On the Z-E photoisomerization of chiral 2-pentenoate esters: Stationary irradiations, laser-plash photolysis studies, and theoretical calculations

Chiral pentenoates 1-3 in both Z and E isomeric forms underwent stationary irradiations in several solvents and in the presence of different photosens itizers. The photostationary-state ratio has been determined for each Z/E c ouple showing a predominance of the thermodynamically more stable isomer fo r 1 and 3. Moreover, transient species were generated by pulsed laser excit ation and detected by their characteristic ultraviolet absorptions, being t he first time that enoate-originated triplets are detected. Stern-Volmer qu enching studies afforded a quantitative measure for the efficiency of the p hotosensitization processes induced by benzophenone or acetophenone and all owed the determination of the corresponding quenching rate constants. Densi ty functional calculations permitted the determination of the geometries an d the energies of the diastereomeric excited states. Two diastereomeric ort hogonal and two diastereomeric planar structures result as a consequence of the presence of a chiral substituent. The orthogonal triplets are the ener gy minima in all cases, whereas the planar triplets are the transition stat es linking these orthogonal structures, the corresponding energy barriers b eing 8-10 kcal mol(-1) for enoates 1-3. The computed S-0 to T-1 excitation energies show a trend which is consistent with the quenching rate constants . On the other hand, the triplet lifetimes determined for 1 and 2 are unusu ally long (1-20 mus) if compared with the data already described for severa l enones, in the range of nanoseconds. This fact has been rationalized from calculations of spin-orbit coupling at several points of the T-1 potential energy surface; This coupling is maximum for structures with a torsional a ngle close to 45 degrees, which are 4-5 kcal mol(-1) above the minima of T- 1. Calculations done on the hypothetical aldehyde 4 and methyl vinyl ketone show much lower energy barriers, thus accounting for the shorter lifetimes reported for enone triplets.