Alcohols homologation

Hydrido(alkoxo) complexes of late transition metals are postulated as intermediates in the transition metal-catalyzed hydrogenation of ketones (Eq. 6.17), the hydrogenation of CO to MeOH, hydrogen transfer reactions and alcohol homologation. However, the successful isolation of such complexes from the catalytic systems was very rare [32-37]. 

Wender and coworkers conclude that cobalt-catalyzed benzyl alcohol homologation involves the intermediate formation of car-bonium ions (8). However, since the methyl cation (CH3+) is unstable and difficult to form (9), it is more likely that methanol homologation to ethanol proceeds via nucleophilic attack on a protonated methyl alcohol molecule. Protonated dimethyl ether and methyl acetate forms have been invoked also by Braca (10), along with the subsequent formation of methyl-ruthenium moieties, to describe ruthenium catalyzed homologation to ethyl acetate. 

Iodide-promoted reactions in phosphine oxide solvents have been observed under some conditions to produce ethanol from H2/CO with good rates and high selectivities (193-195) (Table XVI, Expts. 1-3). Experimental evidence suggests that the ethanol is a secondary product, although its selectivity is high even after very short reaction times (193). An acid component is believed to be involved in alcohol homologation by this system, which will be described in more detail below. 

The table shows the formula of the first three members of the alcohol homologous scries. 

Substituent Hydrocarbon, % Homologated alcohol, % % Homologated % Hydrocarbon Recovered starting alcohol, % Other products... 

Carbonylation of methanol and nitroaromatics, hydroformylation of olefins and alcohol homologation were among the principal reactions aimed at producing high added value molecules. 

Since the secondary reactions involved in alcohol homologation reactions, strictly speaking, are outside the scope of this review, the effects of various promoters on this reactivity will not be discussed. The presented material is given only to serve as a comparison with the analogous cobalt systems, in which the weakness of the metal-carbon bonds governs this aspect of reactivity. 

In general, the corresponding aldehydes are thought to be the primary products of alcohols homologation. They easily undergo secondary reactions such as hydrogenation and aldol condensation. 

Having discussed the influence of molecular weight and heat of vaporisation at boiling point on volatility and heat of vaporisation at ambient temperature on solvent power, their relationship with some other physical properties will also be reviewed. Examination of the data in Table 2.7 for the normal alcohols homologous series, shows a regular effect of molecular weight on certain physical properties. 

The three series we have looked at so far have contained just the two elements, carbon and hydrogen. However, it is easy to imagine a different series of molecules where one of the hydrogen atoms in the chain is replaced by, say, a halogen atom or a hydroxyl (-OH) group. This latter series of compounds is known as the alcohols. Table 10.4 gives some details of the early compounds of the alcohol homologous series. 

Table 10.4 Some data on the early members of the alcohol homologous series... 

Alcohol Homologation Solvent and promoter effects on the cobalt carbonyl catalysed methanol homologation have been studied under synthesis gas pressure.The main product in a methanol/hydrocarbon two-phase system is 1,1-dimethoxyethane (ca. 70 selectivity).Using similar iodide promoted cobalt catalysts, R2C 0Me)2 and dimethylcarbonate are converted to acetaldehyde with up to 87 selectivity.Ruthenium in the presence of Co, 12 and dppe improves the ethanol selectivity in the homologation of dimethylether. Best results are achieved in inert solvents with high dielectric constants, e.g. sulfolane (e = 44), and with BF3 as activator. 

---