Understanding the reaction mechanism of anti-addition of (NHC)Au(i)–H and (NHC)Au(i)–F across alkyne†

The experimentally observed anti-addition reactions of (NHC)Au(i)–H with dimethyl acetylenedicarboxylate (DMAD) MeOOCCCCOOMe and (NHC)Au(i)–F with phenylacetylene MeCCPh are intriguing and deserve more in-depth study. In this work, with the aid of density functional theory (DFT) calculations and intrinsic bond orbital (IBO) analysis, we systematically investigated the addition reactions of (NHC)Au(i)–X (X = H, Me and halides) with different alkynes. We found that the nature of the two anti-addition reactions is different. The addition of (NHC)Au(i)–H is initiated by a direct nucleophilic hydride attack from (NHC)Au(i)–H, followed by migration of the [(NHC)Au(i)]⁺ moiety to the diagonally opposite side with the aid of the out-of-plane π-bond of the alkyne. However, in the addition of (NHC)Au(i)–F, the [(NHC)Au(i)]⁺ moiety functions as a Lewis acid to initially activate the alkyne, followed by the fluoride attack.