Unusual formation of a thiacyclopent-2-ene by the reaction of 4,4′-dimethoxythiobenzophenone with tetracyanoethylene

Article abstract of DOI:10.1002/hc.1041

The reaction of 4,4′-dimethoxythiobenzophenone (1a) with tetracyanoethylene (TCNE) afforded an unusual type of 2:1 adduct with the structure of thiacyclopent-2-ene (5a). On the other hand, the reaction of thiobenzophenone (1b) with TCNE afforded the corresponding thiophene, the normal 2:1 adduct (2b). The mechanism of the formation of 5a is discussed.

Full text of DOI:10.1002/hc.1041

Heteroatom Chemistry
Volume 12, Number 4, 2001
U
nusual Formation of a Thiacyclopent-2-ene
by the Reaction of 4,4Ј-
Dimethoxythiobenzophenone with
Tetracyanoethylene
Kentaro Okuma and Yoshikazu Tsujimoto
Department of Chemistry, Faculty of Science, Fukuoka University, Jonan-ku, Fukuoka 814-0180,
Japan; kokuma@fukuoka-u.ac.jp
Received 12 December 2000; revised 22 January 2001
afford several types of the corresponding cycload-
ducts [4]. In a previous article, we have reported an
unusual reaction of selenobenzophenones with
TCNE to give a novel type of cycloadducts, selena-
cyclopent-2-enes (4), sharply different from the
above result with thiobenzophenone (Scheme 1) [5].
These results prompted us to investigate the possi-
bility to form thiacyclopent-2-enes from thiobenzo-
ABSTRACT: The reaction of 4,4Ј-dimethoxythio-
benzophenone (1a) with tetracyanoethylene (TCNE)
afforded an unusual type of 2:1 adduct with the struc-
ture of thiacyclopent-2-ene (5a). On the other hand,
the reaction of thiobenzophenone (1b) with TCNE af-
forded the corresponding thiophene, the normal 2:1
adduct (2b). The mechanism of the formation of 5a is
discussed. ᭧ 2001 John Wiley & Sons, Inc. Hetero-
phenones. This article describes a precise reaction
atom Chem 12:259–262, 2001
of thiobenzophenones with TCNE.
INTRODUCTION
RESULTS AND DISCUSSION
Tetracyanoethylene (TCNE) has played an important
role in extensive applications in organic synthesis
owing to its anomalous reactivity [1]. Previously, we
reported the reaction of tropothione with TCNE to
afford 8,8-dicyanoheptafulvene via a [2 ם 2]-type cy-
cloaddition [2]. Huisgen et al. reported that thio-
benzophenone (1) reacted with TCNE to give 2:1 ad-
ducts of thiophene derivatives (2) and six-membered
dithiins (3) in moderate yields [3]. We have isolated
selenobenzophenones, which reacted with olefins to
4,4Ј-Dimethoxythiobenzophenone (1a) reacted with
2 equiv. of TCNE in refluxing benzene to afford yel-
low crystals (5a) along with 2a (25%) (Scheme 2).
The structure of 5a (8%) was found to be 5,5-di-
(4-methoxyphenyl)-2-[di-(4-methoxyphenyl)methyl-
ene]amino-3,4,4-tricyano-thiacyclopent-2-ene by its
spectroscopic analysis. Mass spectrum and elemen-
tal analysis suggested that we are dealing with a 2:1
1
(1a: TCNE) adduct. The H NMR spectrum of 5a
Dedicated to Prof. Naoki Inamoto on the occasion of his 72nd
birthday.
Correspondence to: Kentaro Okuma.
Contract Grant Sponsor: Scientific Research from the Ministry
of Education, Science, Culture, and Sports, Japan.
Contract Grant Number: Grant-in-Aid 06640706.
᭧
2001 John Wiley & Sons, Inc.
SCHEME 1
2
59

2
60 Okuma and Tsujimoto
TABLE 1 The Reaction of Thiobenzophenones with TCNE
Conditions
Thiobenzophen-
one (Ar)
TCNE Time
Solvent (equiv.) (h)
Products
(% Yield)
1
a
p-MeOC H4 Benzene
1
2
2
1
2
2
1
2
8
8
8
8
2a 25 5a
8
15
32
0
0
0
6
Toluene
Xylene
55
33
1b C H5
Benzene
Toluene
Xylene
2b 63 5b
6
45
5
SCHEME 2
Since 5b could not be obtained by using thio-
benzophenone 1b, the electron-donating group
(MeO) of 1 might affect the formation of 5a. To con-
firm this assumption, we then tried the reaction of
shows only two different 4-methoxyphenyl protons.
1
3
The C NMR spectrum of 5a shows four methoxy,
three cyano, two aliphatic (quaternary), one CסN,
two olefinic, and aromatic carbons.
4
,4Ј-difluorothiobenzophenone (1c) with TCNE. In
this case, the product was not a thiacyclopent-2-
ene but a thiophene derivative (2c) obtained in 50%
yield (Scheme 4). Thus, the electron-donating group
evidently plays an important role for the formation
of 5.
Thiacyclopent-2-enes were previously prepared
in many ways: for example, in five steps from ␣-ace-
tyl c-lactones [7], in five steps from cysteine [8], by
the partial hydrogenolysis of thiophene [9], by Mi-
chael reaction of a sulfonium ylide with an unsatu-
rated thioamide [10], by a free radical intramolecu-
lar cyclization of enynes [11], and by the the reaction
of S-3,3-bis(phenylthio)propylthioalkanoates with a
low-valent titanium species [12]. The present result
offers the formation of a thiacyclopent-2-ene from
TCNE by novel types of [2 ם 2] and [4 ם 2] sequen-
tial cycloadditions.
This result is quite different from that reported
by Huisgen et al. [3,6]. They have reported that the
reaction of 1a with TCNE in refluxing chloroform
afforded only 2a in 78% yield, and the reaction of
thiobenzophenone (1b) with TCNE in refluxing ben-
zene afforded 2b (ArסPh) and 3b (ArסPh) in 52%
and 29% yields, respectively. The difference in the
reactivity might be mainly due to the reaction tem-
perature. To confirm this possibility, the reaction
was carried out at an elevated temperature. When
the reaction of 1a with TCNE was carried out in re-
fluxing xylene for 8 hours, the yields of 5a and 2a
were improved to 32% and 33%, respectively. Thus,
the reaction temperature plays an important role in
the formation of 5a.
The reaction of thiobenzophenone (1b) with
TCNE was carried out in a similar manner. However,
the corresponding thiacyclopent-2-ene (5b) was not
obtained even in refluxing xylene for 8 hours (Table
EXPERIMENTAL
General
1
).
As to the formation of 5, we thought as follows:
the reaction between thiobenzophenone and TCNE
proceeded through the initial attack of the thiocar-
bonyl sulfur on the nitrile carbon via a [2 ם 2] man-
ner. The four-membered intermediate is easily con-
verted into the ring-opened thioamide (6). Another
molecule of 4,4Ј-dimethoxythiobenzophenone (1a)
reacted with 6 via a [4 ם 2] manner to afford the six-
membered cyclic disulfide (7), which finally ex-
truded sulfur to give 5a. The formation mechanism
of 2 had already been suggested by Huisgen et al. [6].
The thioamide 6 was further attacked by another
molecule of thiobenzophenone (1a) to yield maleon-
itrile and/or fumaronitrile derivatives (8). Ring clo-
sure of 8 and the final sulfur extrusion formed the
five-membered cyclic thiophene derivatives (2)
All chemicals were obtained from commercial sup-
pliers and were used without further purification.
Analytical TLC was carried out on precoated plates
(Merck silica gel 60, F254) and flash column chro-
matography was performed with silica gel (Merck,
1
13
7
1
0–230 mesh). NMR spectra ( H at 400 MHz; C at
00 MHz) were recorded in CDCl3 solvent, and
chemical shifts are expressed in ppm relative to in-
ternal TMS.
Reaction of 4,4Ј-Dimethoxythiobenzophenone
(1a) with TCNE
To a refluxing solution of 1a (0.258 g, 1.0 mmol) in
(
Scheme 3).
benzene (25 mL) was added a solution of TCNE (0.25

Unusual Formation of a Thiacyclopent-2-ene by the Reaction of 4,4Ј-Dimethoxythiobenzophenone with Tetracyanoethylene 261
SCHEME 3
Reaction of 4,4Ј-Difluorothiobenzophenone with
TCNE
To a suspension of 4,4Ј-difluorobenzhydriltriphen-
ylphosphonium fluoroborate (0.509 g, 1.0 mmol) in
benzene (50 mL) was added butyllithium (0.70 mL,
1
.6 M in hexane, 1.1 mmol) dropwise at room tem-
perature (rt). After the mixture had been stirred for
hour, powder of elemental sulfur (0.11 g, 3.5 mg
SCHEME 4
1
atom) was added to the red suspension, and the re-
action mixture was refluxed for 1 hour to give a deep
purple suspension of 4,4Ј-difluorothiobenzophenone
1c. TCNE (0.13 g, 1.0 mmol) in benzene (10 mL) was
added dropwise to the resulting suspension. After
the mixture had been refluxed for 1 hour, TCNE (0.13
g, 1.0 mmol) was further added to this suspension
portionwise. After refluxing for 8 hours, the reaction
mixture was poured into water, and the organic layer
g, 2.0 mmol) in benzene (10 mL). After refluxing for
1
hour, the reaction mixture was filtered and evap-
orated to give dark purple crystals, which were chro-
matographed over silica gel by elution with dichlo-
romethane to afford 5a (0.025 g, 8%) and 2a (0.077
g, 25%). Thiobenzophenone 1a was recovered in
3
9
5% yield (0.090 g). 5a: Yellow crystals; m.p. 89–
1
0ЊC. H NMR (CDCl , 400 MHz) d ס 3.83 (s, 6H,
3
MeO), 3.88 (s, 6H, MeO), 6.86 (d, 4H, J ס 9 Hz, Ar),
was separated, dried over MgSO , and evaporated to
4
6
7
.93 (d, 4H, J ס 9 Hz, Ar), 7.29 (d, 4H, J ס 9 Hz, Ar),
give dark reddish oily crystals. Chromatography on
silica gel by elution with dichloromethane–hexane
(1:1) afforded the thiophene derivative (2c, 0.141 g,
1
3
.51 (d, 4H, J ס 9 Hz, Ar). C NMR (CDCl , 100
3
MHz) d ס 53.8 (Ar C-S), 55.3 (MeO), 55.3 (MeO),
2
1
5
5.5 (MeO), 55.6 (MeO), 73.7 (C(CN) ), 82.0
50%). Orange crystals, m.p. Ͼ 300ЊC. H NMR
2
(
CסCCN), 111.6 (2 CN), 112.7 (CN), 114.0, 114.1,
27.9, 129.2, 130.8, 132.2, 160.2, 162.9, 170.9 (S–
Cס), 174.1 (CסN). Exact Mass: Found: 612.1822
(CDCl ) d ס 7.00–7.20 (br, 12H, Ar), 7.76 (br s, 4 H,
3
1
3
1
Ar); All of aromatic protons in coalescence. C NMR
(CDCl ) d ס 106.7 (C-3/C-4 of thiophene), 112.4
3
ם
(
M ). Calcd for C H N O S: 612.1831. Anal. Found:
(CN), 115.8 (C-2/C-5 of C H F, J ס 23 Hz), 117.1
3
6
28
4
4
6
4
CF
C, 70.38; H, 4.65; N, 9.15. Calcd for C H N O S: C,
(C-2/C-5 of C H F, J ס 23 Hz), 129.1 (C-1 of C H F),
3
6
28
4
4
6
4
CF
6
4
7
2
0.57; H, 4.61; N, 9.14. 2a: Orange crystals; m.p. 227–
29ЊC. (lit. [6] m.p. 227–229ЊC).
130.4 (C-3/C-4 of C H F), 132.3 (C-3/C-4 of C H F),
6
4
6
4
155.3 (C-2/C-5 of thiophene), 166.9 (CסN). Anal.
Found: C, 68.28; H, 2.65; N, 9.81. Calcd for
C H F N S: C, 68.08; H, 2.86; N, 9.92.
Reaction of Thiobenzophenone (1b) with TCNE
3
2
16
4
4
To a refluxing solution of 1b (0.198 g, 1.0 mmol) in
benzene (25 mL) was added a solution of TCNE (0.25
g, 2.0 mmol). After refluxing for 8 hours, the reaction
mixture was filtered and evaporated to give dark pur-
ple crystals, which were chromatographed over sil-
ica gel by elution with dichloromethane to afford 2b
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