Regioselective synthesis of γ-amino esters, nitriles, sulfones, and pyrrolidinones by nickel-catalyzed reductive coupling of aldimines and activated alkenes

Article abstract of DOI:10.1002/anie.200800825

(Chemical Equation Presented) A couple of alternatives: An efficient method has been developed for the synthesis of γ-amino derivatives and pyrrolidinones from readily available starting materials by using a nickel-phenanthroline complex (see scheme). The reaction proceeds by the formation of a C-C bond at the β-carbon atom, instead of the more usual α-carbon atom, of a conjugated alkene via an azanickelacycle intermediate.

Full text of DOI:10.1002/anie.200800825

Communications
DOI: 10.1002/anie.200800825
Synthetic Methods
Regioselective Synthesis of g-Amino Esters, Nitriles, Sulfones, and
Pyrrolidinones by Nickel-Catalyzed Reductive Coupling of Aldimines
and Activated Alkenes**
Chien-Hung Yeh, Rajendra Prasad Korivi, and Chien-Hong Cheng*
The transition-metal-catalyzed regioselective coupling of two
organic p components via a five-membered metallacycle
3a was observed. Fortunately, when we changed the catalyst
to [NiI2(phen)], [NiBr2(bipy)], [NiCl2(bipy)], or [NiBr2-
(phen)] (phen = 1,10-phenanthroline, bipy = 2,2’-bipyridine),
the expected product 3a was formed in yields of 55, 75, 37, or
99%, respectively. The yields of 3a were determined by
À
intermediate is one method for the construction of C C
bonds and the synthesis of molecules with multiple functional
groups. Among these reactions, the reductive coupling of
alkyne/alkyne[1a] alkene/alkyne,[1b,c,i,j] and alkene/alkene[1a]
couples has been widely explored by us and others. Similarly,
the coupling of alkene/carbonyl[1d] and alkyne/carbonyl[1e–h]
couples catalyzed by metal complexes has more recently also
attracted attention. In comparison, the use of an imine
derivative as one of the p components has only rarely been
explored.[2,3] Rhodium-[2a–c] and organo-catalyzed[2d,e] coupling
reactions of conjugated alkenes with activated imines were
1
integration of the H NMR signals, using mesitylene as the
internal standard. The nickel complex [NiBr2(tmeda)]
(tmeda = tetramethylethylenediamine), which has an elec-
tron-rich nitrogen ligand, did not give the desired product.
Also, phenanthroline or zinc alone did not catalyze the
reaction. Surprisingly, these studies show that phosphines are
not suitable ligands in the present catalytic reaction, but that
bipyridine-type ligands, particularly phen, are essential.
We studied the scope of the reductive coupling reaction
under the optimized reaction conditions shown in Table 1.
The reaction of imines 1b, 1a, and 1c with acrylate derivatives
À
found to give Baylis–Hillman-type products in which a C C
bond was formed between the a-carbon atom of the alkene
and the carbon atom of the imine. There is no report of an
analogous metal-catalyzed coupling reaction occurring at the
b-carbon atom of the alkene.[3] Our efforts to develop
methods for the reductive coupling of two p components[1b,c,4a]
prompted us to investigate the coupling reaction of conju-
gated alkenes and imines. Herein, we report an efficient ene–
imine reductive coupling reaction catalyzed by a nickel–1,10-
phenanthroline complex to give various substituted g-amino
esters,[5i,j] g-aminonitriles,[5k,l] g-aminosulfones, and pyrrolidi-
nones.[5] These products are all g-aminobutyric acid (GABA)
derivatives, which are known to exhibit a wide range of
biological properties[6] and have found various industrial
applications.[6] In contrast to the base-[2d] and metal-catalyzed
reactions, our ene–imine coupling reaction yields saturated
Table 1: Nickel-catalyzed reductive coupling reaction of imines and
conjugated alkenes.[a]
Entry Imine: R1, R2
Ene: R3, R4
3
Yield
[%][b]
1
1a: Ph, 4-FC6H4
1b: Ph, Ph
1a
1c: Ph, 4-MeOC6H4
1d: 4-BuC6H4,
4-MeOC6H4
1e: 4-MeOC6H4, Ph
1e
2a: CO2Et, H
2a
2b: CO2tBu, H 3c
2a
2a
3a
3b
85
73
61
74
88
2[c,d]
3
À
4[c,e]
5[c,e]
3d
3e
products in which a C C bond is formed at the b-carbon atom
of conjugated alkenes.
The success of the reductive ene–imine coupling reaction
depends greatly on the choice of the ligand and metal. When
4-fluorobenzaldimine 1a was treated with ethyl acrylate in
acetonitrile in the presence of [CoI2(dppe)] (dppe = 1,2-
bis(diphenyphosphanyl)ethane) or [NiBr2(dppe)], Zn, and
H2O at 808C, only a trace of the reductive coupling product
6[e]
7
2c: CO2Me, H
2d: CO2Me,
3 f
3g
71
65
Me
8
1 f: 4-MeOC6H4, cyclohexyl
1a
2c
3h
3i
3j
3k
3l
3m 64
52
86
84
75
79
9[c]
10[c]
11[c]
12
13
14
15
16
17
2e: CN, H
2e
2e
2 f: CN, Me
2e
2g: SO2C6H4, H 3n
2e
1e
1g: R1 =R2 =4-MeOC6H4
1e
1 f
1e
[*] C.-H. Yeh, Dr. R. Prasad Korivi, Prof. Dr. C.-H. Cheng
Department of Chemistry, National Tsing Hua University
Hsinchu, 30043 (Taiwan)
67
63
65
68
1h: 4-MeOC6H4, H
3o
3p
3q
1i: 4-MeOC6H4, CꢀC(CH2)4CH3 2e
Fax: (+886)3572-4698
E-mail: chcheng@mx.nthu.edu.tw
1j: 4-MeOC6H4, 2-furyl 2e
Homepage: http://mx.nthu.edu.tw/~chcheng/
[**] We thank the National Science Council of the Republic of China
(NSC 96-2113-M-007-020-MY3) for support of this research.
[a] All reactions were carried out under nitrogen (1 atm) using imine
(0.25 mmol), acrylate (0.75 mmol), [NiBr2(phen)] (0.0125 mmol), H2O
(0.50 mmol), and Zn (0.75 mmol) at 808C for 20 h. [b] Yield of isolated
product. [c] No chromatography procedures were required for purifica-
tion of the product. [d] Reaction time: 12 h. [e] Reaction time: 16 h.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or fromthe author.
4892
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Table 2: Formation of pyrrolidinone derivatives.[a]
proceeded smoothly to give amino ester derivatives 3b–d in
good yields (Table 1, entries 2–4). When imines 1d and 1e
were used, the corresponding g-amino derivatives 3e and 3 f
were obtained in yields of 88 and 71%, respectively (Table 1,
entries 5 and 6). Replacement of H2O by D2O in the reaction
of 1e with 2c gave deuterated product [D]-3 f with 68%
deuteration at the carbon atom a to the ester group. Treat-
ment of methyl methacrylate (2d) with imine 1e afforded a
diastereomeric mixture of the corresponding amino ester
derivatives in 65% yield (diastereomeric ratio 2:1; Table 1,
entry 7). When imine 1 f derived from cyclohexanecarbox-
aldehyde was used with acrylate 2c, product 3h was obtained
in 52% yield Table 1, entry 8).
Entry
1
R1, R2
2
Prod. 4 Yield
[%][b]
1
2
1a Ph, 4-FC6H4
1b Ph, Ph
1c Ph, 4-MeOC6H4
1g R1 =R2 =4-MeOC6H4
1e 4-MeOC6H4, Ph
2c 4a
2c 4b
2c 4c
2c 4d
2c 4e
2c 4 f
2c 4g
75
64
74
85
77
45
49
3[c]
4[c]
5[c]
6
1 f
1j
4-MeOC6H4, cyclohexyl
4-MeOC6H4, 2-furyl
7
The scope of the reaction was further extended by using
acrylonitrile derivatives in place of the acrylates. When imines
1a, 1e, and 1g were employed in the reaction with acrylo-
nitrile 2e, g-aminonitriles 3i, 3j, and 3k were obtained in
yields of 86, 84, and 75%, respectively (Table 1, entries 9–11).
Treatment of methyl-substituted acrylonitrile 2 f with 1e gave
a mixture of g-aminonitrile diastereomers 3l in yields of 37
and 42% (Table 1, entry 12). The reaction of imine 1 f with 2e
resulted in the corresponding nitrile derivative 3m in 64%
yield (Table 1, entry 13). These results indicate that acrylo-
nitrile derivatives give higher product yields than their
acrylate analogues (Table 1, entry 8 versus 13). Nitrile deriv-
atives can also be used for the preparation of acids, aldehydes,
and amines. Enzyme-based catalytic[7] methods have been
developed to prepare various chiral derivatives from nitriles
for industrial application. Vinyl sulfone 2g reacted smoothly
with imine 1e to provide 3n in 67% yield (Table 1, entry 14).
The reaction of aldimine 1h derived from formaldehyde, as
well as of 1i and 1j with 2e also proceeded smoothly to
provide g-aminobutyronitriles 3o–q in good yield (Table 1,
entries 15–17). The present study shows that a variety of
imines can be used along with a wide range of conjugated
alkenes.
8
1k
2c 4h
87
4-MeOC6H4,
4-MeOC6H4,
9
1k
1l
2d 4i, 4i’
35, 42
62
10
4-MeOC6H4, 3,4,5-(MeO)3C6H4 2c 4j
[a] The reaction was carried out under similar conditions as shown in
Table 1 for 20 h, then the catalyst, zinc, and acetonitrile were removed.
The residue was heated at 1208C in toluene (2.0 mL) and PTSA
(0.0250 mmol) for 4 h. [b] Yield of isolated product. [c] Chromatography
was not necessary for the isolation of the products.
entries 6 and 7). The aromatic imine derived from piperonal
gives the desired product 4h in excellent yield (Table 2,
entry 8). Under similar reaction conditions, methyl meth-
acrylate reacted with 1k to yield a mixture of the trans- and
cis-pyrrolidinone derivatives trans-4i and cis-4i’ in yields of 35
and 42%, respectively, (Table 2, entry 9). These two isomers
could be separated by column chromatography and the
structures were assigned by comparing the chemical shifts in
the NMR spectra with those of reported[8a,b] compounds with
similar skeletons.
The present catalytic reaction was utilized for the syn-
thesis of pyrrolidinones (Table 2). After completion of the
catalytic reductive coupling reaction of 1 with acrylate 2c, the
mixture was filtered, and the acetonitrile was removed under
vacuum. The resulting crude product was heated in toluene
with para-toluenesulfonic acid (PTSA) to give the corre-
sponding pyrrolidinone. The 4-FC6H4CHO imine 1a con-
verted smoothly into pyrrolidinone 4a in 75% yield (Table 2,
entry 1). When N-phenylbenzaldimine 1b was used, the
corresponding diphenylpyrrolidinone 4b was isolated in
64% yield (Table 2, entry 2). Imines 1c and 1g derived
from 4-MeOC6H4CHO reacted with 2c under similar reaction
conditions to give pyrrolidinones 4c and 4d in yields of 74 and
85%, respectively (Table 2, entries 3 and 4). When imine 1e
prepared from 4-MeOC6H4NH2 was used for the reaction, the
expected pyrrolidinone derivative 4e was obtained in 77%
yield (Table 2, entry 5). These results indicate that slightly
better yields of the pyrrolidinones can be obtained when the
phenyl groups on the imine are substituted with a p-methoxy
group. Apart from aromatic benzaldehydes, imines 1 f and 1j
prepared from cyclohexanecarboxaldehyde and 2-furylalde-
hyde, respectively, can be utilized in the reaction, albeit with
lower yields of the corresponding pyrolidinones (Table 2,
The catalytic reaction can also be applied to the synthesis
of g-aminonitrile 5a and Boc-protected derivative 5b
(Scheme 1) in yields of 64 and 85%, respectively, by following
the reported procedure[9] after the catalytic reaction. The Boc
group in the latter can be removed to give 5a in excellent
yield.[9]
A possible mechanism for the formation of g-amino
derivatives in this reaction is shown in Scheme 2. The reaction
is initiated by the reduction of NiII to Ni0 by zinc powder.
Coordination of the imine and acrylate to the nickel center
results in the formation of azanickelacycle A, which under-
goes hydrolysis to give the g-amino derivative and a NiII
species. The nickel(II) species is further reduced by zinc to
Scheme 1. Synthesis of protected and free nitrogen compounds.
a) [NiBr2(phen)], Zn, H2O, CH3CN, 808C; b) CAN; c) CAN, (Boc)2O.
CAN=cerium ammonium nitrate, Boc=tert-butoxycarbonyl.
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Communications
completion of the reaction, the mixture was cooled and diluted with
dichloromethane. Et3N (0.05 mmol) was added and the mixture was
filtered and then concentrated. Separation on a column of silica gel
using hexane/EtOAc as the eluent gave pure diphenylpyrrolidinone
1
product 4b in 64% yield. H NMR d = 1.97–2.02 (m, 1H), 2.58–2.64
(m, 2H), 2.72–2.77 (m, 1H), 5.22–5.25(m, 1H), 7.04 (t, J = 7.2 Hz,
1H), 7.19–7.24 (m, 5H), 7.29 (t, J = 7.2 Hz, 2H), and 7.39 ppm (d, J =
.8.0 Hz, 2H); 13C NMR d = 29.2, 31.2, 63.9, 122.2, 124.9, 125.9, 127.7,
128.6, 128.9, 138.1, 141.2, and 174.9 ppm; HRMS (EI+) 237.1158
(calcd for C16H15NO: 237.1154); IR (KBr): 1381, 1497, 1697, and
2947 cmÀ1
.
Received: February 20, 2008
Published online: May 21, 2008
Keywords: cross-coupling · homogeneous catalysis · N ligands ·
.
nickel · pyrrolidinones
Scheme 2. Plausible pathway for the formation of g-amino derivatives.
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two new stereocenters in the products, and thus is potentially
useful for enantioselective synthesis. Studies in this direction
are underway.
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