Supported Gold Complex Catalysts

Au111 supported on nanocrystalline ceria efficiently catalyses the homocoupling of boronic acids to give quantitative yields of biaryls in the presence 

Since the reactivity correlates with the amount of Au111 on the surface, it is assumed that the reaction is initiated by a twofold transmetallation from boron to Aum followed by reductive elimination of the biaryl compound. The catalytic cycle is completed when Au1 is re-oxidized to Au111. The reaction takes place in the absence of oxygen, and hydrogen can be detected by Raman spectroscopy. It also takes place in the absence of potassium carbonate but the catalyst is less stable. The TOF was at least 20 (calculated as the moles of boronic acid converted divided by two and by the moles of gold in the catalyst per hour). 

This is in contrast to the situation when palladium is used for the similar Suzuki reaction when the predominant products are from cross-coupling reactions. 

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Guzman, J. and Gates, B.C. (2003) Structure and reactivity of a mononuclear gold-complex catalyst supported on magnesium oxide. Angewandte Chemie International Edition, 42, 690-693. 

A mononuclear gold complex catalyst supported on MgO spectroscopic characterization during ethylene... 

Guzman, J. and B. C. Gates, Structure and Reactivity of a Mononuclear Gold-Complex Catalyst Supported on Magnesium Oxide , Angew. Chem. Int. Ed., vol 42, Issue 6, pp 690-3. 

Guzman J, Gates BC (2004) A mononuclear gold complex catalyst supported on MgO Spectroscopic characterization during ethylene hydrogenation catalysis. J Catal 226 111... 

MgO-supported mono-nuclear gold complex catalysts have been evaluated for ethene (or ethylene) hydrogenation at 353K [464]. The results provided evidence of the stability of the metal complex, complex-support interactions... 

Kozlova, A.P. Sugiyama, S. Kozlov, A.I. Asa-kura, K. Iwasawa.Y. (1998) Iron oxide supported gold catalysts derived from gold phosphor complex Au(PPh3) (NO3) State and structure of the support. J. Catalysis 176 426-438... 

The triethoxysilyl endgroup is a popular functional group to bind the catalyst to a polymeric support [238]. Polymeric supports include silica gel, MCM-41 (mesoporous silica gel) and ITQ-2 (delaminated zeolite) [247]. Corma et al. used this approach to synthesise gold(I) and palladium(II) NHC complexes for Suzuki cross-coupling reactions between iodobenzene and various arylboronic acids (see Figure 4.78) [247]. The results were very modest at 35-80% dependent upon the substitution pattern of the arylboronic acid. Yields with gold(I) catalysts were marginally better than those for palladium(II) complexes. 

In the book, the section on homogeneous catalysis covers soft Pt(II) Lewis acid catalysts, methyltrioxorhenium, polyoxometallates, oxaziridinium salts, and N-hydroxyphthalimide. The section on heterogeneous catalysis describes supported silver and gold catalysts, as well as heterogenized Ti catalysts, and polymer-supported metal complexes. The section on phase-transfer catalysis describes several new approaches to the utilization of polyoxometallates. The section on biomimetic catalysis covers nonheme Fe catalysts and a theoretical description of the mechanism on porphyrins. 

In this chapter, we have briefly reviewed the development of novel gold catalysts for WGS and PROX. These new catalysts are not those composed of gold and a pure oxide support. Instead, they usually contain relatively complex metal-support interfaces. These catalysts can be classified into three categories (1) a regular oxide support is modified by a metal oxide additive, followed by loading gold (2) a mixed... 

Espinet and coworkers examined gold complexes of ADC ligands containing a hydrogen-bonded backbone - denoted hydrogen-bond supported heterocyclic carbenes (HBHCs) - in cyclizations of 1,6- and 1,7-enynes and observed unusual selectivities [27a]. Catalysts 24a and 24b were found to fevor the endo cycliza-tion product in reactions of enyne 23, whereas previously known phosphite-(25) and NHC-based (26) catalysts favored the exo product (Scheme 16.9). Notably, the differing steric bulk of 24a and 24b did not appear to impact the exo endo ratio, suggesting that the observed selectivity depends primarily on the donor properties of the HBHCs. 

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