Α-oxo Gold Carbene Research Articles

Tempering the reactivities of postulated α-oxo gold carbenes using bidentate ligands: implication of tricoordinated gold intermediates and the development of an expedient bimolecular assembly of 2,4-disubstituted oxazoles.

2,4-Oxazole is an important structural motif in various natural products. An efficient modular synthesis of this structure has been achieved via a [3 + 2] annulation between a terminal alkyne and a carboxamide using a gold-catalyzed oxidation strategy. The postulated reactive intermediate, a terminal α-oxo gold carbene, previously known to be highly electrophilic and hence unlikely to be trapped by stoichiometric external nucleophiles, is coerced to react smoothly with the carboxamide en route to the oxazole ring by a P,N- or P,S-bidentate ligand such as Mor-DalPhos; in stark contrast, often-used ligands such as monodentate phosphines and N-heterocyclic carbenes are totally ineffective. The role of these bidentate phosphines in this reaction is attributed to the formation of a tricoordinated gold carbene intermediate, which is less electrophilic and hence more chemoselective when reacting with nucleophiles. The success in using bidentate phosphine ligands to temper the reactivities of in situ-generated gold carbenes is likely to open many new opportunities to apply oxidative gold catalysis to the development of novel methods, and the implication of tricoordinated gold intermediates in homogeneous gold catalysis should stimulate further advances in gold catalysis.

Electrophilicity of α-oxo gold carbene intermediates: halogen abstractions from halogenated solvents leading to the formation of chloro/bromomethyl ketones

α-Oxo gold carbenes generated via intermolecular oxidation of terminal alkynes are shown to be highly electrophilic and can effectively abstract halogen from halogenated solvents such as 1,2-dichloroethane or 1,2-dibromoethane. Chloro/bromomethyl ketones are prepared in moderate efficiencies in one step using Ph(3)PAuNTf(2) as the catalyst and 8-methylquinoline N-oxide as the oxidant.

Umpolung reactivity of indole through gold catalysis.

Umpolung reactivity of indole through gold catalysis.

Gold-Catalyzed Synthesis of Dihydrofuran-3-ones from Homopropargylic Alcohols

Reported is the gold-catalyzed synthesis of dihydrofuran-3-ones 3 from homo­propargylic alcohols 1. The reaction is tolerant to both alkyl- and aryl-substituted alcohols, although it was observed that more electron-rich substrates gave higher yields. Despite the mildly acidic nature of the reaction medium, both the OMOM and NHBoc groups survive. The reaction is proposed to proceed via oxidation of the terminal alkyne by the pyridine-N-oxide to give an α-oxo gold carbene intermediate 2, a mechanism which is supported by the isolation of minor byproducts arising from trapping of the carbene by MsOH or pyridine.