Lewis Acid Catalysis of Photochemical Reactions. 6. Selective Isomerization of β-Furylacrylic and Urocanic Esters

The spectroscopic properties and thermal and photochemical isomerization reactions of (E)- and (Z)-β-furylacrylic and urocanic esters in the presence and absence of Lewis acids have been investigated. As is the case for cinnamic esters, complexation of β-furacrylic esters occurs on the carbonyl oxygen and results in a large red shift in the electronic absorption spectra and photostationary states enriched in the thermodynamically less stable Z isomer. Unlike cinnamic and β-furlacrylic esters, the Z isomer of methyl urocanate is more stable than the E isomer due to the formation of a moderately strong intramolecular hydrogen bond from the imidazole N₁-H to the carbonyl oxygen. Complexation of both E- and Z-methyl urocanate with BF₃ occurs on the imidazole N₃ rather than carbonyl oxygen and causes a large blue-shift in their electronic absorption spectra. The free esters undergo reversible E,Z photoisomerization whereas their BF₃ complexes undergo one-way E Z photoisomerization. The failure of the complexed (Z)-urocanate to photoisomerize is attributed to an increase in the intramolecular hydrogen bond strength upon complexation.