Cobalt(II) phthalocyanine-catalyzed highly chemoselective reductive amination of carbonyl compounds in a green solvent

Article abstract of DOI:10.1002/adsc.201100645

Cobalt phthalocyanine has been employed for the highly chemoselective reductive amination of aldehydes and ketones in ethanol as a green solvent. A large range of functional groups such as nitro, acid, amide, ester, nitrile, halogen, lactone, methoxy, hydroxy, alkene, N-benzyl, O-benzyl and heterocyclic rings were well tolerated under the present reaction conditions. Copyright

Full text of DOI:10.1002/adsc.201100645

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DOI: 10.1002/adsc.201100645
Cobalt(II) Phthalocyanine-Catalyzed Highly Chemoselective
Reductive Amination of Carbonyl Compounds in a Green
Solvent
Vishal Kumar,a Upendra Sharma,a Praveen K. Verma,a Neeraj Kumar,a,*
and Bikram Singha,*
a
CSIR – Institute of Himalayan Bioresource Technology (Council of Scientific & Industrial Research), Palampur,
Himachal Pradesh – 176 061, India
Fax : (+91)-1894-230-433; phone: (+91)-1894-230-426; e-mail: neerajnpp@rediffmail.com or bikram_npp@rediffmail.com
Received: August 11, 2011; Revised: November 29, 2011; Published online: March 13, 2012
IHBT communication no. 2234.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100645.
Abstract: Cobalt phthalocyanine has been employed rings were well tolerated under the present reaction
for the highly chemoselective reductive amination of conditions.
aldehydes and ketones in ethanol as a green solvent.
A large range of functional groups such as nitro,
acid, amide, ester, nitrile, halogen, lactone, methoxy, Keywords: chemoselectivity; cobalt phthalocyanine;
hydroxy, alkene, N-benzyl, O-benzyl and heterocyclic diphenylsilane; green chemistry; reductive amination
Introduction
tion of carbonyl compounds.[14] This limitation was
overcome by employing organosilanes as mild and
Amines and their derivatives are highly versatile environmentally benign reducing agents, such as
building blocks for various bioactive organic sub- Et3SiH-TFA,[15] Bu3SnH-DMF,[16] InCl3-Et3SiH,[17]
strates such as pharmaceuticals[1] and agrochemicals.[2] InCl3-Zn
G
FeCl3-PMHS,[19]
Amines are widespread among natural products and PhSiH3-ReIO CHTUGNTRENNUNG
(PPh3)2.[20] The use of excess amounts
the pharmaceutical industries abound with drugs of catalyst, hydrogen source and non eco-friendly sol-
based on amine moieties.[3] Consequently, the synthe- vents limit the scope of these methods.[17,18]
sis of amines is a very active field in medicinal
Recently, cobalt-based catalysts have been em-
chemistry and modern organic synthesis. The direct ployed for the reduction of carbonyl[21] and nitro[22]
reductive amination of carbonyl compounds is the compounds, hydroformylation,[23] amidocarbonyla-
most attractive method for the preparation of amine tion[24] reactions, etc., but have not explored much for
derivatives in organic synthesis.[4] Several reagents the reductive amination of carbonyl compounds.[25]
including
acacia-Pd
catalytic
hydrogenation,[5]
gum
In continuation of our work on the development of
nanoparticles,[6]
H2-Fe2O3/Au efficient catalytic processes,[22,26,27] here, in the present
nanoparticles,[7] Pd/C/HCOO-NH4,[8] triazole-derived work, we have successfully applied cobalt(II)phthalo-
iridium(I) carbine complexes,[9] [Ir
catalysts,[11] Pd(PhCN)2Cl2-BQC-H2,[12] NaBH3CN,[13a] ficient reductive amination of carbonyl compounds
NaBH
(OAc)3,[13b] and NaBH4-H3PW12O40
(cod)2],[10] Rh(I) cyanine (CoPc) for the highly chemoselective and ef-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[13c]
A
have using diphenylsilane as hydrogen source in ethanol.
been developed for reductive amination. However, This is the first report on the use of CoPc/Ph2SiH2 for
most of these reagents have one or more drawbacks chemoselective reductive amination of carbonyl com-
in terms of functional group tolerance, side reactions pounds in ethanol.
and harsh reaction conditions. Reductive amination
with NaBH4 and Lewis acids requires an excess of the
amine (up to five-fold) in order to suppress the reduc-
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Cobalt(II) Phthalocyanine-Catalyzed Highly Chemoselective Reductive Amination
Table 2. Optimization of hydrogen source for the reaction.
Results and Discussion
To optimize the best reaction conditions, the reduc-
tive amination of 4-bromobenzaldehyde with 4-me-
thoxyaniline was carried out by using different cata-
lysts, hydrogen sources and solvents under varying
temperature conditions. The progress of the reaction
was monitored by TLC and GC-MS. Among the dif-
ferent catalysts cobalt phthalocyanine was found to
be the most active catalyst at 708C (Table 1, entry 9),
however, no reaction was observed at room tempera-
ture. Furthermore, in order to confirm whether the
corresponding metal salts from which the metal
phthalocyanines were prepared could be able to cata-
lyze the reaction, FeSO4·7H2O, CoCl2·6H2O,
Entry
Hydrogen source
Yield [%][a]
1
2
3
4
5
6
7
8
9
10
–
0
HCOONH4
HCOOK
NaBH4
Ph2SiH2
PhSiH3
PMHS
(CH3)2ClSiH
Et2SiH2
NH2NH2·H2O
15
17
46[b]
95
34
0
CoSO4·7H2O,
NiCl2·6H2O,
NiSO4·7H2O
and
27
0
CuSO4·5H2O were also used. No difference was ob-
served in the catalytic activity of FePc and
FeSO4·7H2O (Table 1, entries 2 and 8), and yield was
slightly decreased with CuPc as compared to
CuSO4·5H2O (Table 1, entries 7 and 11). Surprisingly,
a significant increase in yield was observed in case of
CoPc and NiPc as compared to corresponding metal
salts (Table 1, entries 3–6, 9 and 10). In the absence of
catalyst, the desired product was obtained in <5%
yield (Table 1, entry 1), which showed the necessity of
the catalyst.
0[c]
[a]
Yield was determined by GC-MS analysis.
The alcohol was obtained in 29% yield.
The hydrazone derivative was observed in 88% yield.
[b]
[c]
Table 3. Optimization of the solvent for the reaction.
Furthermore, an investigation of the role of differ-
ent hydrogen sources revealed diphenylsilane as the
best hydrogen source with 95% yield of the desired
product (Table 2, entry 5). The use of other organosi-
lanes as reducing agents did not provide comparable
Entry
Solvent
Yield [%][a]
1
2
3
4
–
<5
95
89
92
0
EtOH
Table 1. Optimization of the reaction conditions for different
catalytic systems.
MeOH
MeCN
H2O
5
6
7
8
9
10
11
EtOH:H2O (1:1)
DMSO
DMF
1,4-dioxane
THF
42
12
<5
20
5
toluene
0
Entry
Catalyst
Yield [%][a]
[a]
Yield was determined by GC-MS analysis of the reaction
mixture.
1
2
3
4
5
6
7
8
–
<5
19
<5
10
35
7
36
18
95
67
18
FeSO4·7 H2O
CoCl2·6H2O
CoSO4·7H2O
NiCl2·6H2O
NiSO4·7H2O
CuSO4·5H2O
FePc
CoPc
NiPc
CuPc
yields (Table 2, entries 6–9). Ammonium and potassi-
um formate gave the desired product in very low
yields of 15 and 17%, respectively (Table 2, entries 2
and 3). When NaBH4 was used as hydrogen source
the reductive amination product was obtained in only
46% yield along with alcohol as by-product (Table 2,
entry 4). In hydrazine hydrate, no reductive amination
product was observed and the corresponding hydra-
zone of the aldehyde was observed as the only prod-
9
10
11
[a]
Yield was determined by GC-MS analysis of the reaction
mixture.
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Table 4. CoPc-catalyzed reductive amination of different aldehydes and amines.[a]
Entry
Aldehyde
Amine
Product
Yield [%][b]
87
1
2
3
4
90
79
92
5
95
6
7
8
9
88
85
80
70
10
11
12
13
71
93
95
86
14
77
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Cobalt(II) Phthalocyanine-Catalyzed Highly Chemoselective Reductive Amination
Table 4. (Continued)
Entry
Aldehyde
Amine
Product
Yield [%][b]
15
93
16
91
17
18
89
96
19
20
82[c]
92
21
89
22
23
24
25
93
89
75[d]
90
26
27
28
13[d]
0[d]
95
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Table 4. (Continued)
Entry
Aldehyde
Amine
Product
Yield [%][b]
88
29
30
31
32
91
79
32
[a]
Reaction conditions: aldehyde (1 mmol), amine (1 mmol), CoPc (1 mol%), EtOH (5 mL) at 708C for 12 h.
Yield of isolated product.
Two equivalents of aniline were used.
[b]
[c]
[d]
Yield based on GC-MS analysis of reaction mixture.
uct (Table 2, entry 10). Also, no product was obtained 29). One of the major problems associated with re-
without a hydrogen source (Table 2, entry 1).
ductive amination is intolerance of C=C bonds,[5] how-
The change in solvent had a significant influence on ever, in the present case high chemoselectivity was
the yield. Comparative yields were obtained in etha- observed in the reductive amination of aldehydes con-
nol, methanol and acetonitrile (Table 3, entry 2–4). taining conjugated as well as isolated C=C bonds (98
As ethanol is considered among the green solvents, it and >99%, respectively) with excellent yields
was considered as the solvent of choice. Very low (Table 4, entries 18 and 31). The reaction of 2-carb-
yields (5–20%) of desired product were observed in
ACHTUNGTRENoNGNU xybenzaldehyde and aniline afforded N-phenylisoin-
DMSO, DMF, 1,4-dioxane and THF (Table 3, en- dolone as the only product which is of biological im-
tries 7–10), whereas no product was observed in water portance (Table 4, entry 20).[28] The reaction of ben-
and toluene (Table 3, entries 5 and 11).
zene-1,4-dicarboxaldehyde with two equivalents of
With best reaction conditions in hand, the scope of aniline lead to reductive amination at both aldehyde
the method was explored by the reactions of various groups (Table 4, entry 19). The nitrile group was also
substituted aldehydes and amines. The observation of tolerated and a good yield of product was obtained
high yields and chemoselectivity in most of the cases (Table 4, entry 17). Poly-substituted derivatives of
(Table 4) clearly demonstrated the generality of this benzaldehyde were also well tolerated and the corre-
method. Recently, Sousa and Farnandes have report- sponding products were obtained in very good yields
ed that Re2IO2ACHTUNGTRENNUNG(PPh3)2 catalyzed the highly chemose- (Table 4, entries 15 and 21). A relatively low yield
lective reductive amination of aldehydes.[20] The use was recorded from 2,6-dimethylaniline in comparison
of non eco-friendly solvent (THF) and release of to 2-methylaniline indicating the effect of steric hin-
toxic PPh3 from catalyst limit the scope of the drance (Table 4, entries 23–26). The most unfavorable
method.
combination such as 2,3,4-trimethoxybenzaldehyde
The reductive amination of chloro- or bromo-sub- and 4-nitroaniline was also considered, but no product
stituted aldehydes proceeded smoothly and the de- was observed (Table 4, entry 27). Good to excellent
sired products were obtained in good yields (Table 4, yields were obtained in the reductive amination of ali-
entries 9–12). Recently, we have reported the CoPc- phatic aldehydes (Table 4, entries 18, 31 and 32).
catalyzed reduction of aromatic nitro compounds to
The scope of the method was further extended to
proceed chemoselectively using hydrazine hydrate as other amines. Halogen-substituted amines such as 4-
reducing agent in ethylene glycol.[22] In the present fluoro- and 4-iodoaniline were efficiently utilized for
case, using diphenylsilane in ethanol left the aromatic the reductive amination of benzaldehyde and 4-me-
nitro group unaltered (Table 4, entries 3 and 16). thoxybenzaldehyde, respectively (Table 4, entries 2
Also, very good yields of products were obtained and 14). The reaction of 6-aminobenzothiazole and
when hydroxy- and/or methoxy-substituted aldehydes benzaldehyde was also very efficient (Table 4,
were used as substrates (Table 4, entries 13–15, 21 and entry 4). One of the major advantages of the present
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Cobalt(II) Phthalocyanine-Catalyzed Highly Chemoselective Reductive Amination
Table 5. CoPc-catalyzed reductive amination of aldehydes and aliphatic amines.[a]
Entry
Aldehyde
Amine
Product
Yield [%][b]
85
1
2
3
4
88
53[c]
94
5
93
[a]
Reaction conditions: aldehyde (1 mmol), amine (1 mmol), CoPc (1 mol%), EtOH (5 mL) at 708C for 12 h.
Yield of isolated product.
Yield based on GC-MS analysis of reaction mixture.
[b]
[c]
method is the tolerance of functional groups such as ever, when acetophenone was treated with 4-nitroani-
lactone, carboxylic acid, amide and ester on amines, line, no reaction was observed (Table 6, entry 12).
which are generally not considered in reductive ami-
To study the mechanism of the reaction, the model
nation due to their lower reactivity towards aldehydes reaction was carried out using Ph2SiD2 in ethanol.
and incompatibility with reducing agents.[8–11] In the The clear insertion of deuterium on the carbon of the
present case, these groups were well tolerated and the double bond (as confirmed by 1H NMR) indicated
desired products were obtained in good yields (80– that hydrosilylation of the imine to give an intermedi-
95%) with high chemoselectivity (Table 4, entries 5– ate N-silylamine occurred, followed by solvolysis with
8).
ethanol or a trace amount of water (Scheme 1).[20]
High chemoselectivity was recorded with O-benzyl- The formation of diethoxydiphenylsilane as a by-
and N-benzyl-substituted amines (Table 4, entries 9 product confirmed the solvolysis of N-silylamine with
and 10). The reductive amination of benzaldehyde ethanol. No change in oxidation state (as monitored
and 4-methoxybenzaldehyde with secondary anilines by UV-VIS spectrophotometry) and color of CoPc
like N-methylaniline afforded high yields (Table 4, en- was observed during the reaction and thus ruled out
tries 28 and 29). However, a low yield was observed the involvement of any hydridocobalt species as pre-
in the case of 4-nitrobenzaldehyde (Table 4, entry 30). viously reported.[21,22]
Aliphatic primary as well as secondary amines also af-
forded good yields (Table 5, entries 1–5).
However, the exact role of CoPc is not clear, the
Lewis acidic character of CoPc[30] might be responsi-
Various aromatic and aliphatic (cyclic as well as ble for imine activation via a Lewis acid-base interac-
acyclic) ketones were also tested for the reductive tion. In order to verify this, a competitive reaction of
amination and the desired products were obtained in two imines having different electronic characters was
good yields (Table 6). Aliphatic ketones showed good carried out using our standard reaction condition
reactivity and the corresponding products were ob- (Scheme 2). The higher yield of product was observed
tained in 75–95% yields (Table 6, entries 1–6, 13 and from the more electron-rich imine due to its greater
14). Acetophenones are generally considered to be Lewis basic character. This indicated the possible role
difficult substrates for reductive amination reactions, of a Lewis acid-base type interaction in catalyzing the
resulting in no or nerely low conversions depending reaction (Figure 1).
on the reaction conditions,[13b,29a] or requiring en-
This observation was further supported by the fact
hanced quantities of catalytic activators like that no reaction took place in the presence of a stron-
Bu2SnCl2.[29b] In our hands, using standard conditions ger Lewis base, triethylamine (1.0 equiv.) which may
allowed for the realization of low to moderate yields be due to a preferred interaction of CoPc with
of the desired products (Table 6, entries 7–11). How- triethylACTHUNRTGNEaGNU mine as compared to imine. A similar Lewis
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Table 6. CoPc-catalyzed reductive amination of different ketones and amines.[a]
Entry
Ketone
Amine
Product
Yield [%][b]
93
1
2
3
4
90
91
86
5
6
7
8
89
91
62
65
9
11[c]
14[c]
10
11
12
13
52[c]
0[c]
75
14
95[c]
[a]
Reaction conditions: ketone (1 mmol), amine (1 mmol), CoPc (1 mol%), EtOH (5 mL) at 708C for 12 h.
Yield of isolated product.
Yield based on GC-MS analysis of reaction mixture.
[b]
[c]
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Cobalt(II) Phthalocyanine-Catalyzed Highly Chemoselective Reductive Amination
Scheme 1. Hydrosilylation of imine using CoPc/Ph2SiD2 in ethanol.
Scheme 2. Competitive reaction of electronically different imines with Ph2SiH2 in ethanol.
vantages of this methodology include high isolated
yields, clean reactions and easy work-up procedure.
Experimental Section
General Experimental Procedure for Reductive
Amination of Carbonyl Compounds Catalyzed by the
CoPc/Ph2SiH2 System
To a stirred suspension of CoPc (0.01 mmol) in ethanol
(5 mL) were added carbonyl compound (1.0 mmol), amine
(1.0 mmol) and diphenylsilane (1.5 mmol) at room tempera-
ture and then the temperature was raised to 708C. On com-
pletion of the reaction (as monitored by TLC), the reaction
mixture was filtered and passed through anhydrous Na2SO4.
The crude product was purified by column chromatography
over silica-gel (60–120) mesh with an appropriate mixture of
n-hexane and ethyl acetate.
Figure 1. Activation of an imine by CoPc.
acid-base type mechanism was also proposed by Raja-
gopal et al. for the AlPc-catalyzed cyanosilylation of
aldehydes.[31]
Acknowledgements
Conclusions
Authors are grateful to CSIR and the Director of the Institute
for providing necessary facilities. Mr. VK, Mr. US and Mr.
PKV also thank the UGC and CSIR for granting senior re-
search fellowship. Thanks are also due to Mr. Shiv Kumar
and Mrs. Vijaylata Pathania for analysis of samples.
In conclusion, cobalt phthalocyanine has been estab-
lished for the first time as a catalyst for the efficient
and chemoselective reductive amination of carbonyl
compounds to generate higher amines. Use of a green
solvent with a low loading of catalyst under ambient
reaction conditions makes the present method superi-
or to earlier reported methods. Other remarkable ad-
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Vishal Kumar et al.
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