AN O-18-LABELING STUDY OF THE BETA-(NITROXY)ALKYL AND BETA-(TRIFLUOROACETOXY)ALKYL RADICAL MIGRATIONS - FURTHER EXAMPLES OF A 1,2-SHIFT MECHANISM

The results of an O-18 labeling study of the beta-(nitroxy)alkyl and beta-(trifluoroacetoxy)alkyl radical migrations are presented. Phenacyl bromide dimethyl acetal was hydrolyzed with (H2O)-O-18 water to give labeled phenacyl bromide and, following borohydride reduction, O-18 labeled styrene bromohydrin. Nitration and trifluoroacetylation gave the radical precursors which were allowed to react with tributyltin hydride and AIBN in benzene at reflux. After cleavage to 2-phenylethanol, the rearrangement products were examined by GC-MS. The beta-(nitroxy)alkyl migration is found to occur, in benzene to the extent of 64% through a 1,2-, as opposed to a 2,3-shift, mechanism. The beta-(trifluoroacetoxy)alkyl migration occurs 7% by the 1,2-pathway. It is suggested that, in general, faster ester migrations occur to a greater extent through the 1,2-shift pathway.