Isotope effects in the hydroformylation of olefins with cobalt carbonyls as catalysts

The stoichiometric hydroformylation reaction has been shown to consist of two variations: conducted in the presence of stoichiometric amounts of HCo(CO) ₄ and CO (Type I) or in the presence of Co ₂(CO) ₈ and H ₂ (Type II). The main difference between these two types of stoichiometric hydroformylation reactions was believed to be the nature of the cobalt carbonyl precursor and the number of steps necessary to the onset of reaction. In both types of reactions, however, the source of the hydrogen in the hydrogenolysis step was considered to be HCo(CO) ₄ and not H ₂. In this article, we examine the conventional assumptions regarding the nature and mechanism of the hydrogenolysis step for both types of stoichiometric hydroformylation reactions. This examination stems from inconsistencies observed during the study of the mechanism of the second type of the stoichiometric hydroformylation reaction, which cast some doubt on the identification of HCo(CO) ₄ as the hydrogen-donor species, under conditions in which molecular hydrogen is present. The presence of isotope mixtures of H ₂/D ₂ in the gas phase during the various steps of the reaction showed that the ratio of H/D isotopes in the hydrocarbon portion of the aldehydic product correlates with the HCo(CO) ₄/DCo(CO) ₄ ratio in solution, while the R(CO)H/R(CO)D product ratio correlates with the H ₂/D ₂ in the gas phase. Hence, the dominant reaction pathway for the hydrogenolysis step in this type of stoichiometric hydroformylation reaction is the direct reaction of hydrogen or deuterium with the acyl complex intermediate.