Methanol formation, using transient

The use of MeOD (instead of MeOH, the proton source) gives 90% RCHDOH and 10% RCH2OH. This is in agreement with the protonation by methanol of a transient carbanion. A minor pathway is hydrogen abstraction from THF by the hydroxy-alkyl radical. Since pinacol formation is mainly observed in aprotic THF it should be ascribed to the coupling of the initial anion radical. 

Type IV includes chiral phases that usually interact with the enantiomeric analytes through the formation of metal complexes. There are usually used to separate amino acid enantiomers. These types of phases are also called ligand exchange phases. The transient diastereomeric complexes are ternary metal complexes between a transitional metal (usually Cu +), an amino acid enantiomeric analyte, and another compound immobilized on the CSP which is able to undergo complexation with the transitional metal (see also the ligand exchange section. Section 22.5). The two enantiomers are separated based on the difference in the stability constant of the two diastereomeric species. The mobile phases used to separate such enantiomeric analytes are usually aqueous solutions of copper (II) salts such as copper sulfate or copper acetate. To modulate the retention, several parameters—such as the pH of the mobile phase, the concentration of the copper ion, or the addition of an organic modifier such as acetonitrile or methanol in the mobile phase—can be varied. 

Alonsono and co-workers [146] have used substituted 1-naphthyl and phe-nylphosphonium chlorides as precursors for the generation of the corresponding arylmethyl radicals and cations in both nanosecond LFP and product studies. For instance, the salt 101 has a quantum yield for cation formation of 0.71 in methanol and the sole product observed was the corresponding methyl ether. No transient radical was observed in this solvent. In contrast, in 5% acetonitrile in dioxane, the radical was observed but now the cation was absent. No fluorescence was observed in either solvent suggesting that Si is very reactive. Redox potentials indicate that the conversion of the radical/radical ion pair to the cation/triphenyl-phosphine pair would be exothermic by some 25 kcal/mol. Therefore, both heterolytic cleavage from Si or homolytic cleavage followed by electron transfer were suggested as possible pathways for cation formation. 

The greater activity of Pd for methanol decomposition reaction was also found by using the steady state isotopic transient kinetic analysis (SSITKA) method over noble metal (Pt, Pd, Rh)/ceria catalysts. Their activity increased in the order Rh < Pt < Pd, while the by-products were (i) methane, carbon dioxide, water, methyl formate and formaldehyde in most cases and (ii) ethylene and propylene, formed only over Rh/Ce02, at 553 K. SSITKA measurements indicated that two parallel pools exist for the formation of CO (via formation and decomposition of formaldehyde and methyl formate). The difference in the activity order of noble metal/ceria catalysts seems to correlate with the surface coverage of active carbon containing species, which followed the same order. The latter implies that a part of these species is formed on the ceria surface or/and metal-ceria interface. ... 

Based on these experiments, mechanistic steps were suggested, which explain, independent of the metal used (M = Co, Rh), the formation of the typical reaction products such as formaldehyde, methyl formate, methanol, and the desired ethylene glycol (Scheme 6.115) [11]. A key role in this mechanism is played by formaldehyde, which is produced by the hydrogenolysis of a metal formyl intermediate. Either it reacts afterwards to methyl formate via a methoxy complex or, alternatively, a transient hydroxymethyl complex is formed which becomes the starting point for next transformations. In the absence of CO, preferentially methanol is released. Only by coupling with a further CO equivalent the C2-unit is constructed. Upon hydrolysis, ethylene glycol is released. 

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