Investigations on photochemistry of o-allyloxy-/crotyloxyacetophenones: formation of unexpected intramolecular arene–olefin addition products on n–π∗ excitation of ketones

Photochemistry of o-allyloxy-/crotyloxyacetophenones () has been investigated under different conditions. Irradiation of in dry benzene, under N2 atmosphere (Pyrex filter), led to the isolation of (syn)-2-ethenyl/propenyl-3-hydroxy-3-methyl-2,3-dihydrobenzofurans (∼10%) as the sole product. Irradiation of in dry benzene in the presence of 0.1–0.4 mol equiv. of triethylamine resulted in slightly increased formation of , besides pinacols ( and ) and triethylamine addition products ( and ). However, the formation of is suppressed with increasing molar ratios of triethylamine, with increased formation of products (). On the other hand, irradiation of in dry acetonitrile results in the formation of both syn-() as well as anti-isomers of benzodihydrofuranols (), besides some highly unexpected intramolecular arene-olefin addition products (); formation of these intramolecular arene–olefin addition products is quenched in the presence of 0.1 mol equiv. of triethylamine. With increasing molar ratios of triethylamine, in acetonitrile solvent, the formation of , is also suppressed with increased formation of products () derived from photoreduction of carbonyl function through electron transfer from TEA. No product derived from intramolecular interaction of the ketone derived anion radical/ketyl radical with an olefinic moiety has been detected. A plausible mechanistic rationalization of the results obtained is presented.