Hydrogenation of aromatic aldehydes

Carbon-Nitrogen Bond Formation Based on Hydrogen Transfer 123 Table 5.9 Transfer hydrogenation of aromatic aldehydes with HCOONa in water. ... 

In contrast to phenolic hydroxyl, benzylic hydroxyl is replaced by hydrogen very easily. In catalytic hydrogenation of aromatic aldehydes, ketones, acids and esters it is sometimes difficult to prevent the easy hydrogenolysis of the benzylic alcohols which result from the reduction of the above functions. A catalyst suitable for preventing hydrogenolysis of benzylic hydroxyl is platinized charcoal [28], Other catalysts, especially palladium on charcoal [619], palladium hydride [619], nickel [43], Raney nickel [619] and copper chromite [620], promote hydrogenolysis. In the case of chiral alcohols such as 2-phenyl-2-butanol hydrogenolysis took place with inversion over platinum and palladium, and with retention over Raney nickel (optical purities 59-66%) [619]. 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

The catalytic hydrogenation of benzaldehyde is a model reaction of hydrogenations of aromatic aldehydes. The principal reaction is ... 

X. Xu, H. Vonk, A.I.J.M. van de Riet, A. Cybulski, J.A. Moulijn, and A. Stankiewicz, Monolithic catalysts for selective hydrogenation of aromatic aldehyde. Proceedings of the Conference on Catalysis Science and Technology, Tianjin, China, September 12-15, 1995. 

Tim, B. T., Cho, C. S., Kim, T.-J., Shim, S. C. Ruthenium-catalyzed transfer hydrogenation of aromatic aldehydes with dioxane under KOH. Assistance of Cannizzaro reaction. J. Chem. Res., Synop. 2003, 368-369. 

From a practical point of view, Au addition to Pd in carbon-supported catalysts does not give any advantage as to activity in hydrogenation of aromatic aldehydes. Possible effects on the resistance to deactivation are presently being investigated. 

Platinum oxide-Fe or Cu-containing catalysts allow hydrogenation of furfural to furfurylalcohol". Ruthenium catalysts (Ru—C, RuOj) are successful in this specific case they have an activity well preserved through reuses. Otherwise Ru exhibits little activity in the heterogeneous hydrogenation of aromatic aldehydes. Other heterogeneous catalysis include platinized (PtC ) Raney Ni and copper chromite. 

Under homogeneous conditions, RUCI2 [P(Ph)3]3 is an effective catalyst for hydrogenation of aromatic aldehydes to benzyl alcohols at 50-80°C and at an H2 P of 10 kPa . Aromatic aldehyde hydrogenations are generally performed in ethanol, acetic acid, acetone, or ethyl acetate, with an order of efficiency depending on the catalyst. Acetal formation, catalyzed by traces of acid, are observed in methanol but not in ethanol . 

Selective Hydrogenation of Aromatic Aldehydes Using Precious Metal Catalysts on New High Surface-Area TiO Supports... 

PM catalysts on new high surface-area TiO supports were employed in the selective hydrogenation of aromatic aldehydes. Depending on the choice of catalyst and substrate, the selectivity of the hydrogenation can be... 

We shall discuss the process for the example of the hydrogenation of benzalde-hyde in various reactors [15]. The heterogeneously catalyzed hydrogenation of ben-zaldehyde is a model reaction for the hydrogenation of aromatic aldehydes. The main reactions are shown in Equation 13-19. 

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