The photochemistry of bicyclo[3.1.0]hexanones, pyrrolinium salts and pyridinium salts

Abstract

The gas-phase photodecarbonylation and photofragmentation reactions of substituted bicyclo[3.1.0]hexan-3-ones have been studied in detail. Photolysis of these ketones yields 1,3-dienes, vinylcyclopropanes and 1,4-dienes as detectable products. The possible mechanisms for these reactions are discussed in light of the regiochemical and stereochemical results obtained. In addition, methyl substitution at C-6 and C-2 of these ketones has been shown to have a pronounced effect on both product ratios and overall reaction efficiency. These effects are discussed in terms of stereoelectronic and electronic controls of rates of cyclopropane ring opening of intermediate acyl, cyclopropylcarbinyl diradicals. Irradiation of 2-phenyl-1-pyrrolinium salts in solutions of alcohols and ethers containing α-hydrogens leads to production of addition products. Alcohols and ethers having low ionization potentials and α-hydrogens serve as efficient quenchers of fluorescence from these salts. Deuterium isotope effects on fluorescence quenching rate constants appear to implicate electron transfer mechanisms in photoaddition and quenching pathways. Irradiation of 1-(3-methyl-2-butenyl)pyridinium perchlorate in methanol followed by basic work-up results in the initial build up followed by a loss of hexahydroindolizidine product. This reaction is explained in terms of electron transfer chemistry.