On the Z−E Photoisomerization of Chiral 2-Pentenoate Esters:  Stationary Irradiations, Laser-Flash Photolysis Studies, and Theoretical Calculations

Abstract

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.

Flavin Photocatalysts for Visible‐Light [2+2] Cycloadditions: Structure, Reactivity and Reaction Mechanism

“…The substrate scope is limited to substrates with at least one “activated” double bond with a lowered triplet state energy by suitable substitution, for example, with a phenyl or benzoyl group. Even so, in some cases, the energy transfer seems to be slightly endothermic, which can be explained by non‐vertical energy transfer …”

Cycloheximide congeners produced by Streptomyces sp. SC0581 and photoinduced interconversion between (E)- and (Z)-2,3-dehydroanhydrocycloheximides

“…Starting from these energy minima, other equilibrium points rotating around the C6–Cα double bond in S 0 , S 1 , and T 1 states were located by relaxed potential energy surface scans, except for geometries around torsion angles (C1–C6–Cα–Cβ, Φ) of 90° and −90° in the S 1 surface, for which optimizations failed to converge. Due to the presence of the stereogenic center C4 in 2a and 3a , the twisted structures around Φ 90º and −90° are diastereomeric in chirality [15]. Thus, two twisted minima, P and P′ (Φ = 77º and −84°, respectively) in the T 1 surface and two energy maxima (Φ = 90° and −90°, respectively) in S 0 surface were located.…”

Comparison of the Photochemistry of Acyclic and Cyclic 4-(4-Methoxy-phenyl)-4-oxo-but-2-enoate Ester Derivatives

“…Furthermore, Garci ́a-Expośito et al demonstrated that the intermolecular sensitization of simple alkene ester derivatives can also be used to form triplet 1,2-biradicals (Scheme 1). 9 These authors demonstrated that the twisted biradicals decay by intersystem crossing to the ground state, forming the corresponding cis-or trans-isomers. The energy gap between the triplet biradical and the singlet ground state is minimized for the twisted conformer of the 1,2-biradical.…”