A near-paradox of E1cb-like elimination in the 2-phenylethyl system

Abstract

The leaving group selectivity coefficient, k_OTs/k_Br, is discussed for eliminations in the 2-phenylethyl system. Interestingly, and almost paradoxically, for eliminations in the E2-“nearly-E1cb” region of transition states, a change to more C–H bond-breaking may also induce an attenuated increase in C–X bond-breaking, provided that the leaving group remains constant. Thus, it may be understood why pure E1cb eliminations are virtually non-existent in typical alkyl substrates. Primary toluene-p-sulphonates may actually eliminate faster than the corresponding bromides, if the β-hydrogens are rendered more acidic by electron-withdrawing groups.