Ruthenium-catalyzed hydrogenation reaction conditions

Fig. 44.6 Product inhibition in the ruthenium-catalyzed hydrogenation of cinnamaldehyde. Reaction conditions 0.1 mmol RuCI3, 0.5 mmol TPPTS, H20 Toluene ratio 5 mL 5 mL.

Trost et al 2 briefly explored using non-enone enophiles. Simple alkenes led to the formation of complex mixtures of isomers due to the presence of an additional set of /3-hydrogens. Many other types of substrates were incompatible with reaction conditions. Vinyl ketones were, therefore, the only coupling partners shown to be effective in the ruthenium-catalyzed Alder-ene couplings of allenes and alkenes. 

Mitsubishi have reported several processes based on Ru-catalyzed hydrogenation of anhydrides and acids. Succinic anhydride can be converted into mixtures of 1,4-butane-diol and y-butyrolactone using [Ru(acac)3]/trioctylphosphine and an activator (often a phosphonic acid) [97]. Relatively high temperatures are required ( 200°C) for this reaction. The lactone can be prepared selectively under the appropriate reaction conditions, and a process has been developed for isolating the products and recycling the ruthenium catalyst [98-100]. 

An alternative enzyme/transition metal combination employs transfer hydrogenation catalysts that are capable of racemizing secondary alcohols. The racemization procedure temporarily converts the alcohol into an achiral ketone, which is reduced back to the racemic alcohol. Coupling this racemization procedure to an enzyme-catalyzed acylation reaction affords a dynamic resolution process (Fig. 9-12). Several enzyme/transition metal combinations have been shown to be effective for these reactions, although ruthenium complexes 1-3 appear to be especially effective for the in situ racemization of the alcohol. The product esters are not prone to racemization under the reaction conditions. Early results employing transfer hydrogenation catalysts to effect the racemization of alcohols required the use of added ketone 21, 22. However, it was subsequently shown that added ketone was not required when appropriate transition metal complexes were used as catalysts. Furthermore, the use of 4-chlorophenyl acetate as the acyl donor afforded improved results. 

Although Pd, Ni, Ru, and Pd catalyze hydrogenation of ketones and aldehydes, a platinum catalyst is usually the best choice because the yields are better and there are fewer side reactions. Platinum oxide (Pt02)—sometimes called Adam s catalyst after Roger Adams (United States 1889-1971)—is commonly used. The choice of catalyst depends on the reaction conditions, the nature of the carbonyl compound, etc. The reaction of heptanal with hydrogen gas in the presence of a ruthenium catalyst (Ru/C), in 80% aqueous ethanol, gave 1-heptanol in near quantitative yield. ... 

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