Au-catalyzed reactions

A spiroketal bisphosphine (R R,R)-38a derived chiral Au complex was found to be an efficient catalyst for asymmetric cyclopropanation of diazooxindoles with a broad range of aUcenes, providing a highly diastereo- and enantioselective approach for spiro cyclopropyloxindoles (Scheme 41) [47]. These results further demonstrate the special advantage of rigid spiro ligands in Au-catalyzed reactions. 

Au-Catalyzed reaction of propargyl vinyl ethers furnished tri- and tetrasubstituted furans in high yields. The reaction proceeded through cyclization of 2-allenyl-13-dicarbonyl intermediates produced from propargyl-Claisen rearrangement <05OL3925>. 

While trying to use allylic alcohols as nucleophiles in Au-catalyzed reactions, electrophilic reactivity was unexpectedly observed and the potential for a variety of mechanistic pathways responsible for this reactivity was intriguing. Au-complexes are typically reported as soft, carbophiUc 7t-acids, but for the observed reaction, it seemed more reasonable that a cationic mechanism whereby the Au-complex functioned as a more traditional oxophilic Lewis acid ° was operative. This piqued our interest, and we decided to change the goals of the project to see where it would take us. Fortunately, pursuing these Au-catalyzed dehydrative transformations developed from a single observation into a research program (vide infra). 

Pig. 7. Substrate reduction catalyzed by Mo-nitrogenase. Mo-nitrogenase is depicted as the box containing the Fe protein (Fe-p) and MoFe protein (MoFe-p). The required inputs for catalyzed reduction are shown at left. The electron donor is usually sodium dithionite (8204 ) in vitro and either ferredoxin (Fd) or flavodoxin (Fid) in vivo. Other mandatory inputs are protons (H" ) and magnesium adenosine triphosphate (MgATP). The most commonly used substrates (above the horizontal line) and their products (below the hne) are shown at right. The numbers in parentheses are the number of electrons required to effect each reduction shown. The horizontal arrow indicates that CO is an inhibitor of aU catalyzed reactions except for the production of H2 from protons. 

The same group demonstrated that a variety of mono- and disubstituted 1-oxyindolizine derivatives 3S2 could be readily synthesized via a facile Ag-catalyzed cydoisomerization of skipped propargylpyridines 3S1 (Scheme 9.121) [300, 301]. It was suggested that this Au-catalyzed reaction involved a 5 -endo-d cyclization of the alkyne 351 activated by a jt-philic metal. Formation of the indolizine product 352 was accomplished via a subsequent proton transfer in cyclic vinylmetal zwitterion 354 (Scheme 9.122). It should be noted that a variety of transition metals, such as Au(I), Au(III), Cu(I), Pt(II), and Pd(II), were shown to catalyze this transformation with variable degrees of efficiency. 

The support clearly affects the rate of some Au-catalyzed reactions. The support can play various roles. First, the support can change the nature of the metal particle adhesion to the surface, and thus change the metal particle size that forms, as was discussed above. Second, the support can act to strain the metal-metal bonds, which would significantly change the electronic properties of the metal atoms near the interface and thus their catalytic properties. Third, there can be electron transfer between the metal and the support, which would change the electronic properties of the metal. Neutral and positively and negatively charged Au clusters have all been proposed to be catalytically active for specific reactions in the literature. Lastly, the interface between the metal and the support can act to create unique bifunctional sites which demonstrate enhanced reactivity. We discuss the last three effects below. The effect of particle size on the catalytic performance was discussed in detail in the previous section. 

Jimenez-Nunez, E. Claverie, C. K., Nieto-Oberhuber, C., Echavarren, A. M. (2006). Prins cyclizations in Au-catalyzed reactions of enynes. Angewandte Chemie International Edition, 45, 5452-5455. 

FIGURE 14.7 Substrate saturation curve for au euzyme-catalyzed reaction. The amount of enzyme is constant, and the velocity of the reaction is determined at various substrate concentrations. The reaction rate, v, as a function of [S] is described by a rectangular hyperbola. At very high [S], v= Fnax- That is, the velocity is limited only by conditions (temperature, pH, ionic strength) and by the amount of enzyme present becomes independent of [S]. Such a condition is termed zero-order kinetics. Under zero-order conditions, velocity is directly dependent on [enzyme]. The H9O molecule provides a rough guide to scale. The substrate is bound at the active site of the enzyme. 

The reaction was studied for all coinage metal nanoparticles. In the case of GMEs the rate follows zero-order kinetics with IT for all the coinage metal cases. The observed IT for the Cu catalyzed reaction was maximum but its rate of reduction was found to be minimum. Just the reverse was the case for Au and an intermediate value was obtained for the Ag catalyzed reaction (Figure 7). The adsorption of substrates is driven by chemical interaction between the particle surface and the substrates. Here phe-nolate ions get adsorbed onto the particle surface when present in the aqueous medium. This caused a blue shift of the plasmon band. A strong nucleophile such as NaBH4, because of its diffusive nature and high electron injection capability, transfers electrons to the substrate via metal particles. This helps to overcome the kinetic barrier of the reaction. 

Besides the already described Pd-, Rh- and Ru-catalyzed transformations, many other transition metals have also been used in domino processes, albeit to a lesser extent. Co- and also Ni-catalyzed transformations constitute the largest group in this section, though other examples include Cu, W, Mo, Fe, Ti, Cr, Au, Pt, Zr, and Lanthanide-catalyzed reactions. 

A very new example for the combination of an Au -catalyzed [3,3]-rearrangement and a Nazarov reaction has been disclosed by Zhang and coworker. Thus, cyclopen-tenones could be easily achieved by converting en-ynyl acetates in the presence of AuCl(PPh3)/AgSbF6 [320]. 

Another example of Au-catalyzed was reported using alkynyl cyclopropyl ketones as a starting material. Trisubstituted furans were given in high yields under mild condition via a domino reaction process and an example is given below <06AG(I)6704>. 

Hypothetical (carbene)gold(i) structures of intermediates and reaction coordinates have been calculated (B3LYP/ 6-31G and LAN2DZ levels) for (H3P)Au+-catalyzed cyclization reactions of terminal enynes. The endocyclic skeletal rearrangement reactions were found to proceed exclusively via cyclopropylcarbene complexes.240... 

Hydroarylations of alkynes are catalyzed by gold complexes and these bear some resemblance to the Fujiwara Pd-catalyzed reaction. In general, when using gold chemistry, better Z/E selectivities are observed compared with palladium, lower catalyst loadings and milder conditions (neutral not TFA) are used. The mechanism involves the attack of ArH on the Au-coordinated alkyne. Flowever, electron-poor acetylenes only appear to work with palladium chemistry (Equations (75) and (76)).72... 

The absolute configurations of many branched products prepared by Ir-catalyzed allylic substitution have been determined. The steric course of aU corresponding reactions follows the general rule described in Scheme 9.8. 

Stereochemical probes of the specificity of substrates, products, and effectors in enzyme-catalyzed reactions, receptor-ligand interactions, nucleic acid-ligand interactions, etc. Most chirality probe studies attempt to address the stereospecificity of the substrates or ligands or even allosteric effectors. However, upon use of specific kinetic probes, isotopic labeling of achiral centers, chronfium-or cobalt-nucleotide complexes, etc., other stereospecific characteristics can be identified, aU of which will assist in the delineation of the kinetic mechanism as well as the active-site topology. A few examples of chirality probes include ... 

Figure 6.3 Progress curve of the DERA-catalyzed reaction of ClAA with two equivalents AA (as analyzed by NMR spectroscopy). The y-axis shows the relative molar concentration of the reaction components expressed as arbitrary units (AU). The products and by-products are...

The Au-catalyzed glycerol oxidation was influenced by the kind of support, the size of Au particles and the reaction conditions such as concentration of glycerol, p02 and molar ratio of NaOH to glycerol. As metal oxide supports showed inferior selectivity to glyceric acid compared to carbons, due to successive oxidation and C—C bond cleavage to form di-adds such as tartronic acid and glycolic acid, research has focused on Au NPs supported on carbon, as in the case of ethylene glycol oxidation [182]. Indeed, the catalytic activity was influenced by the kind of carbon support in terms of porous texture [183]. 

The phosphine-substituted derivative Co3( i-dppm)(CO)7 2( i3 p3-CC=CC) (291) was initially isolated from the Cul-catalyzed reaction between Co3( i3-CBr)( i-dppm)(CO)7 and Au(C=CC=CH)(PPh3), and can be prepared in 39% yield if Au(PPh3) 2(p-C=CC=C) is used. Presumably the basic solvent allowed dissociation of both the H and Au(PPh3) groups... 

Au-catalyzed cyclization of alkoxyalkyl o-ethynylphenyl sulfides 115 affords benzothiophenes 117. The reaction involves Au-mediated intramolecular cyclization and subsequent migration of alkoxyalkyl group of the resulting 116 (Scheme 26) <2006AGE4473>. 

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