A Photochemical Long-Range Pinacol Rearrangement. Mechanistic and Exploratory Organic Photochemistry

Article abstract of DOI:10.1021/jo00374a022

In a previous study we uncovered a rearrangement in which a phenyl group migrated 1,4 from a benzhydryl carbon to an excited diphenylvinyl chromophore.The present study aimed at generalizing this bizarre rearrangement and also in ascertaining if an aryl migration would occur from a carbinol carbon.Indeed, the irradiation of 1,1,5,5-tetraphenyl-3,3-dimethyl-4-penten-1-ol led smoothly to 1,4,5,5-tetraphenyl-3,3-dimethyl-1-pentanone.The overall mechanism involves a transannular phenyl migration in a process reminiscent of a pinacol rearrangement.At conversions above 20percent and at wavelengths other than 280 nm, a secondary process was observed wherein a type II fragmentation afforded 1,1,2-triphenyl-3-methyl-2-butene and acetophenone.The initial rearrangement and primary process was shown to proceed via the excited singlet of the tetraphenyldimethylpentenol while the corresponding triplet proved unreactive.An alternate mechanism leading directly from the tetraphenyldimethylpentenol to acetophenone and the triphenylbutene was considered as possibly accounting for part of the overall fragmentation reaction.However, this was precluded by a kinetic study using our dynamic isotope dilution method.An analogous transannular migration was observed for the corresponding methyl ether, which rearranged to afford 1-methoxy-1,4,5,5-tetraphenyl-3,3-dimethyl-1-pentene.Here there was no complicating type II secondary process.Another aspect studied was the facility of migration of different aryl groups.Thus it was observed that when one phenyl group of the methyl ether was replaced by p-cyanophenyl, only the cyanophenyl group migrated.With one phenyl group replaced by anisyl, anisyl migrated with a 2:1 preference relative to phenyl.As in the case of the tetraphenyldimethylpentenol, the singlet excited state was the reactive species.Quantum yields were determined.The tetraphenyldimethylpentenol had an effciency of 0.0026 which was ca. 2.5 times that of our previously studied rearrangement lacking the hydroxyl group.The corresponding methyl ether had a quantum yield of 0.0041.The cyanophenyl migration efficiency was 0.0038, and the total quantum yield of the anisyl analogue was 0.0032.Also, singlet excited state reaction rates were determined by single photon counting.Finally, in the case of the tetraphenyldimethylpentenol rearrangement, deuteration studies showed that the initially formed diradical, resulting from completion of the phenyl migration, leads to product both by hydrogen transfer from the hydroxyl oxygen as well as from the adjacent carbon.