EXCITED-STATE CARBON ACIDS - PHOTOCHEMICAL C-H BOND HETEROLYSIS VS FORMAL DI-PI-METHANE REARRANGEMENT OF 5H-DIBENZO[A,C]CYCLOHEPTENE AND RELATED-COMPOUNDS

The possible excited-state carbon acidity behavior of 5H-dibenzo[a,c]cycloheptene (2) and related compounds has been investigated in aqueous CH3CN solution. In non-basic or aprotic solvents such as CH3CN or cyclohexane, direct photolysis of 2 gave exclusively dibenzonorcaradiene (10) (ϕ = 0.087 in CH3CN), via both singlet (initial 1,7-hydrogen shift of the methylene hydrogen, followed by electrocyclic ring closure) and triplet (di-π-methane) pathways. However, in aqueous CH3CN or alcohol solutions, the main deactivational pathway from S1 is C-H bond heterolysis from the methylene 5-position. Thus photolysis of 2 in 70% D2O-CH3CN gave 5-deuterio-5H-dibenzo[a,c]cycloheptene (11) as the major product (ϕ = 0.028), in addition to 10 (ϕ = 0.010), which was found to arise exclusively via a residual triplet (di-π-methane) pathway. Two related derivatives that are somewhat less rigid (1,2-and 3,4-benzotropilidenes, 7 and 5, respectively) underwent formal di-π-methane rearrangement (via initial 1,7-hydrogen shift) exclusively on photolysis, even in aqueous solution, with no evidence for carbon acid behavior. Fluorescence quenching of 2 by added water in CH3CN solution gave a linear Stern-Volmer plot, with kq = 2.05 ⨯ 108 M-1 s-1. The fluorescence emissions of 7 and 5 were not quenched to any observable extent on addition of water to CH3CN solutions of these substrates: quenching of fluorescence by H2O appears to be a useful experimental criterion for testing the possibility of excited-state carbon acid behavior of these substrates. © 1990, American Chemical Society. All rights reserved.