4796 J . Org. Chem., Vol. 61, No. 14, 1996
Clennan and Greer
(3)
[Et2SO]
2kSkX[MeOH][Et2S]
(kPhOkS[Et2S] + kPhOkPhOH[Ph2SO] + kPhOHkX[MeOH])[Ph2SO]
) 1 +
[Ph2SO2]
G. This rotational isomer collapsed to a thiadioxirane
[Et2SO]
at the computationally higher MP2/6-31G* level.9 Con-
sequently, formation of the activated complex F in highly
protic alcoholic media must involve concomitant rotation
and complexation of persulfoxide G.
)
[Ph2SO2]
2kSkX[MeOH][Et2S]
1 +
(3a)
(4)
(kPhOkS[Et2S] + kPhOHkX[MeOH])[Ph2SO]
kPhO
kPhOH
1
)
+
slope
2kX[MeOH] 2kS[Et2S]
the mechanism which has been suggested for the pho-
tooxidation of thietane16 and also provides an explanation
for the observation that even in the presence of more than
18 equivalents of methanol (Table 2) 16% of the singlet
oxygen is still physically quenched by Et2S.
The magnitude of kPhO/kX is inconsistent with any
mechanism which involves proton or hydrogen bond
activation of the persulfoxide to form a hydroperoxy
sulfonium ion, E or E′, as shown in Scheme 4. The rates
The ultimate formation of a sulfurane is consistent
with a large body of earlier work with hydroxy-tethered
substrates.18-20 In particular, photooxidation of the 17O
labeled γ-hydroxy sulfide (Scheme 5) resulted in forma-
Sch em e 5
Sch em e 4
tion of unusual olefinic sulfones in which the hydroxy
group transferred to sulfur. This observation is consis-
tent with a sulfurane intermediate, H, which collapses
to a novel hydrated sulfone which serves as the precursor
to the olefin.
of protonation and/or hydrogen bond formation are
expected to be close to the diffusion limit17 and as a
consequence, and in contrast to the experimental obser-
vation, kPhO/kX is expected to be small for any mechanistic
scenario involving these steps.
Con clu sion
The most reasonable explanation for the experimental
data is a “concerted” formation of the sulfurane as
depicted in Scheme 2 via the activated complex F . This
mechanism is consistent with: (1) the observed depen-
dence of 1/kr on 1/[ROH], (2) the large value of kPhO/kX,
and (3) the incipient transfer of a hydrogen to the
persulfoxide oxygen in the activated complex which
provides an explanation for the experimentally observed
dependence on the pKa of the alcohol. In addition, if
proton transfer is advanced in comparison to sulfur-
oxygen bond formation little if any steric interaction with
the alkyl group on oxygen would be expected (Table 3).
J ensen and Foote8 pointed out that at the HF/3-21G-
(*) ab initio level the persulfoxide has Cs symmetry with
the oxygen-oxygen bond bisecting the R-S-R angle, G.
However, a 120° rotational isomer with C1 symmetry was
also located with an energy only 1.2 kcal/mol higher than
Our results demonstrate that Et2S is converted to its
oxidized products by different mechanisms in benzene
and in benzene/methanol. In both solvents 1O2 reacts
with Et2S to give a persulfoxide whose ultimate destiny
is a function of solvent. In aprotic solvents like benzene
it is converted to a second intermediate which is most
likely a thiadioxirane.16,21-28 In the protic solvent metha-
nol, however, it is converted to a hydroperoxy-methoxy
sulfurane via “concerted” addition of the alcohol which
involves rotation of the persulfoxide to the reactive
conformation followed by 1,3-dipolar addition across the
O-H bond of the alcohol. This concerted addition com-
(18) Clennan, E. L.; Yang, K. J . Am. Chem. Soc. 1990, 112, 4044-
4046.
(19) Clennan, E. L.; Yang, K.; Chen, X. J . Org. Chem. 1991, 56,
5251-5252.
(20) Clennan, E. L.; Yang, K. J . Org. Chem. 1992, 57, 4477-4487.
(21) Clennan, E. L.; Zhang, H. J . Am. Chem. Soc. 1994, 116, 809-
810.
(22) Clennan, E. L.; Wang, D.; Zhang, H.; Clifton, C. H. Tetrahedron
Lett. 1994, 35, 4723-4726.
(23) Clennan, E. L.; Zhang, H. J . Org. Chem. 1994, 59, 7952-7954.
(24) Clennan, E. L.; Zhang, H. J . Am. Chem. Soc. 1995, 117, 4218-
4227.
(25) Clennan, E. L. In Advances in Oxygenated Processes; Baum-
stark, A. L., Ed.; J AI Press: Greenwich, CT., 1995; Vol. IV, pp 49-80.
(26) Clennan, E. L.; Chen, M.-F. J . Org. Chem. 1995, 60, 6444-
6447.
(27) Clennan, E. L. Sulfur Rep. 1996, 18, in press.
(28) For an alternative viewpoint on the identity of the second
intermediate in benzene see: Watanabe, Y.; Kuriki, N.; Ishiguro, K.;
Sawaki, Y. J . Am. Chem. Soc. 1991, 113, 2677-2682.
(15) Foote and co-workers have reported values of kPhO/kX in benzene
where kX is the rate of interconversion to the thiadioxirane.1 Their
values of 43 and 62 obviously cannot imply that the rate of intercon-
version to the hydroperoxy sulfurane is slower than the conversion to
the thiadioxirane. The difference in the two ratios could be due in large
part to kPhO which is undoubtedly different in the two solvents. In
particular, hydrogen bonding to the sulfoxide oxygen might make it a
better trapping agent in methanol/benzene mixtures than in pure
methanol.
(16) Clennan, E. L.; Dobrowolski, P.; Greer, A. J . Am. Chem. Soc.
1995, 117, 9800-9803.
(17) Denisov, G. S.; Bureiko, S. F.; Golubev, N. S.; Tokhadze, K. G.
In Molecular Interactions; Ratajczak, H., Orville-Thomas, W. J ., Eds.;
J ohn Wiley & Sons Ltd: New York, 1980; Vol. 2; pp 107-141.