Butyllithium addition to alpha-chiral compounds: Solvent mixture effects on diastereofacial selectivity

Temperature-dependent selectivity in nucleophilic additions is affected by the solvent. In this context, we investigated the effect on diastereoselect ivity of solvent mixtures with respect to pure solvents. Binary systems of THF/hexane and of four different hydrocarbon mixtures were employed in BuLi addition to 2-phenylpropanal, (2S)-2-[(tert-butyl)dimethylsilyloxy]-2-phen ylethanal, and (2S)-2-[(tert-butyl)dimethylsilyloxyl]-N-(trimethylsilyl)pro pan-1-imine. A 5-mol-% of THF in hexane affects the isomer ratio by reducin g both Delta Delta H-not equal and Delta Delta S not equal contributions, a nd suppresses T-inv. On the contrary, in hydrocarbon binary mixtures, the T -inv is still observed and occurs at a higher temperature than in pure solv ents. Studying the dependence of T-inv on the hexane/decane mixture composi tion, we propose the formation of a peculiar solvation cluster that is unaf fected by the composition of the bulk reaction solvent.