D. Wang et al. / Journal of Molecular Catalysis A: Chemical 400 (2015) 14–21
15
H gas
2
2. Results and discussion
Cat.
2
.1. Investigation of the optimal reaction conditions for transfer
O
OH
hydrogenation of carbonyl compounds
Hydrogenation
R1
R2
R1
R2
Cat.
Hydrogen Donor
In a preliminary experiment, 4-chloroacetophenone 1a was cho-
sen as a test substrate to identify the optimal reaction conditions.
The transfer hydrogenation was initially carried out using EtOH
(2 mL) as hydrogen source and solvent, in the presence of 2 equiv
Chart 1. Hydrogenation of carbonyl compounds yielding alcohols.
◦
of NaOH at 80 C. The conversion increased with increased reaction
time in the range of 1–15 h, providing 95% conversion (Fig. 1a). It
was further found that the amount of NaOH is one of the most
crucial factors for the formation. Within 15 h, fleetly decreased
conversion was detected with the reduction of NaOH; 0.5 equiv of
NaOH promoted transfer hydrogenation producing only 23% con-
version (Fig. 1b). When 0.25 equiv of NaOH was used in the reaction,
78% conversion was obtained within 60 h, which revealed that
NaOH played the role of catalyst in this transfer hydrogenation.
Furthermore, the decreased conversion caused by the temperature
reduction, was demonstrated (Fig. 1c). The secondary alcohol 2a
was obtained with conversions of 13%, 31%, and 52%, at tempera-
synthesis include in particular H O, glycerine, EtOH, some ionic
2
liquids, and supercritical CO . As one of renewable and cheapest
2
reagents, ethanol, usually produced by fermenting starch, has the
potential to be ideal an alternative to 2-propanol and formate in
transfer hydrogenation [9]. However, the successful application of
ethanol as hydrogen source was rarely reported [10], mainly due
to its ability to produce stable transition metal complexes con-
taining carbonyl with the catalysts that are used for the transfer
hydrogenation process.
Nitro derivatives are a major family of polluants. Their reduction
products of nitroarenes, functionalized anilines are important pre-
cursors and intermediates for the manufacture of pharmaceuticals,
agrochemicals, pigments, dyes, rubbers, polymers, rubbers, corro-
sion inhibitors and photographic developers [11,12]. Reduction of
poisonous nitroarenes [13] based on catalytic hydrogenation, metal
mediated reductions and electrolytic reduction is the traditional
synthesis methods for anilines [14]. Recently, catalytic transfer
hydrogenation has emerged as a green and efficient route for the
formation of anilines, however, the uses of expensive transition
metals and/or ligands are necessary in the transformation [15], in
addition, the product contamination by these noble metals restricts
the application of such systems in several fields, and especially in
biomedicine. Thus, it is highly desirable to develop more economic
and pharmaceutically safe methodologies for synthesis of anilines,
as well as degradation of nitroarenes.
◦
◦
◦
tures of 50 C, 60 C, and 70 C, respectively.
Then we examined the influence of different bases and hydrogen
sources on the transfer hydrogenation of 4-chloroacetophenone
1a. As shown in Table 1, the reaction did not proceed at all
◦
when K2CO3 and Na2CO3 were employed at 80 C in EtOH
(Table 1, entries 1 and 2). K3PO4 was able to promote the same
reaction, albeit in poor conversion (9%) within 15 h. The trans-
fer hydrogenation by EtOH also took place in the presence of
other alkali hydroxides such as KOH and CsOH, the same cation
effect being revealed: both KOH (33%) and CsOH (57%) being
farther less active than NaOH (95%). The organic base Et3N
was not efficient at all for this transformation (entry 6). These
results are in agreement with those reported by the groups of
Ouali et al. [8] using 2-PrOH as solvent and hydrogen donor. The
model reaction was further carried out in various alcoholic solvents
◦
(hydrogen sources), with NaOH as catalyst at 80 C for 15 h. It was
Herein, we report that abundant and cheap NaOH pro-
motes transfer hydrogenation of carbonyls including ketones and
aldehydes compounds forming primary and secondary alcohols,
respectively, using EtOH as both hydrogen source and solvent
under relatively mild conditions. Additionally, nitroarenes are
hydrogenated to form anilines and azobenzenes based on the
NaOH-catalyzed transfer hydrogenation protocol with 2-PrOH as
both hydrogen donor and solvent.
found that the reaction did not occur in H2O nor in MeOH (entries
7 and 8). On the contrary, the transfer hydrogenation proceeded
perfectly in EtOH or 2-PrOH, and the latter is more active than the
former. From the point of view of the goal of an economic and envi-
ronmentally friendly reaction solvent, renewable and cheap EtOH is
clearly more favorable than 2-PrOH. In addition, the use of another
primary alcohol, n-BuOH, produced the desired product 2a with the
conversion of 16%.
Table 1
a
Screening of bases and solvents (hydrogen sources) for the transfer hydrogenation of 4-chloroacetophenone promoted by NaOH.
O
OH
base (2 equiv)
0 C, EtOH (2 mL), 15h
Cl
Cl
o
8
1a (1 mmol)
2a
Entry
Base (2 equiv)
Solvent (1 mL)
Conversion (%)b
1
2
3
4
5
6
7
8
9
K2CO3
Na2CO3
K3PO4
CsOH
KOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
H2O
MeOH
EtOH
i-PrOH
n-BuOH
0
0
9
56
33
0
0
0
95
>99
16
Et3N
NaOH
NaOH
NaOH
NaOH
NaOH
1
1
0
1
a
◦
The reaction was carried out with 4-chloroacetophenone (1 mmol) in the presence of bases (2 mmol) in alcohols (2 mL) at 80 C under a nitrogen atmosphere for 15 h.
Conversion was determined by NMR.
b