Homologation syngas

In this paper we disclose the syngas homologation of carboxylic acids via ruthenium homogeneous catalysis. This novel homologation reaction involves treatment of lower MW carboxylic acids with synthesis gas (C0/H2) in the presence of soluble ruthenium species, e.g., Ru02, Ru3(C0)12, H4Ru4(C0)12, coupled with iodide-containing promoters such as HI or an alkyl iodide (1). 

Where acetic is the starting acid (eq. 1), homologation selectively yields the corresponding C3+ aliphatic carboxylic acids. Since acetic acid is itself a "syngas" chemical derived from methanol via carbonylation (2,3), this means the higher MW carboxylic acids generated by this technique could also be built exclusively from C0/H2 and would thereby be in-depent of any petroleum-derived coreactant. 

Effect of Operating Conditions. Yield data, summarized in Figures 1 and 2, point to acetic acid homologation activity being sensitive to at least four operating variables, viz. ruthenium and methyl iodide concentrations, syngas composition and operating pressure. 

Figure 2. Acetic acid homologation (%) acetic acid, propionic acid, (A) butyric acids, and (Y) valeric acids, ( Z ) ethane A, effect of syngas composition (operating conditions acetic acid, 833 mmol Ru(IV) oxide, 4.0 mmol Mel, 40 mmol 220°C 272 atm initial pressure) B, effect of operating pressure (operating conditions acetic acid, 417 mmol Ru(IV) oxide, 2.0 mmol Mel, 20 mmol 220°C constant pressure CO/H2 = 1/1)...

As in the case of the linear carboxylic acids, the principal by-products are water, C02 and the corresponding hydrocarbons. Substantial quantities of iso-butane are formed for example during iso-butyric acid homologation (see Experimental Section) while 2-methylpentane accompanies the formation of 2,2-dimethylvaleric acid during syngas treatment of 2-methylvaleric acid. 

Deuteration studies with acetic acid-d4 (99.5% atom D) as the carboxylic acid building block, ruthenium(IV) oxide plus methyl iodide-d3 as catalyst couple and 1/1 (C0/H2) syngas, were less definitive (see Table III). Typical samples of propionic and butyric acid products, isolated by distillation in vacuo and glc trapping, and analyzed by NMR, indicated considerable scrambling had occurred within the time frame of the acid homologation reaction. 

Some of the more important features of our novel syngas homologation of aliphatic carboxylic acids, catalyzed by ruthenium, include ... 

Syngas Homologation of Acetic Acid. To a N2-flushed liquid mix of acetic acid (50.0 gm) and methyl iodide (5.67 gm, 40 mmole), set in a glass liner is added 0.763 gm of ruthenium(IV) oxide, hydrate (4.0 mmole). The mixture is stirred to partially dissolve the ruthenium and the glass liner plus contents charged to a 450 ml rocking autoclave. The reactor is sealed, flushed... 

At prolonged reaction times, increasing amounts of high molecular weight condensates are funned. In the gas phase, products like methane, dimethylether and CO2 are found in addition to the syngas components. All reaclion products have been identified by GC/MS measurements and by comparison of CjCproduct composition of a typical methanol homologation run obtained by a cobalt/iudine catalyst is given in Table H. 

Reactions based on syngas, in analogy to hydroformylation, have been performed as well. These include aminomethylation [85], amidocarbonylation (see Section 2.1.2.4), homologation of acids [86] and alcohols, (cf. Section 3.2.7) [87] or silyl-formylation (cf. Section 2.6) [88]. All these reactions are far beyond the scope of this chapter and are not discussed further here. 

Within this scope, homologation reactions are all variants of enlarging the carbon chain in a oxygen-containing molecule by one C atom with the use of syngas. 

The elongation of the carbon chain of these molecules by a CH2 unit derived from CO/H2 is called homologation. Without any doubt, the syngas homologation of methanol to ethanol, once achieved on an industrial scale, would have enormous commercial potential, since it represents the key step in a syngas route to ethylene (249). 

Ichikawa reported that addition of methanol to syngas increased considerably the formation of ethanol on Rh carbonyl cluster catalysts. This experiment showed that surface species made from methanol reacted further to give ethanol. However, one still does not know whether this is by a mechanism involving carbenes e.g., mechanisms (1), (2), and (5) above] or by homologation of intermediates such as methoxy or methyl [mechanism... 

A number of researchers have now examined ruthenium-cobalt catalyzed methanol homologation to ethanol (60-62). Doyle concludes (56) that the ruthenium and cobalt moieties act independently, with the cobalt species responsible for the formation of 2-oxygenates like acetaldehyde and ruthenium reducing the aldehyde intermediate to ethanol. However, in our work - even with CO-rich syngas (8) -acetaldehyde is never more than a trace product. Mixed ruthenium-cobalt carbonyls are now well documented (56,62), but in these melt studies there is no direct spectroscopic evidence for their formation. 

Other methods for the syngas-free synthesis of aldehydes from olefins, for example, a one-pot reaction consisting of hydroboration-homologation and final oxidation (Scheme 3.1), are outside the scope of this book and are therefore not considered in detail here [6]. 

Scheme 3.1 Syngas-free hydroformylation of olefins by a one-pot hydroboration-homologation and final oxidation.

Chelation control has also been observed by Perlmutter and Jackson [10] in the hydroformylation of terminal olefins bearing phosphine groups at 28bar syngas pressure. In comparison to homologous substrates, superior results were observed with 4-diphenylphosphino-but-l-ene (Scheme 4.104). Noteworthy,... 

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