Unmodified Rhodium Research Articles

Composition of catalyst resting states of hydroformylation catalysts derived from bulky mono-phosphorus ligands, rhodium dicarbonyl acetylacetonate and syngas

The paper describes the composition of the resting states of several catalysts for alkene hydroformylation derived from bulky monophosphorus ligands. The results presented assess how bulky ligands compete with CO for the rhodium, and hence the role of ‘unmodified’ catalysts in alkene hydroformylation in the presence of these ligands. High Pressure Infra-Red (HPIR) spectroscopy was carried out at the rhodium and syngas concentrations typically used during catalysis experiments. These HPIR studies revealed that two ligands previously studied in Rh-catalysed hydroformylation react with [Rh(acac)(CO)2] and H2/CO to give the unmodified rhodium cluster, [Rh6(CO)16], as the only detectable species. Both less bulky phosphoramidites, and 1,3,5,7-Tetramethyl-6-phenyl-2,4,8-trioxa-6-phosphaadamantane, on the other hand, do not show the presence of [Rh6(CO)16], and hence catalysis proceeds by purely ligand modified species under normal conditions. In the case of the Rh/phosphaadamantane catalysts, anecdotal evidence that this only forms a particularly useful catalyst above a certain pressure threshold can be understood in terms of how the catalyst composition varies with pressure. The ligands discussed have all been assessed in the hydroformylation of propene to separate their innate branched selectivity from their ability to isomerise higher alkenes to internal isomers.

ChemInform Abstract: Recent Progress in Rhodium‐Catalyzed Hydroaminomethylation

AbstractReview: [52 refs.

On the origin of diastereofacial selectivity in the interaction of β-pinene with rhodium carbonyl: A density functional study

In this work we have performed electronic structure calculations at the density functional theory (DFT) level in order to understand the structural, electronic and energetic factors involved in the diastereofacial selectivity in the interaction of β-pinene with [HRh(CO) 3], used as a model for the unmodified rhodium catalyst of hydroformylation. Analysis of the nature of the metal–ligand interaction of the π-complexes revealed that the catalyst coordination through the more sterically hindered face of β-pinene generates a stronger π-complex, however, being thermodynamically less stable due to the smaller steric hindrance for the catalyst coordination on the other face of the olefin. Our energetic results show that despite the coordination of the olefin to the more sterically hindered face being thermodynamically less favorable, the lower activation energy (8.5 kcal mol −1), the higher stability of the metal–alkyl product (−10.1 kcal mol −1) and the absence of an isomerization pathway for this coordination mode, are responsible for the dramatic preference for this pathway, observed experimentally.

Mass transfer effects during homogeneous 1-octene hydroformylation in [formula omitted]-expanded solvent: Modeling and experiments

CO 2 -expanded liquids (CXLs) represent a continuum of reaction media that combine the reaction benefits provided by organic solvents and the environmental benefits provided by supercritical CO 2 ( scCO 2 ) in an optimal manner. Homogeneous hydroformylation of 1-octene using an unmodified rhodium catalyst ( Rh ( acac ) ( CO ) 2 ) was successfully demonstrated employing CXLs as reaction media [Jin, H., Subramaniam, B., 2004. Homogeneous catalytic hydroformylation of 1-octene in CO 2 -expanded solvent media. Chemical Engineering Science 59, 4887–4893]. The reported experimental study, however, showed a long induction period of nearly 2 h, following which reaction occurs. A detailed reactor model incorporating reaction kinetics, mass transfer rates and phase equilibrium is presented to systematically investigate the effects of mass transfer and catalyst activation on induction period in 1-octene hydroformylation in CXL. Complementary experiments performed in stirred autoclave reactors equipped with an in situ ReactIR probe validate the cause of the induction period predicted by the model.

Rhodium catalyzed hydroformylation of 1-octene in microemulsion: comparison with various catalytic systems

In this work, we describe how addition of alkylpolyglycol ether type nonionic surfactant affects the hydroformylation of 1-octene in the presence of phosphine modified rhodium catalyst. Influence of different process parameters such as ligand excess and amount of surfactant on the reaction rate and selectivity were discussed. Direct comparison of microemulsion systems with classic processes was achieved by performing the reactions under comparable homogeneous and biphasic conditions. Thus, the experiments were carried out using catalysts such as unmodified rhodium carbonyl HRh(CO)4 and HRh(CO)(PPh3)3 in homogeneous system, Rh–TPPTS complex in two-phase system and in association with co-solvent.

Hydroformylation with rhodium phosphine-modified catalyst in a microemulsion: comparison of organic and aqueous systems for styrene, cyclohexene and 1,4-diacetoxy-2-butene

Use of microemulsion as a reaction media in the hydroformylation of different alkenes, namely styrene, cyclohexene and 1,2-diacetoxy-2-butene have been studied using alkylpolygylcol ether-type nonionic surfactant in the presence of phosphine-modified rhodium catalyst. The combination of the experiments under comparable homogeneous and biphasic conditions were performed in order to make direct comparison of microemulsion with classical systems. Thus, the experiments were also carried out using catalysts such as unmodified rhodium carbonyl H Rh(CO)4 and H Rh(CO)(PPh3)3 in homogeneous system, Rh–TPPTS complex in two-phase system and in association with co-solvent.

Alkyl-rhodium transition state stabilities as a tool to predict regio- and stereoselectivity in the hydroformylation of chiral substrates

A theoretical investigation on the stability of the alkyl rhodium transition states as the key-step determining the regio- and diastereoselective outcomes of the hydroformylation reaction with an unmodified rhodium catalyst (H–Rh(CO) 3) has been carried out. The results obtained employing effective core potentials for Rh in the LANL2DZ valence basis set, with the other atoms described at the B3P86/6-31G* level, have been compared to those computed with B3LYP/SBK(d), using effective core potentials for Rh and main group atoms. A number of contaminations between those levels or additional basis functions have also been used. The substrates considered are three related chiral olefins, namely (1-vinyloxy-ethyl)-benzene ( 1), (1-methyl-but-3-enyl)-benzene ( 2), and (1-methyl-allyl)-benzene ( 3). The structural features of the various possible complexes, which show a second chiral center at the inner olefin carbon upon complexation, do not present major changes among the various computational descriptions for each substrate. Significant differences in relative stabilities of the lowest energy transition states can be detected in the case of the ethereal substrate ( 1), whereas for both chiral alkenes ( 2 and 3) only very small energy gaps have been computed. In the case of 1 and 2, a quantitative agreement with available experimental results is obtained at the B3P86/6-31G* level, that should allow the prediction of regio- and stereoselectivity for chiral olefins not already screened. The B3LYP/SBK(d) values are comparable to the B3P86/6-31G* ones, although in the case of vinylether ( 1) the B3LYP/SBK(d) regioisomeric ratio turns out to be critical.

Homogeneous catalytic hydroformylation of 1-octene in CO 2-expanded solvent media

CO 2-expanded liquids (CXLs) represent a continuum of reaction media that combine the reaction benefits provided by organic solvents and the environmental benefits provided by compressed CO 2 (liquid and scCO 2) in an optimal manner. Homogeneous hydroformylation of 1-octene using an unmodified rhodium catalyst (Rh(acac)(CO) 2 ) was successfully demonstrated employing CXLs as reaction media. At temperatures of 30 and 60 ∘ C , the turnover numbers (TONs) for aldehyde formation in CO 2-expanded acetone were up to fourfold higher than those obtained in either neat acetone or compressed CO 2. The higher TON in CXLs is attributed to enhanced syngas solubility in CXL (compared to neat solvent) while maintaining complete miscibility of the transition metal catalyst complex (due to the presence of the neat solvent). The demonstrated advantages of using CXLs include significant replacement of conventional solvents (up to 80% by volume) by environmentally benign CO 2 and process intensification at relatively mild pressures (compared to those required with scCO 2). The regioselectivity towards linear and branched aldehydes ( n/ iso ratio) remained unaffected by the change in either the solvent media or the temperature. The observed effects of temperature and H 2 concentration in CXL media were similar to those reported in the literature: lower temperatures favored the selectivity toward the aldehydes while an increase in H 2 concentration resulted in a higher reaction rate. These first results point to the need for fundamental investigations of the catalyst and reactant solubilities and of the underlying reaction mechanism in CXLs to better understand and exploit these promising reaction media in catalytic hydroformylations.

Markedly different selectivity in the rhodium catalyzed hydroformylation of vinyl olefins containing a chiral alkoxy or alkyl group: good agreement between theory and experiment

A theoretical investigation on the regio- and stereoselective outcomes of the hydroformylation reaction with an unmodified rhodium catalyst (H–Rh(CO) 3) was carried out at the B3LYP/SBK(d) level on related chiral olefins, namely (1-vinyloxy-ethyl)-benzene ( 1) and (1-methyl-but-3-enyl)-benzene ( 2). A thorough and computationally expensive examination of the various possible H–Rh(CO) 3–olefin complex intermediates was performed, in order to determine, interpret, and eventually predict, the regio- and diastereoselectivity of the aforementioned reactions, whose products are a mixture of the linear aldehyde and of two diastereomers of the branched aldehyde. Regio- and diastereoselectivity of the reaction have been experimentally determined via hydroformylation runs at 20° and 100 °C for 1 and at 20 °C for 2. The theoretical results obtained are in good agreement with the experimental ones, which put forward a high chiral discrimination for chiral vinyl ether substrates in contrast to the lack of regio- and diastereoselectivity observed in the hydroformylation of 2. For the hydroformylation of 1, a regioselectivity ratio ( B: L) of 72:28 and a diastereoselectivity ratio ( b: b ′) of 97:3 have been computed which compare well to the corresponding experimental results (85:15 and 88:12). Therefore, theoretical calculations can give reliable estimates of regio- and diastereoselectivity provided a careful and accurate surface scan is performed for the alkyl-rhodium intermediates and the reaction is carried out at room temperature and, hence, in the absence of branched to linear alkyl isomerization.

The Rh4(CO)12-catalyzed hydroformylation of 3,3-dimethylbut-1-ene promoted with HMn(CO)5. Bimetallic catalytic binuclear elimination as an origin for synergism in homogeneous catalysis.

The bimetallic origins of catalytic synergism were studied using unmodified rhodium and manganese carbonyls as catalyst precursors during the low-temperature hydroformylation of 3,3-dimethylbut-1-ene to 4,4-dimethylpentanal in n-hexane solvent (T approximately 298 K, P(CO) = 1.0-4.0 MPa, P(H2) = 0.5-2.0 MPa). A dramatic increase in the catalytic rate was observed in the experiments conducted when both metals were used simultaneously. Detailed in-situ FTIR measurements indicated the observable presence of only homometallic complexes during catalysis, e.g., RCORh(CO)(4), Rh(4)(CO)(12), Rh(6)(CO)(16), HMn(CO)(5), and Mn(2)(CO)(10). The kinetics of product formation show a distinct linear-bilinear form in observables: k(1)[RCORh(CO)(4)][CO](-1)[H(2)] + k(2)[RCORh(CO)(4)][HMn(CO)(5)][CO](-1.5). The first term represents the classic unicyclic rhodium catalysis, while the second indicates a hydride attack on an acyl species. These spectroscopic and kinetic results strongly suggest that the origin of synergism is the presence of bimetallic catalytic binuclear elimination and not cluster catalysis. This appears to be the first detailed evidence for such a catalytic mechanism, and its implications for selectivity and nonlinear catalytic activity are accordingly discussed.

ChemInform Abstract: Hydroformylation with Unmodified Rhodium Catalysts: Reaction Mechanism and Origin of Regioselectivity

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Olefin Insertion into the Rhodium−Hydrogen Bond as the Step Determining the Regioselectivity of Rhodium-Catalyzed Hydroformylation of Vinyl Substrates: Comparison between Theoretical and Experimental Results

A comparison between experimental and theoretical data on regioselectivity concerning the hydroformylation of several vinyl substrates (propene, 2-methylpropene, 1-hexene, 3,3-dimethylbutene, fluoroethene, 3,3,3-trifluoropropene, vinylmethylether, allylmethylether, styrene) with unmodified rhodium catalysts is reported. Various H−Rh(CO)3-olefin complexes are examined at the B3P86/3-21G or /6-31G* level (LANL2DZ for Rh) and compared to the adducts with modified catalysts, such as HRhPH3(CO)2. The computed geometries are in satisfactory agreement with the X-ray ones. The activation energies for the alkyl rhodium intermediate formation, computed at either level along the pathways to branched or linear aldehydes, allow one to predict the regioselectivity ratios, since they are in very good agreement with the experimental ones evaluated for the isomeric aldehydes.

The competitive and non-competitive hydroformylation of conjugated dienes starting with tetrarhodium dodecacarbonyl. An in-situ high-pressure infrared spectroscopic study

It is well known that the liquid-phase homogeneous unmodified rhodium catalysed hydroformylation of alkenes is poisoned by the presence of trace quantities of conjugated dienes. Nevertheless, some hydroformylation of conjugated dienes is possible with unmodified rhodium, and this reaction is in general slower than alkene hydroformylations at comparable reaction conditions. In the present contribution, we examined (A) the catalytic behaviour of alkenes in the presence of trace conjugated diene impurities and (B) the catalytic behaviour of a variety of dienes using Rh 4(CO) 12 in n-hexane solvent at 293 K under 1.0–4.0 MPa CO and 0.5–2.0 MPa H 2. The analytic method was in-situ high-pressure infrared spectroscopy. It was observed that (I) in the hydroformylation of poisoned alkenes, most of the rhodium reacts with the trace quantity of conjugated dienes and not the alkenes in this competitive situation and (II) the metal carbonyl spectra of the hydroformylation of a variety of dienes are very similar. The primary absorbance maxima observed in the hydroformylations of conjugated dienes occur at circa 2109, 2091, 2087, 2064, 2049, 2037, 2030, 2020, 2012, 1999, and 1990 cm −1. Given the known chemistry of Rh 4(CO) 12 under syngas, and the very well documented chemistry of Rh 4(CO) 12 under alkene hydroformylation conditions, the lack of bridging carbonyls in the present experiments strongly suggested that the new infrared vibrations are due to mononuclear rhodium species. Preliminary analysis suggests the presence of at least three new species. In particular, the formation of observable η 3 allyl rhodium tricarbonyl species, σ allyl rhodium tetracarbonyl species and even acyl rhodium tetracarbonyl species RCORh(CO) 4 (R=alkenyl and/or formylalkyl) seems probable. Characteristic wavenumbers of 2108, 2064, 2037, 2020 and 1700 cm −1 are tentatively assigned to the latter. The reduced hydroformylation activity in the competitive hydroformylation of alkenes arises due to the much higher affinity of rhodium complexes for conjugated dienes than for alkenes under otherwise similar reaction conditions.

Diastereoselective Hydrogenation of Substituted Aromatics on Supported Metal Catalysts

The purpose of this work was to hydrogenate disubstituted aromatic compounds on supported metal catalysts to produce enantiomers of the corresponding cyclohexane derivatives using a diastereoselective approach, i.e., associating the organic substrates with chiral auxiliaries. The diastereoselective catalytic hydrogenation of N -(2-methylbenzoyl)-( S )-proline methyl ester was performed on carbon-supported metal catalysts, optionally modified by the adsorption of EDCA (ethyldicyclohexylamine). The formation of cis -diastereoisomers predominated and a cyclohexenic compound was formed transiently. After cleavage of the proline ester auxiliary, optically active 2-methylcyclohexane carboxylic acids were obtained. The influence on reaction kinetics and diastereoselectivity of various factors were investigated on Rh/C catalysts; these factors were pressure, substrate concentration, particle size, and thermal pretreatments of the rhodium catalysts modified with an amine. The reaction rate and the diastereoisomeric excess were found to be very sensitive to these parameters as well as to the presence of water on the catalyst. The importance of the nature of the metal (Rh, Ru, Pt, Pd) was clearly demonstrated. Pt and Pd showed very low activity. Without addition of amine, unmodified rhodium catalysts were nonselective as long as the aromatic substrate was present, whereas the ruthenium catalysts produced moderate selectivity. In the latter case, the carbonyl and carboxyl groups of the molecule could interact with the surface, thus determining which face of the aromatic ring would be preferentially adsorbed. Then, because of the consecutive hydrogenation of the cyclohexenic compound, diastereoselective excesses (d.e.) of 17 and 32%, respectively, were observed in favor of the (1R,2S,2S) isomer. In the presence of an achiral amine (e.g., EDCA) on rhodium catalysts, d.e. attaining ca. 50% were measured in favor of the (1S,2R,2′S) isomer. No inversion of configuration was observed with modified ruthenium, probably because the aromatic substrate is more strongly bonded to the surface than in the case of rhodium and thus displaces the amine. In contrast, the aromatic substrate is weakly bonded to platinum and palladium compared to the EDCA molecules which block the surface. In the absence of amine, the weaker adsorption of the aromatic substrate is responsible for the formation of trans products.