Rhodium catalysis isomerization

In contrast to triphenylphosphine-modified rhodium catalysis, a high aldehyde product isomer ratio via cobalt-catalyzed hydroformylation requires high CO partial pressures, eg, 9 MPa (1305 psi) and 110°C. Under such conditions alkyl isomerization is almost completely suppressed, and the 4.4 1 isomer ratio reflects the precursor mixture which contains principally the kinetically favored -butyryl to isobutyryl cobalt tetracarbonyl. At lower CO partial pressures, eg, 0.25 MPa (36.25 psi) and 110°C, the rate of isomerization of the -butyryl cobalt intermediate is competitive with butyryl reductive elimination to aldehyde. The product n/iso ratio of 1.6 1 obtained under these conditions reflects the equihbrium isomer ratio of the precursor butyryl cobalt tetracarbonyls (11). 

Branching at an olefinic carbon atom inhibited the reaction markedly, the most dramatic case being that of 2,3-dimethyl-2-butene. It should be noted that the product in this case is nearly exclusively 3,4-dimethylpen-taldehyde for either cobalt or rhodium catalysis (7). Thus, a general rule that products containing a formyl group attached to a quaternary carbon atom are not formed (49) remains valid. Hydroformylation proceeds only after isomerization has occurred. 

Similar cyclotrimerizations were accomplished with irradiation in 3 hours, but in lower yield <95CC(2)179>. Rhodium catalysis yielded pyridines without irradiation in slightly longer times, but the unsymmetrical alkynes used led to isomeric products, such as (2) and (3), (Scheme 2) <95JO(488)47>. 

Cyclopropyl ketones are readily isomerized to dihydrofuran derivatives thermally or under catalytic conditions.For example, cyclopropyl ketones 2 and 4 yield dihydrofurans 3 and 5, respectively, thermally or under rhodium catalysis. Such rearrangements occur normally under acid catalysis whereas thermolysis favors the vinylcyclopropane to cyclopentene rearrangement, except for highly functionalized (R = SOjPh) cyclopropanes. 

That Rh-allyl complexes can also act as nucleophiles in addition to aldehydes has been demonstrated by Oshima et al. in 2006 [198]. Retro-aUylation of the homoallyl alcohol 191 under rhodium catalysis generates a nucleophilic aUylrhodium species that reacts with aldehydes 190 to give the corresponding secondary alcohols 192 in situ (Scheme 12.94). Subsequent isomerization of these alcohols proceeds under the reaction conditions to furnish the corresponding saturated ketones 193 in modest to good yields. 

The reaction of crotyl bromide with ethyl diazoacetate once again reveals distinct differences between rhodium and copper catalysis. Whereas with copper catalysts, the products 125 and 126, expected from a [2,3] and a [1,2] rearrangement of an intermediary halonium ylide, are obtained by analogy with the crotyl chloride reaction 152a), the latter product is absent in the rhodium-catalyzed reaction at or below room temperature. Only when the temperature is raised to ca. 40 °C, 126 is found as well, together with a substantial amount of bromoacetate 128. It was assured that only a minor part of 126 arose from [2,3] rearrangement of an ylide derived from 3-bromo-l-butene which is in equilibrium with the isomeric crotyl bromide at 40 °C. 

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]. 

Chauvin, Y., Mussmann, L., and Olivier, H., A novel class of versatile solvents for two-phase catalysis hydrogenation, isomerization, and hydroformylation of alkenes catalyzed by rhodium complexes in liquid 1,3-dialkylimidazolium salts, Angew. Chem. Int. Ed., 34, 2698-2700,1996. 

For the hydroformylation, (PPli j) Rli( H)(CO) with host 11 was used as the catalyst. An excess of PPhj (stemming from the catalyst precursor) was needed to avoid isomerization, as was found when phosphine-free precursors were used (at the concentrations used even bidentates should be added in excess to prevent substantial exchange with carbon monoxide). Linear to branched ratios of 2 1 were obtained and no isomerized alkene could be detected. These results are similar to those obtained by Kalck and coworkers [41]. As expected, catalysis for 11 is slower than that for (PPh3)3Rh(H)(CO) as the host is a bidentate phosphine catalysis with (PPh3)3Rh (H)(CO) strongly depends on the concentrations of rhodium and PPh3 and comparison of the rates of the two systems does not make sense. 

The same reaction scheme can be written for (Z) -2-phenyl-2-butene, except that paths B and E would lead to erythro and threo aldehydes. In cobalt catalysis this isomerization could explain both the lack of stereospecificity and the lack of influence of the sterochemistry of the starting olefin on the distribution of aldehydes 26 and 27. This hypothesis agrees well with results with a-ethylstyrene. On the other hand, when rhodium is used, extensive isomerization occurs less readily probably because of a better stability of alkyl- and acylrhodium carbonyls, and one can thus achieve a high degree of stereospecificity. 

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). 

In the case of rhodium as a catalyst metal for the hydroformylation of methyl oleate, lower pressure and lower temperature have to be compared to cobalt catalysis [20, 21], The use of rhodium is also advantageous because of the lower isomerization. Frankel showed that with a rhodium triphenylphosphine catalyst, hydroformylation occurs only on the ninth and tenth carbon atoms of the methyl oleate [22]. 

Although the hydridorhodacarborane is formally a rhodium (III) derivative, it functions as a facile catalyst in alkenc isomerization, hydrogenation, hydroformylation, and hydrosilylation reactions 80). This catalyst system is extremely stable and may be recovered quantitatively from alkene isomerization and hydrogenation reactions. In addition to these reactions, the hydridorhodacarborane is very effective in the catalysis of deuterium exchange at terminal BH positions 59). These discoveries may soon lead to industrially useful metallocarborane catalysts. 

Asymmetric catalysis undertook a quantum leap with the discovery of ruthenium and rhodium catalysts based on the atropisomeric bisphosphine, BINAP (3a). These catalysts have displayed remarkable versatility and enantioselectivity in the asymmetric reduction and isomerization of a,P- and y-keto esters functionalized ketones allylic alcohols and amines oc,P-unsaturated carboxylic acids and enamides. Asymmetric transformation with these catalysts has been extensively studied and reviewed.81315 3536 The key feature of BINAP is the rigidity of the ligand during coordination on a transition metal center, which is critical during enantiofacial selection of the substrate by the catalyst. Several industrial processes currently use these technologies, whereas a number of other opportunities show potential for scale up. 

Molecular hydrogen can reduce cyclic peroxo complexes, for example Reaction 30 (111), and Sheldon and Van Doom (33) had envisaged epoxide production via a similar process, Reaction 31. A mechanism based on such reactivity (with subsequent epoxide isomerization) is a possibility for our catalysis using H2/02, although Reaction 31 did not occur under mild conditions (33). Isolation of a related peroxometallocyclic rhodium complex, (Ph3As)2Rh[02C2(CN)4] + (92), allows for a testing of Reaction 31 at a Rh center. 

We have already alluded to the diversity of oxidation states, the dominance of oxo chemistry and the cluster carbonyls. Brief mention should be made too of the tendency of osmium (shared also by ruthenium and, to some extent, rhodium and iridium) to form polymeric species, often with oxo, nitrido or carboxylato bridges. Although it does have some activity in homogeneous catalysis (e.g. of m-hydroxylation, hydroxyamination or animation of alkenes, see p. 558, and occasionally for isomerization or hydrogenation of alkenes, see p. 571), osmium complexes are perhaps too substitution-inert for homogeneous catalysis to become a major feature of the chemistry of the element. The spectroscopic properties of some of the substituted heterocyclic nitrogen-donor complexes may yet make osmium an important element for photodissociation energy research. 

The regioselectivity in the hydroformylation of monohydroxyben-zenes was the same for the host catalyst and a triphenylphosphine complex of rhodium (with a molar product ratio linear branched = 2 1) (35). No isomerization was observed. Rates of the conversion of monohydrox-ybenzenes when host-guest catalysis is employed are lower than when a triphenylphosphine complex of rhodium is used. In contrast, dihydroxy substrates, which are more strongly boimd to the host, react at higher rates, with an initial increase by a factor of 4 relative to catalysis by the bare rhodium complex. Dihydroxybenzenes also gave the selectivities to linear products (linear branched molar product ratio > 20 1, see below). At 30% conversion, product inhibition took place. 

Ammonia N-donor Ligands Asymmetric Synthesis by Homogeneous Catalysis Decarbonylation Catalysis Hydride Complexes of the Transition Metals Hydroboration Catalysis Hydrogenation Isomerization of Alkenes Hydrosilation Catalysis P-donor Ligands Rhodium Organometallic Chemistry. 

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