Formate, active methyl groups from

The exact mechanism of the formation of the catalytically active species is not established it is possible that at the initial stages, MAO abstracts the methyl group from zirconium yielding cationic zirconium species that is subsequently stabilized by transfer of the methyl group from aluminum... 

Extracts of Methanobacterium transfer an active methyl group to mercury (21). Inhibition of methane formation from methylcobalamin upon the addition of mercury is presented in Figure 7. From such a reaction mixture small amounts of methylmercury and dimethylmercury... 

The most popular lands of the diols for asymmetric synthesis are bis-secondary diols that have a C2 axis of symmetry [212]. The presence of the symmetry axis avoids the formation of diastereoisomeric esters or acetals [213], (1R, 27 )-Cyclohexanediol 1.34 (n = 1) has been used as an auxiliary in asymmetric cyclopropanation [214] and (IS, 2S)-cycloheptanediol 1.34 (n = 2) in 1,4-addition of cuprates[157], Dioxolane derivatives of 1.34 have been used for asymmetric P-ketoester alkylations [215] and cuprate 1,4-additions [216]. Linear 1,2-diols 1.35 (R = Me, i-Pr, c-CgH j, Ph) and functionalized 1,2-diols 1.36 (Y = COOalkyl, CONR 2, CH2OR ) are readily available from optically active tartaric acids 1.36 (Y = COOH). Acetals derived from these diols are valuable reagents m asymmetric synthesis [173, 213, 217], as the related 1,3-diols 1.37. Acetals of 1,3-butanediol 137 (R = Me, R = H) have also been used. When these acetals are formed from aldehydes under thermodynamic conditions, one 1,3-di-oxane stereoisomer often predominates. In this favored isomer, the substituent from the aldehyde and the methyl group from 1.37 are both in equatorial orientar... 

Biosynthesis of thymine nucleotides. This is shown in Fig. 2. Since thymine is a constituent of DNA, the corresponding nucleotides contain 2-deoxyribose. Thymidylic acid (TMP) is therefore more correctly dTMP (deoxythymidine 5 -monophosphate). The reaction sequence is CMP - CDP - dCDP -v dCMP ->dUMP- TMP (dTMP)- TDP (dTDP)- TTP (dITP). Methylation of dUMP to TMP is catalysed by th idylate synthase (EC 2.1.1.45). The cofiictor, AP,A/ -methylenetetrahydrofolic acid, transfers the active Cl unit to C5 of dUMP, and it also functions as a reducing agent in the formation of the methyl group from the active Cl unit. 

In toluene disproportionation, two toluene molecules react to yield benzene and xylenes by transferring a methyl group from one toluene to the other. This reaction has to be done at moderate temperatures to avoid cracking and dealkylation of toluene. Therefore, reactions are mostly carried out at around 400 °C. Rhodes and Rudham [154] studied this reaction over steam-and EDTA-dealuminated Y zeolites and found initially an increase of conversion with time, followed by a maximum after 5 to 20 min. Subsequently a complete deactivation of the catalysts occurred after 10 to 25 minutes. This behavior resulted from the formation of catalytically active coke during the reaction. The maximum activity over all catalysts was linearly related... 

Few of the methyltransferses under discussion have been extensively purified, so that little can be said as to whether this extensive group of enzymes share in common any physicochemical properties, amino acid sequences, or features of their active sites. No evidence has been adduced that these enzymes act as intermediate carriers of the methyl group. Such evidence might be obtained either directly, by demonstration of formation of methylated enzyme from S-adenosylmethionine, or indirectly, by demonstration of an exchange of S-adenosylhomocysteine into S-adenosylmethionine, catalyzed by a methyltransferase. No such evidence has... 

With electrons flowing from ethylene to zirconium the Zr—CH3 bond weakens the carbons of ethylene become positively polarized and the methyl group migrates from zirconium to one of the carbons of ethylene Cleavage of the Zr—CH3 bond is accom panied by formation of a ct bond between zirconium and one of the carbons of ethylene m Step 3 The product of this step is a chain extended form of the active catalyst ready to accept another ethylene ligand and repeat the chain extending steps... 

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