Thiol complexes, metal

Although there is little doubt that the electron transfer reaction (Reaction 2) is involved in the over-all reaction (21), the suggestion that quantitative yields of disulfide (13) arise from the dimerization of thiyl radicals is inconsistent with the observed behavior of other free radicals (24). It seems preferable to suggest that some kind of coordination occurs as a prerequisite to the transfer of electrons (12,15). In this case, metal-thiol complexes should be formed as intermediates in the oxidation, in which the metal acts not only as an electron acceptor but also to locate the resultant thiyl entities in close proximity, thereby favoring dimerization reactions and producing disulfide. The electrons gained by the metal may then be passed on to an oxygen molecule. The over-all reaction may be represented as... 

Some evidence to support this scheme has been obtained. Thus the catalytic activity of metals has been found to be associated with the formation of soluble metal-thiol complexes (13), and the geometric configuration of thiols has been found to affect the over-all rate of oxidation,... 

Further evidence has been obtained to support the contention that the active catalysts are metal complexes dissolved in solution. With experiments reported in Table II, the kinetics of oxidation under standard conditions in the presence of various metal salts are compared with the rates of reaction when solid residues have been filtered from solution. The agreement between the rates in Cases 1 and 3 of Table II (where the amount of metal available is dictated by the solubility of metal complexes) shows that solid precipitates play little or no part in catalysis in all the systems studied. The amount of metal in solution has been measured in Cases 2 and 3 metal hydroxide complexes (Case 2) are not as soluble as metal-thiol complexes, and neither is as soluble as metal phthalocyanines (19). The results of experiments involving metal pyrophosphates are particularly interesting, in that it has previously been suggested that cobalt pyrophosphates act as heterogeneous catalysts. The result s in Table II show that this is not true in the present system. 

In the light of the above discussion, it is necessary to redefine the criteria useful for describing catalytic activity. The coordination atmosphere of any given metal may be expected to affect the catalytic activity by influencing the solubility of the metal. If the metal complex, added to the reactant solution, can be replaced by thiyl entities, colored metal-thiol complexes may be produced, and the rate of reaction in all cases should correspond to Case 3 for adding simple metal salts (Table II). If the metal complex cannot be replaced, the rate of reaction may be quite different and will depend on the ease with which an electron... 

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... 

Early work on the catalytic autoxidation of carboxythiols confirmed the effectiveness of manganese, iron, cobalt, copper, and arsenic, but the first major assault on the mechanism of the reaction was due to Michaelis and Barron [123,124]. The oxidation of cysteine at pH 7—8 was found to be zero order in cysteine and to involve metal—cysteine complexes as active intermediates. Several studies of metal—thiol complexes have been... 

Metallic copper and silver both have antibacterial properties and Au thiol complexes have found increasing use in the treatment of rheumatoid arthritis, but only copper of this group has a biological role in sustaining life. It is widely distributed in the plant and animal worlds, and its redox chemistry is involved in a variety of... 

Transition metal isocyanide complexes can undergo reactions with nucleophiles to generate carbene complexes. Pt(II) and Pd(II) complexes have been most extensively investigated, and the range of nucleophilic reagents employed in these reactions has included alcohols, amines, and thiols (56) ... 

In general, the electron transfer reaction (Reaction 2) controls the over-all rate of reaction, and the nature of the catalyst has a profound effect on the kinetics of oxidation (23). Thus Wallace et al. (23) have compared the catalytic efficiencies of metal phthalocyanines with metal pyrophosphates, phosphates, phosphomolybdates, and phosphotungstates. The activity of metal pyrophosphates was ascribed to the ease of electron transfer through the metal coordination shell, the reaction being suggested to occur at the solid pyrophosphate-liquid interface. On the other hand, the catalytic effectiveness of a series of metals, added to solution as simple salts, has been explained in terms of their ability to form soluble complexes containing thiols (13). It was not clear whether the high rates of oxidation were caused by the solubility of metal complexes or by the peculiar nature of the thiol complexes. 

Metal Complexes. The importance of Ni complexes is based on their effectiveness as quenchers for singlet oxygen. Of disadvantage is their low colorfastness and their lower ii-reflectance compared to cyanine dyes (qv) therefore they are used in combination with suitable dyes. Numerous complexes are described in the literature, primarily tetrathiolate complexes of Pt or Ni, eg, dithiolatonickel complexes (3). Well known is the practical use of a combination of benzothiazole dyes with nickel thiol complexes in WORM disks (Ricoh, TDK) (17). 

Yet another possibility involves H2 activation at thiolate ligands. This possibility is suggested by the chemistry of an Fe11 tetrathiolate complex (Scheme 4) (110), which evolves H2 when H+ is added to the system. Intermediates proposed for this reaction include thiol complexes derived from the protonation of the thiolate ligands. A role for a metal cluster in the catalysis is also suggested by the mechanism, which involves the formation of dimeric species in order to provide the two electrons necessary for the production of H2. 

Most 0,P donors such as (37) and (38) readily form stable complexes with platinum group metals. Pd complexes of the soft soft S,P mixed donor ligand (39) are chiral and resolvable into optical isomers an As,S analogue is also known. The bicyclo phosphine thiol (40) is also chiral, binding Pd as an S,P chelate. 

Vinyl sulfides have been prepared by the catalytic addition of the S—H bond of thiols (85) to terminal alkynes (86) under solvent-free conditions using the nickel complex Ni(acac)2 (47). High alkyne conversions (up to 99%) were achieved after 30 min at 40 °C in favor of the corresponding Markovnikov products (87) (equation 23). Other metal acetylacetonate complexes were examined for this reaction, but none showed any improvement over the nickel catalyst. Mechanistic details suggest that alkyne insertion into the Ni—S bond is important to the catalytic cycle and that nanosized structural units comprised of [Ni(SAr)2] represent the active form of the catalyst. Isothiocyanates and vinyl sulfides have been produced in related Rh(acac)(H2C=CH2)2 (6) and VO(acac)2 (35) catalyzed sulfenylation reactions of aryl cyanides and aryl acetylenes, respectively. 

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