Mechanism of the amine-promoted photochemical 1,3-H shift of 1,2- and 3,4-dihydrophenanthrene into 1,4-dihydrophenanthrene

Abstract The quantum yield of the amine-mediated 1,3-H shift of 1,2-dihydrophenanthrene (1,2-DHP) ( 1 ) or 3,4-dihydrophenanthrene (3,4-DHP) ( 2 ) into 1,4-dihydrophenanthrene (1,4-DHP) ( 3 ) obeys first-order kinetics with respect to the amine and also varies linearly with the basicity of the amine. This indicates that deprotonation of excited 1 yielding an [RNH 3 + DHP − ]-type ion pair is the rate-determining step in the reaction mechanism; the possible involvement of intermediate radical ion pairs in the formation of 3 is rigorously excluded by the experimental data. In neat tertiary amines, the photophysics and photochemistry of 1 and 2 are dominated by exciplex formation. In these amines, photoconversion of the starting material into 3 and the formation of radical reaction products from the exciplexes take place with very low quantum yields. The addition of an alkyl alcohol to the tertiary amine restores the efficient formation of 3 . By fluorescence lifetime measurements it is shown that the additional presence of alkyl alcohols inhibits exciplex formation of excited 1 or 2 with Et 3 N by quenching the exciplex precursor, an encounter complex. This process involves proton transfer from excited 1 or 2 to Et 3 N and rationalizes the fact that, in the presence of alkyl alcohols, Et 3 N promotes the title reaction by the same mechanism as primary amines.