Catalytic gold catalysis

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). 

Rearrangements of 1,3-diynes (43) to dienynes (45a) and (45b) have been carried out catalytically a metal-carbene intermediate (44) is likely to be involved.41 Interestingly, gold catalysis provides mainly (45a) whereas platinum catalysis under a CO atmosphere yields predominantly (45b). 

Clearly, it has been shown that gold salts display considerable catalytic activity under moderate conditions and gold catalysis will likely provide for some more surprises. Thus, an extensive development of gold catalysis with numerous new applications is anticipated. 

J. Schwank, Catalytic gold Application of element gold in heterogeneous catalysis. Gold Bull. 16(4) (1983) 103. 

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... 

The analytical data show that gold catalysis and enzymatic catalysis allow fast and selective aerobic oxidation of glucose according to the same stoichiometry characterized by the formation of hydrogen peroxide as the by-product (Eq. (21.1)) [8]. However, it is not surprising that completely different catalytic systems adopt different reaction mechanisms as shown by the kinetic studies on commercial enzymatic preparations containing /wcose oxidase and catalase [13]. The results of the research support a Michaelis-Menten type mechanism where the kinetic... 

One of the most problematic questions in heterogeneous catalysis is the cooperative effect of different phases present in a given catalytic system and, in particular, the so-called metal-support interaction [15]. In the case of gold catalysis, interaction of the metal with an oxidic support seems to be of fundamental importance in determining the extraordinary reactivity observed during the low temperature oxidation of CO [14]. 

One of the most common problems in catalysis is to generate an active catalytic species that lives as long as possible. This goal can be achieved at least by two different ways by running the reaction under the mildest possible conditions or the robustness of the catalyst, for example against thermal decomposition, could be improved. In this section, we discuss three examples where the performance of gold catalysis could be improved, or the catalysis could even be conducted for the very first time, by using NHC-Au(I) complexes. 

The main catalytic systems used in gold catalysis are presented in the following sections. 

The success of homogeneous gold catalysis [1] was accompanied by intensive efforts to understand the mechanisms of the new conversions [2]. None of the many new gold-catalyzed reactions proceeds through only one transition state, we always deal with multistep mechanisms. Many of the reactions have the first steps in common, and then diverge at a later stage of the sequence of elementary steps of the catalytic cycles, which ultimately leads to the manifold of different products that can be obtained. 

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]. 

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