Vinylgold intermediates

Whereas these transformations require stoichiometric gold compounds, catalytic amounts of both gold and palladium are sufficient for the cycloisomerization of allyl allenoates to allyl-substituted butenolides. Blum and co-workers reported this tandem C-O/C-C bond formation, which is initiated by activation of the distal allenic double bond with PhaPAuOTf (Scheme 4-107). This induces cyclization to an allyl oxonium intermediate, which undergoes deallylation in the presence of Pd2dba3. Nucleophilic attack of the resulting a-vinylgold intermediate at the ti-allylpalladium species and reductive elimination furnish the allylated butenolide and regenerate both catalysts. 

Scheme 4.1 Not detected in situ vinylgold intermediates as key intermediates in reactions of alkynes 1.

With a phosphane ligand instead of the NHC ligand, and in the presence of triethylamine, the vinylgold intermediates could be detected by in situ NMR spectroscopy, but all attempts for the isolation failed [19]. 

The vinylgold intermediate 3 (Scheme 4.1) then typically reacts with an electrophile (E) in most of the literature this is a proton. Only a few cases of the use of halogen electrophiles have been reported [25]. This use of halogen electrophiles is problematic because, similar to halolactonization reactions, the electrophilic halide can possibly induce the cyclization without the gold. 

Another possibility would be the use of allylic esters, which after a gold-catalyzed cycloisomerization with the carbonyl oxygen atom as the nucleophile deliver activated allylic intermediates which at the same time contain a vinylgold substructure. After transfer of an allyl cation to palladium(O), an oxidative addition to palladium, the vinylgold intermediate could transfer the organic moiety to palladium(II). A final reductive elimination would close the catalytic cycle. At the same time, no halide that potentially could deactivate the cationic gold(I) catalyst would be present. Indeed, Blum et al. [30] presented such systems. But... 

The use of internal propargylamides also allowed the first isolation of stable vinylgold intermediates derived from alkynes [67]. The simple trick of capturing the proton, which would decompose the vinylgold species by protodeauration, by a simple base as triethylamine, worked beautifully (Scheme 7). This principle could be extended to other reaction types as hydroalkoxylation and hydroaminations [68]. 

This formal [2h-2h-2] alkyne/alkene/carbonyl cycloaddition proceeds through the opening of the cyclopropyl carbene intermediate 1-4 by the carbonyl group to form oxonium cation 1-5, which undergoes nucleophilic attack by the vinylgold intermediates in a Prins-type cyclization to give tetrahydropyranyl cation 1-6. 

The first gold-catalyzed addition reactions of carbon nucleophiles to allenes were only first disclosed in 2006, and the number of examples is still small. Toste and co-workers showed that allenic silyl enol ethers undergo a 5-endo- ng cyclization to hexahydroindenone derivatives in the presence of a cationic gold catalyst (Scheme 4-10). In these transformations, water or methanol is used as an external proton source for protodeauration of an intermediate vinylgold species. In an analogous manner, cyclopentenes were obtained in good yields from allenic P-ketoesters. In the presence of a palladium catalyst and an allylic halide, these substrates afford functionalized 2,3-dihydroflirans. 

Scheme 4.4 Stable intermediates and stoichiometric access vinylgold complexes of N-heterocyclic ligands obtained in the presence of a base.

At the stage of vinylgold or arylgold intermediates, most catalytic cycles show the proto-deauration as the final step. In many of these reactions, rather than a direct proton transfer from the nucleophile which was added, a long-distance proton transfer is necessary. A detailed investigation revealed that gold catalysis not only tolerates water but also utilizes clusters of four or more water molecules as proton shuttles [53]. 

With the electTOTi-poor allenic esters, palladium(0) is able to catalyze the reaction without gold. The reactiOTi then is initiated at the other end, after oxidative addition of the aryl halide to the electrophilic palladium(II) species cycloisomerizes the allenic ester and then forms the product by reductive elimination. With o-alkynylbenzoates, the intermediate vinylgold species contains an enol ether substructure and is able to directly intercept the activated allyl donors, even in the absence of palladium. In both cases, by careful trace analysis (ICP), the presence of the other metal was excluded [78]. 

Gold-catalyzed 5-exo-type cyclization is also useful for oxazole synthesis. It was reported that AuCls is efficient in the electrophilic activation of carbon-carbon triple bonds of A-propargylamides 144 to afford 2,5-disubstituted oxazoles 150 under mild conditions (Scheme 19.36) [59]. The reaction proceeds through stereospecific oxyau-ration to generate vinylgold(III) intermediates 149 [60], which would be converted to the oxazoles 150 by protodeauration and subsequent isomerization. Vinylgold... 

In 2006, Liu and coworkers reported a gold-catalyzed intramolecular cycloaddition of diynes with tethered arenes 118 to synthesize l,3-dihydroindeno[2,l-c]pyran or 2,3-dihydro-l//-indeno[2,l-c]pyridinederivatives 120 (Scheme 12.52) [56]. On the basis of deuterium labeling, they believed that the first reaction step is the intramolecular arylation of one alkyne to form vinylgold(I) intermediate 119,... 

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