An Example of 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), but on how efficiently the Tosyl group can be expelled. The question then becomes whether the expulsion of the Tosyl group is assisted in any way by another group present in the same molecule, i.e.
participation by a neightbouring group. It is at this point that the alignment of the remote nitrogen lone pair with respect to the cleaving C-OTs bond is crucial. If the two decalin diastereoisomers differ in this alignment, then the unimolecular rate constant
k for loss of tosyl 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.
