of Et 3 B, it was possible to selectively reduce 4-acetyl-benzonitrile to
4-acetylbenzylamine.
Using the MACHO ligand as well as analogues with different substituents on
phosphorus (iPr, Cy), four pincer complexes were made based on cobalt by the
Beller group [31]. These cobalt complexes contain halogen ligands; this necessitates
the use of base and/or a reductant to activate the catalyst. Compared with iron
analogue 5a, these complexes require higher H 2 pressure (50 bar) and a higher
catalyst loading (4 mol%) and still need a longer reaction time (6 h) to reach
full conversion. The best catalyst was the one based on the MACHO ligand (11a).
A range of aromatic and aliphatic nitriles were hydrogenated in good yields
(Scheme 11).
Thus far, there is only a single report on the transfer hydrogenation of nitriles
catalysed by pincer complexes. Zhou, Liu and co-workers reported the Co-NNP
pincer complex-catalysed transfer hydrogenation of nitriles to primary, secondary
or tertiary amines using NH 3 BH 3 as hydrogen source (Scheme 12) [32]. At 50
C in
hexane, complex 8 bearing an imidazole moiety selectively converted aromatic and
aliphatic nitriles to primary amines in good to excellent yields (26 examples).
Transfer hydrogenation of aromatic nitriles with pyridine-based PNN cobalt complex 9 in hexafluoroisopropanol (HFIP) at room temperature surprisingly led to the
formation of the symmetric secondary amines in good to excellent yields. A few
aliphatic nitriles also gave the secondary amines, but in mediocre yields. If the same
hydrogenation was performed in the presence of primary or secondary amines, the
R
N
R
NH 2
115
o C,4 bar H 2
toluene, 8h
2 mol% X
6 mol% NaO
t Bu
4 mol% additive
22 samples
up to >99%
Co
Cl
N
N
N
N
Py
Cl
Mes
Mes
Co
N
N
N
N
Py
Mes
Mes
NaHBEt 3
BEt 3
Additive
Co
N
N
N
N
H
Mes
Mes
H
N
R
LA
LA=lewis acid
10
10'
Scheme 10 Use of a CCC-pincer ligand in the Co-catalysed hydrogenation of nitriles to amines
C
NH 2
80
o C, 50 bar H 2
dioxane, 6 h
N
P
PPh 2
Co Cl
Cl
H
R
4 mol% 11a
16 mol% NaBHEt 3 R
11a
Ph 2
N
Scheme 11 Hydrogenation of nitriles to amines catalysed by Co(MACHO)Cl 2 (11a)
332
B. Guo et al.
4-acetylbenzylamine.
Using the MACHO ligand as well as analogues with different substituents on
phosphorus (iPr, Cy), four pincer complexes were made based on cobalt by the
Beller group [31]. These cobalt complexes contain halogen ligands; this necessitates
the use of base and/or a reductant to activate the catalyst. Compared with iron
analogue 5a, these complexes require higher H 2 pressure (50 bar) and a higher
catalyst loading (4 mol%) and still need a longer reaction time (6 h) to reach
full conversion. The best catalyst was the one based on the MACHO ligand (11a).
A range of aromatic and aliphatic nitriles were hydrogenated in good yields
(Scheme 11).
Thus far, there is only a single report on the transfer hydrogenation of nitriles
catalysed by pincer complexes. Zhou, Liu and co-workers reported the Co-NNP
pincer complex-catalysed transfer hydrogenation of nitriles to primary, secondary
or tertiary amines using NH 3 BH 3 as hydrogen source (Scheme 12) [32]. At 50
C in
hexane, complex 8 bearing an imidazole moiety selectively converted aromatic and
aliphatic nitriles to primary amines in good to excellent yields (26 examples).
Transfer hydrogenation of aromatic nitriles with pyridine-based PNN cobalt complex 9 in hexafluoroisopropanol (HFIP) at room temperature surprisingly led to the
formation of the symmetric secondary amines in good to excellent yields. A few
aliphatic nitriles also gave the secondary amines, but in mediocre yields. If the same
hydrogenation was performed in the presence of primary or secondary amines, the
R
N
R
NH 2
115
o C,4 bar H 2
toluene, 8h
2 mol% X
6 mol% NaO
t Bu
4 mol% additive
22 samples
up to >99%
Co
Cl
N
N
N
N
Py
Cl
Mes
Mes
Co
N
N
N
N
Py
Mes
Mes
NaHBEt 3
BEt 3
Additive
Co
N
N
N
N
H
Mes
Mes
H
N
R
LA
LA=lewis acid
10
10'
Scheme 10 Use of a CCC-pincer ligand in the Co-catalysed hydrogenation of nitriles to amines
C
NH 2
80
o C, 50 bar H 2
dioxane, 6 h
N
P
PPh 2
Co Cl
Cl
H
R
4 mol% 11a
16 mol% NaBHEt 3 R
11a
Ph 2
N
Scheme 11 Hydrogenation of nitriles to amines catalysed by Co(MACHO)Cl 2 (11a)
332
B. Guo et al.