Mechanistic Studies of Photocyclizations of Vicinal Allylnaphthols: The Duality of Excited-State Proton-Transfer Complexes

Article abstract of DOI:10.1021/ja00193a009

Photoexcitations of 2-allyl-1-naphthol and 1-allyl-2-naphthol caused cyclization, forming dihydrofuranyl and pyranyl ethers and the secondary photodehydrogenation of the dihydrofuranyl ethers.The quenching experiments of the product formations as well as fluorescence intensities of allylnaphthols by methanol, triethylamine, N-nitrosodimethylamine, and 1,3-dienes confirmed that the cyclization occurred from the singlet excited-state proton transfer (ESPT) of the phenolic group; i.e., kinetic acidity initiated photocyclization.The primary photoprocess is quenched mainly by a static quenching process arising from the ground-state hydrogen-bonding association between allylnaphthols and a quencher, as supported by IR spectroscopy, and to a minor extent by a dynamic quenching of the singlet excited state of the allylnaphthol intramolecular proton-transfer complexes.Kinetic analyses also reveal the presence of two identifiable intramolecular ESPT intermediates, probably conformers of the proton-transfer species; one is the precursor of the furanyl ethers and not quenchable in the dynamic process by Et3N, and the other that of the pyranyl ethers and quenchable in the dynamic process by Et3N.The IR absorption of intramolecular hydrogen bonds between the OH and olefin groups began to be replaced by that of intermolecular hydrogen bonds at tetrahydrofuran concentrations above 10^-2 M; methanol, at about this concentration and above, also quenches the photocyclization.