Photochemistry of substituted 1-naphthylmethyl esters of phenylacetic and 3-phenylpropanoic acid: Radical pairs, ion pairs, and marcus electron transfer

Article abstract of DOI:10.1021/ja00059a012

The ring-subtituted 1-naphthylmethyl esters of phenylacetic (3a-k) and 3-phenylpropanoic (5a-c) acid have been photolyzed in methanol solvent. The major products of these reactions are derived from two critical intermediates, the 1-naphthylmethyl radical/acyloxy radical pair and the 1-naphthylmethyl cation/carboxylate anion ion pair. The radical pair results in formation of the in-cage coupled products 8a-k and 10a-c after loss of carbon dioxide from the acyloxy radical. The ion pair leads to the methyl ethers 6a-k and the carboxylic acids 7 and 9. The competition between the radical and ionic pathways is very dependent upon the substituents on the naphthalene ring. Analysis of these substituent effects results in a proposed mechanism of initial homolytic cleavage of the carbon-oxygen bond of the ester from the excited singlet state. This radical pair then partitions between two pathways: decarboxylation of the acyloxy radical and electron transfer converting the radical pair to the ion pair. The rates of electron transfer are shown to fit Marcus theory in both the normal and the inverted region.