Ligands nucleophilic reaction

The pre.sent account follows a Journey in this arena from solution calorimetric studies dealing with nucleophilic carbene ligands in an organometallic system to the use of these thermodynamic data in predicting the feasibility of exchange reactions to applications in homogeneous catalysis. 

Irreversible inhibition is probably due to the alkylation of a histidine residue.43 Chymotrypsin is selectively inactivated with no or poor inhibition of human leukocyte elastase (HLE) with a major difference the inactivation of HLE is transient.42,43 The calculated intrinsic reactivity of the coumarin derivatives, using a model of a nucleophilic reaction between the ligand and the methanol-water pair, indicates that the inhibitor potency cannot be explained solely by differences in the reactivity of the lactonic carbonyl group toward the nucleophilic attack 43 Studies on pyridyl esters of 6-(chloromethyl)-2-oxo-2//-1 -benzopyran-3-carboxylic acid (5 and 6, Fig. 11.5) and related structures having various substituents at the 6-position (7, Fig. 11.5) revealed that compounds 5 and 6 are powerful inhibitors of human leukocyte elastase and a-chymotrypsin thrombin is inhibited in some cases whereas trypsin is not inhibited.21... 

Figure 11 Scheme 3 — ligand-associated nucleophilic reaction. 

The nucleophilic reaction of hydroxide with carbonyl ligands of transition metal complexes,... 

Nucleophilic reactions with coordinated NO are illustrated by the well known reversible reaction of hydroxide ion with the nitrosyl ligand of... 

Nucleophilic addition to acyclic (diene)Mo+ cations has been examined. For (isoprene) Mo(CO)2L (L = Cp, Cp, In), the regioselectivity for nucleophilic attack has been found to depend on the nature of the nucleophile, the ligand L, the reaction solvent and the temperature21,813 833 193. The generation and in situ reactivity of transoid acyclic (diene)molybdenum and tungsten cations with nucleophiles has been previously mentioned (Section IV.C.2). 

The polarization effects of metals have a substantial impact particularly in two areas the promotion of the hydrolysis and other nucleophilic reactions of chelated ligands and the enhanced ionization of coordinated acidic ligands. Their importance has encouraged extensive attacks on the effects. 

Once the oxidative-addition reaction of dioxygen to metal d -ions has occurred, the essentially electrophihc dioxygen becomes a nucleophilic peroxide ligand. Since the oxidation of substrates is associated with electron transfer from the substrate to the oxidant, i.e. in this case the dioxygen adduct, effective oxygenations require a further activation to transform the nucleophihc peroxide into an electrophihc species prior to the oxygen transfer. 

Notes Ligand. Used for Pd-catalyzed nucleophilic reactions at allylic positions. See also, Dodd. With Br forms a useful brominating agent, l,2-bis(diphenylphosphino)ethane tetrabromide [7726-95-6],1 This reagent is useful for the conversion ... 

The complexation of an alkene to a metal ion strongly affects its reactivity namely, the double bond is activated toward nucleophilic attack. Ligands decreasing the electron density on the double bond accelerate the hydrogenation reaction. Electron-withdrawing substituents similarly facilitate hydrogenation of alkenes (PhCH=CH2 > RCH=CH2).29... 

A major class of sulfur-containing ligands is obtained by the general reaction of carbon disulfide with various nucleophiles. These ligands can be classified into two categories the uninegative 1,1-dithioacids and the dinegativc 1,1-dithiolates (Fig. 1). 

Nucleophilic attack on a rt-allyl ligand of a metal complex occurs in general at one of the terminal carbons to afford allylated products. The attack, however, may be directed to the central carbon atom of the 7i-allyl group to produce cyclopropyl derivatives by appropriate choice of nucleophile, metal ligand and reaction conditions (equation 33). A variety of nucleophiles (pA"a > 20) including ester and ketone enolates and a-sulfonyl carbanions react with... 

Nucleophilic reactions, in particular ligand replacement by EDTA and polyamine ligands, readily occur with Ni and Cu11 peptide complexes, the most reactive nucleophiles being those that contain an amine group to give an associative-type intermediate. The presence of histidine as the third residue in a peptide appears to reduce the susceptibility of Cu complexes to nucleophilic attack. 

This type of map can be used to discuss the different types of nucleophilic displacement reaction. Using the simplified version shown in Fig. 2 we have already seen that SN1 reactions, for instance the solvolysis of triarylmethyl halides, go through the separated ions in the top right-hand corner (Swain et al., 1953 Ritchie, 1971). At the opposite extreme, nucleophilic substitution at centres where the number of ligands can be increased may proceed over the bottom left-hand corner of the diagram. Examples are acyl transfer reactions and substitution at tetrahedral phosphorus centres (Alder et al., 1971) as well as substitution at square planar transition metal compounds (Wilkins, 1974). The nucleophilic reactions studied by Ritchie (1976), for which the rate... 

In contrast, those cluster compounds with an 18-electron environment at the central heterometal do not undergo nucleophilic addition reactions. Ligand substitution reactions at both the central heterometal and the peripheral gold atoms are observed to occur for both 16- and 18-electron cluster compounds, as in the reactions shown below ... 

The yellow CpFe1 (//6-arene) salts (most commonly BF4 or PF6 ) are usually stable up to at least 200 °C, are stable in concentrated sulfuric acid, and are very resistant towards oxidation (until recently, it was believed that they could not be oxidized [23] vide infra). They are not easy to reduce either [23] (vide infra). The chloride salts [CpFe+( f -arene) Cl- are water-soluble they are formed upon hydrolysis following ligand-exchange reactions between ferrocene and the arene in the presence of aluminum chloride [21]. Such aqueous solutions may sometimes be directly used for nucleophilic reactions [22] (vide infra). The BF4- salts are also sometimes quite soluble in water, but the PF6- salts are much less so. Electrophilic reactions that are readily undergone by the free arenes, such as Friedel-Crafts reactions, are no longer possible on the CpFe+( /6-arene) complexes [19, 23]. On the other hand, a range of nucleophilic reactions that are impossible or very difficult to carry out with free arenes become possible under ambient or mild conditions with the CpFe+()/6-arene) complexes (Scheme 2) [16-20]. 

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