The Mechanism of Hydroformylation

The mechanism of hydroformylation is in many respects related to the mechanism of homogeneous hydrogenation and is discussed further in Chapter 31. 

The kinetics of the interconversion of complexes (67) and (68) have also been studied.293 It seems that the mechanism of hydroformylation in the presence of PBun3 is similar to that for... 

After the first successful asymmetric hydroformylation, although in low optical yields, the reaction was further investigated by different groups all over the world. The results have been rather disappointing from a synthetic point of view, as in a few cases only, optical yields as high as 30 to 50% have been achieved. However, some interesting information has been obtained, both on the mechanism of hydroformylation and on the basic aspects of homogeneous asymmetric catalysis by transition metal complexes. 

The mechanism of hydroformylation is well studied and is shown in Scheme 5... 

For hydroformylation over cobalt and rhodium zeolites the active species have not been defined. However, in the case of RhNaY the in situ formation of a rhodium carbonyl cluster has been identified (226) by infrared spectroscopy. Interestingly, this cluster appears to be different from known compounds such as Rh4(CO)12 and Rh6(CO)16. This does suggest that alternative carbonyl clusters may possibly be formed in zeolites due to the spatial restrictions of the intracrystalline cavities. The mechanism of hydroformylation in these zeolites is probably similar to that known for homogeneous catalysis. 

In situ IR spectroscopic studies have been carried out on the C02(C0)8 plus PBu"-based catalytic system with ethylene or n-octene as the olefins. Unlike in the case of Co2(CO)8, where HCo(CO)4 and Co(CO)4(COR) may be observed, in the presence of PBu", no Co(CO)3(PBu3)(COR) or Co(CO)3(PBu3))(R) types of complexes could be seen. This presumably is because of low concentrations of such intermediates under operating conditions. The mechanism of hydroformylation is assumed to be similar to the one shown in Fig. 5.7. Very little direct mechanistic evidence at a molecular level is available on the aldehyde hydrogenation reaction. 

The effect of 1-hexene concentration on the rate of hydroformylation, at a total pressure of 4.08, MPa (CO/H2 = 1), a catalyst concentration of 0.01, kmol/m at 353, K. The results are shown in Figure. 6. An interesting observation here, is the 1.6 order variation with 1-hexene concentration. Since, the mechanism of hydroformylation in general for water soluble Rh-TPPTS catalyst is believed to be similar to that using homogeneous HRh(CO)(TPP)3 (14), it is rather unusual that an order of reaction of 1.6 has been observed. Here, it is important to consider the influence of 1-hexene on the solubility of CO and hydrogen in the organic phase. 

Strong evidence for a dinuclear rhodium species as the active catalyst in hydro-formyladon of styrene was reported for the silica gel-supported species in uation 22 (941 Relative amounts of diphosphine and tetraphosphine rhodium species were determined from UV/visible spwtra. The rates of hydroformylation decreased markedly as % site isolation on the silica surface increased. All of the supported cat ysts were less active than the corresponding soluble catalysts, which were not site isolated. The results were interpreted by a dinuclear elimination step in the mechanism of hydroformylation (Equation 23). 

The mechanisms of hydroformylation with rhodium and cobalt catalysts have been studied in detail and are very similar. We have already learnt that for cobalt the active catalyst precursor is [HCo(CO)4] in the case of the modified rhodium catalyst it is the complex [HRh(CO)(PPh3)3]. 

The mechanism of hydroformylation by Pt/Sn systems has been investigated with the help of model complexes (Scheme 42). Only platinum SnCla complexes react with H2 to give EtCHO and close the cycle. 4-Pentenal is cyclized to cyclopentanone by cationic rhodium catalyst such as [Rh(dppe)2] in nitromethane or dichloromethane at 20 °C. The initiating step of the process is the oxidative addition of aldehyde-CH to the Rh(I) centre, a reversal of the final step in an olefin hydroformylation sequence. The mechanism was probed by deuteration studies direct evidence for the catalytic intermediates by NMR was unobtainable. The intermediates are involved in the reversible formation of side products, although selectivity to cyclopentanone can be as high as 98%. The essential features of the reaction are outlined in Scheme 43. ... 

Cobalt. Solvent effects on hydroformylation of propene and of pent-l-ene catalysed by CoH(CO)4 have been investigated by product distribution analysis. Effects of temperature and pressures of hydrogen and carbon monoxide on the mechanism of hydroformylation of propene in the presence of Co2(CO)8(PBu8)a have similarly been probed by product analysis. The reaction of (36) with methanol or ethanol (R OH) produces CHR(COaRi)2. ... 

Alemdaroglu NH, Penninger ML, Oltay E (1976) Study of the mechanism of hydroformylation at industrial reaction conditions. Monatsh Chem 107(4) 1153-1165... 

The cobalt-catalyzed hydroformylation reaction was originally discovered in 1938 by Otto Roelen and was used by the Germans in WWII to convert alkenes, Hj, and CO into aldehydes, which could then be used to make alcohols. The process is still used today, most commonly to convert propylene into butyraldehyde. Using HCo(CO)4 as the catalyst, the mechanism of hydroformylation, which is shown in Figure 19.30,... 

Many studies have been conducted on the mechanism of hydroformylation catalyzed by HRh(CO)2(PPh3)2. Some of these studies have focused on identifying the complexes in the catalytic system, and others have focused on obtaining kinetic data. Together, these data have provided mechanistic insight. However, hydroformylation involves many different steps, the formation of at least two isomeric products, and the intermediacy of species that can exist in several isomeric forms. Thus, this catalytic system is very complex, and only broad conclusions are drawn here. 

The basic steps of the mechanism of hydroformylation catalyzed by the combination of Rh and PPhj are similar to those of hydroformylation catalyzed by HCo(CO). However, the number of possible intermediates is much larger because many combinations of the number of phosphines and geometric orientation are possible in each intermediate complex. For example, phosphines in a five-coordinate complex can occupy the apical or equatorial positions, and these complexes are typically stereochemically nonrigid. Similarly, two phosphines in a four-coordinate intermediate can be located cis or trans to each other. A five-coordinate hydridorhodium-alkene complex containing diequatorial phosphines might be expected to convert to a four-coordinate Rh alkyl complex containing trans phosphines, but it could also lead to an alkyl complex containing cis phosphines. 

The mechanism of hydroformylation has been intensively investigated by Drent and coworkers (Scheme 5.29) [135]. Frequently, aldehydes are immediately reduced to the corresponding alcohols under hydroformylation conditions. 

In papers on the mechanism of hydroformylation, it has been suggested that alkyltetracarbonylcobalt compounds may rearrange to acyltricarbonyl-cobalt compounds followed by formation of HCo(CO)3 by reaction with hydrogen or alcohols without CO being taken up from the gas phase [35] (see Eq. 6). 

A study involving Co(77 -C3Hs)(CO)2PCy3, ra-hydrogen, and NMR spectroscopy provided direct evidence for the mechanism of hydroformylation catalysis as it allowed the detection of linear and branched cobalt acyl intermediates in conjunction to the corresponding alhedydes. ... 

---