POLAR ADDITIONS TO STYRENE AND 2-BUTENE SYSTEMS .2. MEDIUM DEPENDENCE OF BROMINATION PRODUCTS

The product distribution and stereochemistry of bromine addition to several substituted styrenes has been investigated in acetic acid with added lithium bromide, sodium acetate, and lithium perchlorate. The presence of lithium bromide suppresses acetoxy bromide formation and increases the stereoselectivity of dibromide formation to give 95% trans addition. Both sodium acetate and lithium perchlorate increase acetoxy bromide formation. Lithium perchlorate markedly decreases the stereoselectivity of acetoxy bromide formation from cw-β-methylstyrene but has little effect on the stereochemistry of dibromide formation from either of the β-methylstyrenes or from the 2-phenyl-2-butenes. The stereochemistry of dibromide formation from cis- and trans-β-methylstyrenes was determined in six solvents varying in polarity from dioxane to nitrobenzene. The additions in all solvents were nonstereospecific. The stereoselectivity of addition to both olefins is solvent dependent, particularly for the cis olefin which ranges from 73% trans to 80% cis addition. Only in dioxane are the stereochemical results approximately independent of the starting olefin. The results are interpreted in terms ot a mechanistic scheme involving both intimate and solvent-separated ion pairs, in which the cationic portions resemble weakly bridged, rather than open, benzylic carbonium ions. In marked contrast, additions to cis- and tranj-2-butenes were found to give almost exclusively dibromides, which were formed completely stereospecifically and trans, under all conditions investigated. This is taken as evidence that the bridging interaction in the intermediates from these aliphatic olefins is distinctly different from any which may be present in the styrene intermediates, and that only the 2-butenes react by way of strongly bridged cyclic bromonium ions. The temperature dependence of some of these additions is discussed. © 1969, American Chemical Society. All rights reserved.