2 Frustrated Lewis Pair Catalyzed Asymmetric Reactions
75
O
S NH 2
+ HB(C 6 F 5 ) 2
O
S
NH 2
B
H
C 6 F 5
C6F5
NH 3 ·BH 3
O
S N
B(C 6 F 5 ) 2
Ar
Me
NHAr'
Ar
Me
NAr'
B H
C
N
H
H
N
S
O
t Bu
C 6 F 5
Me
Ar'
Ar
C 6 F 5
Fig. 2.5 Plausible catalytic pathway
were obtained predominantly in 58–93% yields with 28/72–75/25 dr (trans:cis) and
up to 99% ee (Table 2.35, entries 4–8) [62].
Optically active β-amino acids and their derivatives are very useful building
blocks in synthetic and medicinal chemistry. The FLP of Piers’ borane and (R)tert-butylsulfinamide was also effective for the asymmetric transfer hydrogenation
of β-N-substituted enamino esters. The desired products were afforded in moderate
to good yields with up to 91% ee (Table 2.36) [63]. Enantio-pure β-lactam could be
easily constructed via a single-step Grignard reaction.
Hantzsch esters are one of the most widely utilized hydrogen transfer reagents
for transfer hydrogenation. A highly efficient transfer hydrogenation of imines with
Hantzsch esters as hydrogen source was successfully realized in a FLP manner. Only
with 0.1 mol % of B(C 6 F 5 ) 3 as catalyst, the desired products could be afforded in
80–99% yields. Moreover, using chiral boranes derived from chiral dienes and Piers’
borane HB(C 6 F 5 ) 2 , the asymmetric reaction was realized to give the amine products
with up to 38% ee (Table 2.37) [64]. The reaction may involve an activation of
Hantzsch esters by borane catalysts instead of substrate-activation.
2.5 Miscellaneous
A frustrated Lewis acid/Brønsted base complex catalyzed α-amination of unactivated carbonyl compounds was developed by Wasa and coworkers [65]. B(C 6 F 5 ) 3 ,