Dealkylation of a 1,2-bis(benzylthio)benzene derivative: Generation of benzodithiete or its equivalent via a dithia dication

Article abstract of DOI:10.1039/b004940i

The reaction of 1,2-bis(4-methylbenzylthio)benzene s-oxide with triflic anhydride in CH₃CN-CH₂Cl₂ in the presence of alkynes affords 1,4-benzodithiin derivatives and N-(4-methylbenzyl)acetamide after quenching with H₂O; this suggests the generation of benzodithiete or its equivalent.

Full text of DOI:10.1039/b004940i

Dealkylation of a 1,2-bis(benzylthio)benzene derivative: generation of
benzodithiete or its equivalent via a dithia dication†
Kenji Kobayashi,* Emiko Koyama, Mariko Goto, Chikako Noda and Naomichi Furukawa*‡
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan.
E-mail: kenjinor@staff.chem.tsukuba.ac.jp
Received (in Cambridge, UK) 20th June 2000, Accepted 21st July 2000
The reaction of 1,2-bis(4-methylbenzylthio)benzene s-oxide
with triflic anhydride in CH3CN–CH2Cl2 in the presence of
alkynes affords 1,4-benzodithiin derivatives and N-(4-
methylbenzyl)acetamide after quenching with H2O; this
suggests the generation of benzodithiete or its equivalent.
KOH (86% yield) followed by the oxidation of the resulting bis-
sulfide with MCPBA (73% yield).
The reaction of 1 with triflic anhydride (Tf2O) was monitored
by 1H NMR spectroscopy. When 1 eq. of Tf2O was added to a
solution of 1 in CD3CN-CD2Cl2 (v/v 1+1) at 275 °C, 1
immediately disappeared and a sole peak due to the N-benzyl–
acetonitrile adduct appeared at d 5.21 in the region of benzyl
groups.13 This result indicates that both benzyl groups of 1 are
completely eliminated at 260 °C within a minute, although we
have no direct evidence for the generation of dithia dication 4
and benzodithiete 5. After warming up to rt, the reaction
mixture gave a white precipitate. Treatment of the supernatant
with H2O afforded N-(4-methylbenzyl)acetamide (57% yield),
and treatment of the white precipitate with NaBH4 in THF
followed by addition of iodomethane gave 1,2-bis(methylthio)-
benzene (42% yield).
In order to elucidate the generation of benzodithiete 5
indirectly, the reaction of 1 with Tf2O was carried out in the
presence of alkynes 2 as a trapping reagent (Scheme 2).§¶ After
complete dealkylation of 1 with Tf2O in CH3CN–CH2Cl2 (v/v
1+1) at 260 °C, 2 was added to the reaction mixture. From
diarylacetylenes 2a–c, 2,3-diaryl-1,4-benzodithiins 6a–c were
obtained in 44, 64, and 20% respective yields, and from
arylacetylenes 2d–e, 2-aryl-1,4-benzodithiins 6d–e were pro-
duced in 18 and 21% respective yields. Thus, the formation of
6 strongly suggests the generation of 5 or its equivalent in the
course of the reaction of 1 with Tf2O. Diarylacetylenes serve as
a more effective trapping reagent of 5 than arylacetylenes.
However, dimethyl acetylenedicarboxylate2,5 did not give
2,3-bis(methoxycarbonyl)-1,4-benzodithiin, although N-(4-me-
thylbenzyl)acetamide and 1,2-bis(methylthio)benzene were ob-
tained in 59 and 20% yields, respectively, after as treatment
mentioned above. This result implies that a concerted mecha-
nism may not be involved in the reaction of 5 or its equivalent
with alkynes.
The development of new methodologies for the generation of
reactive species is an important goal in organic chemistry.
Benzodithiete 5 is a highly reactive transient species, which
occurs in thermolysis1–3 and photolysis4–6 of appropriate
precursors, and is trapped by alkynes to form 1,4-benzodithiin
derivatives.7,8 Benzodithiete 5 is estimated to be 17 kcal mol–1
more stable than o-dithiobenzoquinone by calculations at the
MP2 level9 and decomposed at more than 290 °C.1 s-Bonded
dithia dications have attracted considerable attention in hetero-
atom chemistry from the viewpoint of structural interests.10 The
strained dication of 1,4-dithiane reacts with alkynes to give
dithiabicyclo[2.2.2]octene derivatives.11 Recently, we have
demonstrated the facile dealkylation of dithia dications with a
flexible conformation to afford thiasulfonium salts;12 in partic-
ular, the dithia dication obtained from the reaction of 2,2A-
bis(benzylthio)biphenyl s-oxide with triflic anhydride (Tf2O) is
decomposed even at 220 °C to afford biphenyl disulfide and a
benzyl cation.13 Herein we report the reactions of 1,2-bis(4-
methylbenzylthio)benzene mono-s-oxide 1 with Tf2O in the
presence of alkynes 2 and alkenes 3, which produce 1,4-benzo-
dithiins 6 and 2,3-dihydro-1,4-benzodithins 7, respectively. The
key reaction for the generation of benzodithiete 5 or its
equivalent is based on the dealkylation of 1 via a dithia dication
4 (Scheme 1).
The substrate 1 was prepared by the reaction of benzene-
1,2-dithiol with 4-methylbenzyl chloride in the presence of
The reaction of 1 with Tf2O in the presence of alkenes 3 was
also conducted under the same conditions (Scheme 3).§¶ The
stereochemistry of the product reflects the reaction mechanism.
trans-Stilbene (trans-3a) gave trans-2,3-diphenyl-2,3-dihydro-
1,4-benzodithiine (trans-7a) (29% yield), whereas cis-stilbene
(cis)-3a) afforded a mixture of trans-7a (17% yield) and (cis)-
7a (10% yield). The formation of two stereoisomers (trans)- and
Scheme 1
† Electronic supplementary information (ESI) available: spectral data of
compounds. See http://www.rsc.org/suppdata/cc/b0/b004940i/
Scheme 2
DOI: 10.1039/b004940i
Chem. Commun., 2000, 1667–1668
This journal is © The Royal Society of Chemistry 2000
1667

dication 4. The present study opens the way to the synthetic
application of dithia dications.
This work was supported in part by grants-in-aid from the
Ministry of Education, Science, Sports and Culture, Japan (No.
11440186) and University of Tsukuba (TARA project fund).
Notes and references
‡ Present address: Foundation for Advancement of International Science,
586-9 Akatsuka-Ushigahuchi, Tsukuba, Ibaraki 305-0062, Japan.
§ To a solution of 1 in CH3CN–CH2Cl2 (v/v 1+1) at 260 °C under Ar were
added 1 eq. of Tf2O and, after about 30 s, 2 eq. of 2 or 3. The mixture was
stirred at 260 °C for 10 min and then at rt for 1 h, and quenched with
aqueous NaHCO3. Extraction with CH2Cl2 followed by purification with
silica gel chromatography gave 1,4-benzodithiin derivatives 6 from 2 or
2,3-dihydro-1,4-benzodithiin derivatives 7 from 3 and N-(4-methylbenzyl)-
acetamide in modest yields.
¶ Representative data for 6b: wax; 1H NMR (270 MHz, CDCl3) d 2.26 (s,
6H), 6.96 (d, J = 7.8 Hz, 4H), 7.07 (d, J = 7.8 Hz, 4H), 7.25 (dd, J = 3.5
and 5.7 Hz, 2H), 7.46 (dd, J = 3.5 and 5.7 Hz, 2H); 13C NMR (67.8 MHz,
CDCl3) d 21.2, 127.6, 127.9, 128.8, 129.6, 133.8, 135.5, 136.1, 137.5; EI-
MS m/z 346 (M+); Calc. for C22H18S2: C, 76.26; H, 5.24. Found: C, 76.02;
H, 5.49%. For trans-7a: mp 120–121 °C; 1H NMR (270 MHz, CDCl3) d
4.67 (s, 2H), 7.10 (dd, J = 3.8 and 5.9 Hz, 2H), 7.14–7.19 (m, 10H), 7.33
(dd, J = 3.8 and 5.9 Hz, 2H); 13C NMR (67.8 MHz, CDCl3) d 54.6, 125.4,
127.8, 128.0, 128.1, 128.4, 134.2, 138.3; EI-MS m/z 320 (M+); Calc. for
Scheme 3
(cis)-7a) from cis-3a, but not from (trans)-3a, suggests a
stepwise addition mechanism of benzodithiete 5 or its equiva-
lent to carbon–carbon multiple bonds such as an AdE path. Ethyl
(E)-cinnamate (trans-3b) and 4-methylstyrene 3c gave the
corresponding (trans)-7b and 7c in 19 and 32% yields,
respectively.
The redox behavior of 1,4-benzodithiin derivatives has been
studied extensively from the viewpoint of cationic p-conjugated
systems.7,14 Cyclic voltammetry of 1,4-benzodithiins 6 in
CH3CN with Bu4NClO4 (0.1 M) at rt showed two oxidation
waves, where, in all cases, the first ones are reversible and the
second ones are irreversible. The redox potentials of 6a–e
obtained here are summarized in Table 1. The first half-wave
potentials are in the range of E11/2 = +0.93– +1.02 V and the
C
20H16S2: C, 74.96; H, 5.03. Found: C, 74.84; H, 5.19%.
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second oxidation potentials are in the range of Ep2a
=
+1.26– +1.53 V vs. Fc/Fc+. The first oxidation potentials of
1,4-benzodithiin derivatives are heightened with increasing the
number of aromatic groups at the 2- and 3-positions owing to a
p-conjugative effect: unsubstituted 1,4-benzodithiin15 < 6d
(6e) < 6a (6b). The oxidation potentials are lowered as the
electron-donating ability of a substituent at the para-position of
the aromatic ring is increased: 6a > 6b > 6c. A good linear
+
relationship between the first half-wave potentials and their sp
values was obtained with r = 0.12 and a correlation coefficient
r = 0.98.
In conclusion, we have demonstrated the generation of
benzodithiete 5 or its equivalent through the facile dealkylation
of a 1,2-bis(benzylthio)benzene derivative 1 via a dithia
Table 1 Redox potentials of 1,4-benzodithiin derivatives 6a
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therein.
1
1
2
b
b
b
Compound
Epa
E1/2
Epa
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Chem., 1998, 63, 2168.
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15 W. Schroth, R. Borsdorf, R. Herzschuh and J. Seidler, Z. Chem., 1970,
10, 147.
6a
6b
6c
6d
6e
1.06
1.04
0.96
1.03
0.99
1.02
1.00
0.93
1.00
0.96
1.52
1.47
1.26
1.53
1.53
a In CH3CN with Bu4NClO4 (0.1 M) at rt. b V vs. Fc/Fc+; scan rate, 0.1
V s21
.
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Chem. Commun., 2000, 1667–1668