Phosphine-Modified Cobalt Carbonyl Systems

In the Shell process (SHOP) phosphine-modified cobalt-catalyzed hydrofor-mylation is one of the steps in the synthesis of linear alcohols with 12-15 carbon atoms (see Section 7.4.1). Two important characteristics of this reaction should be noted. First, the phosphine-modified precatalyst HCo(CO)3(PBu3) is less active for hydroformylation than HCo(CO)4 but more active for the subsequent hydrogenation of the aldehyde. In this catalytic system both hydroformylation and hydrogenation of the aldehyde are catalyzed by the same catalytic species. Second, the phosphorus ligand-substituted derivatives are more stable than their carbonyl analogues at higher temperatures and lower pressures (see Table 5.1). 

The major species in the catalytic system with added phosphine is different from that in the system lacking added phosphine. Infrared spectroscopic analyses under steady-state conditions for hydroformylation catalyzed by the phosphine-modified complexes provide no evidence for the accumulation of an acylcobalt complex only phosphine-substituted cobalt carbonyl dimers and hydride complexes are observed.- " ... 

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]. 

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