Evidence Against a Hydrogen Abstraction Mechanism in the Photorearrangement of Azoxybenzene to 2-Hydroxyazobenzene

In an attempt to distinguish between ionic and free radical mechanisms for the photorearrangement of azoxybenzene to 2-hydroxyazobenzene, aromatic azoxycompounds carrying C—H functions ortho to the azoxy linkage have been prepared and irradiated. The failure of these weaker C—H bonds to divert the reaction from its normal course argues against a hydrogen abstraction–hydroxyl transfer mechanism. This conclusion is supported by the observation of a 30-fold increase in quantum yield for 2-hydroxyazobenzene formation on changing from a non-polar to a polar solvent and by the kinetic deuterium isotope effect, which is too small for the primary isotope effect required by the abstraction mechanism. It is concluded that the experimental observations to date may most easily be accommodated in the route originally proposed by Badger and Buttery, where the rearrangement is seen as a substitution by oxygen at the ortho ring carbon.