Metal ligand complex, acting nucleophile

Imidazole also acts as a substrate-competitive inhibitor, forming both binary complexes with LADH, and ternary complexes in the presence of coenzyme. X-Ray studies show that imidazole also binds to the. catalytic zinc by displacing the water molecule.1361 The presence of imidazole at the active site also enhances the rate of carboxymethylation14658 of Cys-46 with both iodoacetate and iodoacetamide.1420 This enhancement of alkylation has become known as the promotion effect .1421 Imidazole promotion also improves the specificity of the alkylation.1422 Since Cys-46 is thought to be alkylated as a metal-thiol complex, imidazole, on binding the active site metal, could enhance the reactivity by donating a electrons to the metal atom, which distributes the increased electron density further to the other ligands in the coordination sphere. The increased nucleophilicity of the sulfur results in promoted alkylation.1409... 

Summary The reactions of undecamethylcyclohexasilylpotassium (1) with various transition metal compounds were examined. Depending on the metal conqiound, 1 can act in four ways (a) as a nucleophile, (b) as a donor ligand, (c) as a strong base, and (d) as a reducing agent. Only (a) and (b) lead to transition metal silyl complexes, while (c) and (d) bring on unwanted side reactions. 

Other efficient catalysts for the aryl amination reaction include the (NHC)-Pd(allyl)Cl series that bear the same metal/ligand ratio of 1 1 and allow excellent conversions to products at temperatures as low as room temperature [78]. Alkoxide bases lacking p-hydrogens (amylates and terf-butoxide) have a dual action in this system as they activate the catalyst through nucleophilic attack on the palladium allyl moiety and act as an efficient base for the catalytic process. The complexes were successfully used in the key step of a Cryptocarya alkaloid synthesis [79]. 

However, the formation of the metal-carbene complex was not observed in pure, halide-free [BMIM][Bp4], indicating that the formation of carbene depends on the nucleophilicity of the ionic liquid s anion. To avoid the formation of metal-carbene complexes by deprotonation of the imidazolium cation under basic conditions the use of 2-methyl-substituted imidazolium is frequently suggested. However, it should be mentioned here that strong bases can also abstract a proton to form the vinyl imidazolidene species which may also act as a strong ligand to electrophilic metal centers. 

The idea of a simple metal complex acting as a nucleophile is conceptually relatively simple. All that is required is a metal centre (the metal and its coligands) which is relatively electron-rich, and which has a vacant site, or a potentially vacant site (via rapid dissociation of a ligand). This is typified by the reaction shown in (2) between the five-coordinate, square-based pyramidal [Co(dmgH)2py] (py = pyridine, dmgH = dimethylglyoxime) and RX. 

Overall, late transition metals, such as Au(III), Rh(III), Pd(II), and Pt(II), with a variety of N-donor and O-donor ligands were often found to have electrophilic transition states. Ru(II) and Ir(III) with N-donor and O-donor ligands as well as [Cp (PMe3)IrMe] were found to act in an ambiphilic manner towards methane. Despite substitution-type transition states having both a Lewis acid metal and a Lewis base ligand that interacts with the CH bond of methane, these transition states are not always ambiphilic. For example, in the transition state between d° (Cp )2ScMe and methane, the (Cp )2ScMe complex acts as a nucleophile. Other nucleophilic transition states involved methane reaction with W(II), Ir(I), and Rh (I) complexes. 

In general with a peptide not having polar side chains, two pathways are possible for the hydrolysis of the metal-substrate complex. An external water molecule activates the amide carbonyl or the adjacent coordinated OH group acts as a nucleophile on the amide function. This process corresponds to an intramolecular hydrolysis by a metal-ligand. In other words, process A involves a carbonyl coordination intermediate whereas in process B a carbonyl attack by the coordinated hydroxide takes place. These pathways can be followed with 0-enriched water. 

Numerous examples of nucleophilic attack on coordinated nitrile ligands are found in the literature, particularly when the transition metal is platinum(II).224 The nucleophilic attack of two equivalents of CIO I2CII20 on the electrophilic nitrile carbon atoms of both nitrile ligands in cis-or /r<7H.v-[PtC12(RCN)2] (R = Et, Prn, Pr1, Bu p-CF3C6H4, p- and o-MeC6H4) affords the corresponding A2-l,3-oxazoline complexes ((76) and (77), respectively), in which the heterocycle acts as a... 

Even more interesting is the observed regioselectivity of 37 its reaction with 2, 3 -cCMP and 2, 3 -cUMP resulted in formation of more than 90% of 2 -phosphate (3 -OH) isomer. The postulated mechanisms for 37 consists of a double Lewis-acid activation, while the metal-bound hydroxide and water act as nucleophilic catalyst and general acid, respectively (see 39). The substrate-ligand interaction probably favors only one of the depicted substrate orientations, which may be responsible for the observed regioselectivity. Complex 38 may operate in a similar way but with single Lewis-acid activation, which would explain the lower bimetallic cooperativity and the lack of regioselectivity. Both proposed mechanisms show similarities to that of the native phospho-monoesterases (37 protein phosphatase 1 and fructose 1,6-diphosphatase, 38 purple acid phosphatase). 

The mechanism sketched in Figure 3.18 implies that the starting complex L M has a free coordination site (or a readily replaceable ligand) and can act as an electrophile. Therefore reactions of this type will occur more readily with increasing nucleophilicity of the ylide and increasing electrophilicity of the metal complex... 

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