Mechanisms of Elimination Reactions. 39. Steric and Electronic Effects on Stereochemistry in Eliminations from Primary Alkyltrimethylammonium Salts

Article abstract of DOI:10.1021/ja00262a010

Percentages of syn elimination have been determined by high-field NMR on the products of elimination from R1R2CHCHDX.The results for X=OTs with t-BuO(-)/t-BuOH at 60 deg C were the following (R1, R2, percent syn): p-MeOC6H4, C6H5, 3.7; p-ClC6H4, C6H5, 29; p-ClC6H4, C6H5, 0 in EtO(-)/EtOH.For X=NMe3(+) with OH(-) in 50 molpercent Me2SO-50 molpercentH2O at 60 deg C, the results were as follows (R1, R2, percent syn): p-MeOC6H4, C6H5, 60; p-ClC6H4, C6H5, 72.For Ar(i-Pr)CHCHDNMe3(+) with OH(-) in 50 molpercent Me2SO-50 molpercent H2O at 80 deg C, the results were as follows (Ar, percent syn): m-ClC6H4, 78.6; p-ClC6H4, 69.5; C6H5, 59.6; p-EtC6H4, 58.3; p-t-BuC6H4, 60.5.Overall rates in this series were dissected into syn and anti rates, which fitted the Hammett equation to give ρ_syn=3.69 +/- 0.20 and ρ_anti=3.02 +/- 0.22.This result supports the conclusion that syn elimination has a more carbanionic transition state than anti.The lower percent syn with X=OTs than with X=NMe3(+) is ascribed to the lesser steric requirements of OTs.