Mechanisms hydroformylation reaction

Along with studies of the catalyst solution and stoichiometric reaction mixtures, the hydroformylation reaction was studied online under typical reaction conditions by connecting a pressurized autoclave (20 bar) directly to the mass spectrometer via a splitter. While this allowed them to identify new reaction intermediates they did not extract any kinetic data from the observed intermediates over time. Nevertheless, a new hydroformylation reaction mechanism for self-assembling ligands (in which the ligands play an active role in H2 activation) was considered based on... 

The mechanism of the cobalt-cataly2ed oxo reaction has been studied extensively. The formation of a new C—C bond by the hydroformylation reaction proceeds through an organometaUic intermediate formed from cobalt hydrocarbonyl which is regenerated in the aldehyde-forrning stage. The mechanism (5,6) for the formation of propionaldehyde [123-38-6] from ethylene is illustrated in Figure 1. 

A simplified mechanism for the hydroformylation reaction using the rhodium complex starts by the addition of the olefin to the catalyst (A) to form complex (B). The latter rearranges, probably through a four-centered intermediate, to the alkyl complex (C). A carbon monoxide insertion gives the square-planar complex (D). Successive H2 and CO addition produces the original catalyst and the product ... 

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. 

Hydroformylation of 1-butene in the presence of the Rh catalyst gave pentanal (P) and 2-methyl bntanal as the main products. Just trace amounts of c/5-and trans-1-butene were detected as by-prodncts. No butane was detected in experiments, where a stoichiometric ratio of CO and H2 were used. Based on preliminary considerations of prodnct distribntions, a kinetic model was developed. The kinetic parameters obtained from the model were well identified and physically reasonable. The prodnct concentrations are predicted very well by the kinetic model. The kinetic model can be further refined by considering detailed reaction mechanisms and extending it to the domain of lower partial pressures of CO and H2. 

The mechanism of the hydroformylation reaction suggests that aldehyde regioselectivity is determined in the hydride addition step, which converts the five-coordinated H(alkene)-Rh(CO)L2 into either a primary or a secondary four-coordinated (alkyl)Rh(CO)L2. For the linear rhodium alkyl species, this... 

Figure 2 shows the generally accepted dissociative mechanism for rhodium hydroformylation as proposed by Wilkinson [2], a modification of Heck and Breslow s reaction mechanism for the cobalt-catalyzed reaction [3]. With this mechanism, the selectivity for the linear or branched product is determined in the alkene-insertion step, provided that this is irreversible. Therefore, the alkene complex can lead either to linear or to branched Rh-alkyl complexes, which, in the subsequent catalytic steps, generate linear and branched aldehydes, respectively. 

Diphosphine-based ligands form the basis of current research in hydroformylation. As Figure 9 shows, free energy profiles have been recently proposed [22] to discuss the kinetics and the reaction mechanism. We first see the rapid equilibrium between ee and ea... 

Metal chemical shifts have not found extensive use in relation to structural problems in catalysis. This is partially due to the relatively poor sensitivity of many (but not all) spin 1=1/2 metals. The most interesting exception concerns Pt, which is 33.7% abundant and possesses a relatively large magnetic moment. Platinum chemistry often serves as a model for the catalytically more useful palladium. Additionally, Pt NMR, has been used in connection with the hydrosilyla-tion and hydroformylation reactions. In the former area, Roy and Taylor [82] have prepared the catalysts Pt(SiCl2Me)2(l,5-COD) and [Pt()i-Cl)(SiCl2Me)(q -l,5-COD)]2 and used Pt methods (plus Si and NMR) to characterize these and related compounds. These represent the first stable alkene platinum silyl complexes and their reactions are thought to support the often-cited Chalk-Harrod hydrosilylation mechanism. 

The formation of formate esters in the hydroformylation reaction (90, 64) may be explained by a CO-alkoxide insertion reaction as well as by the CO-hydride insertion mechanism mentioned above. Aldehydes formed in the hydroformylation reaction can be reduced by cobalt hydrocarbonyl (27) presumably by way of an addition of the hydride to the carbonyl group (90, 2). If the intermediate in the reduction is an alkoxycobalt carbonyl, carbon monoxide insertion followed by hydrogenation would give formate esters (90, 64). 

Cobalt hydrocarbonyl is a very reactive compound. It reacts extremely rapidly with triphenylphosphine, probably by a first-order dissociation mechanism, producing cobalt hydrotricarbonyl triphenylphosphine (44). This demonstrates the very ready replacement of one ligand by another. Cobalt hydrocarbonyl also catalyzes the isomerization of olefins. Under conditions of the hydroformylation reaction, olefin isomerization is observed. But there is controversy as to whether or not rearranged aldehydes (aldehydes which cannot be produced by simple addition to the starting olefin) are produced mainly by rearrangement of an intermediate in the reaction (28, 75, 55) or by reaction of isomerized olefins (55). 

An alternative interpretation of some features of the hydroformylation reaction (including the inverse CO dependence), in terms of heterogeneous catalysis by an (unidentified) insoluble cobalt component, has recently been advanced by Aldridge, Fasce and Jonassen (49a). The universal validity of this seems doubtful in the light of the considerable evidence favoring a homogeneous mechanism. 

The investigation of phosphine complexes of rhodium(I) as catalysts (or catalyst precursors) for the hydroformylation reaction continues both to better elucidate the reaction mechanism and to improve catalyst activity. The presence of dioxygen often decreases the catalytic activity (139), but can also, surprisingly, reactivate hydroformylation catalysts... 

Data are presented to identify some of the important factors in aldehyde hydrogenation and to characterize rhodium carbonyl chemistry under hydroformylation conditions. Comparison is made of the effects of monomeric and of polymeric amines, and a possible reaction mechanism is examined in the light of the data. 

The hydroformylation of several olefins in the presence of Co2(CO)8 under high carbon monoxide pressure is reported. (S)-5-Methylheptanal (75%) and (S)-3-ethylhexanal (4.8%) were products from (- -)(S)-4-methyl-2-hexene with optical yields of 94 and 72%, respectively. The main products from ( -)(8)-2,2,5-trimethyl-3-heptene were (S)-3-ethyl-6,6-di-methylheptanal (56.6%) and (R)-4,7,7-trimethyloctanal (41.2%) obtained with optical yields of 74 and 62%, respectively. (R)(S)-3-Ethyl-6,6-dimethylheptanal (3.5% ) and (R)(S)-4,7,7-trimethyloctanal (93.5%) were formed from (R)(S)-3,6,6-trimethyl-l-heptene. (+/S)-l-Phenyl-3-methyl-1-pentene, under oxo conditions, was almost completely hydrogenated to (- -)(S)-l-phenyl-3-methylpentane with 100% optical yield. 3-(Methyl-d3)-l-butene-4-d3 gave 4-(methyl-d3)pentarwl-5-d3 (92%), 2-methyl-3-(methyl-d3)-butanal-4-d3 (3.7%), 3-(methyl-d3)pentanal-2-d2,3-d1 (4.3%) with practically 100% retention of deuterium. The reaction mechanism is discussed on the basis of these results. 

Although the overall reaction mechanisms (catalytic cycles) written for hydroformylation reactions with an unmodified cobalt catalyst (Scheme 1) and the rhodium catalyst (Scheme 2) serve as working models for the reaction, the details of many of the steps are missing and there are many aspects of the reaction that are not well understood. 

Although the enantiomeric excesses, selectivity and nib ratios are dependent on such variables as the H2/CO ratio, the total pressure, the substrate concentration and the ratio of phosphorus to platinum, no consistent pattern emerges to reveal much information as to the reaction mechanism.33 The hydroformyl-ation of a variety of substrates has been carried out, with enantiomeric excesses ranging from 20 to 44%,36 usually with DIOP (12) or CHIRAPHOS (14) as ligands.4 13 The highest enantiomeric excesses (73% and 85%, respectively) obtained from the hydroformylation of styrene were with [(-)-DBP-DlOPJPtCk/SnCh at 40-60 C, 220 bar, H2/CO = 2.437 and (R,/ )-2,3-bicyclo[2.2.2]octane-diylbis(methylene)bis(diphenylphosphine)PtCh/SnCk (equation 33).3 ... 

Asymmetric hydroformylation of prochiral olefins has been investigated both for the elucidation of reaction mechanism and for development of a potentially useful method for asymmetric... 

The isomerization of alkyl- and acylcobalt carbonyls is important in considering the products of the hydroformylation reaction and has been dealt with in part in Section II, A. Equations (9) and (10) give the most likely mechanism for the isomerization. 

Kinetic studies are of limited value for elucidating the mechanism of the hydroformylation reaction. This is because the empirically derived rate expressions are valid only within a narrow range of experimental conditions. For the rhodium-catalyzed reaction, in the absence of phosphine, the following rate expression has been proposed ... 

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