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Lewis Acid Induced [2+2] Cycloadditions of Silyl Enol Ethers with α,β‐Unsaturated Esters: A DFT Analysis
Author(s) -
Arnó Manuel,
Zaragozá Ramón J.,
Domingo Luis R.
Publication year - 2005
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200500188
Subject(s) - chemistry , cycloaddition , methyl acrylate , electrophile , nucleophile , lewis acids and bases , silylation , enol , silyl enol ether , ketene , medicinal chemistry , enol ether , reaction mechanism , photochemistry , catalysis , organic chemistry , copolymer , polymer
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 AlCl 3 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)