Lewis Acid Catalysis of Photochemical Reactions. 5. Selective Isomerization of Conjugated Butenoic and Dienoic Esters

The effects of Lewis and Brønsted acids upon the photoisomerization reactions of several conjugated butenoic and dienoic esters have been investigated. Lewis acids inhibit the photochemical deconjugation of α,β- to β,γ-unsaturated butenoic esters and shift the photoequilibrium between E and Z isomers toward the Z isomer. As such, irradiation of E α,β-unsaturated esters in the presence of EtAlCl₂ provides a convenient method for the preparation of the thermodynamically less stable Z isomer. Irradiation of methyl (E,E)-2,4-hexadienoate and methyl (E,E)-5-phenyl-2,4-pentadienoate in the absence of added catalysts results in nonselective E,Z isomerization to give mixtures of all four stereoisomers in roughly comparable yields. In the presence of the Brønsted acid trifluoroacetic acid, quantitative conversion of methyl 2,4-hexadienoates to methyl 3,4-hexadienoate is observed. The acid serves as a catalyst for the thermal 1,3-hydrogen shift of an allenic enol ester formed via a photochemical 1,5-hydrogen shift of the conjugated esters. Irradiation of the ground-state complexes of the conjugated esters with the Lewis acids EtAlCl₂ or BF₃ results in selective E,Z isomerization about the α,β-double bond in methyl 2,4-hexadienoate and the γ,δ-double bond in methyl 5-phenyl-2,4-pentadienoate. The mechanistic bases for the observed effects of Lewis acids are selective excitation of the more strongly absorbing E complex and more efficient isomerization of the excited E vs. Z complex.