Kinetic ( 18 O and 14 C) and Magnetic ( 13 C) Isotope Effects in the Photo-Fries Rearrangement of 4-Methoxyphenyl Acetate

Article abstract of DOI:10.1021/jo00226a017

Kinetic isotope effects (KIE) were measured for the photorearrangement of 4-methoxyphenyl acetate (1) into 2-acetyl-4-methoxyphenol (2) in ethanol solution.The KIE for labeling the phenolic oxygen atom with 18 O was 1.0000+/-0.0023.The KIE for labeling with 14 C at the α-carbon atom of the acetyl group was measured in two ways: with recovered 1 (0.9988+/-0.0051) and with isolated 2 (1.007+/-0.008).Labeling with 13 C at the α-carbon atom led to a magnetic, inverse isotope effect (0.9511+/-0.0042).The results show that there is not a detectable activation barrier for breaking the ester bond and that 2 is formed by recombination of a caged radical pair which originates from an excited singlet state.Surprisingly, labeling of 1 with 14 C in the ortho position led to a KIE, measured with recovered 1, of 1.0286+/-0.0021.We attribute this to a reaction of 1, as yet unknown, which is not associated with rearrangement into 2.It is noteworthy that rearrangement is not the major reaction pathway.The larger part (over 60percent) of 1 is converted into polymeric material.The origin of the polymeric material lay in the scission product, 4-methoxyphenol (3), which was itself not obtained during the lengthy irradiations of the KIE work.Whether the KIE for ortho labeling is connected with polymer formation is not now known.