Evidence for a mono-electron transfer process in the BF3-promoted reaction of 4'-nitrobenzenesulphenanilide (NBSA)

Article abstract of DOI:10.1016/0040-4039(91)80101-B

ESR spectroscopy shows the formation of a radical species in the BF₃-promoted reaction of NBSA, an acid/base Lewis-type reaction.

Full text of DOI:10.1016/0040-4039(91)80101-B

Tetrahe&on Letters, Vol.32, No 40, pp 5621-5624, 1991
Prinlexl m Great Britam
0040-4039/91 $3 00 + 00
Pergamon Press plc
Evidence for a Mono-electron Transfer Process in the
BF3-Promoted Reaction of
4'-Nitrobenzenesulphenanilide (NBSA).
Loris Grossi* and Pier Carlo Montevecchi*
Dlparfimentod~ Chma~ca Organica "A Mangmi"
Umversl~ di Bologna
Vmle Rlsorglmento, 4 - 40136 Bologna
Italy
Key words: 4~qitrobenzenesulphenantllde, BF3, ET process, thleammyl radical, E.S.R
Abstract : ESR spectroscopy shows the formaUon of a radical species m the BF3-promoted react]on of NBSA, an acid/base Ll~wls-
type react]on
In the recent years attention has been devoted to the chemistry of benzenesulphenanilides, which have been
considered from both synthetic and speculative point of view. In particular, we have reported that BF3 transforms
these sluggish compounds into reactive species which can undergo fast displacement at the sulphur atom by
suitable nucleoplhiles 1. Under these condiuons 4'-nitrobenzenesulphenanilide (NBSA) was found to be a
powerful sulphenyl transfer agent, usefully employed as synthetic intermedmte in the 1,2-dffunctionalization of
alkenes and alkynes1,2. However, in spite of our efforts, no evidence for the actual nature of the sulphenylating
agent was provided, although the tetracoordinate acid/base complex, 4-O2NC6H4N+(SPh)BF3-, was tentauvely
suggested.
We wish now to report evidence obtained from both chemical and ESR spectroscopy studies of BF3-promoted
reaction of NBSA, which suggest that a radical cation, (O2NC6H4NHSPh)"i', could be the sulphenylatmg agent
(1).
When a degassed benzene or methylene chloride soluuon of NBSA was allowed to react with BF3.Et20 (ca. 47%
BF3) (1.5 molar equiv.) directly in the ESR cavity, a quite persistent radical3 was immediately detected and its
structure, N-(4-nitrophenyl)-N-(phenylthlo)aminyl ra&cal (2), was easdy assigned by comparison of its
spectroscopic parameters with those reported in the literature4. When to such a solution a slight excess of a
nucleophile, i.e. anisole, 1-hexene, or 3-hexyne, was added the ESR signal of the radical species (2) was
istantaneously quenched. Actually, these compounds are known to undergo efficiently the sulphenyl transfer in
the promoted reaction of NBSA2,5, therefore the radical (2) itself might be the sulphenylating agent. However,
5621

5622
independent experiments ruled out this hypothesis. In fact, producing the radical (2) by hydrogen abstract~,n with
ButO-, obtamed by thermal decomposition of drier-butyl hypommre (DTBH) at 40°C, in amsole as well m
benzene in the presence of 3-hexyne or 1-hexene, we found that the reaction products were substantially identical
to those reported for the same reaction carried out m benzene m the absence of nucleophllic species6. The lack of
any product denvlng from the sulphenylanon of these nucleophfles leads to conclude that the radical (2) is not
capable to act as sulphenyl transfer agent. In agreement with these chemical evidence, when the radical (2) was
produced from NBSA and DTBH under the same conditions directly m the ESR cavity, it was not quenched by
adding an excess of amsole, 3-hexyne, or 1-hexene Then, these findmgs might suggest that the thioammyl
radical (2) could be m rapid equihbnum wtth the sulphenylatlng agent (1), so that the disappearance of (2) can be
achteved when the species (1) is trapped by nucleophfles. This suggestion was compelled by independent
experiments. In fact, when the BF3-promoted reaction of NBSA was carned out m anlsote m the presence of 2
molar eqmv of mbutyltm hydride, a strong hydrogen donor, column chromatography led to the separation in
nearly quanmanve yield of 4-nltroanihne (3) and of an inseparable mixture of mbutyltm phenyl sulphide (4) and
dlphenyl disulphide (5) m ca. 4. I ratio. No formation of 4-methoxyphenyl phenyl sulphide (6) was evidenced:
this compound was exclusively formed when this reacnon was camed out in the absence of tnbutyltm hydride
through electrophfllc sulphenylation of the nucleophihc aromatic solvent 2c,5 Therefore, it may be referred that
the presence of a radical trap as Bu3SnH can be capable to suppress the sulphenylanng agent (1) by quenchmg the
radical (2). The formanon of the product (3) and (4) can be atmbuted to a chain reaction revolving an SH2
displacement at the sulphur atom of NBSA by stannyl radicals (see Scheme), and the minor amount of chsulptude
(5) probably arose from the sulphide (4), which was found to be unstable under the reaction condmons and
work-up.
Scheme
Bu~SnH
DTBH,40 °C
O 2 N @ N H S P h
O 2 N ~ / ~ S P h
2
NBSA + Bu3Sn
t-BuO.,- t-BuOH
N B S A
1
I
BF3.Et20
O 2 N - - @ r q H + Bu3SnSPh
Y
4
1
Amsole]
~
3-Hexyne or
-Hexene
O 2 N @ N H2
3
PhSSPh
5
Ph S'--~C)'~- OMe
1,2- Adducts
On these bases it seem unhkely that a tetracoordmate complex might be claimed as possible sulphenylatmg agent
and, although we do not have defimtive evidence, ~t seems more sensible that the sulphenylating agent (1) could

5623
be the radical cation (O2NC6H4NHSPh)4-, a species that actually could exist in equilibrium with radical (2)
through a protonation/deprotonation reaction7.
Such a rachcal cation (O2NC6H4NHSPh)~- might arise from a single electron transfer process from NBSA to the
Lewis acid BF3, but unfortunately we were not able to detect it directly by ESR spectroscopy. However, we
found that a SET process involving NBSA can actually lead to the radical (2). In fact, when a benzene solution of
NBSA was allowed to react in a ESR spectrometer cavity with an N-methyl-4-cyanopyridinium salt, an easy
electron acceptor substrate8, the signal due to radical (2) was istantaneously detected; its formation came
undoubtedly from intermediacy of the radical cation (O2NC6H4NHSPh)~ through a deprotonanon reaction.
It has been noted that a SET process was claimed to occur in several reactions involving electron reach substrates9
but, at the best of our knowledge, no examples are reported in which a Lewis acid can act as electron acceptor.
On the other hand the formation of the radical (2) from the reaction of NBSA with Lewis acid appears to be not
confined to the reaction with the BF3 only. Treatment of a benzene solution of NBSA with AIC13 produced the
same radical (2) as BF3 and also in this case the radical was quenched indirectly by nucleophilic substrates, i.e.
anisole, 3-hexyne, and 1-hexene, and directly by tributyltin hydride. Moreover, the reaction products distribution
followed the same pattern as evidenced for the related BF3-promoted reacnon earned out in anisole solution, both
in the absence and in the presence of tributyltin hydride.
Attempts to detect related aminyl radicals in the BF3-promoted reaction of 4'-substituted benzenesulphenanilides
(BSA) other than NBSA, failed. In fact, no paramagnetic species were found by treatment of a degassed benzene
solunon of 4'-methylbenzenesulphenanilide (BSA, R=Me) with 1.5 molar equiv, of BF3-Et20 directly in the
cavity of the ESR spectrometer, whereas under the same conditions 4'-chloro (BSA, R=C1) and 4'-
methoxybenzenesulphenanilide (BSA, R--OMe) gave radical species that were attributed to some reaction of BF3
with respectively N-(4-chlorophenyl)-(7, R=C1) and N-(4--methoxyphenyl)-N'-(phenylthio)-p-quinonedi-imine
(7, R---OMe)10, ultimate reaction productsl t.
BSA
7
8
In this light the single electron transfer process between a Lewis acid and benzenesulphenanilides seems to be
confined to NBSA only. On the other hand it has been reported that m the BF3-promoted reactions of BSA (other
than NBSA), these compounds can themselves act as nucleophiles towards the sulphenylating agent, initially
leading to the corresponding N,N-bis(phenylthio)aniline (8)11. Therefore, the failure in the detection of
thioaminiy radical as (4-RC6H4NSPh). (R=CI, Me, OMe) does not reject apriori that an electron transfer process
might be involved with BSA too; in fact, the resulting radical cations might be immediately quenched by the
nucleophilic BSA.
Acknowledgment. We gratefully acknowledge C.N.R.-Progetto Finalizzato Chimica Fine for financial
support.

24
R e f e r e n c e s and N o t e s
1
a) Benati, L ; Montevecchi, P.C.; Spagnolo, P.; Tetrahedron, 1986, 42, 1145; b) Benat~, L.; Montevecchx,
P.C., Spagnolo, P.; J Chem Soc ,Perkin Trans I, 1987, 2815; c) Benati, L ; Montevecchl, P C.; Spagnolo,
P, J Chem Soc., Perkin Tnas I, 1989, 1105;
2. a) Benata, L.; Montevecchl, P C.; Spagnolo, P.; J Chem Soc ,Chem.Commun, 1987, 1050; b) Benati, L.;
Casanm, D.; Montevecchi, P.C., Spagnolo, P., J Chem Soc, Perkin Trans. I, 1989, 1113; c) Benau, L.,
Montevecch~, P.C.; Spagnolo, P.; Gazz Chtm It.,1989, 119, 609; d) Benati, L.; Montevecchi, P C,
Spagnolo, P ; J.Chem Soc, Perkm Trans 1, 1990, 1691.
3. The radical signal was detected also after 24 hours
4. Mmra, Y. , Kmoschita, M.,Bull.Chem Soc Jpn., 1977, 50, 1142.
5. Benatx, L.; Montevecchi, P.C.; Gazz Chtm It, 1991, m press
6. Benati, L.; Montevecchl, P.C.; Spagnolo, P., J Chem Soc, Perkin Trans. I, 1985, 1577.
7. NBSA itself might act as Broensted base.
8
Grossa, L, Mamscl, F.; Pedulh, G.F, J Chem Soc, Perkin Trans H, 1977, 943; ibtd, 1977, 948.
9~ ~)A~s~k~y~EC;B~Wets.~I~ÈI~R~;JAmChemS~-98~-~3~2242;h)~stday~EC.~`C~e~D~;
Gamasa, M., J Org.Chem., 1987, 52, 4097, and refences c~ted thereto.
10. The radtcal obtained from 4'-methoxy BSA was identical to that obtained from the reaction of BF3 with N-(4-
methoxyphenyl)-N'-(phenylthlo)-p-qumonedl-lrrune.
11. Benat~, L.; Montevecch~, P.C.; Spagnolo, P.; J Chem Soc, Perkm Trans 1, 1985, 2261.
(Recewed in UK 16 July 1991)