Photochemical Ring-Opening and Intramolecular Hydrogen Shift Reactions in Sulfur Analogues of α-Pyrone

A combined matrix isolation FTIR and theoretical DFT(B3LYP)/6-311++G(d,p) study of the photochemistry
of sulfur analogues of α-pyrone [2H-thiopyran-2-one (TP) and 2H-pyran-2-thione (PT)] was carried out.
The vibrational spectra of monomers of the compounds isolated in low-temperature argon matrixes were
studied experimentally and assigned completely on the basis of theoretical calculations. UV irradiation (λ >
337 nm) of the studied compounds isolated in low-temperature matrixes results mainly in the ring-opening
reaction by means of the cleavage of the α-bond. Other photoprocesses, not involving the α-bond-cleavage
step (such as generation of Dewar valence isomer), correspond to the minor reaction channels in both studied
compounds. The ring-opening photoreaction in PT represents the first reported case of an α-bond cleavage
in a compound with a CS group attached to a six-membered ring, in which the internal strain practically
does not exist, whereas the corresponding reaction in TP (a cleavage of a C−S bond in the α position with
respect to a carbonyl group) is now reported for the first time. Following the ring-opening reactions,
isomerization processes and intramolecular hydrogen shift reactions were observed, enabling production of
TP from PT and vice versa. A detailed study of such processes was undertaken, and kinetical and mechanistical
data are presented and discussed.