Influence of cation, solvent and temperature on Z–E isomerisation of radical ion pairs derived from (Z)- 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

Abstract

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 metals and magnesium amalgam in ethereal solvents has been studied by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and TRIPLE resonance spectroscopies. The structure of the radical ion pairs obtained from 1 exhibits strong dependence upon cation size, solvent polarity and temperature. It was found that (Z)-isomeric ion pairs 1^˙– M⁺ are favoured under conditions involving more ion association, for example, in systems with small counterions, low solvating power of the solvent and at high temperatures. Z–E isomerization occurred 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 final paramagnetic species, referred to as D in the text, has strikingly distinct hyperfine data which could be ascribed to a structurally different ion pair resulting from intramolecular cyclization that may occur in the (E)-exo-endo isomeric ion pair. The arguments in favour of such a structure are corroborated by INDO calculations.