Nickel phthalocyanine assisted highly efficient and selective carbonyl reduction in polyethylene glycol-400

Article abstract of DOI:10.1007/s10562-012-0832-2

Nickel phthalocyanine with polyethylene glycol- 400 is described as a reusable green catalytic system for highly chemo- and regioselective reduction of carbonyl compounds to corresponding alcohols at room temperature. The catalytic system showed wide substrate scope covering aromatic, hetero aromatic and aliphatic carbonyl compounds with high turnover number and frequency. In the present study, 1,3- and 1,4-benzenedicarbaldehydes have been regioselectively reduced to corresponding alcohols for the first time. The catalyst was reused up to seven times without any significance loss in activity. Springer Science+Business Media, LLC 2012.

Full text of DOI:10.1007/s10562-012-0832-2

Catal Lett (2012) 142:907–913
DOI 10.1007/s10562-012-0832-2
Nickel Phthalocyanine Assisted Highly Efficient and Selective
Carbonyl Reduction in Polyethylene Glycol-400
•
Praveen Kumar Verma Upendra Sharma
•
•
•
•
Neeraj Kumar Manju Bala Vishal Kumar
Bikram Singh
Received: 18 January 2012 / Accepted: 22 April 2012 / Published online: 12 May 2012
Ó Springer Science+Business Media, LLC 2012
Abstract Nickel phthalocyanine with polyethylene gly-
col-400 is described as a reusable green catalytic system
for highly chemo- and regioselective reduction of carbonyl
compounds to corresponding alcohols at room temperature.
The catalytic system showed wide substrate scope covering
aromatic, hetero aromatic and aliphatic carbonyl com-
pounds with high turnover number and frequency. In the
present study, 1,3- and 1,4-benzenedicarbaldehydes have
been regioselectively reduced to corresponding alcohols
for the first time. The catalyst was reused up to seven times
without any significance loss in activity.
Numerous methods have been developed to accomplish
this transformation selectively in the presence of other
reducible functionalities [5, 6] using trialkyl boranes with
ionic liquid [7], and metal such as Au [8–11], Mn [12], Ir
[13–16], Pt [17], Ru [18–31], Rh [32–34], Os [35, 36]. Use
of costly metal, stoichiometric amount of additives and
poor selectivity limits their scope.
Iron and nickel are abundantly available and found to be
effective for carbonyl reduction due to their cost effective-
ness over other frequently used transition metals [37–49]. In
this view, Casey et al. have described the bifunctional iron
complex for chemoselective hydrogenation of ketones under
mild reaction conditions albeit with low TON/TOF [37–45].
Morris et al. have applied diamine based iron complex for
hydrogenation of acetophenones with high conversion
(TON = 225) but high pressure and excess of base were
used [37–45]. Recently, Milstein et al. have efficiently cat-
alyzed the hydrogenation of ketones by iron pincer complex
with high TON (1880). However, less substrate scope, use of
base and low TOF limitsthe scope of these reactions [37–45].
Earlier reported nickel based catalytic systems were less
efficient, require additives and showed low turnover number/
turnover frequency (TON/TOF) [46–49].
Keywords Nickel phthalocyanine Á Carbonyl reduction Á
Chemoselectivity Á Regioselectivity Á Reductive amination
1 Introduction
The selective reduction of carbonyl functionalities is one of
the most important and basic chemical transformation in
organic chemistry. It is an important step in the synthesis of
natural products [1, 2], bulk and fine chemicals [3, 4].
Sodium borohydride (NaBH4) has also been reported for
carbonyl reduction without using any metal under solvent
free conditions [50, 51]. However, use of excess NaBH4
long reaction time and lack of chemoselectivity limits their
scope [50]. Solid acids in combination with NaBH4 have
been shown to affect the reaction time and chemoselec-
tivity [51]. In this case equivalent amount of the solid acids
was required which can be avoided. In these reports the
presence of NaBH4 was not able to reduce carbonyl com-
pounds with –OH group in aromatic ring as they formed
sodium phenolates [50]. Moreover, regioselective reduc-
tion of dicarbaldehydes, which is important, has never been
IHBT Communication No. 2235.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-012-0832-2) contains supplementary
material, which is available to authorized users.
P. K. Verma Á U. Sharma Á N. Kumar (&) Á M. Bala Á
V. Kumar Á B. Singh (&)
Natural Plant Products Division, Council of Scientific and
Industrial Research (CSIR), Institute of Himalayan Bioresource
Technology, Palampur 176 061, HP, India
e-mail: neerajnpp@rediffmail.com
B. Singh
e-mail: bikram_npp@rediffmail.com
123

908
P. K. Verma et al.
reported. While working on the development of metal
phthalocyanine-catalyzed reduction methodologies [52–
54], here, we disclose nickel phthalocyanine (NiPc) cata-
lyzed highly chemo- and regioselective method for reduc-
tion of carbonyl compounds to corresponding alcohols with
high TON/TOF.
(3 9 15 mL). The combined diethyl ether fractions were
dried under reduced pressure. The crude product was
analyzed directly by GC–MS.
3 Results and Discussion
Initially, to access the best reaction conditions various
hydrogen sources were tested in different solvents for the
reduction of 4-nitrobenzaldehyde with nickel based cata-
lysts. As expected, no reaction was observed without
hydrogen source (Table 1, entries 1, 2, 5). Up to 50 % yield
was observed with NaBH4 in the absence of catalyst after
12 h (Table 1, entry 3). In the presence of NiCl2 with
NaBH4 moderate yield (65 %) was observed after 12 h
(Table 1, entry 4). Surprisingly, NiPc enhanced the rate of
reaction and full conversion was observed in just 20 min
(Table 1, entry 6). Borohydrides are the most conventional
reducing agents in organic synthesis due to their stability,
high selectivity and ease of handling. Desired product was
2 Experimental
2.1 Materials and Instrumentation
High grade solvents were used for all reactions. Column
chromatography was carried out with 60–120 mesh silica
gel. Silica (60–120 mesh) and TLC silica gel 60 F254 plates
were purchased from Merck India Ltd. Carbonyl com-
pounds, amines, NMR solvents were purchased from sigma
Aldrich and spectrochem. H NMR and 13C NMR experi-
1
ments were performed on Bruker Avance-300 spectrome-
ter. Chemical shifts are reported in parts per million (ppm)
downfield from an internal standard. The GC–MS analysis
was carried out on a Shimadzu (QP 2010) series GC–MS
(Tokyo, Japan) equipped with a FID, AOC-20i auto-sam-
pler coupled, and a DB-5 capillary column, (30 9 0.25
mm i.d., 0.25 lm).
Table 1 Optimization of reaction conditions for reduction of
4-nitrobenzaldehydea
O
Catalyst (1.7 x 10-3mmol)
H
OH
Hydrogen source (0.5 equiv.)
PEG-400, RT, 12 h
O2N
O2N
2.2 General Procedure for the Reduction of Carbonyl
Compounds
Entry
Catalyst
Hydrogen source
Yieldb (%)
To a mixture of carbonyl compound (250 or 500 or
1,000 mg) and catalyst (1.7 9 10-3 mmol) in PEG-400 (2
or 3 or 4 mL), sodium borohydride (0.5 equiv.) was added.
The reaction solution was stirred at room temperature and
the progress of the reaction was monitored by TLC (silica
gel; hexane/ethyl acetate) and GC–MS. After completion,
the reaction mixture was extracted with diethyl ether
(3 9 15 mL). The combined diethyl ether fractions were
dried under reduced pressure. The crude product was
analyzed directly by GC–MS. To obtain the isolated yield
product mixture was subjected to column chromatography
(silica 60–120; n-hexane/ethyl acetate mixture).
1
–
–
NR
NR
50
2
NiCl2
–
–
3
NaBH4
NaBH4
–
4
NiCl2
NiPc
NiPc
NiPc
NiPc
NiPc
NiPc
NiPc
NiPc
NiPc
NiPc
NiPc
NiPc
65
5
NR
[99c
NR
NR
NR
NR
NR
NR
NR
NR
NR
NR
6
NaBH4
NH2NH2ÁH2O
HCOONH4
HCOOK
DPS
7
8
9
10
11
12
13
14
15
16
PMHS
HMDS
H2O
2.3 General Procedure for the Reductive Amination
HCOOH
TES
To a mixture of carbonyl compound (250 mg), amine
(1 equiv.) and catalyst (1.7 9 10-3 mmol) in PEG-400
(2 mL), was added. Sodium borohydride (0.5 equiv.) was
added after the complete consumption of both reactants to
the imine as monitored by TLC and stirring was continued.
The reaction solution was stirred at room temperature and
the progress of the reaction was monitored by TLC (silica
gel; hexane/ethyl acetate) and GC–MS. After completion,
the reaction mixture was extracted with diethyl ether
CDMS
NR no reaction, DPS diphenyl silane, PMHS poly(meth-
ylhydrosiloxane), HMDS hexamethyl disilane, TES triethyl silane,
CDMS chlorodimethyl silane
a
Reactions were carried out by using 250 mg of substrate, [sub-
strate]/[catalyst] ratio = 250:1 (w/w)
b
Yield was calculated on the basis of GC–MS analysis, yield = -
con. 9 sel./100
c
Reaction time, 20 min
123

Selective Carbonyl Reduction
909
Table 2 Optimization of solvent for reduction of 4-nitrobenzalde-
hydea
Table 3 Reduction of aromatic aldehydes to corresponding alcoholsa
O
O
OH
NiPc (1.7 x 10-3mmol)
NiPc (1.7 x 10-3mmol)
H
NaBH4(0.5 equiv.),
PEG-400, RT
OH
R1
H
R1
H
NaBH4(0.5 equiv.)
O2N
O2N
solvent, RT, 12h
Entry Substrate
t (min) Yieldb (%) TON/TOF (h-1
)
Entry Catalyst Hydrogen source Solvent
Yieldb,c (%)
O
1
2
3
4
5
6
7
8
9
a
NiPc
NiPc
NiPc
NiPc
NiPc
NiPc
NiPc
NiPc
NiPc
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
H2O
48
75
H
Ethanol
i-PrOH
86
R
1
R = H
15
20
20
20
30
25
20
20
55
55
20
35
35
20
25
92
83
89
90e
91
94
84
87f
87
90
92
91g
64
70
94
5,105/20,423c
1,443/4,333d
711/2,137d
1,092/3,280d
2,246/4,492d
1,019/2,444
943/2,833
Methanol
PEG-400
44
[99d
2
R = p-Cl
R = p-Br
Ethylene glycol 23
3
THF
7
4
R = m-OH
R = p-CH3
Toluene
1,4-Dioxane
NR
NR
5
6
R = m-OCH3
R = m-CN
7
Reactions were carried out by using 250 mg of substrate, [sub-
strate]/[catalyst] ratio = 250:1 (w/w)
b
GC yield, yield = conv. 9 sel./100
8
R = o-COOH
R = o-NO2
R = m-NO2
R = p-NO2
R = o-CHO
R = m-CHO
R = p-CHO
855/2,568
9
1,707/1,876d
1,764/1,939d
895/2,688
c
Reaction time, 12 h
10
11
12
13
14
15
d
Reaction time, 20 min
1,001/1,727
1,037/1,778
1,004/3,015
1,813/4,352d
not observed with other tested hydrogen sources (Table 1,
entries 7–16). The change of solvents (Table 2) showed that
the hydrogenation of carbonyl group was facilitated in
protic solvents (Table 2, entries 1–6) and full conversion
was observed in PEG-400 within 20 min (Table 2, entry 5).
PEG is a widely available, non-toxic, biodegradable, ther-
mally stable, non-volatile, low price and recyclable green
solvent. In the recent years, it has been used effectively as
green solvent for number of organic transformations.
With best reaction conditions in hand, the reduction of
various carbonyl compounds was carried to check the scope
of the present catalytic system. A variety of aromatic
aldehydes efficiently hydrogenated to the corresponding
alcohols. The hydrogenation of benzaldehyde was
(TON = 5,105; TOF = 20,423 h-1) and yield (Table 3,
entry 1). To the best of our knowledge such a high TON and
TOF with full conversion was not observed earlier [37–49].
Halogen substituted aldehydes were efficiently reduced
to the corresponding alcohols in good to excellent yields
without any dehalogenation (Table 3, entries 2, 3). The
hydroxy, methyl, methoxy, nitrile and acid functionalities
present in the aromatic ring remained unaffected during the
reduction of the corresponding aldehydes (Table 3, entries
4–8). High selectivity was observed for the hydrogenation
of o-, m- and p-nitro benzaldehydes to corresponding
alcohols (Table 3, entries 9–11) [55, 56]. Also, high
selectivity with good yield was observed in case of com-
plex benzaldehydes (Table 3, entries 15, 16).
O
H
H
HO
OCH3
16
25
89
672/1,639
O
O
O
NO2
a
Reactions were carried out by using 250 mg substrate unless
specified, [substrate]/[catalyst] = 250:1 (w/w)
b
Isolated yield
c
Reactions were carried out by using 1 g of substrate, [substrate]/
[catalyst] = 1,000:1 (w/w)
d
Reactions were carried out by using 500 mg of substrate, [sub-
strate]/[catalyst] = 500:1 (w/w)
e
GC–MS yield
f
Yield of phthalide
g
Yield of 1,2-benzenedimethanol
Regioselective reduction of aldehyde group in case of
benzenedicarbaldehydes was not reported earlier. In the
present study, high regioselectivity ([99%) was observed
for 1,3- and 1,4-benzenedicarbaldehydes in 15 min with
41–67 % conversion (Table 4, entries 1, 3). However,
regioselectivity was reduced to 68–77 % after full con-
version (Table 3, entries 2, 4). Moderate isolated yield was
123

910
P. K. Verma et al.
Table 4 Regioselective reduction of dicarbaldehydesa
Table 5 Reduction of aromatic ketones to corresponding alcoholsa
O
O
NiPc (1.7 x 10-3mmol)
OH
NiPc (1.7 x 10-3mmol)
H
OH
R1
R2
NaBH4(0.5 equiv.),
NaBH4(0.5 equiv.)
PEG-400, RT
R1
R2
PEG-400, RT
R
R
Entry Substrate
t (min) Yieldb TON/TOF
(h-1
)
Entry
R
Time (min)
Conv.b (%)
Sel.b (%)
(%)
1
2
3
4
a
R = m-CHO
R = m-CHO
R = p-CHO
R = p-CHO
15
35
15
20
67
[99
68
O
100
41
CH3
[99
77
100
R
Reactions were carried out by using 250 mg substrate, [substrate]/
1
2
3
4
5
6
R = H
15
30
25
30
20
70
94
89
86
87
92
89
4,621/18,484
663/3,317
1,899/4,632c
1,817/1,817c
3,305/9,927d
725/621
[catalyst] = 250:1 (w/w)
b
On the basis of GC–MS analysis
R = p-Br
R = m-OH
R = p-OCH3
R = p-NO2
O
OH
NiPc (1.7 x 10-3mmol)
H
O
H
H
H
NaBH4 (0.5 equiv.)
PEG-400, RT, 20 min.
CH3
OCH3
O
O
H3CO
Scheme 1 Regioselective reduction of dicarbaldehydes
7
8
50
60
78e
93
708/849
634/634
O
CH2Br
obtained in both these cases (Scheme 1, Table 3, entries
13, 14). The reduction of both aldehyde groups was
observed in case of 1,2-benzenedicarbaldehyde (Table 3,
entry 12).
O
To further expand the scope of the method the reduction
of various aromatic ketones was carried out. The excellent
yield was observed under the same reaction conditions.
Full conversion of acetophenone was observed within
15 min with good yield and high TOF (18,484 h-1),
(Table 5, entry 1) which is more than the earlier reports
[37–45]. Bromo, hydroxy, methoxy and nitro substituted
acetophenones were efficiently converted into corre-
sponding alcohols without affecting other functional
groups (Table 5, entries 2–6). The present system selec-
tively (78 %) catalyzed the hydrogenation of 2-bromo-
acetophenone with high yield (Table 5, entry 7). Also,
4-chloro benzophenone and 3,4-methylenedioxy aceto-
phenone were selectively reduced to corresponding alco-
hols with good yield (Table 5, entries 8, 9).
Cl
9
45
95
855/1,900
O
O
O
CH3
a
Reactions were carried out by using 250 mg substrate unless
specified, [substrate]/[catalyst] = 250:1 (w/w)
b
Isolated yield
c
Reactions were carried out by using 500 mg of substrate, [sub-
strate]/[catalyst] = 500:1 (w/w)
d
Reactions were carried out by using 1 g of substrate, [substrate]/
[catalyst] = 1,000:1 (w/w)
e
GC–MS yield
In the present method high chemoselectivity was
observed in case of cyclic as well as acyclic aliphatic
carbonyl compounds with high TON/TOF (Table 6, entries
1–4). Chemoselective hydrogenation of unsaturated car-
bonyl compounds is a very challenging task in organic
chemistry, and, generally, reduction of C=C bond also
occurs [35, 36], however, in present method selective
reduction of aldehyde group was observed without affect-
ing conjugated or isolated C= bond (Table 6, entries 5, 6).
The present catalytic system was also able to catalyze
the hydrogenation of heteroaromatic aldehydes to corre-
sponding alcohols with high TON and TOF. Furaldehyde
and its derivative were efficiently reduced to corresponding
alcohols (Table 7, entries 1, 2). Pyrrole-2-carboxaldehyde
was reduced to corresponding alcohol in good yield (93 %)
(Table 7, entry 3).
123

Selective Carbonyl Reduction
911
Table 6 Reduction of aliphatic carbonyl compounds to corresponding alcoholsa
NiPc (1.7 x 10-3mmol)
O
OH
R2
R1
R2
R1
NaBH4 (0.5 equiv.),
PEG-400, RT
Entry
1
Substrate
t (min)
Yieldb (%)
88c
TON/TOF (h-1
)
60
3,063/3,063
O
O
R
2
3
4
5
R = H
60
30
30
25
81c
87c
88c
89
2,531/2,531
2,292/4,585
2,322/4,645
1,704/4,096
R = 3-CH3
R = 4-CH3
O
H
H
5
6
25
93
2,078/4,997
O
a
Reactions were carried out by 500 mg substrate unless specified, [substrate]/[catalyst] = 500:1 (w/w)
b
c
Isolated yield
GC–MS yield
Table 7 Reduction of heteroaromatic carbonyl compounds to cor-
responding alcoholsa
O
NH2
NiPc (1.7 x 10-3mmol)
H
+
NiPc(1.7 x 10-3mmol)
O
OH
N
H
NaBH4 (0.5 equiv.)
PEG-400, RT, 3h
R1
R2
R1
R2
NaBH4 (0.5 equiv.),
PEG-400, RT
GC-MS Yield >99 %
Scheme 2 Reductive amination of benzaldehyde and aniline
Entry Substrate
t (min) Yieldb (%) TON/TOF (h-1
)
1
30
92
2,838/5,676c
O
O
H
2
40
45
87
1,166/1,749
1,444/1,926
O
O
H3C
H
3
93d
O
H
N
H
a
Reactions were carried out by using 250 mg substrate unless
specified, [substrate]/[catalyst] = 250:1 (w/w)
b
Isolated yield
c
Reactions were carried out by using 500 mg of substrate, [sub-
strate]/[catalyst] = 500:1 (w/w)
d
GC–MS yield
Fig. 1 Recyclability of catalytic system
123

912
P. K. Verma et al.
Fig. 2 Formation of crown
ether-type complex and
carbonyl group activation by
NiPc
R1
-
H3 B
H
R2
H
O
C
O
O
O
O
O
Na+
PEG-400
NaBH4 +
N
O
N
N
N
N
Ni
N
N
N
O
the attack of hydride on carbonyl group which is activated
by NiPc through Lewis acid–base interaction (Fig. 2).
H
D
NiPc(1.7x10-3mmol)
H
OH
NaBD4 (0.5equiv.)
PEG-400,RT
O2N
O2N
4 Conclusions
Scheme 3 Reduction of benzaldehyde with NaBD4
In conclusion, a highly efficient, eco-friendly and green
method has been developed for the high chemo- and reg-
ioselective reduction of carbonyl compounds to corre-
sponding alcohols by using inexpensive NiPc with PEG-
400 and NaBH4 as hydrogen source at room temperature.
The present method showed very wide substrate scope with
high TON and TOF tolerating highly sensitive functional
groups such as hydroxyl, carboxyl and olefin. Regioselec-
tive reduction of aldehyde group in case of benzenedicar-
baldehydes was also reported first time. The catalyst can be
reused up to seven times without any significance loss in
activity. Rapid reaction, easy work up procedure and high
efficiency make the present method most suitable for car-
bonyl group reduction. NiPc along with PEG-400 shows
high impact for the replacement of precious transition
metal based catalytic system generally used for the
reduction of carbonyl compounds at pilot scale.
In an extension of the hydrogenation of carbonyl com-
pounds, the present catalytic system was successfully
applied for the reductive amination of benzaldehyde with
aniline in excellent yield (Scheme 2).
Further, the recyclability of the catalytic system was
evaluated using 4-nitrobenzaldehyde as test substrate. After
completion of reaction the product was extracted with
diethyl ether. Successive reactions were carried out by
sequential addition of fresh substrate and NaBH4 to the
crude remains after extracting the product. After fourth
cycle PEG-400 was also added. The catalytic activity of the
system was maintained up to five cycles and slight decrease
in yield was observed after fifth cycle (Fig. 1). At the end
of seven cycles the total turnover number of the reaction
was calculated as 6,610.
To understand the mechanism, the model reaction was
carried out under optimized conditions using NaBD4. The
insertion of deuterium on the carbon of the carbonyl group
Acknowledgments Authors are grateful to the Director of the
institute for providing necessary facility and encouragement. P.K.V.,
U.S. and V.K. are also thankful to CSIR and UGC for fellowship.
1
(as confirmed by H NMR) indicated the hydride attack
followed by protonation with solvent to give the reduced
product (Scheme 3). The hydride attack is thought to be
facilitated by the formation of crown ether-type complex of
PEG-400 with alkali metal of metal borohydrides [57]. No
change in oxidation state (monitored by UV–vis spectro-
photometer) and color of NiPc during the reaction ruled out
the involvement of any Ni(I)Pc and hydridonickel species
as previously reported [52–54, 58]. The exact role of NiPc
in assisting the reaction is not clear, however, it is envi-
sioned that Lewis acidic character of NiPc [59, 60] might
be responsible for carbonyl group activation.
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