Photoinduced electron transfer reaction of tetraazathiapentalenes with trialkylamines

Article abstract of DOI:10.1246/cl.2002.510

The photoinduced electron transfer reaction of tetraazathiapentalenes with trialkylamines gives thiourea derivatives by dialkylamination of isothiocyanate derivatives, through radical anions of tetraazathiapentalenes and radical cations of trialkylamines.

Full text of DOI:10.1246/cl.2002.510

510
Chemistry Letters 2002
Photoinduced Electron Transfer Reaction of Tetraazathiapentalenes with Trialkylamines
Hirokazu Hayashi, Noboru Matsumura, and Kazuhiko Mizunoꢀ
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531
(Received January 24, 2002; CL-020088)
The photoinduced electron transfer reaction of tetraazathia-
pentalenes with trialkylamines gives thiourea derivatives by
dialkylamination of isothiocyanate derivatives, through radical
anions of tetraazathiapentalenes and radical cations of trialk-
ylamines.
Recently, much attention has been focused on the character-
istic properties and reactivities of hypervalent compounds having
3-center of 4-electron bond.1 However, little is known about the
photoinduced electron transfer reaction of these compounds. We
have previously reported the formation of charge transfer (CT)-
complex between tetraazathiapentalene derivative (1a) bearing
hypervalent sulfur and tetracyanoquinodimethane (TCNQ),
where 1a serves as an electron donating molecule.2 We now
report a novel photoinduced electron transfer reaction of 1a–c
with trialkylamines to give thiourea derivatives, where 1a–c serve
as electron accepting molecules. Although several ꢀ-diethyl-
aminoethylation and ethylation by use of triethylamine (2a) have
been reported previously,3{5 to the best of our knowledge, there is
no example about diethylamination via photoinduced electron
transfer.
Irradiation of a benzene solution containing 1a (1 mM) and a
large excess of triethylamine (2a) through Pyrex filter
(ꢁ > 280 nm) under Ar atmosphere gave N,N-diethyl-N0-
methylthiourea (3a) in a 73% isolated yield accompanying 4a,
5, and acetaldehyde. The structures of these products were
determined by their spectral properties and by direct comparison
with those of authentic samples.
Similar irradiation of 1a with trialkylamines such as
tripropylamine (2b), tributylamine (2c), and tribenzylamine
(2d) afforded the corresponding dialkylaminated products 3b–
d, respectively. Triisobutylamine (2e) also gave the diisobutyla-
minated product 3e, but the yield was low. In addition, the
photoreaction of 1b–c with 2a afforded the corresponding
diethylaminated thiourea derivatives (3ab and 3ac). Similar
irradiation of 1a with diethylamine (2i) in benzene afforded 3a,
but 2i is less reactive than 2a. The photoreaction did not occur in
the case of ethylamine (2j). In the cases of aromatic amines (2f–
h), the photoreaction did not proceed or gave a complex mixture.
These results are summarized in Table 1.
From the mechanistic viewpoints, the effects of solvents and
additives were examined. The photoreaction also proceeded
smoothly in CH3CN and CH3OH, to give 3, 4, and 5 in good yields
(Entry 2–3). The photoreaction of 1a with 2a was not sensitized
by triplet sensitizers such as benzophenone and Michler’s ketone.
However, interestingly, 1,4-dimethoxynaphthalene (DMN) sen-
sitized the photoreaction of 1a with 2j to give N-ethyl-N0-
methylthiourea (3j).
Scheme 1.
Table 1. Photoinduced electron transfer reaction of tetraazathia-
pentalene derivatives (1a–c) with amines (2a–j)a
Entry
1
Amine
Product yields/%b
3
4
5
1
1a
1a
1a
1a
1a
1a
1a
1a
Et3N (2a)
2a
2a
3a (73) 4a (trace) (41)
3a (64) 4a (50) trace
3a (81) 4a (68) trace
3b (94) 4a (trace) trace
3c (92) 4a (trace) trace
3d (93) 4a (trace) trace
3e (40)
(0)
—
2c
3d
4
n-Pr3N (2b)
n-Bu3N (2c)
(PhCH2)3N (2d)
i-Bu3N (2e)
Ph3N (2f)
5
6
7
8
9f
10
11
12
13
14
15g
e
e
—
—
(0)
—
(0)
—
1a dimethylaniline (2g)
1a diethylaniline (2h)
(0)
4a (0)
e
(0)
e
1b
1c
1a
1a
1a
2a
2a
3ab (37)
3ac (50)
3a (60)
3j (0)
(16)
—
—
—
—
e
e
e
e
Et2NH (2i)
EtNH2 (2j)
2j3j
—
4a (0)
—
(0)
—
e
e
—
aReaction conditions; ½1aŠ ¼ 1:0  10À3 M ½2aŠ ¼ 0:1 M.
Irradn time; 24 h through Pyrex filter (>280 nm) in benzene.
bIsolated yields. cIn CH3CN. dIn CH3CN-MeOH (1 : 99).
eNot determined. A complex mixture was obtained. g1,4-
f
Dimethoxynaphthalene (DMN) was added as a sensitizer.
isothiocyanate. In fact, diethylamine efficiently reacts with
methyl isothiocyanate at ambient temperature in the dark,
although both of them were not detected in the photoreaction
mixture of 1a and 2a. In addition, the photoreaction of 1a in the
absence of 2a in benzene did not produce any product at all and 1a
was recovered quantitatively.
In the dark overnight, 1a and 2a were completely recovered
even under reflux conditions. The formation of 3a can be
reasonably explained by the reaction of diethylamine and methyl
On the basis of these results, we propose the mechanism for
the formation of 3a as shown in Scheme 2. The first step is the
photoinduced electron transfer from 2a to the excited singlet state
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
511
ꢁ
of 1a to form the radical cation of 2a (2aþ ) and the radical anion
ꢁ
of 1a (1aÀ ). The single electron transfer reaction mechanism
from amines to 1aꢀ is supported by the facts that the tertiary and
secondary amines are susceptible to the dialkylamination, but
primary amine is not. The second step is an elimination of a proton
ꢁ
on ꢀ-carbon of 2aþ to give the radical (6). This radical 6 may
produce an enamine (7) or an iminium cation (8) by dispropor-
tionation of 6 or one-electron oxidation by 1a, as suggested by
Whitten.5 The hydrolysis of 7 or 8 gives diethylamine and
acetaldehyde. Methyl isothiocyanate is probably produced from
ꢁ
1aÀ , because the direct photolysis of 1a does not give methyl
isothiocyanate. Finally, diethylamine reacts with methyl iso-
thiocyanate smoothly in the dark.
Scheme 3.
Scientific Research from the Ministry of Education, Science,
Sports, and Culture of Japan. The authors are indebted to Prof. M.
Ohashi (Kanagawa University) and Dr. C. Pac (Kawamura
Institute of Technology) for helpful discussion.
References and Notes
1
2
3
G. C. Pimentel, J. Chem. Phys., 19, 446 (1951); R. J. Hack and
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(1990).
N. Matsumura, M. Tomura, H. Inoue, H. Naito, K. Kurosawa,
and M. Okuda, The 1st International Symposium on
Chemistry on Functional Dyes (ISCFD), Osaka, June 1989,
Abstr., p 79; N. Matsumura, M. Tomura, M. Yoneda, and K.
Toriumi, Chem. Lett., 1986, 1047.
M. Bellas, D. Bryce-Smith, and A. Gilbert, Chem. Commun.,
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Commun., 1967, 862; D. Bryce-Smith, M. T. Clarke, A.
Gilbert, G. Kuunklin, and C. Mannig, Chem. Commun., 1971,
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M. T. Clarke, A. Gilbert, G. Kuunklin, S. Krestonosich, C.
Mannig, and S. Wilson, J. Chem. Soc., Perkin Trans. 1, 1977,
2571.
Scheme 2.
In the presence of DMN, the photoinduced electron transfer
ꢁ
from the excited singlet state of DMN to 1a occurs to give 1aÀ
ꢁ
and DMNþ . The former affords methyl isothiocyanate, which
reacts with 2j (Scheme 3). In this stage, the fate of DMNþꢁ is not
ꢁ
clear, because a hole transfer from 2j to DMNþ is probably
unfavorable.
4
5
K. Tsujimoto, S. Suwa, and M. Ohashi, J. Chem. Soc., Chem.
Commun., 1976, 386; M. Ohashi, S. Suwa, and K. Tsujimoto,
J. Chem. Soc., Chem. Commun., 1976, 404; M. Ohashi and K.
Miyake, Chem. Lett., 1977, 615; M. Ohashi, K. Miyake, and
K. Tsujimoto, Bull. Chem. Soc. Jpn., 53, 1683 (1980).
R. S. Davidson, Chem. Commun., 1969, 1450; J. A. Barltrop
and R. J. Owers, Chem. Commun., 1970, 1462; P. J. Delaive,
T. K. Foreman, C. Giannotti, and D. G. Whitten, J. Am. Chem.
Soc., 102, 5627 (1980); M. A. Fox and M.-J. Chen, J. Am.
Chem. Soc., 105, 4497 (1983); O. Ishitani, I. Namura, S.
Yanagida, and C. Pac, J. Chem. Soc., Chem. Commun., 1987,
1153.
In conclusion, we have found that a novel photoinduced
electron transfer reaction of hypervalent sulfur compounds 1 with
trialkylamines such as 2a–e occurs to give dialkylaminated
products 3 of alkyl isothiocyanates. From these results, we have
clarified that 1 have both electron donating and electron accepting
properties depending on the conditions. We believe that this is a
rare example about the trapping of dialkylamines in the
photoinduced electron transfer reaction of trialkylamines in the
presence of an electron acceptor.
This work was partially supported by a Grant-in-Aid for