from 77 to 98%, whereas aromatic nitriles could be reduced with isolated yields
between 68 and 99%. Interestingly, the usually problematic ester-substituted
benzonitrile was selectively reduced (94% isolated yield) leaving the ester group intact.
Ru NNP pincer complexes bearing an imidazole moiety were also developed by
the same group [23]. Different substituents on the phosphorus atom were investigated, but complex 3 carrying tert-butyl substituents gave the best results. A large
range of aromatic, heteroaromatic and aliphatic nitriles was hydrogenated at 30 bar
hydrogen (Scheme 5). For most substrates, a temperature of 50
C was sufficient, but
for more sluggish substrates, temperatures between 70 and 150
C were used. In all
cases the nitrile was completely converted, and the primary amines were obtained in
GC yields of 75–99%.
R C N
Ru cat
H 2
R
NH
H 2
R
NH 2
RCHO
-H 2 O
-RCHO
H 2 O
R
N
R
-NH 3
R
NH 2
R
N
H
R
NH 2
-H 2 O
1
Scheme 3 Role of water in the Ru pincer complex 1 catalysed hydrogenation of nitriles
R
C
N
R
NH 2
1 mol% 4
100
o C, 30 bar H 2
i
PrOH, 3 h
N
P
Ph 2
PPh 2
Ru
CO
H
HBH 3
H
2
Scheme 4 Hydrogenation of nitriles using Ru-MACHO-BH
R
C
N
R
NH 2
0.5-1 mol% 3
50-150
o C, 30 bar H 2
i PrOH, 3 h
N
P
t Bu 2
Ru
HBH 3
H
CO
H
N
N
3
Scheme 5 Hydrogenation of nitriles using Ru PNN complex 3
326
B. Guo et al.