Metal ions promoted reactions

Metal Ion-Promoted Reactions of Thiols. Metal ion-promoted reactions of thiols have been reviewed (53). The bulk of the coverage concerns metal ion promoted aspects of sulfur chemistry. The main topics of interest are the formation of sulfenamides, sulfides, and disulfides using metal-mediated reactions. 

Metal-ion-promoted reactions of organo-sulphur compounds. D. P. N. Satchell, Chem. Soc. Rev., 1977,6, 345-371 (65). 

Bob s research interests and knowledge across chemistry were great. Throughout his career he retained an interest in biomimetic chemistry, specifically the study of metal ion-promoted reactions and reactions of molecules activated by metal ion coordination. His early interests in carbohydrate chemistry inspired him to study metal ion catalysis of both peptide formation and hydrolysis as well as studies in inorganic reaction mechanisms. He was particularly interested in the mechanisms of base-catalyzed hydrolysis within metal complexes and the development of the so-called dissociative conjugate-base (DCB) mechanism for base-catalyzed substitution reactions at inert d6 metal ions such as Co(III). 

Rigorously most of the reactions described ab we should not be called metal ion-catalyzed reactions but rather metal ion-promoted reactions, since the metal ions are consumed stoichiometrically in most instances. 

Compounds of aza macrocycles are prepared by three main types of procedure (1) conventional organic synthesis of the ligand, (2) metal-ion-promoted reactions, involving condensation of non-cyclic components in the presence of a suitable metal ion, and (3) modification of a compound prepared by methods (1) or (2). 

A great variety of aza macrocycle complexes have been formed by condensation reactions in the presence of a metal ion, often termed template reactions . The majority of such reactions have inline formation as the ring-closing step. Fourteen- and, to a lesser extent, sixteen-membered tetraaza macrocycles predominate, and nickel(II) and copper(II) are the most widely active metal ions. Only a selection of the more general types of reaction can be described here, and some closely related, but non metal-ion-promoted, reactions will be included for convenience. The reactions are classified according to the nature of the carbonyl and amine reactants. 

Although this was one of the earliest reported metal-ion-promoted reactions yielding aza macro-cycle complexes, the compounds can often be made without the mediation of metal ions. Thus, salts of the macrocycle (6) are formed rapidly by reaction of [enH]+ with acetone, or with 2-methylpent-3-en-2-one (mesityl oxide), and this reaction has been generalized to the reaction of a variety of a,j3-unsaturated carbonyl compounds, yielding [14]4,ll-dieneN4 macrocycles with varied substituents on the six-membered chelate rings. The [14]4,14-dieneN4 macrocycle compound (4) has also... 

Metal ions are Lewis acids and as such catalyze many reactions which are also subject to specific acid catalysis by the proton. Reactions in which metal ions are involved are often best described as metal ion-promoted reactions as the products of the reaction often remain bound to the metal ion. Although scattered references to metal ion-promoted reactions are to be found in the early literature it was not until the late 1950s that such reactions began to be studied in detail. A strong driving force has been the realization that some 30% of enzymes are metalloenzy-mes or require metal ions for activity. Many of the reactions dealt with in this article have been studied in an attempt to delineate possible mechanisms for enzymic processes. 

Some interesting conclusions can be drawn from the thermodynamic parameters for base hydrolysis of the MA+ complexes (Table 19). The rate enhancements in the metal ion-promoted reactions arise from more positive values of AS41 and, in general, lower enthalpies of activation. In addition, there is a close correspondence between values of log XMA+ and AG (Figure 5). The general trends in AH and AS in the metal-promoted reactions can be rationalized in terms... 

Some highly selective metal ion-promoted reactions have been observed with phenyl phos-phatosulfate (70).263 In both aminolysis and methanolysis reactions it was found that Mg11 promoted P—O bond fission, while Fein promoted S—O bond fission (Scheme 15). The selectivity is almost 100% in each case. 

Soft metal ions such as Hg11 and Ag1 have long been known to react readily with organo sulfur compounds, and a review of the metal ion-promoted reactions which can occur is available.17... 

Hendry, Philip, and Sargeson, Alan M., Metal Ion Promoted Reactions of... 

Heintz, Robert A., see Dunbar, Kim R. Helton, Matthew E., see Kirk, Martin L. Hendry, Philip, and Sargeson, Alan M., Metal Ion Promoted Reactions of Phosphate Derivatives 38 201... 

Metal ion promoted reactions are important-Zn +-catalyses of hydrolysis of a range of organic substrates, for example, phosphate and carboxylate esters and Schiff bases, have recently been reviewed, as has their relevance to biological catalysis, particularly of reactions of phosphates. ... 

Most metal ion-promoted reactions in which labile metal ions are employed involve a reaction scheme of the type shown in equations (2)-(5). There is a rapid pre-equilibrium formation of... 

Some of the earliest kinetic studies on metal ion-promoted reactions were carried out on metal ion-promoted decarboxylations of j8-oxo acids. The literature on this topic up to about 1974 has been reviewed. Much of the work has centred on oxaloacetic acid (HO2CCOCH2CO2H = H20xac) and its derivatives, a,a-dimethyl oxaloacetic acid and fluorooxaloacetic acid. Studies have also been made with acetonedicarboxylic acid (3-oxoglutaric acid), " dihydroxyfumaric acid, dihydroxytartaric acid, acetosuccinic acid, oxalosuccinic acid and 2-oxalopropionic acid (Figure 6). The decarboxylation of )3-oxo acids is of considerable biological importance, and in a number of cases metalloenzymes are involved. Similarities in the enzymatic and chemical processes stimulated early interest in these reactions as models for the enzymatic systems. 

Soft metal ions such as Hg and Ag have long been known to react readily with organo sulfur compounds, and a review of the metal ion-promoted reactions which can occur is available. The hydrolysis of thiol esters (equation 25), thione esters (equation 26) and thiolbenzimidate esters (equations 27 and 28) are all strongly promoted by Hg" in aqueous solution at pH <3... 

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