Hydroformylation mechanistic studies

Rh s(CO)iis they revealed the presence of an unidentified complex which was suggested to be the previously unknown species Rh4(a-CO)i2-The BTEM protocol is an extremely powerful technique to recover pure component spectra of unknown species, even when present at very low concentrations. This was illustrated by a detailed mechanistic study of the promoting effect of HMn(CO)5 on the Rh4(CO)i2 catalyzed hydroformylation of 3,3-dimethylbut-l-ene [22], A dramatic increase in the hydroformylation rate was found when both metals were used simultaneously. Detailed in situ FTIR measurements using the BTEM protocol indicated the presence of homometallic complexes only during catalysis. The metal complexes that were identified under catalytic conditions were RC(0)Rh(C0)4, Rh4(CO)i2, Rh5(CO)i5, HMn(CO)s, and Mn2(CO)io (see Figure 6.6). The kinetics of product formation showed an overall product formation rate, Eq. (3) ... 

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. 

Ru3(CO)i2/Talkylbenzimidazoles showed high selectivity for ethylene glycol [13]. A mechanistic study of this reaction showed that RuH2(CO)3(l-methylbenzimid-azole) is formed, and this complex is considered to be the active species. 1-Methyl-benzimidazole enhances both the rate of the formation of formaldehyde from syngas and the rate of the hydroformylation of formaldehyde [14]. 

For Rh4(CO)12 as hydroformylation catalyst, several mechanistic studies including isotope labeling and kinetics have been undertaken. Thus, the deuteroformylation of 2,3,3-trimethylbut-l-ene and of styrene in the presence of Rh4(CO),2 have been studied In the first case, the 3,4,4-trimethyl-pentanal formed was found to be deuterated exclusively in the formyl position (238). In the latter case, the aldehydes formed were deuterated in the formyl and in the corresponding a-position ( > and (Zy PhCH=CHD and PhCH=CD2 were also observed (239). The mechanistic implications of these findings are not entirely clear. However, a kinetic study of the Rh4(CO)12-catalyzed low-pressure cyclohexene hydroformylation provides some evidence in favor of intact cluster catalysis A mechanism proposed on the basis of these findings includes fission of one Rh-Rh bond, whereby the tetranuclear cluster framework remains intact (Scheme 12) (240). 

Mechanistic studies of rhodium-catalyzed hydroformylation of olefins have shown that the basic feature of the catalyst cycle is more or less the same as that of the cobalt-catalyzed reaction.When unmodified rhodium carbonyls, e.g., Rh4(CO)i2 and RhefCOlie, are used as catalysts, there is an equilibrium among Rh4(CO)i2, Rh6(CO)i6, and HRh(CO)n (n = 3 or 4) in the presence of carbon monoxide and hydrogen, which complicates the mechanistic study. Nevertheless, HRh(CO)n (n = 3) is postulated as the active catalyst species, and the... 

Extensive mechanistic studies have been performed on reactions catalyzed by rhodium and platinum complexes containing enantiopure C2-symmetric diphosphine ligands.As discussed above, (1) the formation of the Tr-olefin-Rh(H) complex 19, (2) stereospecific cis addition of the hydridorhodium to the coordinated olefin to form the alkyl-Rh complex 20 (and then 2, and (3) the migratory insertion of a carbonyl ligand giving the acyl-Rh complex 17 with retention of configuration, have been established in the hydroformylation of 1-alkenes or substituted ethenes. Thus, it is reasonable to assume that the enantioselectivity of the reaction giving a branched aldehyde is determined at the diastereomeric (1) TT-olefin-Rh complex 19 formation step, (2) alkyl-Rh complex 20 formation step, or (3) acyl-Rh complex 17 formation step. 

In 1961 Heck proposed what is now generally considered to be the correct monometallic mechanism for [HCo(CO)4]-catalyzed hydroformylation [10]. He also proposed, but did not favor, a bimetallic pathway involving an intermolecular hydride transfer between [HCo(CO)4] and [Co(acyl)(CO)4] to eliminate aldehyde product (Scheme 2). Most proposals concerning polymetallic cooperativity in hydroformylation have, therefore, centered on the use of inter- or intramolecular hydride transfers to accelerate the elimination of aldehyde product. Bergman, Halpem, Norton, and Marko have all performed elegant stoichiometric mechanistic studies demonstrating that intermolecular hydride transfers can indeed take place between metal-hydride and metal-acyl species to eliminate aldehyde products [11-14]. The monometallic [HCo(CO)4] pathway involving reaction of the acyl intermediate with H2, however, has been repeatedly shown to be the dominant catalytic mechanism for 1-aUcenes and cyclohexane [15, 16]. 

The range of olefins screened in platinum-catalyzed hydroformylation is rather narrow. As seen already above, mostly styrene or 1-olefins were investigated in mechanistic studies with the aim of establishing the structure of catalytic intermediates or to find structure-activity-regio/stereoselectivity relationships. Usually, Pt/Sn catalysts operate under rather mild conditions (10-100 bar syngas, 50-130 °C) [53]. Pt/S ratios of up to 2000 1 have been realized. 

Preliminary mechanistic studies by Fernandez, Peris, and Crudden provided evidence that, under smooth hydroformylation conditions, NHCs remain ligated in the coordination sphere of rhodium [55, 68]. A similar conclusion was drawn by Scholten and Dupont for hydroformylation in ionic liquids [69]. A serious problem sometimes faced is the reductive elimination of carbene ligand from the metal as imidazohum salt under the effect of H2, as found in hydrogenation reactions [70]. 

Ligand Design and Mechanistic Studies for Ni-Catalyzed Hydrocyanation and 2-Methyl-3-Butenenitrile Isomerization Based upon Rh-Hydroformylation Research... 

PHIP effects were also used to detect reaction intermediates in the mechanistic studies of homogeneously catalyzed hydroformylation reactions [46 - 50]. Interestingly, it was observed that such processes can lead to the so-called one-H PHIP effect when incorporation of only one H atom of pH2 into the product molecule can lead to an observation of hyperpolarization of the aldehyde H atom [46,49], which is most Ukely the result of the nuclear spin evolution in the preceding dihydride intermediates. 

The catalysts used in hydroformylation are typically organometallic complexes. Cobalt-based catalysts dominated hydroformylation until 1970s thereafter rhodium-based catalysts were commerciahzed. Synthesized aldehydes are typical intermediates for chemical industry [5]. A typical hydroformylation catalyst is modified with a ligand, e.g., tiiphenylphoshine. In recent years, a lot of effort has been put on the ligand chemistry in order to find new ligands for tailored processes [7-9]. In the present study, phosphine-based rhodium catalysts were used for hydroformylation of 1-butene. Despite intensive research on hydroformylation in the last 50 years, both the reaction mechanisms and kinetics are not in the most cases clear. Both associative and dissociative mechanisms have been proposed [5-6]. The discrepancies in mechanistic speculations have also led to a variety of rate equations for hydroformylation processes. 

---