A novel reductive cyclization of arylmethylidenemalononitrile promoted by samarium diiodide

Article abstract of DOI:10.1039/a802510j

The intermolecular and intramolecular reductive coupling reactions of arylmethylidenemalononitriles induced by SmI₂ have been studied. The configuration of the cyclodimerization products, 4,5-trans-isomers of 2-amino-1,3,3-tricyano-4,5-diarylcy

Full text of DOI:10.1039/a802510j

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A novel reductive cyclization of arylmethylidenemalononitrile
promoted by samarium diiodide
Longhu Zhou† and Yongmin Zhang*
Department of Chemistry, Hangzhou University, Hangzhou, 310028, P.R. China
The intermolecular and intramolecular reductive coupling reactions of arylmethylidenemalononitriles
induced by SmI2 have been studied. The configuration of the cyclodimerization products, 4,5-trans-isomers
of 2-amino-1,3,3-tricyano-4,5-diarylcyclopentenes, has been confirmed by X-ray analysis and a possible
reaction mechanism is proposed.
Carbon᎐carbon bond formation is the essence of organic syn-
thesis and the reductive dimerization of carbonyl derivatives is
a most valuable method for establishing carbon᎐carbon bonds.
Since the time Kagan demonstrated a simple preparation of
samarium diiodide from samarium metal and 1,2-diiodo-
ethane,1 SmI2 has been developed as a mild, neutral and
versatile single electron transfer reductant. It has been widely
used in synthetic reactions, and is especially useful for
ring closure reactions, carbon᎐carbon bond formation and in
reaction stereocontrol.2 The reactivity of samarium diiodide
towards various nitrogen compounds including nitro com-
pounds,3 azocompounds,4 hydrazones,3b,5 oximes,3b imines,3b
azides6 and hydroxylamines7 has been examined.
knowledge, no literature examples for the reduction of a
cyano group to give an amine have been reported with this
reagent. Here, we wish to describe our preliminary results on
novel intermolecular reductive cyclodimerization and intra-
molecular reductive cyclizations of arylmethylidenemalono-
nitriles by SmI2, which differ from substituted malononitriles,9
diphenylmethylidenemalononitrile11 and nitrobenzonitrile10 to
give substituted cyclopentamines or 2-aminoquinoline deriv-
atives respectively.
When arylmethylidenemalononitriles 1 were treated with
SmI2 in THF, the intermolecular reductively coupled cycliz-
ation products, the 4,5-trans-isomers of 2-amino-1,3,3-tri-
cyano-4,5-diarylcyclopentenes 2, were formed (Scheme 1).
Nitro groups are known to be easily reduced by samarium
diiodide. The cyano group however, is relatively more stable to
samarium diiodide than a nitro group and could not be reduced
by this reagent. Souppe and Kagan8 reported that aromatic
and aliphatic nitriles are inert in the presence of SmI2 and m-
or p-nitrobenzonitrile could be selectively reduced to the
corresponding cyanoanilines in almost quantitative yields.
Kang and Hong9 reported the decyanation of α-cyano sub-
stituted nitrile derivatives by samarium diiodide. However,
no cyclization products were observed. It has been known
that α,β-unsaturated carboxylic acid derivatives are reduced
with SmI2 to the corresponding saturated compounds.10
Recently, Yacovan and Hoz11 reported that 1,1-diphenyl-2,2-
dicyanoethylene can be quantitatively reduced to (diphenyl-
methyl)malononitrile without any dimeric products or
reduction products of the cyano group. To the best of our
NH2
Sml2 (1.1 equiv.)
NC
ArCH
C
CN
CN
THF, r.t.
NC
Ar
CN
Ar
1
2
Scheme 1
Table 1 summarizes our results on the cyclodimerization of a
number of substrates. In the reactions, the cleavage takes place
at the carbon᎐nitrogen triple bond and the carbon᎐carbon
double bond. The chloro, bromo and alkoxy groups of the
substrates could not be reduced under the reaction conditions
and have no influence on the rate of cyclodimerization. All the
reactions were completed rapidly within a few minutes and
afforded the corresponding substituted cyclopentenes in good
yields. The effect of HMPA (hexamethylphosphoramide) to
promote the reaction of various organic compounds with
† Permanent address: Department of Chemistry, Xuzhou Normal
University, Jiangsu, 221009, China.
Table 1 Cyclodimerization of arylmethylidenemalononitriles induced by SmI2
Yield (%)a
Entry
Ar
Reaction time/min
In THFb
In THF–HMPAc
a
b
c
d
e
f
g
h
i
4-ClC6H4
5
5
5
5
5
5
5
10
5
84
83
80
78
73
68
70
67
75
86
82
84
76
73
67
72
69
77
4-CH3OC6H4
4-CH3C6H4
4-BrC6H4
3-BrC6H4
C6H5
3,4-OCH2OC6H3
2-furyl
3,4-(OCH3)2C6H3
a Isolated yield. b 10 ml THF was used. c 10 ml THF and 1 ml HMPA were used.
J. Chem. Soc., Perkin Trans. 1, 1998
2399

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R1
R2
CH
C
CN
R1
R2
CN
Sml2 (6 equiv.)
THF, r.t., 2 h
CN
NO2
N
NH2
3
4
a: R1 = R2 = H
b: R1 = R2 = OCH2O
4a: 87%
4b: 84%
Scheme 3
In summary, SmI2 promotes a novel reductive cyclization of
arylmethylidenemalononitriles, which has not been achieved
previously by other reductants. Further studies to develop other
new reactions using SmI2 are now in progress.
Experimental
General details
Tetrahydrofuran (THF) was distilled from sodium–
benzophenone immediately prior to use. All reactions were
conducted under a nitrogen atmosphere. Melting points are
uncorrected. Infrared spectra were recorded on Perkin-Elmer
Fig. 1 Molecular structure of 2a showing the atom numbering scheme
1
683 spectrometers in KBr with absorptions in cmϪ1. H NMR
SmI2, is well known.12 However, in this case, the reactivity of the
starting materials and the yields of the reaction were the same
either in the presence of HMPA or without HMPA. As shown
in Table 1, the reactions are very stereoselective, as only
one isomer was obtained and careful analysis of the reaction
mixture indicated the absence of any other stereoisomer.
This was confirmed by an X-ray crystal structure analysis
of a single crystal of 2aؒEtOHؒH2O, which clearly illustrates
the trans-stereochemistry of the product (Fig. 1).
Spectra were determined on Bruker AC-80 or JEOL PMX-60
spectrometers as CDCl3 solutions. J Values are in hertz.
Chemical shifts are expressed in ppm downfield from internal
tetramethylsilane. Mass spectra were recorded on HP 5989A
or Finnigan MAT GC–MS spectrometers. Microanalyses were
carried out on a Perkin-Elmer 240C instrument.
General procedure for the cyclodimerization reactions
A solution of arylmethylidenemalononitrile 1 (1 mmol) in
anhydrous THF (3 ml) was added to a solution of SmI2
(1.1 mmol) in THF (10 ml) at room temperature under a dry
nitrogen atmosphere. The reaction was completed in a few
minutes and the deep blue green colour of samarium di-
iodide changed to a brownish yellow. The reaction was
quenched with dilute HCl (1 , 1 ml) and extracted with diethyl
ether (3 × 40 ml). The combined extracts were washed with
saturated aqueous Na2S2O3 (15 ml), saturated aqueous NaCl
(15 ml), and dried over anhydrous Na2SO4. After evaporating
the solvent under reduced pressure, the crude product was
purified by preparative thin layer chromatography [eluent, ethyl
acetate–light petroleum (bp 30–60 ЊC) (1:2)].
Though the detailed mechanism of the above cyclodimeriza-
tion has not been clarified yet, the cyclopentene formation
can be explained by the possible mechanism presented in
Scheme 2.
CN
Ar
Ar
Sml2
–
•
1
–
–
CN
CN
ArCH
C
CN
ArCH
C
CN
Sml2
CN
CN
CN
1
A
B
Compound 2a. Mp 158–160 ЊC; νmax(KBr)/cmϪ1 3370, 3200,
2220, 1680, 1630 and 1500; δH 3.74 (1H, d, J 9.5, ArCH), 4.51
(1H, d, J 9.5, ArCH), 5.30 (2H, br s, NH2), 6.85–7.40 (8H, m,
2 × ArH); m/z 382 (M ϩ 4, 12.1%), 380 (M ϩ 2, 62.6), 378 (Mϩ,
84.6), 377 (42.7), 343 (85.3), 308 (22.3), 278 (16.9), 189 (18.4),
155 (100) (Found: C, 63.12; H, 3.08; N, 14.83. C20H12N4Cl2
requires C, 63.33; H, 3.19; N, 14.77%).
NH2
NH
N –
NC
NC
NC
NC
NC
NC
H3O +
–
CN
CN
CN
Ar
Ar
Ar
Ar
Ar
Ar
2
D
C
Crystal data and structure refinement for 2aؒC2H5OHؒH2O.
Crystals suitable for X-ray analysis were obtained by slow
evaporation of an ethanol solution of 2a. C22H20O2N4Cl2, M =
Scheme 2
¯
443.33, triclinic, space group P1, a = 11.033(4), b = 12.199(3),
In the initial step, an electron is transferred from SmI2
to substrate 1 to form radical anion A, which then attacks
another molecule of substrate to form a carbon᎐carbon bond
and generate intermediate B. Intermediate B then reacts
intramolecularly to result in the formation of a new carbon᎐
carbon bond and produce intermediate C. Then C is converted
to D which isomerizes to give product 2.
However, the reductive cyclization process is even more
surprising when contrasted with the reaction of substrate 3
with SmI2 in THF. When a solution of 1 equiv. of substrate 3
in anhydrous THF was allowed to react with 6 equiv. of SmI2
in dry THF under the same reaction conditions, an entirely
different type of intramolecular reductive cyclization took place
and afforded the 2-aminoquinoline derivative 4 (Scheme 3). The
cyclodimerization product 2 was not detected.
c = 10.732(3) Å, α = 114.46(2), β = 118.33(3), γ = 81.56(3)Њ,
V = 1155.1(7) Å3, Z = 2, Dc = 1.275 g cmϪ3, F(000) = 460.00,
µ(Mo-Kα) = 3.05 cmϪ1, colorless prismatic crystals, crystal size
0.20 × 0.20 × 0.30 mm.
Intensity data were collected at 293 K on a Rigaku AFC7R
diffractometer
with
graphite-monochromated
Mo-Kα
radiation (λ = 0.701 69 Å): 3033 independent reflections were
collected using ω Ϫ 2θ scan mode in the range of 6Њ <
2θ < 49.4Њ, of which 2063 intensity data with [I > 2σ(I)] were
observed. The corrections for Lp factors were applied. The
structure was solved by direct methods13 and expanded using
Fourier techniques.14 Some non-hydrogen atoms were refined
anisotropically, while the rest were refined isotropically.
Hydrogen atoms were included in F value calculations but fixed
during the structural refinement. A full-matrix least-squares
2400
J. Chem. Soc., Perkin Trans. 1, 1998

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Compound 2g. Mp 166–168 ЊC; νmax(KBr)/cmϪ1 3380, 3240,
Table 2 Selected bond lengths (Å) and angles (Њ)
2210, 1675, 1620 and 1510; δH 4.26 (1H, d, J 8.8, ArCH), 4.43
(1H, d, J 8.8, ArCH), 5.27 (2H, br s, NH2), 5.98 (4H, s,
2 × OCH2O), 6.30–6.93 (6H, m, 2 × ArH); m/z 399 (M ϩ 1,
15.4%), 398 (Mϩ, 100), 371 (16.5), 155 (43.5), 154 (26.7)
(Found: C, 66.52; H, 3.78; N, 13.82. C22H14N4O4 requires C,
66.33; H, 3.54; N, 14.06%).
Cl1᎐C12
Cl2᎐C18
N1᎐C6
N2᎐C2
N3᎐C7
N4᎐C8
C1᎐C2
C1᎐C5
C1᎐C6
C2᎐C3
C3᎐C4
C3᎐C7
C3᎐C8
C4᎐C5
1.752(6)
1.757(6)
1.141(8)
1.338(7)
1.133(7)
1.137(7)
1.332(7)
1.514(8)
1.436(9)
1.539(8)
1.570(7)
1.472(8)
1.470(8)
1.531(7)
C2᎐C1᎐C6
C5᎐C1᎐C6
N2᎐C2᎐C1
N2᎐C2᎐C3
C1᎐C2᎐C3
C2᎐C3᎐C4
C2᎐C3᎐C7
C2᎐C3᎐C8
C4᎐C3᎐C7
C4᎐C3᎐C8
C7᎐C3᎐C8
C3᎐C4᎐C5
C1᎐C5᎐C4
N1᎐C6᎐C1
N3᎐C7᎐C3
N4᎐C8᎐C3
123.2(5)
122.5(5)
133.1(6)
118.9(5)
108.1(5)
101.4(4)
114.7(4)
107.8(5)
112.3(4)
111.7(4)
108.9(4)
103.1(4)
100.2(4)
178.8(7)
176.9(6)
177.3(7)
Compound 2h. Mp 125–127 ЊC; νmax(KBr)/cmϪ1 3360, 3240,
2210, 1680, 1630 and 1510; δH 4.23 (1H, d, J 9.0, ArCH), 4.60
(1H, d, J 9.0, ArCH), 5.61 (2H, br s, NH2), 6.23–7.53 (6H, m,
2 × furan-H); m/z 291 (M ϩ 1, 21.3%), 290 (Mϩ, 100), 263
(15.5), 155 (37.4) (Found: C, 66.43; H, 3.61; N, 19.41.
C16H10N4O2 requires C, 66.20; H, 3.47; N, 19.30%).
Compound 2i. Mp 138–140 ЊC; νmax(KBr)/cmϪ1 3360, 3220,
2220, 1670, 1620 and 1525; δH 3.74 (1H, d, J 9.5, ArCH), 3.80
(6H, s, 2 × OCH3), 3.88 (6H, s, 2 × OCH3), 4.48 (1H, d, J 9.5,
ArCH), 5.40 (2H, br s, NH2), 6.60–7.15 (6H, m, 2 × ArH);
m/z 431 (M ϩ 1, 15.2%), 430 (Mϩ, 100), 403 (25.7), 293 (21.3),
266 (19.3), 156 (18.4), 155 (13.2) (Found: C, 67.21; H, 5.01;
N, 13.20. C24H22N4O4 requires C, 66.97; H, 5.15; N, 13.02%).
C2᎐C1᎐C5
114.1(5)
refinement gave final R = 0.082 and Rw = 0.098 with w = 1/
σ2(F0), S = 2.81. The maximum peak in the final difference
General procedure for the intramolecular reductive cyclization
reactions
Fourier map is 0.42 e ÅϪ3 and the minimum peak is Ϫ0.52 e ÅϪ3
.
In the final circle refinement the largest parameter shift (∆/σ)max
is 0.04. All calculations were performed using the TEXSAN
program package.15 Selected bond lengths and angles are shown
in Table 2.‡
A solution of nitro cyano olefins 3 (1 mmol) in anhydrous
THF (3 ml) was added to a solution of SmI2 (6 mmol) in THF
(40 ml) at room temperature under a dry nitrogen atmosphere.
The mixture was stirred under these conditions until the
reaction was complete. The reaction mixture was poured into
10% aqueous K2CO3 (50 ml) and extracted with diethyl ether
(4 × 30 ml). After usual work-up, the crude product was
purified by preparative thin layer chromatography [eluent, ethyl
acetate–cyclohexane (1:1)].
Compound 2b. Mp 95–97 ЊC; νmax(KBr)/cmϪ1 3380, 3240,
2210, 1670, 1620 and 1520; δH 3.68 (3H, s, OCH3), 3.74 (3H, s,
OCH3), 4.17 (1H, d, J 9.5, ArCH), 4.50 (1H, d, J 9.5, ArCH),
5.40 (2H, br s, NH2), 6.70–7.48 (8H, m, 2 × ArH); m/z 371
(M ϩ 1, 30.3%), 370 (Mϩ, 100), 369 (35.2), 355 (21.3), 339 (35),
225 (16.4), 199 (31.7), 184 (32.5), 171 (68.6), 155 (50.4), 142
(31.4), 114 (35.5) (Found: C, 71.62; H, 4.71; N, 15.30.
C22H18N4O2 requires C, 71.34; H, 4.90; N, 15.13%).
Compound 4a. Mp 226–227 ЊC; νmax(KBr)/cmϪ1 3400, 3160
and 2230; δH 5.46 (2H, br s, NH2), 7.32–7.71 (4H, m, ArH),
8.30 (1H, s, hetero-ArH); m/z 170 (M ϩ 1, 21.5%), 169 (Mϩ,
100), 144 (73.2), 143 (33.5), 117 (20.3), 116 (32.5) (Found: C,
70.86; H, 4.36; N, 24.78. C10H7N3 requires C, 70.99; H, 4.14;
N, 24.84%).
Compound 2c. Mp 190–192 ЊC; νmax(KBr)/cmϪ1 3375, 3240,
2210, 1670, 1620 and 1520; δH 2.23 (3H, s, CH3), 2.30 (3H, s,
CH3), 3.77 (1H, d, J 9.5, ArCH), 4.51 (1H, d, J 9.5, ArCH),
5.35 (2H, br s, NH2), 6.95–7.35 (8H, m, 2 × ArH); m/z 339
(M ϩ 1, 15.1%), 338 (Mϩ, 100), 325 (15.6), 324 (28.8), 155 (15),
91 (10.4) (Found: C, 77.97; H, 5.21; N, 16.73. C22H18N4 requires
C, 78.08; H, 5.36; N, 16.56%).
Compound 4b. Mp 280 ЊC (decomp.); νmax(KBr)/cmϪ1 3400,
3160 and 2230; δH 6.14 (2H, s, OCH2O), 6.66 (2H, br s, NH2),
6.91 (1H, s, ArH), 7.12 (1H, s, ArH), 8.37 (1H, s, hetero-ArH)
(Found: C, 62.21; H, 3.15; N, 19.50. C11H7N3O2 requires C,
61.97; H, 3.31; N, 19.71%).
Compound 2d. Mp 145–147 ЊC; νmax(KBr)/cmϪ1 3380, 3215,
2210, 1680, 1630 and 1500; δH 3.70 (1H, d, J 9.0, ArCH), 4.54
(1H, d, J 9.0, ArCH), 5.41 (2, br s, NH2), 6.97–7.63 (8H, m,
2 × ArH); m/z 470 (M ϩ 4, 48.3%), 468 (M ϩ 2, 100), 466 (Mϩ,
51.3), 391 (23.1), 390 (36.5), 389 (74.2), 388 (30.4), 309 (35.2),
308 (47.1), 155 (43.9) (Found: C, 51.49; H, 2.77; N, 11.70.
C20H12Br2N4 requires C, 51.31; H, 2.58; N, 11.97%).
Acknowledgements
We are grateful to the National Natural Science Foundation
of China (Project No. 294938004 and 29672007) and the
Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Science for
financial support. We thank Professor Weimin Lu for helpful
discussions about the X-ray analysis for compound 2a.
Compound 2e. Mp 116–118 ЊC; νmax(KBr)/cmϪ1 3380, 3210,
2210, 1670, 1630, 1600 and 1580; δH 3.60 (1H, d, J 9.2, ArCH),
4.50 (1H, d, J 9.2, ArCH), 5.33 (2H, br s, NH2), 6.78–7.60 (8H,
m, 2 × ArH); m/z 470 (M ϩ 4, 46.2%), 468 (M ϩ 2, 100), 466
(Mϩ, 50.3), 390 (20.3), 389 (49.1), 388 (32.6), 308 (27.3), 155
(15.4), 154 (41.7) (Found: C, 51.61; H, 2.31; N, 12.26.
C20H12Br2N4 requires C, 51.31; H, 2.58; N, 11.97%).
References
Compound 2f. Mp 128–130 ЊC; νmax(KBr)/cmϪ1 3380, 3200,
2210, 1670, 1630 and 1505; δH 3.80 (1H, d, J 9.2, ArCH),
4.58 (1H, d, J 9.2, ArCH), 5.20 (2H, br s, NH2), 7.10–7.70 (10H,
m, 2 × ArH); m/z 311 (M ϩ 1, 21.5%), 310 (Mϩ, 100), 283
(15.3), 156 (15.1), 155 (41.4), 128 (18.4) (Found: C, 77.57;
H, 4.38; N, 18.20. C20H14N4 requires C, 77.40; H, 4.55; N,
18.05%).
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Paper 8/02510J
Received 2nd April 1998
Accepted 5th June 1998
2402
J. Chem. Soc., Perkin Trans. 1, 1998