Mechanisms Involving Cationic Gold

We may distinguish four main types of mechanism (1) the reaction proceeds only on metallic gold particle, (2) it requires the simultaneous availability of metallic gold and cationic gold species, (3) it involves collaboration between metal and support, with the possible assistance of cationic species and (4) it proceeds solely via cationic species on the support. In the next section we examine mechanisms 1 and 2 then in Section 6.6.3, we look at mechanisms 3 and 4. 

Periana et al. reported the Pd-catalyzed oxidative functionalization of CH4 to CH3COOH via C—H activation [117]. The conversion of 2 equiv. of methane into acetic acid, which is also an important petrochemical, brightens the future use of natural gas. Recently, the same group also reported that cationic gold catalyzes the selective and low-temperature oxidation of methane to methanol in a strongly acidic solvent using SeVI ions as the stoichiometric oxidant (Equation 11.54) [118]. The reaction does not appear to proceed through free radicals and DFT calculations indicate that Au1 or Auln species are both viable catalysts. Possibly the mechanism involves overall electrophilic C—H activation and oxidative functionalization. 

Starting from tetrahydrocyclopenta[f)]furan-2-one 342, enyne 343, the substrate for the domino reaction, was prepared in 12 steps and with an overall yield of 45%. Exposure of 343 to the electron-rich gold(I) complex (t-Bu)2P(o-biphenyl)AuCl at room temperature afforded cis-hydrindanone 344 in 78% yield as a single stereoisomer (Scheme 14.54). The postulated mechanism involved Au(I) activation of the alkyne to initiate the cationic olefin cyclization of 346 to give carbocation 347, which then underwent a pinacol rearrangement to the final product 344. An originally attempted Lewis acid-catalyzed domino Prins/pinacol rearrangement of... 

The gold(I)-catalyzed intramolecular Schmidt reaction of azido alkynes 49 provides easy entry to a series of pyrroles 54 with a variety of substitution patterns. The proposed mechanism involves gold(I)-induced activation of the alkyne toward addition by the proximal nitrogen of the azide. Subsequent loss of nitrogen leads to cationic intermediate 52, which is... 

In 2010, Malacria and coworkers developed a novel Au-catalyzed cyclization of 1,6-enyne 9, furnishing allene-substituted tetrahydrofuran 11 in excellent yield. The proposed mechanism involves an initial enyne cyclization to generate the cationic gold-vinyl complex 10 and subsequent unexpected [l,5]-hydride shift. Deuteration experiments support the [l,5]-hydride shift and rule out the inter-molecular scrambling. This cascade reaction is unusual because the cyclization precedes followed by an uncommon [l,5]-hydride shift (Scheme 12.5) [9]. 

A synthesis of cyclopentenones somewhat related to those shown earlier was found from conjugated 1,3-enynes (equation 19) Cyclopentenones are obtained by the hydrolysis of the enol acetates. This transformation involves a 1,3-migration of the acetate to form pentadienyl cation and the formation of the gold carbene after a Nazarov-type cyclization. DFT calculations support this mechanism and provide interesting insight into the mechanism of the final stages of the process. 

---