Methylated metals reaction mechanisms

In this review we shall not deal with the synthesis of this coordination complex, but we shall deal with the chemical properties of B 12-coenzymes with special emphasis on how these properties relate to Bi2-enzyme mechanisms. Also, we shall show how B -catalyzed methyl-transfer reactions have special significance in the biosynthesis of methylated heavy metals in the aqueous environment, and how the synthesis of these organometallic compounds has special relevance to problems concerned with continuing global environmental health hazards. 

A number of examples have been reported documenting the use of palladium phosphine complexes as catalysts. The dialkyl species [PtL2R2] (L2 = dmpe, dppe, (PMe3)2 R = Me, CH2SiMe3) catalyze the reaction of [PhNH3]+ with activated alkenes (acrylonitrile, methyl acrylate, acrolein).176 Unfunctionalized alkenes prove unreactive. The reaction mechanism is believed to proceed via protonation of Pt-R by the ammonium salt (generating PhNH2 in turn) and the subsequent release of alkane to afford a vacant coordination site on the metal. Coordination of alkene then allows access into route A of the mechanism shown in Scheme 34. Protonation is also... 

The activated methyl groups of 5,7-dimethyl-l,8-naphthyridin-2-one (106) can be selectively metallated, reaction with two equivalents of butyllithium giving (107), while with two equivalents of sodamide in liquid ammonia (108) is obtained. Both (107) and (108) gave appropriate products on treatment with electrophiles (81JOM(213)405>. A possible explanation of this result involves a coordination mechanism for the formation of (107) and an acid-base mechanism giving (108). 

Methyl-metal bond cleavage also occurred in the reactions of gold(I), gold(III), and platinum(II) compounds with thiols and selenols (207-209). The reactions of gold(I) and platinum(II) complexes were much faster than those of gold(III), due to the operation of a radical chain mechanism in the former cases. For the reactions with diphenylphosphine, however, the following order of reactivity was found (209) ... 

Model reactions have contributed significantly to our understanding of biological processes. Both pyridoxal phosphate (vitamin B6) and Bi2-coenzymes have proved useful in mechanism studies. Methyl transfer reactions to various metals are of environmental significance. In 1968 it was shown that methylcobalamin could transfer a methyl carbanion to mercury(II) salts in aqueous solutions. Recent research on interaction between B12-coenzymes and platinum salts has shown that charged Ptn salts labilize the Co—-C bond. Secondly, the B12-coenzymes are unstable in the presence of platinum salts this observation correlates with the fact that patients who have received cw-platin develop pernicious anemia. 

Both mechanisms are predicted to show syn addition of hydride and caiboxylate to the alkene. In the metal hydride mechanism (equation 36) alkene insertion is syn and CO insertion proceeds with retention of configuration at carbon. In the metal carboxylate mechanisms (equation 37) alkene insertion is syn and cleavage of the metal-carbon bond can take place with retention at carbon. The palladium-catalyzed hydroesterification reaction produces the erythro ester from (Z)-3-methyl-2-pentene (equation 38) and the threo ester from ( )-3-methyl-2-pentene (equation 39).w... 

Many studies on the direct reaction of methyl chloride with silicon-copper contact mass and other metal promoters added to the silicon-copper contact mass have focused on the reaction mechanisms.7,8 The reaction rate and the selectivity for dimethyldichlorosilane in this direct synthesis are influenced by metal additives, known as promoters, in low concentration. Aluminum, antimony, arsenic, bismuth, mercury, phosphorus, phosphine compounds34 and their metal complexes,35,36 Zinc,37 39 tin38-40 etc. are known to have beneficial effects as promoters for dimethyldichlorosilane formation.7,8 Promoters are not themselves good catalysts for the direct reaction at temperatures < 350 °C,6,8 but require the presence of copper to be effective. When zinc metal or zinc compounds (0.03-0.75 wt%) were added to silicon-copper contact mass, the reaction rate was potentiated and the selectivity of dimethyldichlorosilane was enhanced further.34 These materials are described as structural promoters because they alter the surface enrichment of silicon, increase the electron density of the surface of the catalyst modify the crystal structure of the copper-silicon solid phase, and affect the absorption of methyl chloride on the catalyst surface and the activation energy for the formation of dimethyldichlorosilane.38,39 Cadmium is also a structural promoter for this reaction, but cadmium presents serious toxicity problems in industrial use on a large scale.41,42 Other metals such as arsenic, mercury, etc. are also restricted because of such toxicity problems. In the direct reaction of methyl chloride, tin in... 

The initiation step is normally fast in polar solvents and an initiator-free living polymer of low molecular weight can be produced for study of the propagation reaction. The propagation step may proceed at both ends of the polymer chain (initiation by alkali metals, sodium naphthalene, or sodium biphenyl) or at a single chain end (initiation by lithium alkyls or cumyl salts of the alkali metals). The concentration of active centres is either twice the number of polymer chains present or equal to their number respectively. In either case the rates are normalized to the concentration of bound alkali metal present, described variously as concentration of active centres, living ends or sometimes polystyryllithium, potassium, etc. Much of the elucidation of reaction mechanism has occurred with styrene as monomer which will now be used to illustrate the principles involved. The solvents commonly used are dioxane (D = 2.25), oxepane (D = 5.06), tetrahydropyran D = 5.61), 2-methyl-tetrahydrofuran (D = 6.24), tetrahydrofuran (D = 7.39) or dimethoxy-ethane D = 7.20) where D denotes the dielectric constant at 25°C. 

During the course of biomethylation the methyl group is most likely transferred as a bridging intermediate rather than a free entity. Such an intermediate is assumed to form during an associative mechanism [4]. The methyl group may be electrophilic (cationic), radical or nucleophilic (anionic), depending on the specific donor moiety. A broad variety of methyl transfer reactions are therefore possible. Besides the two biological donors, methylcobalamin (see below) and S-adenosyl-methonine (1) nonenzymatic transmethylation is also possible in the natural environment, probably also very important for the formation and decomposition of metal methyl compounds [3b],... 

Amino acid esters may coordinate as a monodentate (amino N) or as a bidentate (N,0) ligand. In the latter case (17) significant polarization of the ester bond results which is of likely importance in the reaction mechanism (Scheme 6). Alternatively, hydroxide attack at the metal centre may occur (equation 15). Hydroxide ion rather than water is the predominant nucleophile, even at pH 5. Direct evidence for metal-ester bond formation comes from isolated Co" complexes which contain such a bond. Rate enhancements, compared to the free amino acids, are often of the order of lO -lO . For methyl histidinate, however, rate enhancement by Cu" is only of the order of265 this is ascribed to there being no Cu-ester bond formation. The order of decreasing reactivity in this study was established as being CuE " > Ni > CuE > NiE " > EH" > CuEA > CuEOH > NiEA > E, where E is L-HisOMe and A is L-HisO . The relative effectiveness of protons and metal ions as catalysts has been discussed by Martin. ... 

When polystyrenelithium is aminated by a reagent prepared from methoxyamine and methyl-lithium, two reaction mechanisms are possible. One may proceed via nitrene intermediates and the other via electophilic nitrenium ions. Many other reactions of polystyrenelithium can be found in the literature. Sodium metalated polystyrene reacts in a similar manner ... 

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