9082 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Communications to the Editor
of only 30° in 9 and 10 (Table 1), allow access to the disulfide
linkage and are responsible for the ability to form the persul-
foxide. In addition, formation of the persulfoxides from 9 and
1
0 relieves a significant amount of destabilizing lone-pair-lone-
1
pair interaction. Disulfides 6 and 8 are chemically inert to O2
under our reaction conditions, consistent with their inability to
form the persulfoxide. Disulfides 4 and 7 on the other hand
give oxidation products, but only under forcing conditions. For
example, after 30 min of irradiation under identical conditions,
6
3% of 3 but only 2% of 4 is converted to product.5
We have determined the chemical rate constant, kr, for the
1
Figure 1. Log(k
of singlet oxygen by hydrazines.
T
) versus the taft steric parameter, E
S
, for the quenching
reaction of 9 with O2 by competition with 2-methyl-2-pentene
5
-1 -1
(
kT ) 7.2 × 10 M s ) using the method of Higgins, Foote,
and Cheng. The rate constant, kr ) (5.87 ( 0.75) × 10 M
s , is considerably smaller than kT. The difference, 1.38 ×
0 M s , represents the rate constant for physical quenching
of singlet oxygen by 9. This value is much larger than the rate
1
4
6
-1
-
1
8
-1 -1
1
5
-1 -1
constant for charge transfer quenching, 3.05 × 10 M s , as
predicted by the linear regression line (point a in Figure 2a).
Consequently, the predominate mechanism for physical quench-
ing by 9 is via decomposition of the persulfoxide (Scheme 1b).
Spin-orbit coupling (kisc in Scheme 1a) does not control the
charge-transfer quenching efficiencies of the disulfides which
fall on the linear regression line (Figure 2a), since this is
4
expected to increase as Z , contrary to to the experimental
results (compare the disulfides and hydrazines). We suggest
Figure 2. Log(k
potentials of the disulfides 3-10. Point a is the extrapolation of the
points 9 and 10 to the linear regression line and represents the CT
quenching by these substrates. (b) Log(k ) versus IPeV for several
T
substrates in the gas phase superimposed on the data from Figure 2a.
T
) versus photoelectron spectroscopy ionization
15
instead that it is the magnitude of Keq (Scheme 1a), which is in
1
turn dictated by the magnitude of δ CT, which controls the
quenching efficiencies. Furthermore, the lack of a steric effect
on the charge-transfer quenching of the disulfides is consistent
with a weakly bound charge-transfer complex (small Keq) with
a large intracomplex distance between the disulfide and singlet
oxygen.
Scheme 1
The results presented here demonstrate that disulfides in
extended conformations (>RSSR ≈ 85°) are ineffective at
1
providing protection from O2 damage in biological systems.
Exposed disulfides (>RSSR ≈ 30° or smaller), however,
1
physically deactivate O2 with rate constants comparable to that
16
reported for several important biological molecules, and much
1
7
more rapidly than others, such as nucleosides.
deviating significantly from the linear regression line (Figure
Acknowledgment. We thank the National Science Foundation and
the donors of the Petroleum Research Fund, administered by the
American Chemical Society, for their generous support of this research.
2a). This linear relationship is consistent with a charge-transfer
mechanism, since, to the extent the charge transfer states are
mixed with the ground states of the complex, log(kT) should be
1
1,12
JA9720568
directly related to the ionization potential of the donor.
Consistent with this suggestion is the fact that kT values
13
measured in the gas phase for methanethiol, dimethyl disulfide,
dialkyl sulfides, and thiophene also fall remarkably close to the
correlation line (Figure 2b). Charge-transfer quenching is likely
to be the dominant mechanism for quenching by these substrates,
since the buildup of charge necessary to form a persulfoxide
(14) Higgins, R.; Foote, C. S.; Cheng, H. In AdVances in Chemistry
Series; Gould, R. F., Ed.; American Chemical Society: Washington, DC,
1
968; Vol. 77, pp 102-117. The measured kr value is a composite of all of
the rate constants which contributed to the formation of the oxidized products
1 and 2. Consequently, if a nonsinglet oxygen process is contributing to
the magnitude of kr, the physical quenching rate constant (kq ) kT - kr) in
Table 1 represents lower limits to its true value.
(A) is unlikely to be tolerated in the gas phase.
(
15) Corey, E. J.; Khan, A. U.; Ha, D.-C. Tetrahedron Lett. 1990, 31,
1
389-1392.
We suggest that the enhanced reactivities of 3 and the two
(16) Wilkinson, F.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref.
1
,2-dithiolanes 9 and 10 reflect their ability to form the
Data 1995, 24, 663-1021.
(17) Prat, F.; Hou, C.-C.; Foote, C. S. J. Am. Chem. Soc. 1997, 119,
5051-5052.
corresponding persulfoxides. Cophotooxidations of 9 with Ph2-
1
SO, which under the reaction conditions is inert to O2, resulted
(
18) Bock, H.; Wagner, G. Angew. Chem., Int. Ed. Engl. 1972, 11, 150-
in formation of Ph2SO2 and is consistent with trapping of the
persulfoxide intermediate. The small steric demands of the
methyl groups in 3, and the constrained >CSSC dihedral angle
1
51.
(19) Sutter, D.; Dreizler, H.; Rudolph, H. D. Z. Naturforsch. 1965, 20a,
1
676-1681.
(
20) Yokozeki, A.; Bauer, S. H. J. Chem. Phys. 1976, 80, 618-625.
(
21) Rindorf, G.; Jørgensen, F. S.; Snyder, J. P. J. Org. Chem. 1980,
(
12) Tsubomura, H.; Mulliken, R. S. J. Am. Chem. Soc. 1960, 82, 5966-
45, 5343-5347.
5
974.
(22) Jørgensen, F. S.; Snyder, J. P. J. Org. Chem. 1980, 45, 1015-1020.
(23) Guimon, M.-F.; Guimon, C.; Pfister-Guillouzo, G. Tetrahedron Lett.
1975, 441-444.
(13) Ackerman, R. A.; Rosenthal, I.; Pitts, J. N., Jr. J. Chem. Phys. 1971,
5
4, 4960-4961.