Gold -activated allenes

Gold-activated allene can also be employed as hydride acceptor. Gagosz et al. demonstrated that a phosphite gold complex 154-catalyzed intramolecular hydroalkylation of allenes 151, which afforded the spiro compound 152 and undesired fused bicyclic compound 153 in 30 and 61 % yields, respectively (Scheme 56) [128]. The selectivity could be reversed if HNTfa was exploited. 

Reactions by Other Nucleophiles As in the case of the formal cycloadditions of alkenes to allyl cations, the addition of alkenes to gold(I)-activated allenes generates intermediates that determine which cycloaduct formed. Based on this hypothesis, Toste et al. recently developed enantiorich bicycle-[3.2.0] structures by [2+2]-cycloaddition reaction catalyzed by chiral biarylphosphinegold(I) complexes [51]. 

Similar to the abovementioned silver nhc coordination compounds, carbene chemistry has also been dominant in the field of gold organometallic chemistry. Noteworthy examples include a Au(PPh3)-compound derived from tetraaminoallene, that can be rationalised in terms of a dicarbene with ylide character and which, owing to the electron-rich character of the central carbon atom, offers the potential for dimetallation products.108 Non-activated allenes and alkynes have been found by Lavallo to be readily aminated by cationic carbene gold complexes.109 For this purpose, a 2,6-diisopropylphenyl functionalized cyclic alkylaminocarbene gold(I) complex... 

The reactions described below are categorized according to the nature of the nucleophile that attacks the gold-activated alkynes/ allenes. While complex 1 is emphasized, other cationic Au(I) complexes could in general catalyze these reactions as well. 

The activation of allenes is a rather new, but particularly promising area of gold catalysis.381,400 The first example for such a transformation is the cycloisomerization of allenic ketones 480 to furans 482 which probably occurs via intermediate 481 (Scheme 147). Hashmi et /.401,401a showed that this reaction proceeds much faster when gold(m) chloride in acetonitrile is employed as the precatalyst instead of the traditionally used silver salts (cf. Section 9.12.3.2). The products are usually contaminated by substituted furans originating from a Michael addition of aurated 482 to the substrates 480, thereby indicating that the gold catalyst is also capable to activate C-H bonds of furans. 

The coordination of olefins to gold species is well known, despite the preference for alkynes, allenes, and carbonyl groups. This is reflected in the fact that synthetic applications based on olefin activation by gold have been developed to a lesser extent than aUcyne or carbonyl activation. 

For alkynes (and in part, allenes), synthetically useful protocols for Markovnikov and anti-Markovnikov selective hydrations, hydroalkoxylations (mainly intramolecular), and hydrocarboxylations are available and find increasing applications in organic synthesis. In the past decade, the research focus on cationic gold(l) complexes has led to new additions to the catalysis toolbox. It can be predicted that a further refining of such tools for alkyne functionalization with respect to catalytic activity and functional group tolerance will take place. 

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. 

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]. 

Cationic gold(I) complexes favor the formation of six- and seven-membered rings by 6-endo-dig, 6-exo-dig, and 1-exo-dig cyclization. However, indoloazo-cines V are selectively obtained with AuCls via %-endo-dig cyclization. Internal alkynes are also active in the intramolecular process leading to allenes VI and tetracyclic compounds VII (Scheme 1.10). In Scheme 1.10, the proposed mechanism for the formation of the different products is shown. Nucleophilic attack of the indole on the activated alkyne affords intermediate VIII, which arises from a 1,2-shift of the initially formed seven-membered ring iminium cation. Proton loss from VIII forms azocine V, while protonation of intermediate VIII leads to an open intermediate IX, which rearranges to the final allene VI or the tetracyclic compound VII via Michael-type addition of the XH group in intermediate X. 

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