Abstract
An original concerted antarafacial mechanism for the addition of diorganosilyl-zinc reagents across the C-C triple bond in ynamides is computationally investigated using DFT calculations. This concerted mechanism, leading to a trans-product in only one step, results in the formation of a Si-C and a Zn-C σ-bonds on opposite sides of the π-system. We demonstrate that the mechanism going through a η2-vinyl intermediate and the proposal of a radical chain pathway are energetically unsustainable. The retained concerted antarafacial pathway is tested on experimental selectivities: the regioselectivity, in favor of the silyl β-addition in ynamide, and stereoselectivity, which is cis- with (Me2PhSi)2Zn but trans- with [(Me3Si)3Si]2Zn, are well reproduced by DFT calculations. The regio- and stereoselectivity are discussed using the activation strain model and a chemical bonding analysis.
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