Reductive elimination hydroformylation

In contrast to triphenylphosphine-modified rhodium catalysis, a high aldehyde product isomer ratio via cobalt-catalyzed hydroformylation requires high CO partial pressures, eg, 9 MPa (1305 psi) and 110°C. Under such conditions alkyl isomerization is almost completely suppressed, and the 4.4 1 isomer ratio reflects the precursor mixture which contains principally the kinetically favored -butyryl to isobutyryl cobalt tetracarbonyl. At lower CO partial pressures, eg, 0.25 MPa (36.25 psi) and 110°C, the rate of isomerization of the -butyryl cobalt intermediate is competitive with butyryl reductive elimination to aldehyde. The product n/iso ratio of 1.6 1 obtained under these conditions reflects the equihbrium isomer ratio of the precursor butyryl cobalt tetracarbonyls (11). 

Scheme 8.4 Catalytic cycle for the hydroformylation of C=C bonds using HRh(CO)2(PPh3)2. Step 1, ligand dissociation step 2, ligand association step 3, /J-hydride transfer step 4, ligand dissociation step 5, CO insertion step 6, oxidative addition of H2 steP 2, reductive elimination step 8, ligand association...

This reaction is a variation of the hydroformylation reaction. Transmetallation of Rh(I)(acac) with the alkylmercury(I) compound gives ClHg(acac) and an alkylrhodium(I) compound. Oxidative addition of H2 gives a Rh(III) compound, and coordination and insertion of CO gives the acylrhodium(IH) compound. Reductive elimination then gives the product and regenerates Rh(I) — but as a Rh-H, not as Rh(acac). 

Olefin formation, by reductive elimination of 3-hydroxysulfones, 72, 2 Olefins, hydroformylation of, 56, 1 Oligomerization of 1,3-dienes, 19, 2 Oligosaccharide synthesis on polymer support, 68, 2 Oppenauer oxidation, 6, 5 Organoboranes ... 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

Rate-determining step, hydroformylation, 163 Reactivity, enantiomers, 286 Recognition, enantiomers, 278 Reduction and oxidation, 5 Reductive coupling, dissolving metal, 288 Reductive elimination, 5, 111 Resolution. See Kinetic resolution Rhenium-carbene complexes, 288 Rhodium-catalyzed hydrogenation, 17, 352 amino acid synthesis, 18, 352 BINAP, 20... 

Hydroformylation (the oxo process) involves the addition of H2 and CO to an olefin to form aldehydes (eq. 2.8), which have a number of important industrial applications. Extensive mechanistic studies have shown that this reaction involves migratory insertion of a bound alkyl group (formed by insertion of an olefin into a metal hydride) into a bound CO, followed by reductive elimination of the aldehyde. The rate-limiting step for the hydroformylation in liquids is either the reaction of olefin and HCo(CO)4 or the reaction of the acyl complex with H2 to liberate the product aldehyde. The high miscibility of CO in sc C02 is therefore not necessarily a major factor in determining the rate of the hydroformylation. Typically, for a-olefins, linear aldehydes are preferred to branched products, and considerable effort has gone into controlling the selectivity of this reaction. 

The processes as well as the potential problems of hydroformylations are depicted in Scheme 1. Catalytically active is the 16-electron complex 4, which, after coordination of the olefin 1 to the r-complex 5, forms in a stereospecific syn addition the <7-complex 8. Subsequently, coordination and insertion of CO gives rise to the acyl complex 9. After addition of H2 to the Rh(III) complex 7, a reductive elimination to... 

An alternate bimetallic pathway was also suggested, but not favored, by Heck and Breslow (also shown in Scheme 1). The acyl intermediate could react with HCo(CO)4 to undergo intermolecular hydride transfer, followed by reductive elimination of aldehyde to produce the Co-Co bonded dimer Co2(CO)s. A common starting material for HCo(CO)4-catalyzed hydroformylation, Co2(CO)g is well-known to react with H2 under catalysis reaction conditions to form two equivalents of HCo(CO)4. The bimetallic hydride transfer mechanism is operational for stoichiometric hydroformylation with HCo(CO)4 and has been proposed to be a possibility for slower catalytic hydroformylation reactions with internal alkenes.The monometallic pathway involving reaction of the acyl intermediate with H2, however, has been... 

The hydrocarboxylation reactions discussed above have been proposed to involve direct addition of water to the metal center prior to elimination of the product, analogous to the oxidative addition of hydrogen to a metal center at the end of a hydroformylation catalytic cycle. Another class of hydrocarboxylation reactions is more analogous to the haUde-promoted Monsanto acetic acid process, where one has a reductive elimination of an acyl halide species that is rapidly hydrolyzed with free water to generate the carboxylic acid and HX. 

A second example is the reaction of the acylcobalt complex with hydrogen equivalents and the successive reductive elimination of the aldehyde (step 7). This reaction was studied in great detail and has been controversial [93]. For this very last step of the hydroformylation cycle several pathways may be imagined. The two most plausible pathways are shown in eq. (4). 

Two pathways have been proposed for step e, which may be the rate-limiting stage in the cycle (this notion is supported by the observed overall rate law for hydroformylation, which is typically first-order in H2 concentration). Equation 9.10 illustrates an oxidative addition followed by reductive elimination sequence, as originally proposed. Since Heck and Breslow s work, a bimolecular process described by equation 9.11 has been suggested.23... 

Oxidative-addition reactions (see Section 10) play a widespread role in catalytic processes and constitute one of the principal routes for the activation and dissociation of saturated bonds, including H—H, C—(X = halogen), strained C—C bonds, and in certain cases, C—H bonds. Correspondingly, the reverse process, i.e., reductive elimination, constitutes a widespread route for the formation of such bonds. The latter process frequently is the product-forming step in catalytic reactions such as hydrogenation, car-bonylation and hydroformylation ". ... 

Both methods can also be used in intramolecular ring closure reactions to form cyclic ketones. Similar, but not identical reaction mechanisms are assumed. The first reaction resembles hydroformylation and requires carbon monoxide insertion and an additional metal acyl alkene insertion step, while in the second reaction the carbon monoxide unit is already present in the substrate. This reaction starts with an oxidative addition to the aldehyde C-H bond, forming an acyl metal hydride, which then undergoes alkene insertion and reductive elimination. 

Side reactions such as double-bond migration and others are observed, similar to hydroformylation. Mechanistically, hydrocarboxylation is related to hydroformylation up until the metal acyl formation stage13. The presence of an acidic compound shifts the reaction towards formation of carboxylic acid derivatives and suppresses reductive elimination which forms aldehydes. The mechanism of the final steps is unclear13. 

---