Milstein and co-workers developed the low-pressure hydrogenation of nitriles to
primary amines catalysed by pyridine-based pincer Ru complex 4 in which the
pyridine is dearomatised [24]. With lower catalyst loading (0.3 mol%), lower
temperature (110
C) and no extra additives, aromatic nitriles could be reduced
with high selectivity and conversions (Scheme 6). The authors assume a ligandassisted mechanism (Scheme 7), in which the catalyst 4 rapidly reacts with the nitrile
to form nitrile complex I, which is hydrogenated to the dihydride complex II in
which the aromaticity is restored and the side arm protonated. Next, in an outerAr C N
0.3 mol% 4
Ar
NH 2
100
o C, 5 bar, 24-60 h,
Benzene
N
t Bu 2 P
P t Bu 2
Ru
H CO 4
Scheme 6 Hydrogenation of nitriles to amines by Ru pincer complex 9
N
t Bu 2 P
P
t Bu 2
Ru
H CO
N
t Bu 2 P
P
t Bu 2
Ru
H CO
N
C
R
RCN
N
t Bu 2 P
P
t Bu 2
Ru
H CO
H
H
H
N C R
N
t Bu 2 P
P
t Bu 2
Ru
H CO 4
H
H
H
N
t Bu 2 P
P
t Bu 2
Ru
H CO
NH
R
RCN
H 2
-RCN
R
H
C NH
H 2
RCH 2 NH 2
I
II
III
IV
Scheme 7 Proposed mechanism of nitrile hydrogenation using a dearomatised Ru PNP pincer
complex
Catalytic Conversion of Nitriles by Metal Pincer Complexes
327