Influence of a Very Bulky N-Heterocyclic Carbene in Gold-Mediated Catalysis

The syntheses of the free carbene IPr* (IPr* = 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) and related gold complexes [Au(IPr*)Cl] (C1) and [Au(IPr*)(NTf2)] (C2) were achieved in high yields. The % VBur of IPr* for both gold complexes was measured, revealing IPr* as one of the bulkiest NHCs on gold complexes reported to date. In addition, the catalytic activity of C1 and C2 in several reactions, typically catalyzed by AuI complexes, was investigated. Examples include the tandem alkoxylation/lactonization of γ-hydroxy-α,β-acetylenic esters, the [3,3]-rearrangement of propargylic acetates leading to the formation of conjugated enones and substituted indenes, and the rearrangement of allylic acetates. These studies revealed a strong solvent effect on the catalytic activity with 1,2-dichloroethane as the solvent of choice. The screening of C1 and C2 demonstrated only slightly diminished activities in comparison to [Au(NHC)(L)] complexes bearing bulky ligands such as IPr and SIPr.