Neighbouring Group Participation in Reactions of similar molecularity
If the elimination reaction show on the right proceeds by initial loss of the Tosyl group (E
1) then the rate of the reaction will
not depend on the concentration of base (or indeed the basicity of BH
4), and entropic differentiation between the two reactions cannot be sustained. The question then becomes whether the expulsion of the Tosyl group is assisted in any way by another group present in the same molecule. It is at this point that the alignment of the nitrogen lone pair with respect to the cleaving C-OTs bond is crucial. If the two diastereoisomers differ in this alignment, then the unimolecular rate constant
k will differ for the two reactions. A good
antiperiplanar alignment will increase the rate constant
k (reduce ΔG
‡ by reducing ΔH
‡) with respect to a less efficient alignment.
