Gold alkyne functionalizations

Selective activation of alkyne functions of enynes to give products either of alkoxy-cyclization or of exo- and endo-skeletal rearrangement can be achieved by using alkynophilic cationic gold(I) complexes. The endocyclic cyclization catalysed by gold(I) proceeds via a mechanism different from those known for Pd(II), Hg(II), or Rh(I) catalysts.118... 

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. 

In contrast to C(2)-linked terminal alkynes 145, gold-catalyzed alkylation of C(3)-linked tetrahydrofurans bearing terminal alkyne functions 148 mainly led to the formation of major product exo-methylene cyclopentanes 149 and minor products 150 (Scheme 55). This reversed selectivity might be explained by the relative stability of intermediates III and V. Steric constrains should be weaker for the fused bicyclic intermediate V (in Scheme 55) than intermediate HI (in Scheme 54), thus allowing a rapid [l,2]-hydride shift, which leads to VI rather than a [1,2]-alkyl shift, which leads to III. 

Finally, a last example of enantioselective gold promoted rearrangement is the intramolecular cyclopropanation introduced by Toste et al. as a tool for the synthesis of medium-sized rings (Fig. 10.51) [70]. Favoured substrates are 1,7- and 1,8-enynes with terminal alkyne functions and an acetoxy substituent at the propargylic position. They are converted into bicyclic structures containing a cyclopropane unit. 

Alkynes are by far the most popular and studied functional group in gold catalyzed reactions since Thomas et al. chose them for their research in 1976 [53, 84]. 

A gold monohydride species was also suggested in the report by Ito and Sawamura et al. on the dehydrogenative silylation of alcohols by HSiEt3 and a diphosphine gold(I) complex. Reaction was selective for the silylation of hydroxy groups in the presence of alkyl halides, ketones, aldehydes, alkenes, alkynes and other functional groups [193]. 

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 Lewis acidity of gold, related to relativistic effects, allows not only the coordination to carbon itt-systems but also other functional groups. Among them, the most exploited field is the activation of carbonyls and imines toward different nucleophiles. In these cases, Au(ni) species are many times the catalysts of choice over Au(I). An explanation for this may be found in the thermodynamic coordination preference of AuCft for aldehydes over other moieties such as alkynes, accounting for the fimctional group discrimination in these reactions. Nevertheless, Au(I) is still used in some of the transformations described in this section. 

Alabugin, I. V., Gold, B. (2013). Two Functional Groups in One Package Using Both Alkyne 7t-bonds in Cascade Transformations. Journal of Organic Chemistry, 78, 7777-7784. 

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