On the Z-E photoisomerization of chiral 2-pentenoate esters: stationary irradiations, laser-flash 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 photosensitizers. The photostationary-state ratio has been determined for each Z/E couple showing a predominance of the thermodynamically more stable isomer for 1 and 3. Moreover, transient species were generated by pulsed laser excitation and detected by their characteristic ultraviolet absorptions, being the first time that enoate-originated triplets are detected. Stern-Volmer quenching studies afforded a quantitative measure for the efficiency of the photosensitization processes induced by benzophenone or acetophenone and allowed the determination of the corresponding quenching rate constants. Density functional calculations permitted the determination of the geometries and the energies of the diastereomeric excited states. Two diastereomeric orthogonal and two diastereomeric planar structures result as a consequence of the presence of a chiral substituent. The orthogonal triplets are the energy minima in all cases, whereas the planar triplets are the transition states linking these orthogonal structures, the corresponding energy barriers being 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 unusually long (1-20 micros) if compared with the data already described for several 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 angle 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.

1,3-Dipolar cycloadditions of diazomethane to chiral electron-deficient olefins: the origin of the pi-facial diastereoselection

The stereochemical outcome of diazomethane cycloadditions to several chiral electron-deficient olefins has been investigated in order to establish the origin of the pi-facial diastereoselection. Nitro olefins, vinyl sulfones, enoates, and 2-amino enoates have been used for such a purpose. These substrates have been prepared from D-glyceraldehyde acetonide through Wittig-type condensations and present an alkoxy substituent, provided by the bulky dioxolane ring, attached to the stereogenic allylic carbon. Syn-adducts have been obtained in all cases as the major isomers, independently of the Z/E stereochemistry of the double bond and the number and the nature of the substituents, the chirality of the asymmetric allylic carbon being the only thing responsible for the diastereoselection. Theoretical calculations show that steric hindrance due to the bulky dioxolane group is the main factor governing the preference for the syn-attack of diazomethane to the olefinic double bond.