Elimination and addition reactions. Part 32. Discrimination between concerted and stepwise processes in activated elimination reactions

In 1,2-elimination reactions activated by phenylsulphonyl, cyano, and benzoyl groups with leaving groups SO2Ph, SPh, and OPh, comparison of ionisation rates with elimination rates confirms that the (E1cB)R mechanism operates in the sulphonyl and cyano activation series but in the benzoyl series the mechanism for all leaving groups is (E1cB)I. This difference in behaviour is discussed.For all three activation series, comparison of elimination rates with predicted ionisation rates suggests assignments of (E1cB)I mechanism when the leaving group is Cl, OAc, OTs, or OMes but E2 mechanisms when it is I and possibly Br.In all the cases studied, α- and β-phenyl substituents in chloro-, bromo-, and mesyloxy-sulphones affect elimination rates in a manner paralleled by the effect of these substituents on ionisation rates. The results strengthen assignments of (E1cB)1 mechanisms where these have been made.The results are compared with those obtained in other systems in relation to current views on the relationship between stepwise and concerted processes.