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

Abstract

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.