Extraordinary kinetic behavior of the α-tocopheroxyl (vitamin E) radical

Article abstract of DOI:10.1021/jo00122a026

Rate constants which have been reported for the bimolecular self-reaction of α-tocopheroxyl radicals vary by about 5 orders of magnitude. We have found that the observed bimolecular rate constant can vary by about a factor of 7 during a single, but typical experiment, e.g., in chlorobenzene at 37°C from ca. 7 x 10³ M^-1 s^-1 initially to ca. 1 x 10³ M^-1 s^-1 finally. The overall reaction involves a disproportionation with the transfer of a hydrogen atom from the 5-methyl group of one radical to the phenoxyl oxygen atom of the other radical forming α-tocopherol and an o-quinone methide. In the slow regime (which corresponds to the true reaction of two α-tocopheroxyl radicals) this disproportionation has a deuterium kinetic isotope effect of 3.7. The bizarre kinetic behavior observed with α-tocopheroxyl radicals has been traced to a very minor impurity which will be present in any normal sample of α-tocopherol. The impurity in question is a bisphenol in which two α-tocopherol moieties have become linked through their 5-methyl carbon atoms. This bisphenol is a 'natural' impurity in α-tocopherol since it will be formed upon exposure of α-tocopherol to air. The coupling of two o-quinone methide molecules yields a spiro-dimer which is then reduced to the bisphenol, probably by unoxidized α-tocopherol.