Direct Observation of Aryl Gold(I) Carbenes that Undergo Cyclopropanation, C-H Insertion, and Dimerization Reactions

Abstract

Mesityl gold(I) carbenes lacking heteroatom stabilization or shielding ancillary ligands have been generated and spectroscopically characterized from chloro(mesityl)methylgold(I) carbenoids bearing JohnPhos-type ligands by chloride abstraction with GaCl₃ . The aryl carbenes react with PPh₃ and alkenes to give stable phosphonium ylides and cyclopropanes, respectively. Oxidation with pyridine N-oxide and intermolecular C-H insertion to cyclohexane have also been observed. In the absence of nucleophiles, a bimolecular reaction, similar to that observed for other metal carbenes, leads to a symmetrical alkene.

Keywords: carbenes; carbenoids; density-functional calculations; gold; structure elucidation.

Scheme 1

General concepts and reactivity of [JohnPhosAuCHPhCl] (1) carbenoid. Tf=trifluoromethanesulfonyl, TMS=trimethylsilyl.

Scheme 2

Synthesis of the chloro(mesityl)methylgold(I) carbenoids 1 a–e.

Scheme 3

Generation of the mesityl gold(I) carbenes 5 a–e. ¹H NMR spectrum and selected region of ¹H‐¹³C HSQC spectra of 5 c in CD₂Cl₂ at −90 °C.

Figure 1

Top: Optimized geometry for 5 c′. Bottom left: plot for the LUMO orbital for 5 c′. Bottom right: plot for the NLMO associated to Au‐to‐C1 π‐backdonation for complex 5 c′. Cutoff: 0.05.

Scheme 4

a) Synthesis of the phosphonium‐ylide complexes 7 a,b. b) Oxygen transfer from pyrine N‐oxide. Yields determined by ¹H NMR spectroscopy using Ph₂CH₂ as an internal standard. Mes=2,4,6‐trimethylphenyl.

Scheme 5

Gold(I)‐promoted C−H insertion. Yields determined by ¹H NMR spectroscopy using Ph₂CH₂ as an internal standard. Mes=2,4,6‐trimethylphenyl.

Scheme 6

Calculated reaction profile for the bimolecular coupling of 5 b′. L=MeJohnPhos. DFT calculations were carried out at the B3LYP‐D3/6‐31G(d,p) + SDD on Au and Ga. CH₂Cl₂ was represented with the PCM. Free energies in kcal mol⁻¹.