Methylated metals methyl transfer

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. 

Metal complexes, the nucleophilicity of towards organic molecules, 23,1 Methyl transfer reactions, 16,87... 

The E/Z stereoselection can be rationalized by assuming metal-centered pericyclic chairlike transition states 1 13,10 , 12 and 13. In this model proton transfer and metal ion transfer are assumed to occur simultaneously. When R is a bulky group, the nonbonded steric interaction between this group and the methyl group becomes strong and the Z-enolate will be the predominating isomer under kinetic control. 

For the purposes of this chapter, we shall use the term biological methylation (in recent years this has frequently been contracted to biomethy-lation ) in Challenger s first sense to describe methyl-transfer (transmethylation) reactions involving metals and metalloids in biological systems. We shall not differentiate between endo- and exocellular transmethylation (5, 6), and we shall include some purely chemical studies of model reactions. The relationship of these reactions to the larger area of organometallic chemistry has been reviewed (7, 8). 

Vitamin B]2 is a red crystalline, cobalt-containing compound that can be isolated from the liver. It has a functional role in preventing pernicious anaemia and also serves as a coenzyme in hydrogen and methyl transfer reactions (Co appears to be the only metal in vivo catalyzing C transfer reactions O and N transfers are more common). Vitamin B12 is also a growth-promoting factor for several microorganisms. 

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. 

Transition-metal catalyzed transfer of acylcarbenes to nitriles leads to 1,3-oxazoles via nitrile ylide intermediates123. The corresponding nitrile ylide chemistry derived from acyl(silyl)carbenes still awaits a closer look, but it has been shown that the rhodium-catalyzed decomposition of 198 in the presence of methyl cyanoformate and benzaldehyde provides 1,3-oxazole 221 (equation 71) exclusively120. This implies that the carbene moiety has been transferred only to the nitrile but not to the aldehyde. 

Tetracyanoquinodimethane (TCNQ) and many of its derivatives are easily reduced to anions of the type TCNQ-, which form salts with various cations. With many cations, e.g., tetrathiafulvalene cations (TTF+), and N-methyl phenazinium cations (NMP+), the TCNQ- anions form electronically conducting salts (- molecular metals, -> charge-transfer complexes) that can be used as electrodes, especially because of their electrocatalytic properties (- biosensors, -> electrocatalysis, -> molecular metals) [i,ii]. TCNQ undergoes insertion electrochemical reactions (-> insertion electrochemistry) leading to TCNQ salts [iii, iv]. Polymers... 

Other processes also contribute to chain growth termination under special conditions. In particularly crowded catalysts, fi-methyl transfer to the metal centre can occur instead of p-H transfer. When other reaction paths are blocked, a-bond metathesis, i.e. transfer of an H atom from a monomer to the metal-bound alkyl C atom can release a polymer with a saturated chain end with formation of a new unsaturated metal-bound chain start. Saturated chain ends will also result when H2 gas is added to a catalyst system thus leading to the production of shortened polymer chains. Such an H2 addition will often also cause an increase in overall catalyst activity, since H2 will predominantly react with species - such as occasional 2,1-inserted units - which are rather... 

Photolysis of Cp2Ti(CH3)2 and Cp2Ti(CH3)Cl in the presence of another transition metal complex has been shown to result in methyl transfer from titanium to the second transition metal and also to other ligand transfer reactions (32a). Photolysis in the presence of certain transition metal halides gave simple metathetical exchange of halide for methyl [e.g., Eq. (26)]. The authors postulated titanium-carbon bond... 

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