SYNTHESIS OF MODELS FOR NEIGHBORING PROLINE AMIDE AND ARYL PARTICIPATION IN ELECTRON TRANSFER FROM THIOETHERS

Abstract

A series of 6-endo-(methylthio)-bicyclo[2.2.1]heptane-2-endo-proline amides was synthesized to study the neighboring proline amide participation in electron transfer from thioethers. The thioether with endo-pyrrolidine amide formed a two-center three-electron SO bond after one electron oxidation and the oxidation potential of the thioether was lowered by 530 mV and 330 mV compared to the corresponding exo-pyrrolidine amide and the primary amide analogues, respectively. The thioether with a proline methyl ester showed the oxidation potential of 410 mV higher than that of the pyrrolidine amide. The basis for this surprising result was revealed by an X-ray crystallographic structure study of the diastereomerically pure proline methyl ester which showed amide carbonyl n → methyl ester π* interaction which removes electron density from the neighboring amide which results in less effective neighboring amide participation in thioether oxidation. This accounts for the electrochemical result. A potent synthetic route for S-tert-butyl m-terphenyl thioethers was developed and a series of such thioethers was synthesized. Electrochemical studies showed through-space S∙∙∙π interaction with lower oxidation potentials for thioethers with more electron rich aromatic groups and higher oxidation potentials with electron withdrawing aromatic groups. Selective Suzuki reactions were discovered in which mono-coupling of the precursor dibromides could be achieved. A second coupling was then possible in which two different aromatic rings are attached to the central aryl thioether ring. This enabled the synthesis of a two-sulfur three-aromatic ring extended m-terphenyl thioether as a potential electron conductor. In support of this possibility this compound showed an oxidation potential of +0.99 V which is less positive than the +1.09 V measured for the mono-sulfur analog.