2 Frustrated Lewis Pair Catalyzed Asymmetric Reactions
35
up to 99% ee (Table 2.3). DFT studies suggested that both the repulsive steric and
stabilizing intermolecular noncovalent forces were very important in the stereodetermining hydride transfer step.
A ferrocene-based phosphane/borane frustrated Lewis pair catalyst 15 was
reported by Erker and coworkers [31], which was applied in the asymmetric hydrogenation of imines, giving the corresponding products in moderate yields and with
up to 69% ee [32] (Table 2.4). With α-phosphanyl ferrocenecarbaldehydeas starting
material, chiral alkene 14 was obtained after a Wittig–Horner olefination. The phosphonium/hydridoborate 15 was then obtained through the hydroboration with Piers’
borane and a following reaction with H 2 , and the major stereoisomer (pS,R)-15 could
easily be obtained in diastereomerically pure form by crystallization (Scheme 2.6).
To shorten the synthetic steps for chiral alkenes, the Du group developed a novel
type of chiral olefins 17, which were easily obtained by the reaction of chiral binaphthols 16 with 3-chloro-2-(chloromethyl)prop-1-ene in one step (Scheme 2.7). Chiral
boron Lewis acids 18 generated in situ from these chiral alkenes were found to
be effective for the asymmetric hydrogenation of imines, giving the corresponding
optically active amines in 84–99% yields with 45–89% ee’s (Table 2.5) [33].
Very recently, Wang and coworkers developed a novel type of chiral C 2 -symmetric
bisborane catalysts derived from chiral bicyclic[3.3.0] dienes 19 with HB(C 6 F 5 ) 2 or
HB(p-C 6 F 4 H) 2 [34]. Interestingly, tuning the reaction temperature could afford two
diastereomeric catalysts from the same diene precursor (Scheme 2.8). At 25 °C, a
kinetically controlled process predominated, and a thermodynamically controlled
hydroboration occurred at 80 °C. These bisboranes exhibited both excellent catalytic
activity (up to 200 TONs at −40 °C) and high enantioselectivity (up to 95% ee) for
the hydrogenation of imines (Scheme 2.9 and Table 2.6).
A variety of NHC–boranes and triazolium-based carbene–boranes were synthesized, the chirality is located either on the carbene or alternatively on the
borane(Fig. 2.2) [35]. Some of these species were effective precursors for the asymmetric hydrogenation of imine, but only giving very low enantioselectivities (1–20%
ee) (Table 2.7).
In 2019, the Fuchter group developed a chiral N-heterocyclic carbene (NHC)stabilized borenium ions for the asymmetric reduction of N-alkyl ketimines [36]. The
borenium catalysts could be prepared from hydride 25 by the treatment with HNTf 2
to give 26 in 73% yield on Gram scale (Scheme 2.10). For the asymmetric hydrogenation of N-alkyl ketimines, moderate to good enantioselectivities were achieved
(Table 2.8).
Vicinal diamines widely exist in natural products and biologically active
compounds, and are also very important building blocks in synthetic chemistry.
Catalytic hydrogenation of vicinal diimines provides a straightforward way for
the synthesis, the vicinal diamines. Du and coworkers reported the first metal-free
hydrogenation of 1,2-diaryl-1,2-diimines using B(C 6 F 5 ) 3 as catalyst, a variety of
cis-1,2-diaryl-1,2-diamines were afforded in 92–99% yields as single isomers [37]
(Table 2.9). For the asymmetric reaction, only 10% ee was obtained with the diene
29-derived chiral borane catalyst (Scheme 2.11).
35
up to 99% ee (Table 2.3). DFT studies suggested that both the repulsive steric and
stabilizing intermolecular noncovalent forces were very important in the stereodetermining hydride transfer step.
A ferrocene-based phosphane/borane frustrated Lewis pair catalyst 15 was
reported by Erker and coworkers [31], which was applied in the asymmetric hydrogenation of imines, giving the corresponding products in moderate yields and with
up to 69% ee [32] (Table 2.4). With α-phosphanyl ferrocenecarbaldehydeas starting
material, chiral alkene 14 was obtained after a Wittig–Horner olefination. The phosphonium/hydridoborate 15 was then obtained through the hydroboration with Piers’
borane and a following reaction with H 2 , and the major stereoisomer (pS,R)-15 could
easily be obtained in diastereomerically pure form by crystallization (Scheme 2.6).
To shorten the synthetic steps for chiral alkenes, the Du group developed a novel
type of chiral olefins 17, which were easily obtained by the reaction of chiral binaphthols 16 with 3-chloro-2-(chloromethyl)prop-1-ene in one step (Scheme 2.7). Chiral
boron Lewis acids 18 generated in situ from these chiral alkenes were found to
be effective for the asymmetric hydrogenation of imines, giving the corresponding
optically active amines in 84–99% yields with 45–89% ee’s (Table 2.5) [33].
Very recently, Wang and coworkers developed a novel type of chiral C 2 -symmetric
bisborane catalysts derived from chiral bicyclic[3.3.0] dienes 19 with HB(C 6 F 5 ) 2 or
HB(p-C 6 F 4 H) 2 [34]. Interestingly, tuning the reaction temperature could afford two
diastereomeric catalysts from the same diene precursor (Scheme 2.8). At 25 °C, a
kinetically controlled process predominated, and a thermodynamically controlled
hydroboration occurred at 80 °C. These bisboranes exhibited both excellent catalytic
activity (up to 200 TONs at −40 °C) and high enantioselectivity (up to 95% ee) for
the hydrogenation of imines (Scheme 2.9 and Table 2.6).
A variety of NHC–boranes and triazolium-based carbene–boranes were synthesized, the chirality is located either on the carbene or alternatively on the
borane(Fig. 2.2) [35]. Some of these species were effective precursors for the asymmetric hydrogenation of imine, but only giving very low enantioselectivities (1–20%
ee) (Table 2.7).
In 2019, the Fuchter group developed a chiral N-heterocyclic carbene (NHC)stabilized borenium ions for the asymmetric reduction of N-alkyl ketimines [36]. The
borenium catalysts could be prepared from hydride 25 by the treatment with HNTf 2
to give 26 in 73% yield on Gram scale (Scheme 2.10). For the asymmetric hydrogenation of N-alkyl ketimines, moderate to good enantioselectivities were achieved
(Table 2.8).
Vicinal diamines widely exist in natural products and biologically active
compounds, and are also very important building blocks in synthetic chemistry.
Catalytic hydrogenation of vicinal diimines provides a straightforward way for
the synthesis, the vicinal diamines. Du and coworkers reported the first metal-free
hydrogenation of 1,2-diaryl-1,2-diimines using B(C 6 F 5 ) 3 as catalyst, a variety of
cis-1,2-diaryl-1,2-diamines were afforded in 92–99% yields as single isomers [37]
(Table 2.9). For the asymmetric reaction, only 10% ee was obtained with the diene
29-derived chiral borane catalyst (Scheme 2.11).