Enantioselective conjugate addition of an α,α-dithioacetonitrile with nitroalkenes using chiral bis(imidazoline)-Pd complexes

Article abstract of DOI:10.1039/c9cc02443c

The enantioselective conjugate addition reaction of an α,α-dithioacetonitrile with nitroalkenes was catalysed by chiral bis(imidazoline)-palladium pincer-type complexes. The reaction was applicable to various nitroalkenes to afford products in good yield with high enantioselectivity. The obtained products can be converted to γ-lactam and biologically active rolipram.

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Enantioselective conjugate addition of an α,α-
dithioacetonitrile with nitroalkenes using chiral
bis(imidazoline)-Pd complexes
Shuichi Nakamura,*,a,b Akari Tokunaga,a Hikari Saito,a Masaru Kondoa,b
nitroalkenes as electron-deficient alkenes using Phebim-Pd
complex (Scheme 1).9
The enantioselective conjugate addition reaction of an α,α-
dithioacetonitrile with nitroalkenes was catalysed by chiral
bis(imidazoline)-palladium pincer-type complexes.
The
reaction was applicable to various nitroalkenes to afford
products in good yield with high enantioselectivity. The
obtained products can be converted to γ-lactam and
biologically active rolipram.
C-C bond formation reaction through the conjugate addition of
nucleophiles to electron-deficient alkenes is one of the most
important synthetic methods for organic compounds.
Furthermore, the enantioselective version of conjugate addition
has attracted considerable attention in synthetic chemistry.1
Therefore, expanding the scope of catalytic stereoselective
conjugate addition of α,α-dithioacetals, which are well known as
“synthetic chameleon”,2 would be highly desirable.3 Koga and
Tomioka first reported the enantioselective reaction of α-
carbanion of α,α-dithioacetals with α,β-unsaturated esters as
electron-deficient alkenes using a stoichiometric amount of
chiral aminoether ligand to give chiral conjugate addition
Scheme
1 Enantioselective reaction of α,α-dithioacetonitriles as
cyanomethyl and cyanocarbonyl anion equivalents with electron-
deficient alkenes
First, we examined the reaction of various α,α-
dithioacetonitriles
mol% of chiral palladium-pincer complexes
2
(1.5 equiv.) with nitrostyrene 1a using 5
and Ag(acac) at
4
room temperature in ethyl acetate (Table 1). The reaction of 1a
with 1,3-dithiorane-2-carbonitrile 2a using Phebim-Pd 4a gave
product 3aa in good yield with moderate enantioselectivity
(entry 1). In order to improve the yield and enantioselectivity,
catalytic precursor 4b-g having various carbonyl and sulfonyl
groups on the R1 group and 2,4,6-trimethylphenyl (mesityl) and
1-naphthyl groups on the R2 group were examined (entries 2-7).
As a result, complex 4f bearing an acetyl group on the R1 group
and a mesityl group on the R2 group showed good yield with high
enantioselectivity (entry 6). We also examined the reaction of
various α,α-dithioacetonitriles 2b-d with 1a using 4f, but the
reaction did not give better results than the reaction using 2a
(entries 8-10). When the reaction was carried out at a lower
temperature (0 oC), enantioselectivity improved to 98% ee (entry
11).
products.4
Recently, catalytic enantioselective conjugate
addition reactions of 1,3-dithiane-2-thiocarboxylates with
nitroalkenes and α,β-unsaturated carbonyl compounds using a
cinchona alkaloid-thiourea catalyst was reported by Benaglia
and Gaggero’s group.5 Despite the impressive progress achieved
in this type of reaction, there is still room for developing the
catalytic enantioselective conjugate addition of α,α-dithioacetals
to electron-deficient alkenes.6
On the other hand, α,α-
dithioacetonitriles are also an important class of synthetic
intermediate, because they can act as cyanomethyl and
cyanocarbonyl anion equivalents. Recently, we reported the first
highly enantioselective reaction of α,α-dithioacetonitriles with
imines using palladium pincer-type complexes with 1,3-
bis(imidazolin-2-yl)benzene (Phebim).7,8 However, to the best
of our knowledge, there are no reports for the enantioselective
conjugate addition of α,α-dithioacetonitriles to electron-deficient
alkenes. Herein our ongoing interest was extended to the
catalytic enantioselective reaction of α,α-dithioacetonitriles with
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Table 1 Enantioselective reaction of α,α-dithioacetonitriles 2a-d with Table 2 Enantioselective reaction of 2a with various nitroalkenes 1a-o using
nitrostyrene 1a using palladium complexes 4a
-
g
.
4f.
Prod Temp. Time Yield Ee
Entry
1
R
uct
(oC)
0
(h)
48
9
(%)
99
98
78
87
77
80
86
47
59
77
71
76
91
72
80
(%)
98
95
97
97
98
98
96
94
98
98
95
94
97
95
96
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
Ph
3aa
3ba
3ca
3da
3ea
3fa
3ga
3ha
4-FC6H4
3-FC6H4
4-ClC6H4
3-ClC6H4
4-BrC6H4
4-CF3C6H4
4-CH3C6H4
rt
rt
rt
rt
0
48
24
24
48
24
24
72
48
96
48
72
48
24
rt
50
0
rt
0
rt
0
0
9
4-MeOC6H4 3ia
Entry
2
4
Time
(h)
24
24
24
24
24
24
24
24
48
48
48
Yield Ee
10
11
12
13
14
15
2-Naphthyl
2-Furyl
3-Furyl
2-Thienyl
nPr
3ja
3ka
3la
3ma
3an
3oa
(%)
80
93
26
95
72
92
89
85
-
(%)
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
2a
4a
4b
4c
4d
4e
4f
4g
4f
4f
73
72
80
77
94
95
71
89
-
iBu
rt
On the other hand, the reaction of ethyl 1,3-dithiolane-2-
carboxylate 2e with nitrostyrene 1a using 4f-Ag(acac) did not
give any product (Scheme 2). In addition, the reaction of various
α-acetonitrile carbanion equivalents, such as acetonitrile (2f),
9
10
4f
4f
-
-
phenylthioacetonitrile
(2g), dichloroacetonitrile (2h), and
11a
99
98
cyanoacetic acid (2i), did not give products in high yield. These
results show that the nitrile and dithiolane group in 2a were
[a] At 0 oC.
With optimized reaction conditions for the reaction of essential in this C-C bond formation reaction.
nitrostyrene 1a with 2a, we next examined the scope of
nitroalkenes for this reaction (Table 2). The reaction of
nitroalkenes 1b-g bearing electron-withdrawing groups such as
a fluoro, chloro, bromo, or trifluoromethyl group in the para or
meta position by using 4f gave corresponding products 3ba-ga
in high yield and enantioselectivity (entries 2-7, 77-98% yield,
95-98% ee). Electron-rich nitroalkenes 1h,i having a methyl or
methoxy group reacted with 2a and afforded products 3ha,ia
with excellent enantioselectivity but in moderate yield (entries 8
and 9). Furthermore, nitroalkenes having naphthyl group 1j or
Scheme 2 Reaction of ethyl 1,3-dithiolane-2-carboxylate 2e or various
acetonitrile carbanion equivalents 2f-i with nitrostyrene 1a using the 4f-Ag
system
heteroaryl group 1k-m were tolerable substrates in the reaction
giving products 3ka-ma in moderate to good yield (71-90%
yield) with high enantioselectivity (94-98% ee, entries 10-13).
In addition, the reaction of nitroalkenes bearing an alkyl group
The proposed reaction mechanism for the reaction of 2a with
nitroalkenes
acetylacetonate-complex (complex
addition of Ag(acac) to 4f through the exchange reaction of
bromide on 4f to acetylacetonate. The palladium in complex
coordinates to the cyano group in 2a to afford cationic complex
, and is deprotonated by acetylacetonate to give the palladium
ketenimide complex (complex ). Next, the reaction of complex
with nitroalkenes proceeds to give complex , which
subsequently affords the protonation product and regenerated
complex . In order to clarify the proposed reaction cycle, we
checked the ESI-Mass spectroscopic analysis for the mixture of
4f, Ag(acac) and 2a, in a 1:1:1 ratio. Complex could be
observed in this mixture (cation mode, calcd for
C56H62N5O2PdS2 as complex : 1006.3, found: 1006.3, see
1
using 4f is depicted in Figure 1. The palladium
1n,o also gave products 3na,oa with high enantioselectivity
A
) was formed by the
(entries 14 and 15). To our knowledge, these results are the first
examples of the highly enantioselective reaction of α,α-
dithioacetonitrile with electron-deficient alkenes.
A
B
C
C
D
A
B
B
supporting information). This result supports the proposed
catalytic cycle. We also checked the retro-conjugate addition
reaction of product 3aa, in which the treatment of racemic 3aa
with 4f and Ag(acac) did not change the enantiopurity.
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Therefore, the stereoselectivity of the reaction would be acetaldoxime in toluene/THF afforded corresponding amide 5 13
.
kinetically controlled.
Further reduction of the nitro group in
gave cyclized lactam . Reductive desulfurization of
NiCl2·6H2O and NaBH4 afforded lactam in good yield without
5
using iron and NH4Cl
6
6
using
7
the loss of enantiopurity.14 The absolute configuration of 21 was
assigned to an R isomer by comparing with the value of the
specific optical rotation from a previous report.15
Scheme 3 Transformation of 3aa to chiral γ-lactam 7
This procedure can be applied to the synthesis of biologically active
rolipram, which is an anti-inflammatory drug and a family member of
γ-aminobutyric acid (GABA) derivatives.16 Furthermore, rolipram is
known to be a possible antidepressant and has been reported to have
immunosuppressive, and antitumor effects.17 The reaction of 2a with
1p using 5 mol% of 4f afforded conjugate addition product 8 in high
yield with high enantioselectivity (Scheme 4, 88%, 95% ee). Then,
the reaction using InCl3∙4H2O and acetaldoxime gave amide 9, whose
Fig. 1 Assumed reaction mechanism for the reaction of 2a with
nitroalkenes
1 using 4f
nitro group can be reduced to give

-lactam 10
.
The reductive
desulfurization of 10 afforded (
R)-rolipram 11 without the loss of
We next examined the DFT calculation of complex
C
between
4f and the carbanion of 2a using a Gaussian 1610 B3LYP/6-31G*,
enantiopurity.
LANL2DZ(Pd) level (Figure 2).11 Palladium in the complex
coordinated to the cyano group in 2a to form a ketenimide
structure. Natural bond orbital (NBO) analysis12 for complex
C
C
shows the interaction between the two * orbital of the C-S bond
and a p orbital of the anionic carbon for 2a. The stabilization
energies for their interaction was estimated to be 9.86 and 9.84
kcal/mol. In the case of the reaction of 2c and 2d, the n-*
stabilization effects are not so large due to the flexibility of
substituent on sulfur. Based on the absolute configuration of
products, the proposed transition state for the enantioselective
reaction of palladium ketenimide with nitroalkenes
1 is shown in
Figure 2. Nitroalkenes approach from the Re-face of the
1
double bond avoiding steric repulsion between the mesityl group
on imidazoline and nitroalkenes to afford the (R)-isomer of the
product. The fine-tuning of substituent on chiral imidazoline
group changes their transition state slightly.
Scheme 4 Synthesis of optically active (R)-rolipram 11
Conclusions
The first highly enantioselective reaction of α,α-
dithioacetonitriles with nitroalkenes using Phebim-palladium
complexes was developed. The reaction is applicable to various
nitroalkenes. The obtained product was converted to γ-lactam
and (R)-rolipram. Further experiments are in progress to study
the application of the bis(imidazoline)-palladium pincer complex
to other reactions.
Fig. 2 DFT calculation of the complex between 4f and 2a using Gaussian
16 B3LYP/6-31G*, LANL2DZ(Pd) and assumed transition state for the
reaction with nitroalkenes 1. H atoms have been omitted for clarity.
We next examined the transformation of product 3aa to useful
compounds (Scheme 3). The hydrolysis of the cyano group in
3aa using 40 mol% of InCl3∙4H2O and 3.0 equivalent of
This work was partly supported by the Hibi Science
Foundation.
Notes and references
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aDepartment of Life Science and Applied Chemistry, Graduate School of
Engineering, Nagoya Institute of Technology, Gokiso, Showa-ku, Nagoya
466-8555 (Japan). E-mail: snakamur@nitech.ac.jp
bFrontier Research Institute for Material Science, Nagoya Institute of
Technology, Gokiso, Showa-ku, Nagoya 466-8555 (Japan)
Electronic Supplementary Information (ESI) available: See DOI:
10.1039/c000000x/
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