COMMUNICATIONS
3
0 bar of H at 110°C for 24 h (Table 1, entry 1).
2
Complex 2 was reported by us as the first complex of
earth-abundant-metal for the dehydrogenative synthe-
sis of cyclic imides as well as for the hydrogenation of
[13f,15]
esters and organic carbonates.
Interestingly, com-
plex 2 catalyzed the hydrogenation of azobenzene
under the conditions described above in 35% con-
version forming aniline in 27% yield as detected by
GC-MS (Table 1, entry 2). At longer reaction time
(
36 h) and higher temperature (120°C), the azobenzene
conversion increased to 60% and aniline was detected
in 52% yield. Using the Mn-PNN-bipyridyl complex 3
(
3 mol%), previously reported to catalyze the dehydro-
[16]
genative synthesis of hydrazones,
only 15% con-
version of the azobenzene was obtained (entry 4). We
have recently reported complex 4, bearing PPh groups
2
is a very active catalyst for the hydrogenation of urea
and carbamate derivatives. Interestingly, performing
Scheme 2. Reported catalytic hydrogenation of azo compounds
[17]
(above) and the manganese complexes (1-4) employed in this
the catalytic hydrogenation reaction using complex 4
study (present work).
t
(
3 mol%) and BuOK (3 mol%) resulted after 24 h in
65% conversion of azobenzene and aniline was
detected in 56% yield (Table 1, entry 5). After 36 h, in
of this manuscript), another heterogeneous procedure THF, 74% conversion of the azobenzene was obtained
for the catalytic hydrogenation of azobenzene using to form aniline in 68% yield. Moreover, 93% con-
[9]
Mo S clusters has been reported. Cleavage of azo version of the azobenzene was obtained using 3 mol%
3
4
derivatives into the corresponding amine(s) via transfer of complex 4 at 30 bar pressure upon increasing the
hydrogenation or using hydrogen donors were reported reaction temperature (130°C) and time (48 h), forming
but both these reports also need stoichiometric aniline in 87% isolated yield (entry 7). Changing the
[10]
additives.
Most of the literature reports on con- H pressure to 15 bar resulted in a lower conversion
2
version of azo compounds to amines are based on the (74%) and lower yield of aniline (52%) with 1,2-
reduction of azobenzene or hydrazobenzene as an diphenylhydrazine as a minor product (entry 8, Sche-
intermediate
reaction.
of
nitro
Despite all those reports, a straightfor-
ward, green, waste-free, cost-effective homogeneous zene, we attempted to utilize the manganese complex 4
compounds
reduction me 3a).
[
9,10a,11]
Following the successful hydrogenation of azoben-
catalytic system is challenging.
for the catalytic hydrogenation of azobenzene deriva-
In recent years there has been much interest in the
development of catalysts based on earth-abundant-
[12]
metals for a variety of (de)hydrogenation reactions.
Table 1. Manganese catalyzed hydrogenation of azobenzene to
aniline.
We and others have reported on pincer catalysts based
on earth-abundant-metals for the catalytic hydrogena-
tion of several carbonyl compounds such as aldehydes,
ketones, imides, esters, amides, carbamates and
ureas. Despite such strong interest in using earth-
abundant-metal complexes for hydrogenation reac-
tions, catalytic hydrogenation of azo compounds to
amines using an earth-abundant-metal complex is still
unknown. We report here the hydrogenation of azo
compounds to amines catalyzed by a pincer complex
of the earth-abundant manganese.
[a]
entry [Mn] solvent
Temp
T
conv. Aniline
(
°C).
(h) (%)
(%)
[13]
1
2
3
4
5
6
7
8
1
2
2
3
4
4
4
4
Toluene 110
Toluene 110
Toluene 120
Toluene 120
Toluene 120
24
24
36
24
24
36
48
48
–
–
35
60
15
65
74
93
74
27
52
–
[b]
56
68
87
52
THF
THF
THF
120
130
130
[
c]
[d]
[b]
We started our investigation by studying the
catalytic hydrogenation of azobenzene employing
[
a]
Reaction conditions:
Azobenzene (1 mmol), Mn-cat
t
manganese pincer complexes (1-4, Scheme 2) devel- (0.03 mmol), BuOK (0.03 mmol), 2 mL solvent, 30 bar H .
2
[b]
[c]
[d]
oped in our group for various (de)hydrogenation
reactions. Employing the Mn-PNP complex
The rest of product was 1,2-Diphenylhydrazine.
Isolated yield.
15 bar pressure was used. All yields and conversions are
1
(
3 mol%), which is the first manganese complex
1
[14]
calculated using GC-MS and H NMR with mesitylene as
reported for catalytic (de)hydrogenation reactions,
internal standard.
did not result in any conversion of azobenzene under
Adv. Synth. Catal. 2021, 363, 3744–3749
3745
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