Photochemistry of phosphate esters: An efficient method for the generation of electrophiles

Article abstract of DOI:10.1021/ja00172a016

The photochemical cleavage of benzyl diethyl phosphates, 1a-g, has been examined in tert-butyl alcohol, which produces the corresponding benzyl tert-butyl ether as the major solvolysis product upon direct irradiation. The multiplicity of the reactive excited state has been established as the singlet state. Evidence favoring an intermediate benzyl cation-phosphate ion pair formed from photofragmentation includes oxygen-18 scrambling, racemization of chiral benzyl phosphates, and substituent effects on the rate of reaction. The existence of an electrophilic intermediate is further supported by a linear free energy relationship for the rate of disappearance of 1, k_dis, with Hammett σ, which gave a ρ value of -0.90 and for the rate of appearance of 2, k_app, a Hammett ρ value of -0.97. Possible mechanisms including an electron transfer either before or after homolysis or simple heterolysis of the ester bond are evaluated.