α-Bromo carbonyl compounds as promoters for the synthesis of (2-bromoethyl)benzene by the anti-Markovnikov addition of hydrogen bromide to styrene

The synthesis of (2-bromoethyl)benzene by the anti-Markovnikov addition of gaseous hydrogen bromide to styrene has been found to be promoted by α-bromo carbonyl compounds such as 2-bromo-2-methylpropanal. These compounds were found to catalyze the "abnormal" addition in a variety of solvents such as ethyl acetate, heptane, toluene and dioxane. High concentrations of 2-bromo-2-methylpropanal and hydrogen bromide and low concentrations of styrene favor formation of (2-bromoethyl)benzene. Using the free-radical catalyzed cyclization of 6-bromo-1-hexene as a probe we have found that the 2-bromo-2-methylpropanal does not in itself initiate a free-radical chain reaction by thermal formation of radicals. Instead, radicals may react with 2-bromo-2-methylpropanal to form relatively stable 2-methylpropanal radicals. The presence of such radicals increases the effective length or inhibits termination of the free-radical chain reaction (propagation) and in the case of hydrobromination of styrene raises the yield of (2-bromoethyl)benzene.