Medium and substituent effects on the photochemistry of phenanthridine N-oxides. Is an intermediate of diradical character involved in the photorearrangement of heterocyclic N-oxides?

Abstract

The photochemistry of several 6-substituted phenanthridine N-oxides has been investigated, or reinvestigated, in benzene and ethanol. The main processes observed are: (a) 1,2-oxygen and substituent shift to yield N-substituted phenanthridones (2) and (b) ring enlargements to dibenzo[d,f]-1,3-oxazepines (7). With 6-diphenylmethylphenanthridine N-oxide (1b) rearrangement (a) predominates and occurs with 45% substituent loss in benzene (but only 2% in ethanol). With the 6-phenyl derivative (1c) process (a) predominates in ethanol and process (b) in benzene and with the 6-p-nitrophenyl derivative (1d) the latter process predominates in both solvents. With 6-cyanophenanthridine N-oxide (1e) rearrangement (b) predominates in benzene; in the presence of 2,3-dimethylbutene (but not of cyclohexene) addition products are obtained; with dienes deoxygenation is the main process. Medium and substituents may change the nature of the lowest excited singlet state, but more importantly affect the stability of an intermediate of diradical character occurring along the reaction pathway, thus driving it towards rearrangement (a) or (b). Intermediate diradicals are unambiguously indicated only in particular cases [elimination of a diphenylmethyl radical from (1b), alkene addition in the case of (1e)] but their role is probably more general.