Laser flash photolysis study of the photochemistry of o-methylbenzils.

Abstract

The photochemistry of 2-methylbenzil (1) was investigated by steady state and laser flash photolysis techniques. Laser excitation of 1 in benzene leads to a transient, which shows absorption at 370 and 470 nm and is quenched by known triplet quenchers, whereas in methanol or acetonitrile a different transient absorption spectrum was observed. For the latter solvent the absorption bands are located at 350 and 410 nm when the spectrum is recorded 30 micros after the laser pulse. The kinetics associated with these two bands shows biexponential decay, from which lifetimes of 200 and >200 micros were determined. These transients were attributed to a mixture of two possible photoenols. Similar behavior was observed for 2,2'-dimethylbenzil (3), whereas 2,4,6-trimethylbenzil (4) did not show any detectable enol. Lamp irradiation of 1 in benzene leads to 2-hydroxy-2-phenylindan-1-one (2) as the major product, with a quantum yield of 0.09. Compound 3 also gives the corresponding hydroxyindanone as the main product (phi = 0.25), whereas for 4 the cyclization product is formed in a very low quantum yield, i.e., <0.004. From these data two possible mechanisms were suggested to explain the formation of hydroxyindanones from methylbenzils, depending on the nature of the solvent.