Trans‐Concerted Addition to Alkynes: the case of Ynamide Silylzincation

Abstract

AbstractAn original concerted antarafacial mechanism for the addition of diorganosilyl‐zinc reagents across the C−C triple bond of 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‐bond 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.