Gold carbene or carbenoid: is there a difference?

Abstract

By reviewing the recent progress on the elucidation of the structure of gold carbenes and the definitions of metal carbenes and carbenoids, we recommend to use the term gold carbene to describe gold carbene-like intermediates, regardless of whether the carbene or carbocation extreme resonance dominates. Gold carbenes, because of the weak metal-to-carbene π-back-donation and their strongly electrophilic reactivity, could be classified into the broader family of Fischer carbenes, although their behavior and properties are very specific.

Keywords: Fischer carbenes; carbenes; carbenoids; carbocations; gold.

Scheme 1

Carbene and carbocation resonance forms of gold carbenes.

Scheme 2

Most general methods for the generation of gold carbenes.

Scheme 3

Schematic representation of the bonding of gold carbenes.

Scheme 4

Influence of the ligand on the carbocation-like or carbene-like properties of gold carbenes.[18]

Scheme 5

NHC ligands increase the back-donation from gold to carbene.[18]

Scheme 6

NHC ligands favor carbene-type transformations.[18]

Scheme 7

Site-selectivity in the gold(I)-catalyzed nucleophilic attack of indole on a 1,6-enyne.[24b]

Scheme 8

Formation of fluorenes via retro-Buchner reaction.[13b]

Scheme 9

Rotation of the mesityl substituents in [IPr**Au=CMes₂].[34]

Scheme 10

Carbenoid and carbene equilibrium and isolated gold(I) carbenoids.[47b]

Scheme 11

Generation of gold(I) carbenoids in the oxidation of alkynes with pyridine N-oxides.[10]

Scheme 12

The bonding structure of typical Fischer carbenes.