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

Abstract

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.

On the involvement of diazonium ions in the photorearrangement of azoxybenzene

“…2,2'-Azoxytoluene is known to be unusual in that it gives photo products additional to those predicted by [ I ] (3), and similar anoinalous behaviour has also been observed recentiy for other o-substituted azoxybenrenes (4). The extra products are suppressed when 2,2'-azoxytoluene is photolyzed in the presence of 2-naphthol, and it has been suggested ( 5 ) that some intermediate such as 2, originally proposed by Badger and Buttery (6), has the ability to fragment into a diazonium ion.…”

“…Although we have no direct information about the geometry of the excited state, HMO calculations on azoxybenzene d o not indicate a large increase in the C-N n-bond orders upon excitation. Thus the excited state is also likely to twist easily, and this is compatible with the aromatic substitution meclianism for photorearrangement (4,6), in which attack by oxygen on the ortl~o carbon must involve a non-planar transition state such as 4. This would minimize steric hindrance and explain why bulky substituents have only a small effect o n the efficiency of the reaction.…”

“…Photolytic procedures and methods for the separation and identification of photolysis products have been described in detail previously (4). In this work, photolyses were carried out without deoxygenation and with the light of a Hanovia 450 W medium pressure mercury arc filtered through Pyrcx glass, cxccpt as noted otherwise.…”

“…Syntheses of azoxybenzene and 2,2'-azoxytoluene were described previously (4). Several compounds were prepared by reduction of the nitro compound with thalliumethanol (21).…”

Structure of 2,6-dihydroxy-2,6-dimethylheptan-4-one

“…Solution studies on sterically crowded azoxybenzenes, however, have established that a number of side products are formed in lesser amounts implicating diazonium intermediates in the process; as a result, other mechanisms have also been proposed (Goon, Murray, Schoch & Bunce, 1973;Bunce, 1974). In our investigations of the photochemistry of azoxybenzenes, both in solution and in the solid state, we have succeeded in isolating a most unusual and previously unreported product (I) whose structure we now report.…”