Gold chloride, catalysis

Recent developments in gold chloride catalyzed C C and C- 0 coupling reactions provide a fitting opportunity to highlight the current status [8] of homogeneous catalysis with gold salts and to show the potential for further developments. 

In this chapter there are collected short accounts of a number of reactions that do not fit easily into the previous chapters. Section 13.2 deals with the synthesis of vinyl chloride, a reaction of interest because its discovery signalled the resurgence of interest in what gold could do as a catalyst. It is also effective in the formation of C-C bonds (Section 13.3). The reactions mentioned in the other sections serve to demonstrate vividly the versatility of gold in catalysis. 

Hydrochlorination of Alkynes When Thomas and coworkers treated different alkynes in aqueous methanol with HAuC14 and observed the corresponding ketones as major products (Equation 8.28), with less than 5% of methyl vinyl ethers and vinyl chlorides, they were unaware of the fascinating treasure that was in front of them. Some of the most important types of products for gold catalysis were reported in the aforementioned study, but unfortunately at that time this process was believed to be a gold(III) oxidation process, despite the fact that the reaction achieved almost six turnovers. 

Scheme 7. Additional examples of protic catalysis provided by gold (I II) chloride.

However, the ability of gold(III) chloride to provide protic catalysis under exceptionally mild conditions is further demonstated by two recent examples the hydroxyallene 35 bearing a silyl protecting group is efficiently cyclyzed to give the 2,5-dihydrofuran 36 without deprotection [20] other acidic catalysts which in principle sufficiently promote this type of cyclization - such as HC1 gas or Amberlyst 15 resin - are of course much less compatible with acid sensitive functionalities. Also for the formation of macrocycle 39 gold(III) chloride turned out to be the catalyst of choice [21],... 

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. 

Although these results point to the action of chloride ions as nucleophiles, it might be argued (Dahn and Gold, 1963b) that, when the less nucleophilic water molecules are the only available reagents, unimolecu-lar decomposition of the diazonium ion occurs. Measurements of catalysis by bromide and iodide ions (Albery and Hyde, 1966) have allowed... 

A major hurdle towards understanding the unusual low temperature CO oxidation activities of supported Au catalysis is that there is a wide variation in the reported CO oxidation activities over these catalysts [5]. This arises because these catalysts are very sensitive to the preparation procedures. Most supported Au catalysts are prepared with the chloride-containing chloroauric acid precursor, although there have been recent attempts to prepare these catalysts with precursors such as dimethyl gold acetonate [6], gold-phosphine complexes [7], and gold ethylene diamine complexes [8]. This discussion will focus on the complications that arise with chloride-containing precursors. 

The attractive combination of transition metals and iodobenzene diacetate as strong oxidant allowed the subsequent development of additional diamination reactions. For example, in the presence of a conventional ttiphenylphosphino-gold(l)chloride complex, intramolecular diamination using ureas as nitrogen sources proceeds within a gold(l/lll) cycle with results that are comparable to the mentioned paUadium(II/IV) catalysis [99]. However, as the initial step consists of an anti-aminoauration, this reaction provides the opposite stereochemistry with respect to the overall product configuration. 

In 2000, Hashmi reported one of the first applications of gold(m) chloride to homogeneous catalysis. In this report, several examples of intramolecular and intermolecular cyclisations involving, carbon-carbon and carbon-oxygen bond formation, were efficiently promoted by gold at room temperature and at low catalyst loading. The authors reported a cyclo-isomerisation/dimerisation of allenyl ketones and a,(3-unsaturated ketones (Scheme 16.50). This one-pot transformation was catalysed by 1 mol % of gold(m) chloride and proceeded at room temperature. 

Gold(in).—Several papers have appeared on the reactions of [AuClJ" with mono-and di-carboxylic acids. In general the reduction is preceded by a substitution reaction at the oxidant. Preliminary data indicate that the chloride-ion dependence of the rates of reduction of methyl and dimethyl malonates varies markedly between the two substrates. Confirmation of this aspect is provided in a later paper where the catalysis by Cl is attributed to an interaction of the chloride ion with a hydrogen on the central CHX moiety of the substituted malonic acid. Second-order rate constants increase in the manner k = k +k [C ] and a mechanism of reaction involves reaction of [AuCl and [AuCljCOH)]" with the free acid and the chloro adduct. In the rate-determining electron transfer [AuC ]-, Au , and COj are produced. 

Homogeneous catalysis by gold is not new [13,19], however, and even before 2004 there were about a hundred published papers on this topic, the first being the chlorination of naphthalene to octachloronaphthalene using AuCl or AuClj, in 1935 [2]. The gold compounds were among the most active catalysts (but it is now known that other Lewis acid catalysts, such as ferric chloride, are superior for this reaction). 

The first significant practical demonstration of the commercial relevance of catalysis by gold was carried out by Hutchings, then working in Sonth Africa gold catalysts supported on activated carbon were found to be abont three times more active than commercial mercuric chloride catalysts for vinyl chloride prodnction (see Figure 6.5) and to deactivate much less rapidly than other... 

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