Mechanistic Investigation of Aromatic C(sp²)-H and Alkyl C(sp³)-H Bond Insertion by Gold Carbenes

It was recently reported that the gold-carbenes have an unprecedented catalysis toward the functionalization of C(sp²)-H bonds of aromatic compounds. However, the associated mechanisms of C(sp²)-H bonds inserted by gold-carbenes have not been comprehensively understood. We carried out a detailed mechanistic investigation of gold-carbene insertion into the C(sp²)-H bond of anisole by means of theoretical calculations and control experiments. It significantly reveals that the aromatic C(sp²)-H bond activation starts with the electrophilic addition of aromatic carbon toward the carbene carbon and subsequently followed the [1,3]-proton shift to form an enol intermediate. The rearrangement of enol proceeds through the mechanisms of proton transfer assisted by water molecules or enol intermediates, which are supported by our control experiments. It was also found that the C(sp³)-H insertions of alkanes by gold-carbenes proceed through a concerted process via a three-centered transition state. The further comparison of different mechanisms provides a clear theoretical scheme to account for the difference in aromatic C(sp²)-H and alkyl C(sp³)-H bond activation, which is instructive for the further experimental functionalization of C-H bonds by gold-carbenes.