Rhodium complexes oxygen

Rhodium complexes with oxygen ligands, not nearly as numerous as those with amine and phosphine complexes, do, however, exist. A variety of compounds are known, iucluding [Rh(ox)3] [18307-26-1], [Rh(acac)3] [14284-92-5], the hexaaqua ion [Rh(OH2)3] [16920-31 -3], and Schiff base complexes. Soluble rhodium sulfate, Rh2(804 )3-a H2 0, exists iu a yellow form [15274-75-6], which probably coutaius [Rh(H20)3], and a red form [15274-78-9], which contains coordinated sulfate (125). The stmcture of the soluble nitrate [Rh(N03)3 2H20 [10139-58-9] is also complex (126). Another... 

The use of a catalyst such as cadmium oxide increases the yield of dibasic acids to about 51% of theoretical. The composition of the mixed acids is about 75% C-11 and 25% C-12 dibasic acids (73). Reaction of undecylenic acid with carbon monoxide using a triphenylphosphine—rhodium complex as catalyst gives 11-formylundecanoic acid, which, upon reaction with oxygen in the presence of Co(II) salts, gives 1,12-dodecanedioic acid in 70% yield (74). 

In one such procedure a rhodium complex concentrate prepared from a 400 ppm rhodium containing hydroformylation catalyst solution, for which catalytic activity had declined to about 30 percent of its initial value, was concentrated in a wiped-film evaporator to about 27,700 ppm rhodium. This concentrate was oxygenated with tertbu-tylhydroperoxide. After isolation and treatment with triphenylphosphine, a 70% yield of [HRh(CO)(PPh3)3] was obtained.[41]... 

Formation of the methyl-rhodium complex is analogous to the formation of CH3-C(C0)4 from CH30H2 arid Co(C0K as proposed by Wender. The difference here is that the nature of the active rhodium species is not known. Under the present conditions,homologation does not occur because CO is not present however, addition of the methyl-rhodium species to benzaldehyde must occur as shown in (19), metal adds to the oxygen. The product in (19) is then subject to acid catalyzed etherification to obtain the methyl ether. 

The differences between the iridium and rhodium systems were interpreted by considering that Int 1 may open an oxygenation path via intramolecular rearrangement. In this case, the scission of a M-0 bond between the semi-quinone and metal center would be followed by intra-diol insertion of an oxygen and ultimately by the formation of muconic acid and water. The results indicate partial preference of the rhodium complex toward the oxygenation path. 

Another type of chiral rhodium complex [Rh-MeDuPHOS] was also immobilized in [BMIM][PF6] and used in the enantiomeric hydrogenation of related enamides [95] (Fig. 41.5). Geresh et al. focused their research on the stabilization of the air-sensitive catalyst in the air-stable ionic liquid, so that the complex was protected from attack by atmospheric oxygen and recycling was possible. 

The rhodium-diphosphine catalysts are generally sensitive to oxygen, hence the reactions have to be carried out under strictly inert atmospheric conditions. A decrease in the yield or the enantiomeric excess can be due to a lack of sufficient precaution during the procedure or to the inactivation of the catalyst when exposed to oxygen. However, the reactions using rhodium complexes as catalysts give very good results which correlate well with the published material. 

Based on the precedent of Van Leeuwen and Roobeek, livinghouse and co-workers screened a variety of electron-deficient phosphine/phosphite ligands for the rhodium-catalyzed [4-1-2] reaction, which provided an alternative catalyst system for the formation of 5,6- and 6,6-ring systems [13]. The most notable of these was the tris-(hexafluoro-2-propyl) phosphite-modified rhodium complex, which was applicable to both carbon- and oxygen-tethered substrates, and also provided the first example of a facial-directed diastereoselective intramolecular rhodium-catalyzed [4-i-2] reaction (Eq. 4). 

Carbonyl ylides can be viewed as an adduct between a carbonyl group and a carbene and, in fact, some ylides have been prepared this way (see above). The application of carbonyl ylides to the synthesis of complex natural products has been greatly advanced by the finding that stabilized carbenoids can be generated by the decomposition of ot-diazocarbonyl compounds with copper and rhodium complexes. The metallocarbenoids formed by this method are highly electrophilic on carbon and readily add nucleophiles such as the oxygen of many carbonyl derivatives to form carbonyl ylides. This type of reaction is in fact quite old with the first report being the addition of diazomalonate and benzaldehyde (33,34). 

Abstract The purpose of this chapter is to present a survey of the organometallic chemistry and catalysis of rhodium and iridium related to the oxidation of organic substrates that has been developed over the last 5 years, placing special emphasis on reactions or processes involving environmentally friendly oxidants. Iridium-based catalysts appear to be promising candidates for the oxidation of alcohols to aldehydes/ketones as products or as intermediates for heterocyclic compounds or domino reactions. Rhodium complexes seem to be more appropriate for the oxygenation of alkenes. In addition to catalytic allylic and benzylic oxidation of alkenes, recent advances in vinylic oxygenations have been focused on stoichiometric reactions. This review offers an overview of these reactions... 

Most of the catalytic studies on the oxygenation of alkenes were carried out in the 1970s and 1980s in which typical rhodium complexes [9,10] such as [RhCKPPhals] [11], or even Rh Cls [12-14] with or without co-catalysts (Cu [14,15] or Bi [16]), and some iridium compoimds such as [ Ir (p.-Cl)HCl(cod) 2] [17] were used as catalyst precursors. 

Rhodium complexes were generally found to be more effective than iridium, but on the whole they show moderate activity in alkene oxygenation reactions. Significantly, epoxides, a typical product of the oxidation of olefins catalyzed by the middle transition metals, have rarely been evoked as products [18-22]. Although allylic alcohols [23] or ethers [24] have sometimes been described as products, the above cited rhodium and iridiiun complexes are characterized by an excellent selectivity in the oxygenation of terminal alkenes to methyl ketones. 

The inefficiency of the platinum/hydrogen reduction system and the dangers involved with the combination of molecular oxygen and molecular hydrogen led to a search for alternatives for the reduction of the manganese porphyrin. It was, for example, found that a rhodium complex in combination with formate ions could be used as a reductant and, at the same time, as a phase-transfer catalyst in a biphasic system, with the formate ions dissolved in the aqueous layer and the manganese porphyrin and the alkene substrate in the organic layer [28]. 

The reaction of bis-phenylpropargyl ether (321) with tris(triphenylphosphine)rhodium chloride in benzene or toluene led to the formation of the unusual organometallic compound (322), which can be viewed as a derivative of an oxygen-rhodium pentalene system. Reaction of the rhodium complex (322) with sulfur leads to the corresponding 4,6-diphenyl-l,3-dihydro[3,4-c]furan (323). The selenium and tellurium analogs (324) and (325) were made in a similar manner (Scheme 111) (76LA1448). 

These results gave some support to the premise that mixed ligand complexes could be prepared from these oxidized rhodium complexes. Because of the supposed polymeric nature of 3 and the difficulty encountered in determining the nature of the oxygen function present in this species, more attention was devoted to studying the reactions of 2 and 4, in both of which the nature of the oxygen function has been established. 

When the catalyst was used for simple olefin systems, it was not as active as with the amino acid precursors. Table III shows the relative rates for a variety of substrates, special care being taken in each case to purge oxygen. The slow rate of a-phenylacrylic acid was unexpected, but, it may be the result of a stable olefin-rhodium complex similar to the one Wilkinson (15) experienced with ethylene. Such a contention is consistent with the increased speed of hydrogenation with increased pressure. 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

This oxygen-transfer reaction was based on an important observation by Read et al., who found that rhodium complexes such as RhCl(PPh3)3 were able to promote the cooxygenation of terminal... 

In the spectra of the various solutions containing excess triphenylphosphine, the expected singlet signal at 7.2 ppm appeared. Also present in all of the solutions was a sharp singlet signal at +22.1 ppm, due to triphenylphosphine oxide, which always was formed due to oxidation by the traces of molecular oxygen present. The present rhodium complex system is a very effective catalyst for such oxidations. 

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