DFT theoretical study of the chemioselectivity of the crotonic acid dianion and its trimethylsilyl ester silylation. Role of the solvent

The different chemioselectivity observed experimentally during the silylati on of the title species was investigated by means of theoretical methods. T he influence of the solvent on the optimized geometries and relative energi es of different reaction intermediate species was studied by applying the c ontinuum model at the DFT/6-31 + G*//DFT/6-31 + G* level. Two groups of neu tral reaction intermediates were considered: dianion-bislithium intermediat es in the case of crotonic acid and monoanion-lithium intermediates in the case of its silyl ester. It was found in both cases that in the gas phase t he negative charge is better stabilized when delocalized over the entire mo lecular skeleton, while in solution the solute-solvent interactions are mor e important when the charge is localized over the oxygen atoms. For the dia nion-bislithium intermediates, the intramolecular interactions are more imp ortant and the chain-delocalized charge intermediate remains the most stabl e one, even in solution. This is not the case for the monoanion-lithium int ermediates because the solvent effect inverts the gas phase stability order . The differences observed experimentally in solution are thus explained by the differences in the stability order of these reaction intermediate spec ies. (C) 2000 Elsevier Science Ltd. All rights reserved.