Electronic Effects in Migratory Groups. [1,4]- versus [1,2]-Rearrangement in Rhodium Carbenoid Generated Bicyclic Oxonium Ylides.

The variety of alpha-diazo beta-keto esters (3a-f, 8a-f) with varying substituents (ED/EW) on the phenyl ring of the O-benzyl group were prepared. The rhodium(II) acetate catalyzed decomposition of diazo compounds in benzene reflux conditions. The ratio of 1,4 versus 1,2 migration product was determined. It was found that an increase in electron density on the benzylic carbon of the migrating group prefers 1,4 migration products (4, 9) while a decrease in electron density leads to a preponderance of 1,2 migration products (5, 10). The results obtained were correlated to the mechanistic aspect of the product selectivity. The intermediacy of the intramolecular oxonium ylide formation was demonstrated by crossover experiments. The preference for the formation of 2,3 sigmatropic rearrangement product over 1,2 and 1,4 was demonstrated by performing the reaction with alpha-diazo beta-keto esters (13a, 13b) with O-allyl and O-propargyl at C3. The effect of solvent, temperature, and mole percentage of rhodium(II) acetate was also studied.