400
Chemistry Letters 2000
Reduction of Sulfoxides to Sulfides Mediated by Ferrocene and Trifluoroacetic Anhydride
Kenji Kobayashi,* Yasuo Kubota, and Naomichi Furukawa*
Department of Chemistry and Tsukuba Advanced Research Alliance Center, University of Tsukuba, Tsukuba ,Ibaraki 305-8571
(Received January 12, 2000; CL-000041)
The reaction of diaryl sulfoxides with trifluoroacetic anhy-
dride in the presence of ferrocene gives the corresponding sul-
fide. The reduction of sulfoxides composed of the ferrocene-
spacer-methylsulfinyl triad system would proceed via a
through-bond electron transfer rather than a through-space
process.
The reaction of dimethyl sulfoxide 3a and aryl methyl sul-
foxides 3b-d with TFAA (1 equiv) in the presence of ferrocene
(1 equiv) in CHCl3 at room temperature quantitatively gave a
mixture of sulfide 4a-d and α-trifluoroacetoxymethyl sulfide
5a-d (Table 2). The product ratios of 4a-d to 5a-d were in the
range of 20:80 for 3a to 34:67 for 3c. When ruthenocene was
used in place of ferrocene, the reaction of 3a gave 4a and 5a in
a 11:89 ratio. Thus, the competitive reactions between the
reduction and Pummerer rearrangement10 occur for sulfoxides 3
having α-methyl group.
Much effort has been devoted to the reduction of sulfox-
ides to sulfides with low-valent metal salts from the viewpoint
of synthetic application.1-3 One-electron electrochemical or
chemical reduction of sulfonium salts causes the S-C σ-bond
cleavage to provide a sulfide and a carbon radical.4-7 Ferrocene
has been widely used as a redox active probe molecule.8,9
However, the utility of ferrocene as a reducing agent with
respect to sulfoxides and sulfonium salts has not been studied
so far. Herein we report the reduction of sulfoxides 1 and 3 to
sulfides mediated by ferrocene and trifluoroacetic anhydride
(TFAA). We also describe that the reduction of sulfoxides 6,
composed of the ferrocene-spacer-methylsulfinyl triad system,
with TFAA would proceed via a through-bond electron trans-
fer.
Treatment of diaryl sulfoxides 1 with ferrocene (1 equiv)
and TFAA (1 equiv) in CHCl3 for 24 h gave the corresponding
sulfides 2 quantitatively (Table 1). Both ferrocene and TFAA
are essential for the reduction of 1. The reduction of 1a (X =
CH3) and 1b (X = H) proceeded at room temperature, while the
refluxing temperature was required for 1c (X = Cl). However,
1d (X = NO2) was recovered unchanged. This result suggests
that the rate determining step is the formation of trifluoroace-
toxysulfonium salt and that one-electron reduction of the result-
ing sulfonium salt4-7 with ferrocene causes the S-O σ-bond
cleavage to afford 2 and ferrocenium ion.
The electron transfer process of ferrocene-terminated con-
jugated derivatives, through-bond or through-space interaction,
has been intensively studied in the field of electron transfer
devices.11-13 The nature of spacers as a molecular wire affects
the electronic communication between donor and acceptor.14
The present competitive reactions could serve as a probe for the
qualitative evaluation of this process. Thus, we have designed
the ferrocene-spacer-methylsulfinyl triad molecules 6. The
triad molecules 6 were prepared by the palladium-catalyzed
cross-coupling reaction of ferrocenylzinc chloride15 with p-
(methylthio)aryl halides followed by the oxidation of the result-
ing sulfides 7 with m-CPBA.
The reaction of 6 with TFAA (1 equiv) in CHCl3 at room
temperature for 24 h was carried out (Table 3). The sulfoxides
6a-c quantitatively afforded only the sulfides 7a-c as a reduc-
tion product. This result is in marked contrast to the reaction of
3 shown in Table 2 and indicates the intramolecular electron
transfer of the ferrocene moiety to the sulfonium moiety in 6.
The sulfoxides 6d-e having biphenyl and diphenylmethane
spacers quantitatively gave a mixture of 7d-e and the Pummerer
product 8d-e in the ratios of 7d:8d = 50:50 and 7e:8e = 33:67,
respectively. In both cases, the product ratios of 7d-e to 8d-e
increased with decreasing the concentration of 6d-e. It is noted
that the ratio of the reduction product 7 to the Pummerer prod-
uct 8 clearly depends on the nature of spacers such as length
Copyright © 2000 The Chemical Society of Japan