INFLUENCE OF CATION, SOLVENT AND TEMPERATURE ON Z-E ISOMERIZATION OF RADICAL ION-PAIRS DERIVED FROM (Z)-1,2-BIS(2,4,6-TRIMETHYLBENZOYL)ETHENES AND (E)-1,2-BIS(2,4,6-TRIMETHYLBENZOYL)ETHENES BY CHEMICAL-REDUCTION WITH ALKALI-METALS AND MAGNESIUM - A STRUCTURAL, ELECTRON-PARAMAGNETIC-RESONANCE, ENDOR AND TRIPLE-RESONANCE STUDY

The chemical reduction of (Z)-1,2-bis(2,4,6-trimethylbenzoyl)ethene, 1 , and (E)-1,2-bis(2,4,6-trimethylbenzoyl)-ethene, 2, with alkali metal s and magnesium amalgam in ethereal solvents has been studied by elect ron paramagnetic resonance (EPR), electron nuclear double resonance (E NDOR) and TRIPLE resonance spectroscopies. The structure of the radica l ion pairs obtained from 1 exhibits strong dependence upon cation siz e, solvent polarity and temperature. It was found that (Z)-isomeric io n pairs 1. - M+ are favoured under conditions involving more ion assoc iation, for example, in systems with small counterions, low solvating power of the solvent and at high temperatures. Z-E isomerization occur red at temperatures as low as 193 K even for lithium ion pairs in 1,2- dimethoxyethane. The resulting (E)-isomers decayed significantly with increasing temperature, yielding secondary reduction products. The fin al paramagnetic species, referred to as D in the text, has strikingly distinct hyperfine data which could be ascribed to a structurally diff erent ion pair resulting from intramolecular cyclization that may occu r in the (E)-exo-endo isomeric ion pair. The arguments in favour of su ch a structure are corroborated by INDO calculations.