Decarbonylation and Dehydrocarbonylation of Aldehydes

Both reactions deserve attention for the rationalization of unexpected side products observed in hydroformylation reactions, but they also have a large synthetic potential in fine chemistry and for the lab-scale synthesis of complex natural compounds. A special case concerns the decarbonylation of formaldehyde to CO and H2, which is treated separately in Section 3.2. 

First attempts for decarbonylation of aldehydes were conducted with transition-metal complexes in stoichiometric or near-stoichiometric reactions. Later on, also active catalysts and milder reaction protocols could be identified. The reaction proceeds best with electrophilic aldehydes and nucleophilic metal centers [5]. 

Hydrcformylation Fundamentals, Processes, and Applications in Organic Synthes, First Edition. 

An excess of PPhg may inhibit the decarbonylation (but see also below) [5]. This effect has been rationalized by assuming equilibrium between two rhodium(lll) complexes hosting two or three phosphine ligands. Only the former representing a 16e complex is the active decarbonylation catalyst. 

Chiral aldehydes can be decarbonylated under full retention of the configuration, but occasionally partial racemization may take place [10]. In general, decarbonylation with stoichiometric rhodium-arylphosphine complexes can be achieved at ambient temperature, but usually the catalytic version requires more severe conditions. Goldman et al. [11] discovered that trialkylphosphines form significantly more active catalysts, as exemplified with the binuclear complex [Rh(PMe3)(CO)Cl]2. The complex operates even at room temperature. A similar effect was also noted with iridium-phosphine catalysts [12]. 

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Besides isomerization of the olefinic substrates, some other side reactions may complicate the hydroformylation of fatty acids. Such reactions are decarboxylation and decarbonylation (see also Chapter 8) under formation of saturated or unsaturated compounds reduced by one carbon atom (Scheme 6.88) [37]. For example, at high temperatures and long reaction times, the formed aldehydes can undergo dehydrocarbonylation (a). Subsequent hydrogenation produces saturated fatty acids [25, 38]. This reaction sequence may lead to the false conclusion that hydrogenation of the starting olefin has taken place. The same products can suffer decarbonylation (b). On the other hand, decarboxylation of formyl carboxyl acids produces aldehydes (c). 

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