A Study of the Relationship Between Dihedral Angle and Transition State Geometry in Bimolecular Elimination Reactions

Abstract

<p> The nitrogen and hydrogen-deuterium isotope effects have been measured for the base-promoted elimination reactions of cis-and trans-2-phenylcyclopentyltrimethylammonium ions and cis-and trans-2-phenylcyclohexyltrimethylammonium ions. The observations have been interpr eted as indicative of a concerted E2 mechanism for both trans and cis elimination processes. In the cis eliminations, however, proton transfer to base is far advanced at the transition state resulting in a much higher degree of carbanionic character ass ociated with the transition state than for the trans eliminations. </p> <p> The kinetic isotope effect results for these cyclic systems establish that the two bond-rupture processes complement each other; the greater the extent of proton transfer to base at the transition state, the smaller is the extent of C-N bond weakening. These observations are interpreted in terms of the coupling of the motion which extends the lengths of the H-C and C-N bonds at the transition state and the approach to coplanarity of the bonds involved in the elimination process. </p>