Unveiling the mechanisms and secrets of chemoselectivities in Au(i)-catalyzed diazo-based couplings with aryl unsaturated aliphatic alcohols

Abstract

Density functional theory (DFT) calculations have been conducted to unravel the mechanisms and chemoselectivities of Au-catalyzed diazo-based couplings with phenyl unsaturated aliphatic alcohols: the propargyl alcohol Ba resulting in the [4 + 1]-cycloaddition product P_4a and the allyl alcohol Db giving the [2,3]-σ rearrangement species P_5b. P_4a formation involves a catalyst interaction with phenyldiazoacetate, N₂ release, a hydroxyl O nucleophilic attack of Ba, a [1,4]-H shift, coordination isomerization, 5-endo-dig cyclization, a [4,1]-H shift and a H₂O-assisted [1,3]-H shift. After the [4,1]-H shift, the slightly less favorable five-membered ring-opening possibly follows to afford trace P_5a ([2,3]-σ rearrangement product), which would be kept in solution due to subsequent irreversible evolution. In addition, the Ba-involved chemoselectivity was probed and explained as follows: (i) both large H(hydroxyl)⋯C(carbene) electrostatic repulsion and strong three-membered ring strain involved in the TS make the formation of the O–H insertion product P_1a difficult and (ii) the nucleophilic attack from the C² atom of Ba brings about a structural twisting and thus increases the energy penalty forming the cyclopropenation product P_2a. On the other hand, compared with the sp-C² atom of Ba, the sp²-C² atom of Db greatly facilitates the five-membered ring-opening step because of the presence of an extra pπ–pπ orbital overlap and eventually provides P_5b exclusively.