J . Org. Chem. 1998, 63, 3905-3910
3905
P h otoch em ica l Electr on -Tr a n sfer Rea ction s betw een Su lfid es a n d
Tetr a n itr om eth a n e. Oxid a tion vs F r a gm en ta tion of th e Su lfid e
Ra d ica l-Ca tion In ter m ed ia te
Waldemar Adam,† J uan E. Argu¨ello,‡ and Alicia B. Pen˜e´n˜ory*,‡
Institute of Organic Chemistry, University of Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany,
and Departamento de Quı´mica Orga´nica, Facultad de Ciencias Qu´ımicas,
Universidad Nacional de Co´rdoba, Agencia Postal 4-CC 61,5000 Co´rdoba, Argentina
Received December 2, 1997
Oxidation and/or fragmentation products are observed in the photochemical reaction of the alkyl
phenyl sulfides 1a -d with tetranitromethane (TNM). The product distribution depends markedly
on the substrate structure. Thus, methyl phenyl sulfide (1a ) and benzyl phenyl sulfide (1b) give
only the corresponding sulfoxides (oxidation). However, when the radical cation 1b•+ is generated
by chemical oxidation with triarylaminium salts (Ar3N•+) in acetonitrile, in addition to oxidation
fragmentation is also observed, and with an excess of Ar3N•+ oxidation is facilitated and no
fragmentation is produced. For the photoreaction of diphenylmethyl phenyl sulfide (1c) with TNM,
fragmentation is the main reaction, while for triphenylmethyl phenyl sulfide (1d ) only this process
is observed. The ease of C-S bond scission in these sulfur-centered radical cations 1•+ follows the
ease of alkyl cation formation, i.e., Ph3C > Ph2CH > PhCH2 > CH3.
In tr od u ction
(TNM).15 The latter novel oxygen-transfer process is
illustrated for TNM in Scheme 1.
Oxidation of sulfides may be achieved by a variety of
methods; the most common is the two-electron oxidation
of the sulfur with concomitant atom transfer.1 In con-
trast to this electrophilic substitution on the sulfur
center, electron-transfer oxidation produces a sulfur
radical cation as intermediate. These reactive intermedi-
ates, which are important in biochemical transforma-
tions,2 can be generated in different ways: by electro-
chemical oxidation of sulfides,3 by chemical oxidation
with cerium(IV) ammonium nitrate (CAN),4 K5CoW12O40,5
Mn(III),6 Cr(VI),7 cytochrome P-450,8 peroxidases,9 tri-
arylaminium salts (Ar3N•+),10 and antimony pentachlo-
ride,11 by radiolysis,12 by photosensitized oxidation,13 and
by irradiation of the charge-transfer complex of sulfides
with tetracyanoethylene (TCNE)14 and tetranitromethane
These reactions proceed through initial formation of a
charge-transfer complex, followed by light-induced, dis-
sociative electron transfer to lead to the triad of reactive
species, namely the sulfide radical cation, the radical
nitrogen dioxide, and the nitroform anion. Coupling of
the two former and subsequent loss of nitrosyl cation
gives the corresponding sulfoxide (pathway a oxidation,
Scheme 2).
Alternative transformations of the sulfide radical
cation are the pathways b-d in Scheme 2. In pathway
b, C-S bond cleavage16 occurs to produce a thiyl radical
and a carbocation, the radical dimerizes to the corre-
sponding disulfide 3, and the cation is trapped by a
nucleophile to give product 4. During pathway c, CR-H
deprotonation to form a carbon-centered radical takes
place, which after reaction with a nucleophile and oxida-
tion gives product 5. Finally, in pathway d, nucleophilic
attack at the aromatic ring proceeds to form a carbon
radical, which after oxidation and protonation results in
product 6.
In view of this complexity in the potential fate of the
intermediary sulfide radical cations in such photoiniti-
ated electron-transfer, oxidation of sulfide by TNM, it was
of mechanistic interest to conduct a detailed product
study for a related set of alkyl phenyl sulfides. For this
purpose, the sulfides 1a -d were chosen, in which the
alkyl group comprises the series CH3, PhCH2, Ph2CH,
and Ph3C, well suited for mechanistic elucidation of
competitive reaction channels. Presently, we report the
† University of Wu¨rzburg.
‡ Universidad Nacional de Co´rdoba.
(1) March, J . Advanced Organic Chemistry, 4th ed.; J ohn Wiley &
Sons: New York, 1992; p 1201.
(2) For reviews on radical cations of organosulfur compounds see:
(a) Glass, R. S. Xenobiotica 1995, 25, 637. (b) Sulfur Centered Reactive
Intermediates in Chemistry and Biology; Chatgilialoglu, C., Asmus,
K. D., Eds; Nato ASI Series; Plenum Press: New York, 1990.
(3) (a) Uneyama, K.; Torii, S. Tetrahedron Lett. 1971, 329. (b)
Baciocchi, E.; Rol, C.; Scamosci, E.; Sebastiani, G. V. J . Org. Chem.
1991, 56, 5498. (c) Torii, S. Electroorganic Synthesis Part 1: Oxidation;
VCH: Weinhein, 1985; p 205. (d) Severin, M. G.; Farnia, G.; Vianello,
E.; Are´valo, M. C. J . Electroanal. Chem. 1988, 251, 369.
(4) Baciocchi, E.; Intini, D.; Piermattei, A.; Rol, C.; Ruzziconi, R.
Gazz. Chim. Ital. 1989, 119, 649.
(5) Baciocchi, E.; Fasella, E.; Lanzalunga, O.; Mattioli, M. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1071.
(6) Gilmore, J . R.; Mellor, J . M. Tetrahedron Lett. 1971, 3977.
(7) Srinivasan, C.; Chellamani, A.; Rajagopal, S. J . Org. Chem. 1985,
50, 1201.
(8) Watanabe, Y.; Oae, S.; Iyanagi, T. Bull. Chem. Soc. J pn. 1982,
55, 188.
(9) Baciocchi, E.; Lanzalunga, O.; Malandrucco, S.; Ioele, M.; Steen-
ken, S. J . Am. Chem. Soc. 1996, 118, 8973.
(10) Platen, M.; Steckhan, E. Chem. Ber. 1984, 117, 1679.
(11) Kamata, M.; Otogawa, H.; Hasegawa, E. Tetrahedron Lett.
1991, 32, 7421.
(12) Asmus, K. D. In Methods in Enzymology; Parker, L., Ed.;
Academic Press: Orlando, 1984; p 167.
(13) (a) Kamata, M.; Kato, Y.; Hasegawa, E. Tetrahedron Lett. 1991,
32, 4349. (b) Baciocchi, E.; Del Giacco, T.; Ferrero, M. I.; Rol, C.;
Sebastiani, G. V. J . Org. Chem. 1997, 62, 4015. (c) Baciocchi, E.;
Crescenzi, C.; Lanzalunga, O. Tetrahedron 1997, 53, 4469.
(14) Kamata, M.; Miyashi, T. J . Chem. Soc., Chem. Commun. 1989,
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(15) Adam, W.; Gonza´lez Nu´n˜ez, E. Tetrahedron 1991, 47, 3773.
(16) For C-S bond cleavage of the (1-phenyl)ethyl phenyl sulfide
radical cation, see ref 5.
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