Rate of hydroformylation

The rate of hydroformylation increases with increasing hydrogen and decreases with increasing carbon monoxide partial pressures (9), suggesting that rates of hydroformylation would be satisfactory at high H2 and low CO partial pressures. In industrial practice, however, high pressures of both H2 and CO ate required in order to stabilize the HCo(CO)4 catalyst at the temperatures necessary for practical rates (10). Commercial processes, for example, operate at >24 MPa (3480 psi) and >140 C. 

Tetrarhodium dodecacarbonyl can effect carbonylation of an olefin at atmospheric pressure (132). The rate of hydroformylation of an olefin decreases with increasing alkyl substitution. 

The rate of hydroformylation was found to vary in a nonlinear fashion as a function of triphenylphosphine concentration. A maximum in rate was noted at a triphenylphosphine/HRh(CO)(PPh3)3 weight ratio of (5-10) 1, as illustrated in Fig. 7. A maximum in selectivity to linear aldehyde was noted at about a 5 1 ratio, and no significant further increase was noted up to a 50 1 ratio of triphenylphosphine to rhodium complex. 

Since under these conditions the olefin is not isomerized by Cojj(CO)8, any isomerization that occurs must proceed at a rate about equal to or greater than the rate of hydroformylation. 

Mobility of equilibrium (4.2) results in the situation, that the concentration ratio of [RhH(CO)(TPPMS)3] to [RhX(CO)(TPPMS)2] at any time will depend solely on [H ], i.e. on the pH. An inciease of pH will increase the concentration of the immediate catalyst precursor, which, in turn, should result in an increased rate of hydroformylation. 

The rate of hydroformylation was proportional to the concentration of the acyl complex. The apparent activation parameters were Ai-T = 49.3 kj mol" and AS = 121 J moT K". Both the activation parameters and the reaction order are consistent with the hydrogenolysis reaction being rate determining. The low order of 0.1 in alkene suggests that the rate-determining step is not purely the reaction with hydrogen and that either a pre-equilibrium also contributes or one of the earlier steps in the cycle is also somewhat slower. 

The reactions are solvent dependent. Non-polar solvents favour the formation of the neutral hydride (68), whereas in polar media, ionic species predominate. Hydroformylation activity for propylene was observed only under conditions where (68) was formed.291 The dinuclear species [Co2(CO)6(PBu"3)2] (69) also appears to be catalytically inactive.290,291 If PBu"3 is added in excess over cobalt, only complexes (68) and (69) were present. The rate of hydroformylation then became first order in hydrogen, owing to the equilibrium shown in equation (60).292... 

The reaction of dicobalt octacarbonyl with molecular hydrogen Eq. (17), can occur at room temperature and is similarly inhibited by carbon monoxide, again suggesting an unsaturated intermediate (158). Pino et al. (116) suggested that dicobalt octacarbonyl is in equilibrium with a more reactive lower carbonyl. Natta et al. have also shown that Co4(CO)12 and Co2(CO)8, in the absence of hydrogen, coexist at carbon monoxide pressures corresponding to the highest rates of hydroformylation (104, 158). 

Solvent effects on the rate of hydroformylation have been found to be small, however. A small increase in the rate of hydroformylation of cyclohexene has been found in the series methanol > benzene > heptane but the overall increase is only by a factor of 1 5 (158). Alcohols have been reported to increase the yield of hydrogenated products (66, 88). 

Acrylonitrile has now been successfully hydroformylated (76). The j3-cyanopropionaldehyde which was formed, polymerized unless the cobalt carbonyls were decomposed before work-up. Yields were highest in Lewis base solvents, which were said to suppress reduction of acrylonitrile and give higher rates of hydroformylation than hydrocarbon solvents. 

A considerable amount of work has been done on the hydroformylation of alkyl acrylates. The formation of y-oxobutyrates has previously been reported (15). Iwanaga (70) studied the effect of solvent variation and found that the rate of hydroformylation was in the order alcohols > acetone > toluene. Pyridine and some of its homologs also increased the rate of reaction. At higher temperatures and pressures, lactones were formed (32, 35, 135), presumably by reduction of the Oxo ester to the hydroxy ester followed by ring closure with elimination of alcohol. 

Marko (91) studied the effect of olefin structure on the ratio of hydroformylation to hydrogenation products, and concluded that the ratio declined with increasing branching of the olefin. This is not surprising in view of the known decrease in rate of hydroformylation with increased... 

A convenient catalyst precursor is RhH(CO)(PPh3)3. Under ambient conditions this will slowly convert 1-alkenes into the expected aldehydes, while internal alkenes hardly react. At higher temperatures pressures of 10 bar or more are required. Unless a large excess of ligand is present the catalyst will also have some isomerization activity for 1-alkenes. The internal alkenes thus formed, however, will not be hydroformylated. Accordingly, the 2-alkene concentration will increase while the 1-alkene concentration will decrease this will slow down the rate of hydroformylation. This makes the rhodium triphenylphosphine catalyst... 

The rate of hydroformylation depends on the structure of the olefin, the order being as follows straight-chain terminal olefins > straight-chain internal olefins > branched-chain olefins. Cyclohexene reacts more slowly than cyclopentene, cycloheptene, or cyclooctene. 

The coordinatively and electronically unsaturated complex [RhH(CO)(PPh3)2] can add the alkene that is to be hydroformylated. Added phosphine will shift the above equilibrium to the left, resulting in a lower concentration of the catalytically active 16-electron complex. Thus, you can predict that the rate of hydroformylation will be decreased by added phosphine. 

Chaudhari and co-workers [64] showed that the rate of hydroformylation of... 

Highly active unmodified rhodium catalysts for the hydroformylation of various olefins in SCCO2 are formed under mild conditions from [(cod)Rh(hfa-cac)] (8 cod = cis,cis-l,5-cyclooctadiene) and a number of other simple rhodium precursors [24]. Especially for internal olefins, the rate of hydroformylation is considerably higher than using the same catalysts in conventional liquid solvents under otherwise identical conditions. A detailed study of the hydroformylation of 1-octene (Scheme 6) using the online GC setup shown in Fig. 3 revealed a network of competing isomerization and hydroformylation when 8 was used without additional modifiers. As a result, the regioselectivity for the desired linear n-aldehyde varied considerably with conversion. At 60% conversion, the product aldehydes contained almost 80% of nonanal, whereas only 58 % linear aldehyde were present in the final product mixture. 

Table 1. Relative Rates of Hydroformylation of Various Olehn ...

---