Lewis Acid Induced [2+2] Cycloadditions of Silyl Enol Ethers with α,β‐Unsaturated Esters: A DFT Analysis
The Lewis acid (LA) induced cycloaddition of trimethysilyl vinyl ether with methyl acrylate has been studied by DFT methods at the B3LYP/6-31G* level. In the absence of an LA, a [4+2] cycloaddition between the silyl enol ether and methyl acrylate in the s-cis conformation takes place through an asynchronous, concerted bond-formation process. This cycloaddition presents a large activation enthalpy of 21.1 kcal mol–1. Coordination of the LA AlCl3 to the carbonyl oxygen atom of methyl acrylate yields a change of molecular mechanism from a concerted to a two-step mechanism and produces a drastic reduction of the activation energy. This stepwise mechanism is initialized by the nucleophilic attack of the enol ether at the β-position of methyl acrylate in a Michael-type addition. The very low activation energy (7.1 kcal mol–1)associated with this nucleophilic attack can be related to the increase of the electrophilicity of the LA-coordinated α,β-unsaturated ester, which favors the cycloaddition through a polar process. The subsequent ring-closure allows the formation of the corresponding [2+2] and [4+2] cycloadducts. While the [4+2] cycloadduct is formed by kinetic control, the [2+2] cycloadducts are formed by thermodynamic control. The energetic results provide an explanation for the conversion of [4+2] cycloadducts into the thermodynamically more stable [2+2] ones. The cis/trans ratio found for the catalytic [2+2] process is in agreement with the experimental outcome. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)