PHOTOCHEMISTRY OF ALLYLPHENOL DERIVATIVES - ROLE OF THE PHENOLIC AND STYRENIC EXCITED-STATES IN THE BEHAVIOR OF BICHROMOPHORIC CINNAMYLPHENOL

Irradiation of trans-2-cinnamylphenol (2a) in hexane, under argon, led to the cis isomer 2b, 2-benzyl-2,3-dihyrobenzofuran (3), and 2-phenyl-3,4-dihydro-2H-benzopyran (4). The presence of oxygen produced a marked decrease of the 2b yield, with a parallel enhancement of the pathway leading to 3 and 4. In benzene, the cyclization process was enhanced, together with the 3:4 ratio. The latter was less favored in the presence of photosensitizers with lower singlet energies, such as toluene, methyl benzoate, or p-dichlorobenzene. In the acetone-photosensitized reaction, a clean and efficient isomerization to 2b took place. These;results, together with CNDO/S :calculations, suggested involvement of the phenolic or the styrenic singlets in the cyclization to 3 or 4, respectively, and the styrenic triplet (partially quenched by oxygen) in the double bond isomerization to 2b. Addition of methanol to the benzene solutions of 2a partially quenched the photocyclization. By contrast, the formation of 4 was markedly enhanced in the presence of trimethylamine or triethylamine, which were also efficient fluorescence quenchers. The above observations evidenced a new route to product 4 involving electron transfer from the tertiary amine to excited trans-2-cinnamylphenol (2a). The key step in the photocyclization must be intramolecular excited-state proton transfer (EST) and appears to involve pi-complexes of the type I, IIa, and IIB. Accordingly, direct irradiation of the o-acetyl derivative 9a resulted exclusively in isomerization to 9b, owing to chelation of the hydroxy group, which should prevent formation of the pi-complexes and facilitate deactivation of the phenolic singlet, via intramolecular proton exchange.