Hydroformylation experiments

Modified rhodium systems show considerable activity in the hydroformylation of styrene to the branched aldehydes. Chiral diphosphines, diphosphites, and phosphine-phosphites have been the ligands most studied. Hydroformylation experiments have often been performed in situ but the characterization of intermediates has provided an interesting contribution to coordination chemistry.179... 

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... 

The fluorous solvent alone had a minimal effect on the outcome of the reaction. However, with the fluorinated ligand, the rr.iso ratio was found to increase with increasing phosphine concentration, reaching a value of almost 8 1 at a phosphine to rhodium ratio of 103 1. The beauty of this system was demonstrated by its use in a semicontinuous hydroformylation experiment. After each... 

Tab. 6.3 Selected hydroformylation experiments with polymeric amphiphiles (experimental data are listed in Scheme 5.5, reaction time = 3 h).

It is interesting to note that no specific study was devoted to the aqueous biphasic hydrogenation of aldehydes with water-soluble cobalt-phosphine complexes, although such a property has long been known from hydroformylation experiments [199,200]. 

The same type of procedure was applied to the Rh4(CO)i2 / HMn(CO)5 initiated homogeneous catalyzed hydroformylation of 3,3-dimethyl-l-butene at 298 K [94]. A set of e=21 experiments with k >26 spectra and v=4751 MIR spectral chaimels were used. The spectra from the 21 hydroformylation experiments were preconditioned and assembled into one matrix A7i3x295i- This data matrix was then... 

The same authors recently described the synthesis of similar rhodium-complexed dendrimers supported on a resin having both interior and exterior functional groups. These were tested as catalysts for the hydroformylation of aryl alkenes and vinyl esters (52). The results show that the reactions proceeded with high selectivity for the branched aldehydes, with excellent yields, even up to the tenth cycle. The hydroformylation experiments were carried out with first- and a second-generation rhodium-complexed dendrimers as catalysts, with a mixture of 34.5 bar of CO and 34.5 bar of H2 in dichloromethane at room temperature. Each catalyst was easily recovered by simple filtration and was reusable for at least six more cycles without... 

All hydroformylation experiments were performed in a continuously operated reactor. The concentration of the catalyst was varied from 0.1 to 0.4 wt % to keep olefin conversion at the same level (80-90% ) for all reaction temperatures. The total pressure for all experiments was 280 atm, and n-octene was used as typical straight chain olefin of medium chain length. 

The isomerization of the olefin prior to its hydroformylation has been the explanation of this question (3) and the formation of isomeric aldehydes was related to the presence of isomeric free olefins during the hydroformylation. This explanation, however, is being questioned in the literature. The formation of (+) (S) -4-methylhexanal with an optical yield of more than 98% by hydroformylation of (+) (S)-3-methyl-l-pentene (2, 6) is inconsistent with the olefin isomerization explanation. Another inconsistency has been the constance of the hydroformylation product composition and the contemporary absence of isomeric olefins throughout the whole reaction in hydroformylation experiments carried out with 4-methyl-1-pentene and 1-pentene under high carbon monoxide partial pressure. The data reported in Ref. 4 on the isomeric composition of the hydroformylation products of 1-pentene under high carbon monoxide pressure at different olefin conversions have recently been checked. The ratio of n-hexanal 2-methylpentanal 2-ethylbutanal was constant throughout the reaction and equal to 82 15.5 2.5 at 100°C and 90 atm carbon monoxide. 

Materials. The gases used had a purity >99% the carbon monoxide contained <1% hydrogen by GLC silica gel, 4 m X 1/8 inch, 70 °C, Perkin-Elmer F 11 hot wire gas chromatograph and argon as carrier gas. All hydroformylation experiments were performed with a 1 1 mixture of carbon monoxide and hydrogen. The olefinic substrates were Fluka AG pure or very pure grade. a-Ethylstyrene, prepared according to Ref. 21, was > 98% pure by GLC. 

The application of zeolite-entrapped rhodium carbonyl clusters [prepared by exchanging Rh(NH3)6Cl3 into NaY zeolite followed by reduction in (CO H2) mixtures] as catalysts for the liquid-phase hydroformylation of alkenes has been discussed. More recently, infrared spectra of Rh6(CO)i6, supported on NaY zeolite by sublimation and treated with carbon monoxide at 100 C, have been found to be virtually identical to those obtained in the hydroformylation experiments. ... 

Table 3 Comparison of concentrations used in hydroformylation experiments in CO2 and conventional solvents...

The metal precursor Rh(acac)(CO)2 and 19 were loaded onto CPG-240 with a ligand-to-Rh ratio of 10 1 (Scheme 11) [37]. The in situ formed complex 20 was studied in hydroformylation experiments using 1-octene and compared with Rh(TPPTS)3 on silica. Catalyst 20 was very selective in favour of linear aldehydes (n/z=40 l). Even after ten runs, the selectivity remainedhigh. With TPPTS as ligand, only a regioselectivity of riH 3 1 was achieved. But the activity of complex 20 in toluene was poor (1 h . It could be enhanced by about 15 times when neat 1-octene instead of toluene was applied (15 h". Increasing the... 

Hydroformylation. Experiments with supported cobalt catalysts led to the hydroformylation (or 0X0) process for the conversion of olefins and synthesis gas to aldehydes. Homogeneous catalysis followed, and is now used exclusively. The generally accepted mechanism involves the following reactions ... 

The pure component spectra were then fit back onto the original raw multicomponent reaction spectra in order to get the signal contributiOTi of each pure component spectra, and calibration was achieved using the known quantities of Rh and Re moles used in each experiment. The results are sets of smooth concentration profiles in time. A typical profile for a 300 min hydroformylation experiment is shown in Fig. 20. As this figure shows, there is a fast nearly step change in the concentration of HRe(CO>5 after addition, with the immediate formation of significant amounts of RhRe(CO)9. The concentration of the observable precursor Rh4(CO)i2 approaches zero at circa 150 min, and at the same time the concentration of RCORh(CO)4 has been attained at maximum. The induction period (initial curvature) in the profile of aldehyde is minimal. Care was taken to ensure that experiments reported are free from transport control in either H2 or CO. 

This assumption was verified in hydroformylation experiments at unusual mild conditions (25°C, P(H2) = 2.8 MPa, P(CO) = 0.2 MPa) using an acylcobalt tetracarbonyl as a catalyst precursor (214). 

As reported in the same paper, it is likely that this unstable rhodium cluster is converted into the mononuclear rhodium-hydride species HRh(CO)x (x = 3,4), which are usually considered as the true catalyst system in the reaction mixture. These compounds represent extremely unstable intermediates, which would certainly recombine to form higher nuclearity rhodium species if alkene is not present in the reaction mixture. This mechanism is proposed for all the hydroformylation experiments carried out in the presence ofRh4(CO)iz... 

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