A Crossover Mechanistic Investigation of the Wolff vs. photo-Favorskii Rearrangement of Diazo-p-Hydroxyacetophenone: Methoxy Substituent Effects on p-Hydroxyphenacyl Cage; Exploratory Studies of Hydroxyquinoline-Based Phototriggers

Abstract

A series of substituted p-hydroxyphenacyl (pHP) and 8-hydroxyquinoline photoremovable protecting groups were designed, synthesized and their photochemistry explored. The light-induced release of phosphates, carboxylates, phenyl ethers and molecular nitrogen (N2) was studied. It was found that photorelease of diethyl phosphate from o-methoxy substituted pHP derivatives were 50% more efficient than the m-methoxy derivatives. In addition, convenient, high yield, one-pot syntheses of several p-HP chromophores were made from their corresponding benzoic acids via the α-diazo-p-hydroxyacetophenones. Photochemical and photophysical studies of the diazo pHP derivatives provided insight into the mechanism of N2 release. Time-resolved IR spectroscopic measurements, sensitization and quenching studies and product analyses support a mechanism for nitrogen release that proceeds via a photochemical Wolff rearrangement pathway. A preliminary study on the photochemistry of 1,5-substituted 8-hydroxyquinolines showed a strong dependence on the leaving group and the presence or absence of oxygen for this heterocyclic pHP analog.