42
X. Feng et al.
N
N
O
O
R
R
H B
22a: R = i Pr
22b: R = t Bu
N
N
O
O
i Pr
i Pr
H 3 B
22c
N
N
HB
23a: R = R' = Me
23b: R = R' = i Pr
23c: R = Me, R' = Bn
R
R'
OMe
N
N
N
Ph
BH
OMe
N
N
N
Ph
HB
24a
24b
10 : 1
Fig. 2.2 Chiral carbene–borane adducts
Table 2.7 Hydrogenations of imines by catalysts derived from ref. [22–24]
Ph
Me
N
Ph
[Ph 3 C][B(C 6 F 5 ) 4 ] (5 mol %)
chiral carbene borane (5 mol %)
H 2 (102 atm), CH 2 Cl 2
Ph
Me
HN
Ph
3a
5a
Entry precatalyst t (h) T (ºC) yield (%) ee (%)
1
22a
3
25
50
12
2
22b
24
25
0
-
3
22c
3
50
6
7
4
23a
24
−30
5
2 0
5
23b
24
−30
<5
8
6
23c
4
25
47
13
7
24
18
25
100
6
Scheme 2.10 Synthesis of
NHC-stabilized borenium
ions 26
N
N
O
O
B
R
R
NTf 2
N
N
O
O
B
R
R
H
HNTf 2 , toluene
rt, 1 h
26a: R = i Pr
26b: R = Me
26c: R = Bn
25
H