Balancing Lewis Acidity and Carbocation Stability for the Selective Deoxyhalogenation of Aryl Carbonyls and Carbinols

Deoxyhalogenation of aryl aldehydes, ketones, carboxylic acids, and esters has been achieved utilizing an appropriate metal halide Lewis acid acting as a carbonyl activator and halogen carrier in combination with borane-ammonia as the reductant. Selectivity is accomplished by matching the stability of the carbocation intermediate with the effective acidity of the Lewis acid. Substituents and substitution patterns significantly influence the requisite solvent/Lewis acid combination. Logical combinations of these factors have also been applied for the regioselective conversion of alcohols to alkyl halides.