Specifics of the photolysis of alkylated hydroxy-1,2-dihydroquinolines in water and methanol

Abstract

Comparative study of photoinduced reactions of 6-and 8-hydroxy-2,2,4-trimethyl-1,2-dihydroquinolines (6-OH-DHQ and 8-OH-DHQ) in water and methanol was carried out by continuous and pulse photolysis. The products of MeOH addition to the dihydroquinolines under study were isolated and identified. The spectral and kinetic parameters of active transient species generated in the photolysis were measured. The rate constants of the transient carbocations strongly depend on the position of the hydroxy group on the aromatic ring and are an order of magnitude lower for 6-OH-DHQ than 8-OH-DHQ (80 versus 870 s−1, respectively). Two factors that play a significant part in the photolysis mechanism were revealed, namely, an increase in the acidity of the phenolic group in the excited state and the possibility of intramolecular hydrogen bonding between phenolic and amino groups in 8-OH-DHQ, which are in close proximity. The former factor causes the formation of by-products from the deprotonated excited state of OH-DHQ, which increases substantially in water. The latter affects the structure and the composition of transient species in the case of 8-OH-DHQ.