Olefin isomerization rhodium-catalyzed

Olefin isomerization can be catalyzed by a number of catalysts such as molybdenum hexacarbonyl [13939-06-5] Mo(CO)g. This compound has also been found to catalyze the photopolymerization of vinyl monomers, the cyclization of olefins, the epoxidation of alkenes and peroxo species, the conversion of isocyanates to carbodiimides, etc. Rhodium carbonylhydrotris(triphenylphosphine) [17185-29-4] RhH(CO)(P(CgH )2)3, is a multifunctional catalyst which accelerates the isomerization and hydroformylation of alkenes. 

Conventionally, organometallic chemistry and transition-metal catalysis are carried out under an inert gas atmosphere and the exclusion of moisture has been essential. In contrast, the catalytic actions of transition metals under ambient conditions of air and water have played a key role in various enzymatic reactions, which is in sharp contrast to most transition-metal-catalyzed reactions commonly used in the laboratory. Quasi-nature catalysis has now been developed using late transition metals in air and water, for instance copper-, palladium- and rhodium-catalyzed C-C bond formation, and ruthenium-catalyzed olefin isomerization, metathesis and C-H activation. Even a Grignard-type reaction could be realized in water using a bimetallic ruthenium-indium catalytic system [67]. 

Recent Advances in Rhodium(l)-Catalyzed Asymmetric Olefin Isomerization and Hydroacylation Reactions... 

In 2000, we demonstrated that the planar-chiral phosphaferrocene PF-PPhj is a useful ligand for rhodium-catalyzed asymmetric isomerizations of several allylic alcohols, providing the first catalyst system that furnishes the target aldehyde in >60% ee (Eq. 6) [7]. It appears that, in order to obtain high enantiomeric excess (>0% ee), the olefin should bear a relatively bulky substituent (for example, Pr Eq. 6). 

Both the rhodium and the cobalt complexes catalyze olefin isomerization as well as olefin hydroformylation. In the case of the rhodium(I) catalysts, the amount of isomerization decreases as the ligands are altered in the order CO > NR3 > S > PR3. When homogeneous and supported amine-rhodium complexes were compared, it was found that they both gave similar amounts of isomerization, whereas with the tertiary phosphine complexes the supported catalysts gave rather less olefin isomerization than their homogeneous counterparts (44, 45). 

Scheme 8.1 Rhodium-catalyzed hydroformylation of alkenes leading to linear (L) and branched (B) aldehydes and isomerized (IS) olefins.

Olefin isomerization (Continued) industrial applications, 9, 95 ligand synthesis, 96 rhodium-catalyzed, 98 Olefins ... 

Curtin-Hammett principle, 23 industrial applications, 8, 26 mechanism, 21 phosphine ligands, 7, 18 reaction conditions, 18 scope and limitations, 27 Wilkinson complex, 17 Rhodium-catalyzed olefin isomerization ab initio calculations, 110... 

Recently, it has been discovered that catalysis by rhodium compounds is more effective than by the older cobalt catalyst when tris(triphenylphosphine)rhodium chloride is treated with carbon monoxide, the catalyst bis(triphenylphosphine)rhodium carbonyl chloride is formed. This catalyst is very effective under very mild conditions (49-51). It is believed that the tr-ir rearrangement is also important with this catalyst and operates in a manner analogous to that in the cobalt-catalyzed process, since stablization of the cr complex has been shown to lead to olefin isomerization and lower linear selectivity (52). 

Olefin isomerization has been widely studied, mainly because it is a convenient tool for unravelling basic mechanisms involved in the interaction of olefins with metal atoms (10). The reaction is catalyzed by cobalt hydrocarbonyl, iron pentacarbonyl, rhodium chloride, palladium chloride, the platinum-tin complex, and by several phosphine complexes a review of this field has recently been published (12). Two types of mechanism have been visualized for this reaction. The first involves the preformation of a metal-hydrogen bond into which the olefin (probably already coordinated) inserts itself with the formation of a (j-bonded alkyl radical. On abstraction of a hydrogen atom from a diflFerent carbon atom, an isomerized olefin results. 

In order to eliminate the possibility for in situ carbene formation Raubenheimer et al. synthesized l-alkyl-2,3-dimethylimidazolium triflate ionic liquids and applied these as solvents in the rhodium catalyzed hydroformylation of l-hejEne and 1-dodecene [178]. Both, the classical Wilkinson type complex [RhCl(TPP)3] and the chiral, stereochemically pure complex (—)-(j7 -cycloocta-l,5-diene)-(2-menthyl-4,7-dimethylindenyl)rhodium(i) were applied. The Wilkinson catalyst showed low selectivity towards n-aldehydes whereas the chiral catalyst formed branched aldehydes predominantly. Hydrogenation was significant with up to 44% alkanes being formed and also a significant activity for olefin isomerization was observed. Additionally, hydroformylation was found to be slower in the ionic liquid than in toluene. Some of the findings were attributed by the authors to the lower gas solubility in the ionic liquid and the slower diffusion of the reactive gases H2 and CO into the ionic medium. 

Industrially, the rhodium-catalyzed hydroformylation is normally operated at about 100°, at pressures up to 50 atm and in the presence of a large excess of added phosphorus ligand, it can be carried out in molten PPhs. Under these conditions, a terminal olefin can be converted in over 90% yield to linear aldehyde. By-products include branched aldehydes as well as small amounts of alkanes and isomerized olefins. Advantages over the more conventional cobalt catalysts include lower temperatures and pressures, higher ratios of linear to branched products, and less hydrogenation of aldehyde products to alcohols. 

Note added in proof In the preparation of rhodium and iridium complexes from cyclo-octa-1,5-diene, however, all the uncomplexed olefin is converted to the 1,3-diene. The same isomerization is catalyzed by tertiary phosphine complexes of rhodium and iridium, and the 1,4-diene can be detected as an intermediate (117a). 

The intramolecular insertion of a hydride into a coordinated olefin is a crucial step in olefin hydrogenation catalyzed by late transition metal complexes, such as those of iridium, rhodium, and ruthenium (Chapter 15), in hydroformylation reactions catalyzed by cobalt, rhodium, and platinum complexes (Chapter 16), and in many other reactions, including the initiation of some olefin polymerizations. The microscopic reverse, 3-hydride elimination, is the most common pathway for the decomposition of metal-alkyl complexes and is a mechanism for olefin isomerizations. 

Depending on the nature of olefins, hydrosilanes, and catalysts, side reactions can also occur, such as H-Cl exchange in silanes as well as hydrogenation and olefin isomerization (144,145). In some cases, in the reaction catalyzed by some complexes of ruthenium, rhodium, iron, iridium, platinum, and nickel, alkenylsilanes have been obtained as a major product of the dehydrogenative... 

Subsequent to Brookhart s findings, Jun el al. [101] developed a rhodium-catalyzed alkylation of a,P-unsaturated carbonyl compounds in the presence of diethylamine (Scheme 19.70). The proposed mechanism involves the in silu formation of an enamine, followed by C-H activation directed by this enam-ine, olefin hydrometallation, and reductive ehmination. In situ enamine hydrolysis regenerates the carbonyl group. The authors demonstrated that the formation of product 32 originates from the isomerization of initially formed compound 31 with the rhodium catalyst. 

The rhodium-catalyzed alkylation has also been extended to the alkylation of a,P-unsaturated aldimines (Scheme 19.75) [109], An efficient system was found using [RhCl(coe)2]2 in combination with electron-rich Ugands (i.e., PCyj, FcPCyj), allowing milder reaction conditions (i.e., 50°C). Under these conditions, limited ( /Z)-isomerization was observed and the reaction proceeded with olefins substituted with alkyl, aryl, ester, and alkyne groups. 

Many transition metal hydrides and low-valent complexes that can generate an M—H bond by protonation catalyze hydrogen migrations in olefins. Rhodium trichloride or rhodium(I) compounds plus HCl rapidly isomerize 1-butene to an equilibrium mixture of butenes in which trans-2-butene is the largest single component. Most of the complexes that catalyze olefin dimerization also catalyze isomerization. The isomerization mechanism postulated by Cramer is similar to his dimerization mechanism except that no insertion step is involved (185). 

The requisite benzyne is generated in situ by the reaction of ortho-bromo-fluorobenzene with magnesium. These ligands, which are conformationally strongly restricted in all three dimensions, gave in the rhodium-catalyzed hydroformylation high activity, but even more remarkable, they induced almost no olefin isomerization. 

Hydroformylation of Aliphatic Olefins In 1997, Herrmann and coworkers [25] were the first to use NHCs and imidazohum salts, respectively, as hgands or preligands in rhodium-catalyzed hydroformylation. The isolated NHC-rhodium complexes 1 and 2 (Figure 2.55) and the complexes prepared in situ from the water-soluble imidazolium salts 3a-c and rhodium(lll)acetate were tested in the homogeneous and biphasic hydroformylation of propene. The catalyst derived from 1 produced >99% yield of isomeric butanals (CO/H2 = 1 1,10 MPa S/C = 100 000 1, toluene, 60 h). In the biphasic system, after 20 h of reaction time and S/C = 10000 1 in water, rhodium catalysts derived from 2 or based on hgands 3a-c allowed up... 

Scheme 5.13 Competition between isomerization and hydroformylation in relation to the CO partial pressure in rhodium-catalyzed hydroformylation of terminal olefins.

J-Silylallylic alcohols undergo rhodium-catalyzed double bond migration to give the corresponding a-silyl-substituted ketones in excellent yields . Thermolysis of 243 gives the corresponding silacyclopentenes 244 (equation 196). The reaction may proceed via an olefin to a carbene isomerization . ... 

Phenoxaphosphino-modified xantphos-type ligands in the rhodium-catalyzed hydroaminomethylation of internal olefins were found to give linear amines. Hydro-aminomethylation and each of its individual steps were monitored by high-pressure infrared spectroscopy. The results suggest that hydroaminomethylation takes place by a sequential isomerization/hydroformylation/amination/hydrogenation pathway [170]. 

Rhodium (l)-Cata yzed Asymmetric Isomerization of Olefins 83 Tab. 4.1 Rh(l)/PF-P(o-Tol)2-catalyzed isomerization of E-allylic alcohols. 

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