Rate-determining step, hydroformylation

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... 

Most hydroformylation investigations reported since 1960 have involved trialkyl or triarylphosphine complexes of cobalt and, more recently, of rhodium. Infrared studies of phosphine complex catalysts under reaction conditions as well as simple metal carbonyl systems have provided substantial information about the postulated mechanisms. Spectra of a cobalt 1-octene system at 250 atm pressure and 150°C (21) contained absorptions characteristic for the acyl intermediate C8H17COCo(CO)4 (2103 and 2002 cm-1) and Co2(CO)8. The amount of acyl species present under these steady-state conditions increased with a change in the CO/ H2 ratio in the order 3/1 > 1/1 > 1/3. This suggests that for this system under these conditions, hydrogenolysis of the acyl cobalt species is a rate-determining step. 

It is also called dissociative because one of the rate-determining steps is the dissociation of carbon monoxide. The cycle is started by the dissociation of a ligand, which results in the release of the planar 16 electron species (I). In analogy to the cobalt mechanism (see Wiese KD and Obst D, 2006, in this volume), the next step is the addition of an olefin molecule to form the r-complex (II). This complex undergoes a rearrangement reaction to the corresponding reaction steps decide whether a branched or a linear aldehyde is the product of the hydroformylation experiment. The next step is the addition of a carbon monoxide molecule to the 18 electron species (IV). Now, the insertion of carbon monoxide takes place and... 

In 2004 Caporali investigated the hydroformylation of 1-hexene and cyclohexene using HRh(CO)(PPh3)3 [61]. The collected data indicated that the rate-determining step in the hydroformylation cycle depends upon the structure of the olefin. With an alpha-olefin like 1-hexene, the slowest step seems to be the hydrogenolysis of the acyl rhodium complex. In the presence of cyclohexene as a model for an internal olefin, the rate-determining step is the reaction of the olefin with the rhodium hydride complex (intermediate II in Fig. 6). 

If one would be able to derive from the experimental data an accurate rate equation like (12) the number of terms in the denominator gives us the number of reactions involved in forward and backward direction that should be included in the scheme of reactions, including the reagents involved. The use of analytical expressions is limited to schemes of only two reaction steps. In a catalytic sequence usually more than two reactions occur. We can represent the kinetics by an analytical expression only, if a series of fast pre-equilibria occurs (as in the hydroformylation reaction, Chapter 9, or as in the Wacker reaction, Chapter 15) or else if the rate determining step occurs after the resting state of the catalyst, either immediately, or as the second one as shown in Figure 3.1. In the examples above we have seen that often the rate equation takes a simpler form and does not even show all substrates participating in the reaction. 

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. 

Ab initio molecular orbital studies on the whole catalytic cycle of hydroformylation of ethylene catalyzed by HRh(CO)2(PH3)2 has been performed [59,60], which points out the significance of the coordinating solvent—ethylene in this case—and identifies the oxidative addition of molecular hydrogen to the pentacoordinate acyl-Rh complex as the rate-determining step. In fact, this step is the only endothermic process in the catalytic cycle. 

Figure 1 outlines the key intermediates of a catalytic cycle where the rate-determining step is the formation of an n-alkyl derivative of the trans-bisphosphine via a coordinated olefin complex. This presumed catalytic cycle appears to satisfy proposals by Cavalieri d Oro et al. (9), C. V. Pittman et al. (10), and J. Hjortkjaer (II). Although no single mechanism of hydroformylation was established, the cycle is shown here to illustrate the key nature of the equilibrium between the trisphosphine (D) and the trans-bisphosphine (E). 

Orchin and Roos (108) examined the isomerization of allylbenzene by HCo(CO)4 and DCo(CO)4 at ambient temperature and pressure. Both HCo(CO)4 and DCo(CO)4 catalyzed isomerization to propenylbenzene at the same rate, and when DCo(CO)4 was used as catalyst 5% of the propenylbenzene produced was found to contain a deuterium atom. Hydroformylation of propylene with residual DCo(CO)4, after an isomerization of allylbenzene, yielded RCDO with no detectable RCHO. The authors chose to reject a mechanism involving addition of D—Co to the olefinic double bond, on the grounds that the lack of an isotope effect indicated breaking of D—Co, or H—Co, was not the rate-determining step, and that only a relatively minor amount of deuterium was incorporated into the isomerized reaction product. Instead, the authors favored a mechanism expressed as... 

Assuming that only the reactions shown in Fig. 5.1 operate for the hydroformylation of propylene to n-butyraldehyde with 5.1 as the catalyst, and oxidative addition of dihydrogen is the rate-determining step, what should be the rate expression What is the implicit assumption ... 

Hydrogenolysis of metal carbonyls, such as Mnj(CO)jg [250 atm (25 MPa), 200°C] or COjCCOg [250 atm (25 MPa), 110°C], leads to metal-metal bond cleavage, forming HMn(CO)j or HCo(CO) The conversion of CoJiCO to HCo(CO) is the rate-determining step in Co-catalyzed hydroformylations at high P and T. Tertiary amines, nitrogen heterocycles, tertiary phosphorus bases or halide ions enhance the rate of HCo(CO) formation e.g., pyridine can increase the rate of HCo(CO).j formation 300-fold at 40°C. 

This study, performed on the hydroformylation of oct-l-ene, has shown that below 140°C nonanals are the predominant products with linearities of approximately 97% (99% in one run at I00°C), whereas above 180°C nonanol was obtained almost exclusively with high octene conversions (>98% at 200°C) but poor linearities (65%). At high temperatures a 10-fold excess of bipy increases the nonanol linearity (to 76%). This parameter is not very sensitive to the CO or partial pressures as the total pressure is above about 95 bar. The author (40) seems to prefer coordination of the alkenc to a ruthenium center or hydride transfer to form an alkyl ruthenium cluster as the two possible rate-determining steps. Thus, by optimization of the experimental conditions it is possible to reach high linearities (n/iso>100) for the hydroformylation of terminal alkenes by the [Ru3(CO),2]/bidentate nitrogen- or phosphorus-containing ligand/phos-... 

The most detailed and generally accepted kinetic study on triphenylphosphine-modified rhodium catalysts was published in 1980 [109]. It was concluded from the coefficients obtained (Table 2) that the fast alkene insertion is followed by the rate-determining step involving CO or TPP [110]. The apparent activation energy for propene hydroformylation was found to be 84 kJ/mol, very similar to the value obtained for unmodified cobalt catalysts. 

Recently, the activation energy for hydroformylation of 1-decene with HRh(CO)(TPP)3 was determined to be 48 kJ/mol, which is significantly lower [111]. A rate model was developed (eq. (11)) that was similar to Bourne s two-parameter eq. (8). The rates predicted by the model were found to agree within 6-8 % error with the experimental data. This time the oxidative addition of hydrogen was recognized as the rate-determining step. However, the model is not generally applicable as the phosphine concentration was not considered and the reaction temperatures were fairly low T = 50-70 °C). 

An example is the hydroformylation reaction of cyclohexene catalyzed by the unsaturated compound HCo(CO)3 which is formed under reaction conditions from the precursor HCo(CO)4. Following the usual mechanism (see, e. g., [18]), the catalytic cycle is depicted in Scheme 1. Since the oxidative addition of H2 to the acylcobalt complex is the rate-determining step in this case the rate equation follows eq. (2) (cf. Section 2.1.1) ... 

In the hydroformylation of internal olefins, only CoH(CO)4 and CoafCOfs were observed spectroscopically, suggesting that in these cases the rate-determining step is conversion of CoH(CO)4 to the alkyltetracarbonyl [reaction (b )]> with undetectable, steady state concentrations of the other species. The reaction of styrene with CoH(CO)4 and CO yields the a-phenylpropionylcobalt derivative Co(COCHMePh)(CO)4, which partially isomerizes to the -phenylpropionyl derivative Co(COCH2CH2Ph)(CO)4- This sequence establishes that acyl derivatives of cobalt(I) can be prepared from CoH(CO)4, olefin, and carbon monoxide, as for the combined steps of reactions (b ) and (b")-... 

The triaryl phosphine seems to have the right combination of steric (to induce the formation of linear product at the 1,2-insertion stage) and electronic (to donate electron density to metal in order to stabilize CO ligands) properties. Studies indicate that the rate-determining step is likely to be hydrogenation of the acylrhodium intermediate (as with unmodified Co hydroformylation), but the mechanism of this apparent OA-RE step is not completely understood.34 DFT-level theoretical studies have suggested that the selection for linear versus branched aldehydes... 

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