Richard S. Givens, Dominik Heger, Bruno Hellrung, Yavor Kamdzhilov, Marek Mac, Peter G. Conrad, Elizabeth Cope, Jong I. Lee, Julio F. Mata-Segreda, Richard L. Schowen and Jakob Wirz,"The Photo-Favorskii Reaction of p-Hydroxyphenacyl Compounds is Initiated by Water-Assisted, Adiabatic Extrusion of a Triplet Biradical", J. Am. Chem. Soc. 2008, 130, 3307-3309.
Abstract: The p-hydroxyphenacyl group 1 is an effective photoremovable protecting group, because it undergoes an unusual photo-Favorskii rearrangement concomitant with the fast release (<1 ns) of its substrates in aqueous solution. the reaction mechanism of the diethyl phosphate derivative 1a was studied by picosecond pump-probe spectroscopy, nanosecond laser flash photolysis and step-scan FTIR techniques. The primary photoproduct is a triplet biradical with a lifetime of about 0.6 ns. The release of diethyl phosphate determines the lifetime of the triplet state T1(1a), tau(T1) = 60 ps in wholly aqueous solution. Formation of a new photoproduct, p-hydroxybenzyl alcohol (6), was observed at moderate water concentrations in acetonitrile. It is formed by CO elimination from the elusive spirodione intermediate (4), followed by hydration of the resulting p-quinone methide (5). Computational studies show that CO elimination from the spirodione is a very facile process.