Transition state structures in ion-pair base-promoted syn-elimination reactions

Abstract

The temperature dependence of the primary deuterium kinetic isotope effect has been applied as a criterion of transition state (TS^‡) structure in the syn-elimination reaction of bicyclo [2.2.1]heptan-2-exo-yl toluene-p-sulphonate, caused by two different hindered bases in a low dielectric, aprotic solvent. The isotope effect (k_H/k_D= 3.43 ± 0.02) was temperature independent, and identical for the two bases used (t-butoxide and cyclohexylcyclohexyl oxide). These results are interpreted to indicate a non-planar, cyclic TS^‡ involving non-linear hydrogen transfer. A comparison with the analogous Hofmann and amine oxide elimination reactions affords some understanding of the factors controlling TS^‡ structure in cyclic syn-elimination processes.