One-Electron Reduction of Methanesulfonyl Chloride
RS• + O2 h RSOO•
A R T I C L E S
(1/-1)
(2/-2)
RS• + RS- h RSSR•-
RS• + -CHdCH- h -C•H-CH(SR)- (3/-3)
The reversible oxygen addition to thiyl radicals (eq 1/-1)
becomes significant also because of the known reactions: (i) a
rearrangement of the thiylperoxyl radical RSOO• to the sulfonyl
radical RSO2• (eq 4)8 and (ii) a further oxygen addition leading
to the sulfonylperoxyl radical RSO2OO• (eq 5).9 All these sulfur-
oxygen radicals must, therefore, be considered as potential
participants in thiol/thiyl based systems exposed to oxygen. For
instance, they are likely to act as oxidants. The question is also
to what extent they may undergo addition and abstraction
reactions like the RS• radicals.
•
RSOO• f RSO2
(4)
(5)
Figure 1. Absorption spectra obtained from the pulse radiolysis of Ar-
purged aqueous solutions containing 5 mM MeSO2Cl and 0.5 M CH3OH:
(b) 0.1 µs after the pulse; (9) 1.0 µs after the pulse. Inset shows the
absorption-time trace at 320 nm. Optical path ) 5.0 cm; dose per pulse )
39 Gy.
RSO2 + O2 f RSO2OO•
•
ESR and optical absorption spectra of sulfonyl radicals
indicated that they are σ-type species with similar spin density
of the unpaired electron on sulfur and oxygen atoms at the SO2
moiety.10-12 Theoretical calculation supported these findings and
showed that the spin distribution for the unpaired electron in
chloride (eq 6)17 or by oxidation of sulfinic acid (eq 7).9 The
present study focuses especially on the former reaction in an
aqueous environment in the absence or in the presence of oxygen
•
MeSO2 is 42% on sulfur and 44% on the oxygen atoms. For
•
and on the ability of MeSO2 radical to act as oxidant and as
PhSO2 the corresponding values are 43% and 39%.12 Moreover,
•
catalyst in the cis-trans isomerization of unsaturated fatty acids.
the ∆H298(RSO2-Cl) were measured for R ) Me or Ph by
photoacoustic calorimetry and found to be equal (295 kJ mol-1),
eaq- + MeSO2Cl f MeSO2 + Cl-
(6)
(7)
•
•
indicating that the radical chemistry of RSO2 should be
independent of the R group.13
•
HO• + MeSO2- f MeSO2 + HO-
The main synthetic applications of RSO2• radicals are based
on their ability to add to carbon-carbon double bonds.10,14 There
is also a clear evidence that the addition of a sulfonyl radical to
an olefin is a reversible reaction.15 Although the absolute rate
Results and Discussion
Reduction of Sulfonyl Chloride in the Absence of Oxygen.
Until now the generation of the sulfonyl radical, as formulated
in eq 6, has been considered a one-step dissociative electron
•
constants for the addition of RSO2 radicals to double bonds
are absent from the literature, rate constants for the â-elimination
of a p-CH3C6H4SO2• radical from a variety of carbon-centered
radicals have been measured.16
•
attachment reaction. As shown below, the formation of MeSO2
involves, however, an intermediate sulfonyl chloride radical
anion. The bimolecular rate constant k6 ) (3.3 ( 0.3) × 1010
M-1 s-1, evaluated for the reduction of MeSO2Cl from the decay
of the eaq- absorption (e.g., at its maximum at 720 nm at three
different sulfonyl chloride concentrations), thus refers only to
the initial electron attachment. This step is clearly controlled
only by the diffusion of the reactants.
•
In radiation chemical studies, MeSO2 radicals have been
generated by dissociative electron capture using sulfonyl
(8) Sevilla, M. D.; Yan, M.; Becker, D. Biochem. Biophys. Res. Commun. 1988,
155, 405. Sevilla, M. D.; Becker, D.; Yan, M. Int. J. Radiat. Biol. 1990,
57, 65. Razskazovskii, Y.; Colason, A.-O.; Sevilla, M. D. J. Phys. Chem.
1995, 99, 7993.
(9) Sehested, K.; Holcman, J. J. Radiat. Phys. Chem. 1996, 47, 357.
Razskazovskii, Y.; Sevilla, M. D. J. Phys. Chem. 1996, 100, 4090. Flyunt,
R.; Makogon, O.; Schuchmann, M. N.; Asmus, K.-D.; von Sonntag, C. J.
Chem. Soc., Perkin Trans. 2001, 2, 787.
(10) Chatgilialoglu, C. In The Chemistry of Sulfones and Sulfoxides; Patai, S.,
Rappoport, Z., Stirling, C. J. M., Eds.; Wiley: Chichester, U.K. 1988; pp
1089-1113.
(11) Chatgilialoglu, C.; Gilbert, B. C.; Norman, R. O. C. J. Chem. Soc. Perkin
Trans. 1979, 2, 770. Chatgilialoglu, C.; Gilbert, B. C.; Norman, R. O. C.
J. Chem. Soc., Perkin Trans. 1980, 2, 1429.
Tracing the sulfonyl chloride reduction at 320 nm, that is, at
•
the maximum of the MeSO2 radical absorption, revealed the
above indicated, more complex situation. This can be realized
by inspection of the inset in Figure 1, which specifically pertains
to an argon-saturated aqueous solution containing 50 mM
MeSO2Cl and 0.5 M methanol.18 Two distinct processes are
apparent. An initial fast rise occurred practically within the
duration of the pulse and accounted for almost two-thirds of
the final absorption. The second, considerably slower step, made
up for the remaining third.
(12) Chatgilialoglu, C.; Griller, D.; Guerra, M. J. Phys. Chem. 1987, 91, 3747.
Alberti, A.; Chatgilialoglu, C.; Guerra, M. J. Chem. Soc., Perkin Trans.
1986, 2, 1179.
(13) Chatgilialoglu, C.; Griller, D.; Kanabus-Kaminska, J. M.; Lossing, F. P. J.
Chem. Soc., Perkin Trans. 1994, 2, 357. See also: Laarhoven, L. J. J.;
Mulder, P.; Wayner, D. D. M. Acc. Chem. Res. 1999, 32, 342.
(14) Chatgilialoglu, C.; Bertrand, M. P.; Ferreri, C. In S-Centered Radicals;
Alfassi, Z. B., Ed.; Wiley: Chichester, U.K. 1999; pp 312-354. Bertrand,
M. P.; Ferreri, C. In Radical in Organic Synthesis; Renaud, P., Sibi, M.,
Eds.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 2, pp 485-503.
(15) Asscher, M.; Vofsi, D. J. Chem. Soc. 1964, 4962.
(17) Eriksen, T. E.; Lind, L. Radiochem. Radioanal. Lett. 1976, 25, 11.
(18) Radiolysis of neutral water leads to eaq- (0.27), HO• (0.28), and H• (0.062);
the values in parentheses represent the radiation yields in units of µmol
(16) Timokhin, V. I.; Gastaldi, S.; Bertrand, M. P.; Chatgilialoglu, C. J. Org.
Chem. 2003, 68, 3532.
J-1. In the presence of 0.5 M CH3OH, both HO• (k ) 9.7 × 108 M-1 s-1
)
and H• (k ) 2.6 × 106 M-1 s-1) are scavenged efficiently.19
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8717