Reactions Catalysed by Two Metals

Cycloaddition of aUcynes catalysed by transition metals is one of the most efficient and valuable ways to prepare benzene and pyridine systems [12], Among the possible catalytic systems able to catalyse this reaction, cobalt and iron complexes containing NHCs as ligands have shown high catalytic activity in the intramolecular cyclotrimerisation of triynes 36 (Scheme 5.10) [13]. The reaction was catalysed with low loading of a combination of zinc powder and CoC or FeClj with two or three equivalents of IPr carbene, respectively. 

Since many of the transformations undergone by metabolites involve changes in oxidation state, it is understandable that cofactors have been developed to act as electron acceptors/ donors. One of the most important is that based on NAD/NADP. NAD+ can accept what is essentially two electrons and a proton (a hydride ion) from a substrate such as ethanol in a reaction catalysed by alcohol dehydrogenase, to give the oxidized product, acetaldehyde and the reduced cofactor NADH plus a proton (Figure 5.2). Whereas redox reactions on metal centres usually involve only electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer but... 

Instead of adding two hydrogen atoms to an alkynyl silane we could add H and SiMe3 to a simple alkyne by hydrosilylation (addition of hydrogen and silicon). This is a cis addition process catalysed by transition metals and leads to a tram (E-) vinyl silane. One of the best catalysts is chloroplatinic acid (H2PtCl6) as in this formation of the E-vinyl silane from phenylacetylene. In this case photochemical isomerization to the Z-isomer makes both available. Other than the need for catalysis, this reaction should remind you of the hydroboration reactions earlier in the chapter. The silicon atom is the electrophilic end of the Si-H bond and is transferred to the less substituted end of the alkyne. 

An enzymic counterpart of these complex base-catalysed rearrangements of sugars may be the reaction catalysed by 4-phospho-3,4-dihydroxy-2-butanone synthetase. The enzyme catalyses the formation of the eponymous intermediate in secondary metabolism from ribulose 5-phosphate. Labelling studies indicated that C1-C3 of the substrate became C1-C3 of the product, that H3 of the substrate derived from solvent and that C4 was lost as formate. X-ray crystal structures of the native enzyme and a partly active mutant in complex with the substrate are available. The active site of the enzyme from Met ha-nococcus jannaschii contains two metals, which can be any divalent cations of the approximate radius of Mg " or Mn ", the two usually observed. Their disposition is very reminiscent of those in the hydride transfer aldose-ketose isomerases, but also to ribulose-5-phosphate carboxylase, which works by an enolisation mechanism, so the enolisation route suggested by Steinbacher et al. is repeated in Figure 6.14, as is the Bilik-type alkyl group shift, for which an equivalent reverse aldol-aldol mechanism cannot be written. 

For the mechanism of polyesterification reactions catalysed by metallic derivatives (for example titanium or tin compounds) two variants (variant A and variant B) are discussed. 

The recent developments on the metallation chemistry of oxazoles and benzoxazoles, isoxazoles and benzisoxazoles, pyrazoles and indazoles, thiazoles and benzo-thiazoles, and isothiazoles, benzo[c]isothiazoles, and benzoMisothiazoles have been reviewed. The two-decade history of catalytic carbon-carbon bond formation via direct borylation of alkane C-H bonds catalysed by transition metal complexes has been reported. The alkane functionalization via electrophilic activation has been underlined. " Recent advances of transition-metal-catalysed addition reactions of C-H bonds to polar C-X (X=N, O) multiple bonds have been highlighted and their mechanisms have been discussed. The development and applications of the transition metal-catalysed coupling reactions have been also reviewed. - ... 

The discovery of self-splicing introns showed that RNA could catalyse chemical reactions. Yet, unlike proteins, RNA has no functional groups with pKa values and chemical properties similar to those considered to be important in protein-based enzymes. Steitz and Steitz (1993) postulated that two metal ions were essential for catalysis by ribozymes using a mechanism similar to DNA cleavage, in which a free 3 OH is produced. They proposed,... 

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