Photochemical Transformation. 48. The Nonconcertedness of Nucleofuge Loss and anti-Aryl Migration in Photochemical Wagner-Meerwein Rearrangements

Article abstract of DOI:10.1021/ja00203a022

Preparation of the two diastereomeres of 3-(3,5-dimethoxyphenyl)-2-butanol and of their 2-deuterio analogues has been carried out.When the methanesulfonate esters of the diastereomers were solvolyzed by heating at reflux in 50percent aqueous methanol, the resulting alcohols and methyl ethers were formed with complete retention of diastereomeric identity and without measurable hydride migration to give the 2-aryl-2-butyl isomer.Similar treatment of the methanesulfonates of the deuterio analogues gave complete deuterium scrambling, that is, gave equal mixtures of 3-aryl-2-deuterio -2-butyl and 3-aryl-3-deuterio-2-butyl derivatives.These results of ground-state solvolyses are consistent with aryl participation coincident with nucleofuge loss, with bridged phenonium ions as sole intermediates.On the other hand, irradiations of the methanesulfonates with 300-nm light in the same solvent gave photosolvolysis with considerable diastereomeric mixing, although again complete deuterium scrambling was observed here as well.In addition, significant amounts of 2-aryl-2-butanol were formed.The excited-state results require a much more complicated reaction course than the ground-state results, probably involving syn-hydrogen migration to give the tertiary alcohol, along one path, and requiring that the observed aryl migrations, along another path, occur later than, rather than concerted with, nucleofuge loss.