P and also acts as Lewis acid catalyst for the catalytic hydrogenation of
aldimines at room temperature. This system is a rather curious example of a catalyst
containing an ancillary metal center.
Extending the concept of FLPs, Wass among others have developed systems in
which transition metal species are used as the Lewis acid component [79]. On the
other hand, metals can also act as the basic component of an FLP. Examples of
this latter situation are a series of Ni species described by Peters and coworkers [80].
The complexes (ArB(C 6 H 4 PPh 2 ) 2 Ni Ar = Ph, Mes) activate H 2 reversibly, whereby a
boronhydride nickel(II) complex is generated. This species affects the hydrogenation
of olefin substrates under mild conditions.
While efforts to develop new FLPs derived from new, strong Lewis acids continue,
an alternative approach exploited a weaker Lewis acid and a stronger base. Perhaps
the most noteworthy examples of such systems are the salts, MPtBu 2 (M = Li, Na,
K), KH and KN(SiMe 3 ) 2 (Scheme 1.5). In a 2018 report, we [81] showed that these
systems, activate H 2 to generate phosphine and MH reversibly and indeed these
systems are capable of mediating imine and olefin hydrogenation.
1.2.5 Heterogeneous Hydrogenation Catalysis
FLPs have also been exploited to advance the development of heterogeneous hydrogenation catalysts. The groups of Guo and Wang [82] demonstrated that while a
clean gold surface is unreactive to H 2 , its combination with an imine or nitrile in
solution prompted the hydrogenation of C–N bonds. In this system, the Au surface