N-dealkylation of electronically excited coumarins by nitrobenzenes

Abstract

When excited (monochromatically or with λ ⩾ 280 nm) in benzene solution in the presence of nitroaromatic acceptors, several 7-dialkylamino-coumarins undergo moderately efficient mono- N-dealkylation to form 7-alkylaminocoumarins, accompanied by the reduction of the nitro function in the acceptor. For 7-( N,N-diethylamino)-4-methylcoumarin ( 1a) a linear plot of ø P −1 versus the reciprocal of the concentration of the starting material suggests that two molecules of 1a are involved in the dealkylation of irradiated 1a by ground state 3-chloronitrobenzene. When the concentration of 1a is kept constant but that of the acceptor is varied, ø P goes through a maximum at intermediate concentrations. Furthermore, ø P is found to be larger for weaker acceptors (more negative reduction potential) than for stronger acceptors (less negative reduction potential). The rigid dye Coumarin 102 does not show any analogous decomposition. All results are interpreted in terms of the interference of the nitroaromatic with the self-quenching mechanism of coumarin N-dealkylation put forward recently by Jones and coworkers.