Divergent catalytic behaviors of assembled organogold(i) clusters derived from enyne cyclization

Abstract

Homogeneous gold catalysis has attracted much recent attention due to diverse activation modes of gold(I) towards unsaturated organic groups. Because of attractive aurophilic interaction, structural transformations of metalated species into high nuclear clusters are often proposed in gold catalysis, while to date little is known about their assembly behaviors and catalytic activity. In this work, based on stoichiometric Au(I)-mediated enyne cyclization reactions, we achieve a discrete vicinal dicarbanion-centered Au₄ intermediate and three assembled Au₁₁, Au₂₈, and Au₁₄ clusters held together by several aryl dicarbanions. Spectral monitoring, kinetic and theoretical investigations confirm that these discrete and assembled intermediates display four different pathways upon catalyzing the cyclization reaction of the same 1,5-enyne substrate. The discrete Au₄ cluster undergoes a full protodeauration process to generate active [Au(PPh₃)]⁺ species for catalytic use. In contrast, the net-like Au₁₁ cluster experiences a substrate-induced dissociation to generate a semi-stable Au₁₀ unit and an active [alkyne-Au(PPh₃)]⁺ fragment for further transformation. The dumbbell-like Au₂₈ cluster is prone to cleavage of the central Au–Au linkage and each Au₁₄ moiety exposes a coordination unsaturated site to activate a substrate molecule. However, the synthetic closed-Au₁₄ cluster with full ligand protection is no longer catalytically active.