Gold catalysis activation

Hashmi et al. also applied gold catalysis to the isochromene derivatives. This paper also reported a benzylic C-H activation that provided unprecedented dimerization from the formation of eight new bonds [110]. 

Through a combination of experimental and theoretical calculations, Corma and coworkers proposed a mechanism for the hydrogenation of oleftnic molecules by gold catalysis. In this study, Au(III)-Schiffbase complexes proved to be as active as the corresponding Pd complexes. Some gold(III) intermediate species were proposed but were not detected [192]. 

The special nature of gold chemistry and gold catalysis is now known and many applications can be based on the low-temperature activity of supported gold compared to that of other metals [196]. 

Figure 3.1 Four important conditions for catalysis by gold (metal-oxide junction, water, OH- and size of the Au particles or tubes). The figure by each circle is the diameter (nm) of the gold particles or tubes. Gold turns out to be catalytically very active, provided that at least two of the four conditions are fulfilled. For example, in CO oxidation at room temperature even unsupported gold is active in the presence of alkaline (OH-) water (H20).

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. 

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. 

Apart from these two examples, in the rest of the olefin activations, gold coordinates to the olefin, turning it susceptible to nucleophilic attack. The early examples of alkene functionalization by gold catalysis (equation 144) focused on the intermolecular addition of 1,3-diketones to styrenes. " An intramolecular version with ketoamides to yield pyrrolidinones was later developed and followed by the intermolecular addition of phenols (equation 145) and carboxylic acids to double bonds,a work that included an example of intramolecular addition of an aliphatic alcohol to an olefin. 

To increase the effectiveness of recent attempts to combine experimental work using real and model catalysts with theoretical calculations, the effect of moisture should also be considered. Most work in surface science occurs in UHV conditions, while measurements of the activity of real catalysts are conducted in fixed bed flow reactors containing between 1 and 10 ppm moisture. The tolerance of gold catalysis to water is an unexpected advantage, which is particularly valuable in developing new catalyst systems for pollution control and other applications. 

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... 

Despite different reaction mechanisms, similar activation energies (47.0 kj mol" for gold catalysis and 49.6 kJ mol for enzymatic catalysis) were found for the corresponding rate determining steps [20, 22]. 

The proposed reaction mechanism is shown in Scheme 6.75. The nitroalkene moiety of bifunctional ortAo-alkyne-substituted nitrostyrenes 159 is activated through hydrogen bonding with catalyst 160 to incorporate the stereoehemieal information in the first AFC reaction. Then the alkyne is activated under gold catalysis to affect the seeond AFC/ring expansion cascade. 

For years, gold was considered as a poorly active catalyst. In 1987, Haruta and his group discovered that gold particles were exceptionally active in the reaction of CO oxidation at room temperature (RT) and even much below, provided that the particles were smaller than 5 nm and supported on a reducible oxide support. Since then, gold has been extensively studied, and in many other reactions. For more information, we refer to several reviews [4-6], special issues in Applied Catalysis A 291 (2005) and Topics in Catalysis 44(1-2) (2007), and also a book on gold catalysis [7]. 

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