Ligand Effects in Rhodium Catalyzed Hydroformylation

Soon after the initial discovery of the hydroformylation activity of RhH(CO)(PPh3)3 it was found that ligands have a profound influence on the activity and selectivity of the rhodium catalysts. The majority of the numerous 

The catalyst for hydroformylation is a rhodium(I) hydride species, which is clearly distinct from the species that are active for hydrogenation. The hydrogenation catalysts are cationic Rh(I)+ or neutral Rh(I)Cl species. Carbonylation of alcohols also requires an ionic Rh(I) species, e.g. [Rl CO y-- Often rhodium(I) salts are used as the precursor for hydroformylation catalysts. Under the reaction conditions (H2, CO, ligands, temperature 50°C) these salts are converted to a rhodium hydride complex, although there are several papers that seem to invoke cationic rhodium species as the catalysts. Chlorides have a particularly deleterious effect on the activity (i.e. they are not converted into hydrides under mild conditions) and it has been reported that the addition of bases such as amines has a strong promoting effect on such systems  

Rhodium salts of weaker acids are smoothly converted into rhodium hydride without the addition of base  

L3RhA + H2 + CO - L3Rh(CO)H + HA A = acetate, 2,4-pentanedionate, hydrocarbyl sulphide etc. 

Aryl phosphites were among the first ligands to be studied extensively. The results obtained with triphenylphosphine and triphenyl phosphite are strikingly similar at low ligand concentrations. At higher ligand metal ratios phosphites may retard the reaction. The results of the early work by Pruett and Smith [8,9] together with two more recent data [10], are summarized in Table 6.4. We have added the %- and 0-values in this overview, which were not known at the time the catalytic work was reported. 

---