Gold carbenes, generation

Gold-catalysed intramolecular oxidation of terminal alkynes with an arenesulfinyl group as the tethered oxidant has been reported to involve a gold carbene generation via alkyne oxidation. DFT studies suggest that the cyclized product is formed via an intramolecular [3,3]-sigmatropic rearrangement instead of the previously proposed Friedel-Crafts-type cyclization (Scheme 12) ... 

Note Since this article was written, a number of other studies have appeared on the generation and reactions of gas phase reactive metallic species, including the use of decarboxylation reactions to produce organometallic ions (105-107] and the lithium acetate enolate anion (108j silver and silver hydride cluster ions (109) gold carbenes [110, 111] and metal-oxo cations 112]. 

Generation of gold carbene was also shown to proceed via internal redox processes involving homopropargylic sulfoxides (Scheme 11.10). The carbenoid can then be trapped by insertion into an aryl C-H bond or alkyl 1,2-... 

Surface-introduced photoreactive group, aryl diazirine, can generate carbenes by 365 nm UV exposure. However, gold-sulfiir bonds, which support photolinker on gold, can be easily broken by UV light below 300 nm. Therefore, UV transmission filter is essential in UV irradiation. 

The easiest reactions are those in which the nucleophile is the gold-activated species. Examples of this are Au(I)-catalyzed carbene and nitrene transfers (equations 142 and 143) that convert olefins into cyclopropanes or aziridines, respectively. In the carbene transfer, ethyl diazoacetate is the source of carbene and the active NHC-gold cationic catalyst is generated by chloride abstraction with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate NaBAT4. The cyclopropanation is competitive with other carbene insertions with active C H or N H bonds present in the substrate. For the aziridinations of olefins, nitrene formation is accomplished by the oxidation of sulfonamides with PhI(OAc)2 and the catalyst of choice is a gold-(I) triflate with a terpyridine ligand. 

Chapter 4, by Casini et al., focuses on the rapid development over the last few years of gold organometallics as potential metallodrugs. The unusual character of the mechanism of action is revealed, whether for complexes of Au(l) or Au(III) bound to carbenes. In particular, their antiproliferative activity is often linked to their interference in the redox homeostasis of cancer cells. These compounds also have antiparasite potential. Finally, a possible approach to their use as theranostic agents is described. These species merit the wide interest they generate. 

Gagosz et al. reported Au(l) catalyst 147-catalyzed alkylation of alkynyl ethers which produced cyclohexane 146 as major product (Scheme 54) [127]. Theoretically, the electrophilic activation of the alkyne 145 by Au(l) initiates a [1,5]-hydride shift to furnish oxocarbenium ion 1, interaction of which with the pendant nucleophilic vinyl-gold moiety affords cyclopropenium intermediate II. Carboca-tion IV, which would finally collapse into cyclohexene 146 after elimination of the gold(I) catalyst might be generated via a [1,2]-alkyl shift on Au-carbene intermediate III. 

Reaction of o-(ClAuCN) C Hx (25) with o-(HxM)xCcH gives benzlmldasolln-2-ylidene(chloro)gold as the sole product. Carbene c q>lexes have been generated [AuCl C(NRi)i ] from (25) and R(HH. 

Treatment of the 1,5-enyne 66 with a cationic gold complex caused cycliza-tion to generate the spirocyclic carbene complex 67 (Scheme 7.26) [35]. 1,2-Alkyl migration followed to furnish the tricyclic compound 68. 

While different gold catalysts can give rise to the products of the 1,3- as well as the 3,3-rearrangement and sometimes generate mixtures, a special carbene gold catalyst leads to aldehyde 394 exclusively. 

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