Rhodium-phosphine catalyzed hydroformylation, mechanisms

Unruh, J.D. and Christenson, J.R. (1982) A study of the mechanism of rhodium phosphine catalyzed hydroformylation use of l,l -bis(biarylphosphino) ferrocene ligands. Journal of Molecular Catalysis, 14, 19. 

Figure I. Ligand exchange is a proposed key step in the mechanism of phosphine-rhodium complex catalyzed hydroformylation of olefins...

In 1961 Heck and Breslow presented a multistep reaction pathway to interpret basic observations in the cobalt-catalyzed hydroformylation.28 Later modifications and refinements aimed at including alternative routes and interpreting side reactions.6 Although not all the fine details of hydroformylation are equally well understood, the Heck-Breslow mechanism is still the generally accepted basic mechanism of hydroformylation.6,17,19,29 Whereas differences in mechanisms using different metal catalysts do exist,30 all basic steps are essentially the same in the phosphine-modified cobalt- and rhodium-catalyzed transformations as well. 

Industrially the straight chain isomer is generally the most desired product and hence the normal/iso product ratio obtained for a given catalyst is of importance. Further, the hydrogenation activities of catalysts vary considerably such that alcohols can in some cases be obtained in a single step (222). The first catalysts developed for this reaction were based on cobalt carbonyl and later cobalt carbonyl phosphine complexes. However, more recently attention has been focused on the intrinsically much more active rhodium catalysts (222, 223). A simplified mechanism for (223) cobalt- and rhodium-catalyzed hydroformylation has been proposed which involves the following steps ... 

The important discovery by Wilkinson [1] that rhodium afforded active and selective hydroformylation catalysts under mild conditions in the presence of triphenylphosphine as a hgand triggered a lot of research on hydroformylation, especially on hgand effects and mechanistic aspects. It is commonly accepted that the mechanism for the cobalt catalyzed hydroformylation as postulated by Heck and Breslow [2] can be apphed to phosphine modified rhodium carbonyl as well. Kinetic studies of the rhodium triphenylphosphine catalyst have shown that the addition of the aUcene to the hydride rhodium complex and/or the hydride migration step is probably rate-limiting [3] (Chapter 4). In most phosphine modified systems an inverse reaction rate dependency on phosphine ligand concentration or carbon monoxide pressure is observed [4]. 

Kinetic studies are of limited value for elucidating the mechanism of the hydroformylation reaction. This is because the empirically derived rate expressions are valid only within a narrow range of experimental conditions. For the rhodium-catalyzed reaction, in the absence of phosphine, the following rate expression has been proposed ... 

The first calculation of the complete hydroformylation cycle with Rh-phosphine catalysts (substrate = ethylene, model ligand = PH3) was published in 1997 [3]. The QM methods used are HF and MP2, respectively (cf. Section 3.1.2.1). Hybrid DFT methods such as B3LYP [4], however, are more appropriate in terms of both accuracy and efficiency [5, 6] (cf. Section 3.1.2.1). Therefore, the same model system was recalculated [7] on the level B3LYP functional/DZVP basis set [8]/quasi-relativistic pseudopotentials on rhodium [9]. Since homologous Ir catalysts are interesting alternatives from an economic point of view [10], calculations with the central metal Ir were also made. This comparative treatment is supported by the experimental assumption of a common mechanism [11], which equals the Heck-Breslow mechanism of the cobalt-catalyzed reaction [12],... 

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