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 silylation of the title species was investigated by means of theoretical methods. The influence of the solvent on the optimized geometries and relative energies of different reaction intermediate species was studied by applying the continuum model at the DFT/6-31+ G∗//DFT/6-31+G∗ level. Two groups of neutral reaction intermediates were considered: dianion–bislithium intermediates 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 the negative charge is better stabilized when delocalized over the entire molecular skeleton, while in solution the solute–solvent interactions are more important when the charge is localized over the oxygen atoms. For the dianion–bislithium intermediates, the intramolecular interactions are more important and the chain-delocalized charge intermediate remains the most stable one, even in solution. This is not the case for the monoanion–lithium intermediates 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 species.