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S. M. Bonesi et al. / Tetrahedron 62 (2006) 10716–10723
this step, as mentioned above, is again subjected to steric
hindrance). This mechanism is followed in methanol and,
when intramolecular activation is slow, also in a polar apro-
tic solvent such as MeCN. However, this path is less impor-
tant with amines (the barely bonded CT complex is too
short-lived for activation) as well as, for the opposite reason,
with alkenes (the strength of the bonds formed leaves little
room for improvement).
at 1.27 mm in the presence of known amounts of sulfides in
aerated CDCl3. Singlet oxygen was generated by energy
transfer to O2 from the triplet state of TPP, populated by laser
excitation (Nd:YAG laser, 532 nm). The near-IR lumines-
cence of molecular oxygen was observed at 90ꢅ geometry
through a 5 mm thick AR coated silicon metal filter with
wavelength pass >1.1 mm and an interference filter at
1.27 mm by means of a preamplified (low-impedance) Ge-
photodiode cooled at 77 K (Applied Detector Corporation,
Model 403 HS, time resolution 300 ns). Single exponential
analysis of the emission decay was performed with the
exclusion of the initial part of the signal, affected by scat-
tered light, sensitizer fluorescence, and formation profile
of the emission signal itself. The rate of chemical reaction
was determined by comparing the oxidation of octaline
(kr¼1.83ꢂ106 Mꢃ1 sꢃ1) under the same conditions.
In conclusion, the reaction of singlet oxygen with a series
of benzyl sulfides was examined. The total quenching rate
(kT) depends on the nucleophilicity of the sulfide, since the
persulfoxide is formed over an essentially flat surface and
bulky substituents tilt the balance against bonding with
singlet oxygen, drastically lowering kT. The strength of the
S–O bond formed with sulfides is intermediate between
the strong C–O bond formed with alkenes and the weak
N–O bond with amines, resulting in largely variable propor-
tion of chemical quenching (kr/kT). Two types of chemical
reactions contribute. The path leading to sulfoxide via the
protonated persulfoxide observed in a protic medium
depends again on the sulfide nucleophilicity (making the
overall dependence quadratic). The photocleavage reaction
observed in aprotic solvent depends on the strength of the
a-CH bond.
Acknowledgements
Partial support of this work by MURST, Rome, is gratefully
acknowledged.
References and notes
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Rate constants for the quenching of singlet oxygen were ob-
tained from the shortening of the (O2)1Dg emission lifetime