Gold catalysis Reaction

Krause et al. worked on the conversions of 2-hydroxy-3,4-dienoates in the corresponding tri- and tetrasubstituted 2,5-dihydrofuranes by treatment with HCl gas in chloroform. Since this reaction was not accessible with acid-labile substrates [30,31], these conversions were tested through gold catalysis, obtaining better reaction rates and transformations in more difficult substrates compared to the well-established Ag(I)-promoted method [32]. 

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. 

The reaction worked with both internal and terminal alkynes (except silylated alkynes) and in many solvents, even in the neat alcohol added [105]. The mechanism proposed involved two catalytic cycles first, gold catalysis would lead to dihydro-furan by a fast intramolecular reaction then, the subsequent slower intermolecular reaction would be produced by the addition of alcohol to the enol ether to deliver a ketal (Scheme 8.18). 

Dyker et al. reported an effective combination of an Ugi four-component reaction and gold catalysis to build highly functionalized isoindoles and dihydroisoquinolines with relatively good stereoselectivity [116] (Scheme 8.21). 

Transformations achieved by gold catalysis are more and more complex, with a higher number of bonds formed and many tandem reactions. 

The Conia-ene reaction was brought to the held of gold catalysis by Toste et al.29 It provided very mild conditions for this cyclization to a functionalized hve-membered ring (Scheme 12.14). 

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. 

It is interesting to note that although this was one of the first gold-catalyzed reactions, and despite the great boost in the last few years, gold asymmetric catalysis is stiU an underdeveloped field. 

In recent years, the unexpected observation of highly active Au as a low temperature CO oxidation catalyst59,60 has initiated extensive research activity into the use of supported gold for liquid phase oxidation reactions. In general, the adsorption characteristics and catalytic properties of Au depend crucially on particle size, which can be controlled by the preparation method and the support.61-66 The crucial question involving gold catalysis, which as yet has not been fully answered, is the concept of why Au nanoparticles exhibit such radically different behaviour than bulk Au.67-70... 

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