Methyl exchange

SCHEME 5. Rate equations and activation parameters for the methyl exchange in the Me2Zn-Me2Cd system... 

Sodium bismuthate reacts with 1 (47, 52), as does BiON03 (46). Methyl exchange between Cr(II) and 1 follows second-order kinetics (55) the rate constant was 360 30M sec-1, and values for the activation parameters A H and A 5 were 15.9 0.9 kJ mol-1 and -144 J mol-1 K respectively. Ferric compounds are reported to demethylate 1 (30, 31, 37). Cupric nitrate gives no noticeable reaction by itself with 1 (46, 54), but de-methylation proceeds readily in the presence of high chloride or bromide concentrations (>2 M). Methyl chloride and methyl bromide formed as products. When ethanol was used as solvent, methyl ethyl ether also formed as product (54). An unstable intermediate CH3CuC1 may form as... 

Various groups have reported on methyl exchange among metals other than methylcobalamin in aqueous solution. This area has been discussed in some detail by Jewett et al. 51) and by Pratt and Craig (59). Table III lists representative examples of such systems, along with the reported... 

Methyl Exchange hf.i ween Metals in Aqueous Media0... 

Pyrolysis of poly(methylacetylene) shows rather similar behaviour 528>, with mesi-tylene as the major product but substantial yields of methyl and proton-enriched products. Thermal decomposition of this polymer sets in at around 150 °C and the mechanism is postulated to involve chain scission followed by cyclization reactions and both electron-proton and electron-methyl exchanges. Pyrolysis of poly(phenyl-acetylene) has been reported to start at 270 °C in nitrogen 529). 

At lower temperatures the methylene protons and the N-methyl groups decoalesced in two distinct rate processes AG Me = 8-4 kcalmol-1 AG H2 = 11.5 kcalmol-1. This was interpreted by the authors as Si—N dissociation leading to N-methyl exchange, with a... 

The dimethylation of methyl acetate (methylation of the 1,1-dimethoxyethyl cation) was calculated to form two isomeric gitonic dications (303), which are thermodynamically more stable than the neutral ester.578 They are expected to be involved in the observed methyl exchange of 1,1-dimethoxyethyl cation. Rotamer 303b is more stable than the isomeric dication 303a, by 10.0 kcal mol-1 (MP2/6-31G //MP2/6-31G +ZPE level). 

Figure 3.69 (a) DMF methyl exchange mechanism, (b) ll NMR spectroscopic signals for the methyl... 

Mixed disilane derivatives with Si—me and Si—Cl bonds were prepared, usually by the methods of Kumada et al. These methods were recently reviewed368. Two principal methods are known, firstly methyl exchange by H2S04 and subsequent chlorination or fluorination by NH4C1 or NH4F,... 

Rate Constants for Bridge-Terminal Exchange in A12(CH3)6 and for Methyl Exchange between A12(CH3)6 and Ga(CH3)3 in Cyclopentane and Toluene Solutions0... 

Fig. 4. Linear correlation between V-methyl exchange barriers and coordinative 29Si chemical shifts for 4a-4g.14 Reproduced with permission from the American Chemical Society.

Fig. 17. Two schematic routes for the H NMR spectral changes associated with TV-methyl exchange in 46 upon increase of temperature route A exchange within each diastereomer route B exchange between diastereomers.47 Reproduced with permission from the American...

Fig. 38. Selective Inversion Recovery (SIR) H NMR spectra for A-methyl exchange in 70g in toluene-Jg solution at 360 K (a) stack plot of spectra (b) a plot of signal intensity as a function of the variable delay t (c) linear correlation of data (Eq. 27).51 Reproduced with permission from the American Chemical Society.

As the temperature of a collection of molecules increases, the molecules acquire more thermal energy, and the rates of physical processes such as translation, rotation, and torsion increase. At sufficiently high temperature, the rate of rotation of the C-N bond in DMF becomes significant, bringing about reversible exchange of the two methyl groups The cis methyl exchanges position with the trans ... 

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