Photochemical Deuteration ofortho-SubstitutedN-(Diphenylmethylene)acetamides

The alkyl hydrogens of N-(2-alkyldiphenylmethylene)acetamides were found to be readily replaced with deuterium on irradiation in deuteriomethanol (CH3OD). It is proposed that the deuteration proceeds via an enamide isomer produced by intramolecular hydrogen transfer from the o-alkyl group to the excited C=N double bond. An intermolecular hydrogen atom abstraction pathway was excluded because neither N-(4-methyldiphenylmethylene)acetamide in deuteriomethanol(CH3OD) nor diphenylmethane in deuteriomethanol(CH3OD) in the presence of N-(diphenylmethylene)acetamide was deuterated. Intramolecular hydrogen abstraction is essential for the deuteration, because no deuteration occurs on irradiation of 2-methylbenzophenone semicarbazone which undergoes no photoreduction. The difference in reactivities of the ortho-substituted imines between in solution and in a rigid matrix is discussed from the viewpoint of syn-anti isomerization.