Carbon distribution, oxygen-methyl

Figure 7. Water oxygen-methyl group carbon pair distribution, averaged over the three solute methyl groups...

Corypalline (165 R = H)249 could be oxidatively dimerized electrochemi-cally in overall yields ranging from 44 to 85% depending on experimental conditions. Two types of dimers are obtained, the amount of carbon-carbon dimer 166 decreasing when R was varied from H through methyl to ethyl, and the product distribution shifted toward the carbon-oxygen-carbon dimer 167, presumably because of steric hindrance250 [Eq. (103)]. 

The results for the isomerization of n-heptane are presented in Table 20.5. Over all the catalysts the main products are again the 2-methyl (M2H) and 3-methylhexanes (M3H), with a significant contribution from the dimethylpentanes of 12-13% over the platinum and Mo2C-oxygen-modified catalysts, and 21% over the Mo03-carbon-modified catalyst. 3-Ethylpentane always contributes around 3% to the isomer distribution and almost no cyclic products are observed. Increasing the pressure over the... 

These results clearly show that the potential energy surface can contain a series of minima. The fact that selectivity in re-attack by the F ions can be observed indicates that the differences between the energy barriers for the secondary reactions control the distribution of the final products. The multistep character of these processes is further illustrated by the reactions observed when enolate anions are used as reactant ions. The ambident enolate anions may react with methyl pentafluorophenyl ether at the carbon or the oxygen site. If they react with the carbon site at the fluorine-bearing carbon atoms, then the molecule in the F ion/molecule complex formed contains relatively acidic hydrogen atoms so that proton transfer to the displaced F ion may occur. An example is given in (47) where the enolate anion, generated by HF loss, is not observed. An intramolecular nucleophilic aromatic substitution occurs instead and leads to a second F ion/ molecule complex. The F" ion in this complex then re-attacks the substituted benzofuran molecule formed, either by proton transfer or SN2 substitution. 

The reaction is presumed to occur by initial formation of HjfMenO) PO], which then undergoes direct P-alkylation by nucleophilic attack of phosphorus on carbon with displacement of halide. An alternative alkylation mechanism involving nucleophilic attack of oxygen on carbon, followed by a Michaelis-Arbuzov rearrangement i) of a dialkyl phenylphosphonite (C6H5(MenO)POR) intermediate with the alkyl halide was effectively eliminated by the observation that reaction of methyl phenylphosphinate with a tenfold excess of methyl-c/3 iodide gave the product distribution shown in Eq. (2). 

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