Indirect electron-donating effects governing the concerted bimolecular elimination processes

Abstract

The concerted bimolecular β-elimination (E2) reaction of substituted alkanes (X–C α H 2–C β H 2–…) is studied using the semilocalized quantum-chemical approach suggested previously (V. Gineityte, J. Mol. Struct. (Theochem), 588 (2002) 99; V. Gineityte, Int. J. Quant. Chem., 94 (2003) 302) and based on the power series for the one-electron density matrix. Electron density redistributions among separate bonds of both the reactant and the external base ( B ⋅⋅ ) are analyzed and interpreted in this approach in terms of direct (through-space) and indirect (through-bond) interactions of bond orbitals (BOs). The main aim of the study lies in revealing the principal ways of the indirect influence of the base B ⋅⋅ upon characteristics of the heteroatom containing (X–C α ) bond (The direct interactions between respective orbitals are expected to be negligible in the E2 processes). The above-specified indirect influence is shown to consist of two components: First, of the additional indirect electron-donating effect of the initially occupied (bonding) orbital of the H–C β bond upon the X–C α bond, wherein the orbital of the base participates as a mediator, and, second, of the indirect electron-donating effect of the base itself upon the same bond by means of orbitals of the H–C β bond. The known predominance of the trans β elimination over the relevant cis process is accordingly related to higher efficiencies of the above-enumerated indirect electron-donating effects for the case of trans-arranged X–C α and H–C β bonds. The results obtained along with those of an analogous investigation of the S N 2 process (V.Gineityte, J.Mol.Struct. (Theochem), 541 (2001) 1) yield a unified description of both reactions, wherein the direct and indirect attacks of the reagent upon the X–C α bond underly the S N 2 and E2 reactions, respectively. Consequently, the result of competition between these processes is expected to depend upon relative efficiencies of direct and indirect influences of the given reagent upon characteristics of the nucleofuge-containing (X–C α ) bond.