Photochemical α Cleavage of Benzoin Derivatives. Polar Transition States for Free-Radical Formation

The photochemical α cleavage reactions of benzoin, benzoin ethers, benzoin acetate, and several related phenyl ketones have been investigated, α cleavage is the only primary process observed for benzoin and the benzoin alkyl ethers in benzene solution, β cleavage is a minor competing reaction for benzoin phenyl ether. Substituents at the a carbon have little effect on the efficiency (quantum yield) of cleavage; however, they have a pronounced effect on the rate constant for cleavage. Benzoin alkyl ethers are about 10⁵ times more reactive toward α cleavage than deoxybenzoin. Substituents capable of stabilizing an adjacent positive charge are far more effective in accelerating a cleavage than are substituents capable of stabilizing free radical centers. It is concluded that the transition state for α cleavage has considerable ionic character and does not resemble the free radical products.