Base-Induced Proton Tautomerism in the Primary Photocyclization Product of Stilbenes

Article abstract of DOI:10.1021/ja00291a046

The mechanism of the photoformation of 1,4-dihydrophenanthrenes (1,4-DHP) and 9,10-dihydrophenanthrenes from 1,2-diarylethylenes in amine solution is clarified by demonstrating that the amine reacts as a base with the initially formed 4a,4b-dihydrophenanthrene.The predominant formation of 1,4-DHP from stilbene is ascribed to an easy proton transfer from C(4b) to C(4) in 4a,4b-DHP via a deprotonation/protonation step, in which the amine operates as the transferring agent.The product formation in basic methanolic solutions proceeds with another mechanism or with less selectivity.When propyl thiolate, having a weak hydrogen-bonding capability, is used as the base, the solvent-mediated protonation in the deprotonation/protonation step occurs exclusively at C(9) and leads eventually to 9,10-DHP.When the stronger base sodium methoxide is used, solvent-mediated protonation proceeds rather unselectively at C(2), C(4), and C(9) and causes the ultimate formation of a mixture of 1,2-, 1,4-, and 9,10-DHP.Deuteration experiments indicate that 1,2- and 3,4-DHP are intermediates in the formation of 1,4-DHP (Scheme VII).The former compounds isomerize photochemically in the presence of a base.Larger diarylethylenes give only compounds analogous to 9,10-DHP.