Acid-Catalyzed Photoisomerization of 2-Alkylindenes to 2-Alkylideneindanes

Article abstract of DOI:10.1021/jo00345a032

Photolysis of 2-alkylindenes in aprotic media containing hydrochloric acid leads to the formation of 2-alkylideneindanes with concomitant quenching of the previously reported phototransposition reaction.Quantum efficiencies range from 0.02 to 0.1 and, as with the tranposition reaction, are highest for those indenes having short singlet lifetimes.Though there is evidence that the reaction is derived from the singlet excited state, acid has no effect on fluorescence and therefore intervenes by protonating an intermediate formed from S₁.This intermediate (proposed as the bicyclopentene "I") is common to both the transposition and the olefin migration (cf.Scheme III).