Dynamic solvation effects in ethylmagnesium bromide addition to (2S)-O-(tert-butyldimethylsilyl)lactal

The diastereofacial selectivity of (2S)-O-(tert-butyldimethylsilyl)lactal t owards nucleophilic addition of ethylmagnesium bromide is strongly solvent- dependent. We have shown that solute-solvent interactions occurring in a se ries of ethers, such as tetrahydrofuran, tetrahydropyran, diethyl ether, di pentyl ether, tert-butyl methyl ether, diisopropyl ether, diisopentyl ether , and anisole, can govern a diastereomeric switch from the anti to the syn isomer. Determination of the temperature-dependence of the anti/syn ratio h as enabled the selectivity to be analysed in terms of the differential enth alpies and entropies of activation of the two diastereomeric reaction paths . We have demonstrated that the predominance of one isomer over the other i s often due to an entropy effect on the diastereofacial selectivity. The ph enomenon of the inversion temperature was evident in the corresponding Eyri ng plots.