Rhodium complexes hydrocarbons

A number of catalysts are known to effect homogeneous hydrogenation of aromatic hydrocarbons, e.g., some oxidized rhodium complexes (/, p. 238), some rhodium 7r-complexes with phenyl carboxylates (/, p. 283), some Ziegler systems (/, p. 363), and Co2(CO)8 (/, p. 173). However, the catalysts in the first three systems are not well characterized, and the carbonyl systems require fairly severe hydroformylation conditions, although they are reasonably selective, possibly via radical pathways (Section II, C). 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

In the gas phase reaction of Rh with c-CaH, among other processes, elimination of C2H4 has been observed and the resulting metal ion was shown to have the structure of a methylidene-rhodium complex (238) instead of a hydrido-methylidene species (239) . RhCHa (238) reacts readily with H2 and CH4 and represents the first example of methane activation by a cationic mononuclear transition-metal complex in the gas phase. Reactions of both Rh = CH2 and its analogues Fe-CH2 and C0-CH2 with cyclic hydrocarbons were studied, and it is assumed that in the initial step metallacycloalkanes are generated (Scheme 35). 

Diels-Alder reaction to polycyclic hydrocarbons. The structure of the intermediate metallo-adduct has been determined by X-ray studies for a rhodium complex. ... 

The major part of the spin density is delocalized over the hydrocarbon framework of the trop ligand. Inspection of previously reported monomeric rhodium complexes with... 

Rhodium complexes generated from the water-insoluble carboxylated surfactant phosphine 17 (n = 3, 5, 7, 9, 11) were used as catalysts in the micellar hydrogenation of a- and cyclic olefins, such as 1-octene, 1-dodecene, and cyclohexene, in the presence of conventional cationic or anionic tensides such as cetyltrimethylammo-nium bromide (CTAB) or SDS and co-solvents, e.g., dimethyl sulfoxide [15], After the reaction the catalyst was separated from the organic products by decantation and recycled without loss in activity. There is a critical relationship between the length of the hydrocarbon chain of the ligand 17 and the length and nature of the added conventional surfactant, for obtaining maximum reactivity. For example,... 

The intermediacy of rhodium complex 9 structurally similar to 6 has been proposed in the stoichiometric formation of the acylrhodium complex 10 from the hydrocarbon 1 and di-ju.-chloro-tetracarbonyldirhodium (I) (16). Thus both stoichiometric and catalytic trapping experiments have demonstrated the valence isomerization 1 -> 2 is going in a stepwise fashion via an organometallic intermediate. 

However, TRIR has also been applied to more classical coordination compounds. Ford and co-workers have used a combination of ns-TRIR and time-resolved UV/vis spectroscopy to investigate the mechanism of hydrocarbon C—H bond activation with the rhodium complex, trans-RhCl(CO)(PR3)2 (R = Ph, />-tolyl, or Me). Upon photoexdtation, each of these species was found to undergo CO dissociation to form the transient solvated complex, tra 5-RhCl(Sol)(PR3)2 (Sol = solvent). The solvated complexes reacted with added CO to regenerate the parent complex, and also underwent competitive unimolecular C H activation to form the Rh products of hydrocarbon oxidative addition. These were identified from the step scan FTIR spectra, which showed a positive shift in /(CO) relative to the parent complex, which is consistent with oxidation of the metal center. 

The first direct observation of oxidative addition of the C-H bond of a saturated hydrocarbon to a transition metal center was reported in 1982 by Janowicz and Berman and by Hoyano and Graham (Equation 6.27). Jones reported the oxidative addition of alkane C-H bonds to the analogous rhodium complexes at nearly the same time (Equation 6.28). These reports have provided the foimdation for himdreds of subsequent reports of C-H bond cleavage by late transition metal complexes and studies of the mechanism by which a metal can cleave an alkane C-H bond under mild conditions. In fact, cleavage of the strong C-H bond in methane - (104 kcal/mol) occurs even at 12 K in a CH matrix. ... 

The insoluble, polymer-supported, optically-active rhodium complex prepared by Dumont et al. (1973) was used for asymmetric reduction of alkenes, leading to optically active hydrocarbons. Thus, 2-phenylbutene produced (R)-2-phenyl butane in 1.5% optical purity. The catalyst also hydrogenated methyl atropate to (5)-(-i-) methyl hydroatropate with an optical yield 2.5%. With a-acetamidocinnamic acid there was no reduc-... 

As was pointed out in Chapter 5, with reference to the conjugated ligand cyclooctatetraene, and as the structures of the cyClobutenyl nickel complex, 7.IS, and of the rhodium complex 7.18 show, it is not necessary for all the annular carbon atoms of a cyclic unsaturated hydrocarbon ligand to be involved in bonding to a metal. Further examples of this are shown in Figure 41, where in complexes derived from S-, 6- and 7-membered ring systems respectively, only 5 electrons are formally involved in the bonding in each case. 

It is very well established that 16-electron species such as [CsRsML] (M = Rh, Ir) generated in situ undergo facile oxidative addition of CH bonds of saturated hydrocarbons or arenes (see COMC (1995) and COMC (1982)). This situation is likely to be due to the fact that iridium and rhodium complexes are kinetically far more stable than cobalt. However, during the last decade, some CH activations were found to take place on some typical cobalt complexes. 

The use of polymeric catalytic membranes in distributor/contactor-type reactors for hydrogenation or oxidation reactions has been widely described in several former and recent reviews. Mixed-matrix membranes of PDMS hlled with Pd particles, composite membranes of ionic liquid-polymer gels filled with Pd/C, ° ionic liquids containing rhodium complexes and supported in polystyrene sheets in a corrugated configuration, have been used for the selective gas-phase hydrogenation of hydrocarbons in contactor-type membrane reactors. 

The use of silver fluoroborate as a catalyst or reagent often depends on the precipitation of a silver haUde. Thus the silver ion abstracts a CU from a rhodium chloride complex, ((CgH )2As)2(CO)RhCl, yielding the cationic rhodium fluoroborate [30935-54-7] hydrogenation catalyst (99). The complexing tendency of olefins for AgBF has led to the development of chemisorption methods for ethylene separation (100,101). Copper(I) fluoroborate [14708-11-3] also forms complexes with olefins hydrocarbon separations are effected by similar means (102). 

Liquid-phase oxidation of lower hydrocarbons has for many years been an important route to acetic acid [64-19-7]. In the United States, butane has been the preferred feedstock, whereas ia Europe naphtha has been used. Formic acid is a coproduct of such processes. Between 0.05 and 0.25 tons of formic acid are produced for every ton of acetic acid. The reaction product is a highly complex mixture, and a number of distillation steps are required to isolate the products and to recycle the iatermediates. The purification of the formic acid requires the use of a2eotropiag agents (24). Siace the early 1980s hydrocarbon oxidation routes to acetic acid have decliaed somewhat ia importance owiag to the development of the rhodium-cataly2ed route from CO and methanol (see Acetic acid). 

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