Rhodium bimetallic complexes

Hsu et al. [15] applied a bimetallic catalyst comprising rhodium and ruthenium for the hydrogenation to combine the high selectivity of the rhodium complex with the lower cost of the ruthenium complex. When the amount of each metal is identical, the catalytic activity of the bimetallic complex catalyst system was similar to that of the single rhodium-complex catalyst, containing... 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34] ... 

Other recent reports have also indicated that mixed-metal systems, particularly those containing combinations of ruthenium and rhodium complexes, can provide effective catalysts for the production of ethylene glycol or its carboxylic acid esters (5 9). However, the systems described in this paper are the first in which it has been demonstrated that composite ruthenium-rhodium catalysts, in which rhodium comprises only a minor proportion of the total metallic component, can match, in terms of both activity and selectivity, the previously documented behavior (J ) of mono-metallic rhodium catalysts containing significantly higher concentrations of rhodium. Some details of the chemistry of these bimetallic promoted catalysts are described here. 

Mirkin and coworkers reported on catalytic molecular tweezers used in the asymmetric ring opening of cyclohexene oxide. In this case the early transition metal is the catalyst and rhodium functions as the structural inductor metal. The catalyst consists of two chromium salen complexes, the reaction is known to be bimetallic, and a switchable rhodium complex, using carbon monoxide as the switch. Indeed, when the salens are forced in dose proximity in the absence of CO the rate is twice as high and the effect is reversible [77]. 

With the exception of the mono-rhodium complexes of the n-propylsapphyrins (i.e., 5.88 and 5.91), each of the above mono- and bimetallic complexes proved to be... 

In an unusual approach, Rauchfuss and coworkers were able to show tliat Fc3(CO),2 reacts not only with free thiophenes -as described in the preceding section- but also with the n activated thiophene in the rhodium complex Cp Rh(r -T), to yield predominantly a bimetallic product containing a ferrole unit K-bonded to Rh. The reaction was shown to proceed via a stable intermediate containing the T ligand simultaneously bonded t) to Cp Rh and t) to a Fe(CO)4 fragment both complexes... 

A highly active catalytic system for direct arylation reactions of nonactivated arenes relied on a homobimetallic rhodium complex. Thus, treatment of [bis(2-pyridyl) amino]diphenylphosphane (109) with [Rh(cod)Cl]2 led to the formation of a complex 108, which, according to X-ray crystal structure analysis, consisted of [Rh(cod) Cl2] anion and a rhodium cation stabilized by two P,N-ligands (Scheme 9.34) [73]. This bimetallic rhodium complex (108) allowed the direct arylation of benzene (87) with the aryl chloride 106 with a turnover number (TON) of 780 under comparably mild reaction conditions. 

Rhodium, tetrakis(trimethylphosphine)-reactions, 926 Rhodium carboxylates, 903 chemotherapy, 903 Rhodium complexes, 901 applications. 903 bimetallic, 1074, 1076 cationic, 1072,1074... 

Scheme 76. Synthesis of bimetallic tungsten-rhodium complexes with bridging alkylidyne groups (R = C6H4Me-4, Me L = PPh3, X = Pp6 L = 2 cod, X = Bp4).

In 1996, Garcia et al. reported cyclotrimerization of (trifluoromethyl)acetylene 390a using a bimetallic rhodium complex at room temperature to afford l,3,5-/ra (trifluoromethyl) benzene 391 in high yield (Scheme 2-33). Recently, Tanaka et al. reported a chemoselective and regioselective intermolecular cross-cyclotrimerization of two different alkynes using a cationic Rh complex as catalyst (Scheme 2-34). ° ... 

Modified Rh systems show considerable activity in the hydroformylation of styrene to the branched aldehydes. Chiral diphosphine rhodium complexes that lead to high activity and high regioselectivity in branched aldehyde have been studied. These systems do not provide high enantioselectivity [2, 3]. Stanley reported that a tetraphosphine hgand can be used to form a bimetallic complex [8] and provide ee s up to 85% using vinyl esters. 

Synthetic binuclear complexes as catalysts, aiming at high activity, have been subjects of intensive research [3], However, the number of novel bimetallic catalysts that are active and actually operating via a bimetallic mechanism is extremely scarce. Stanley and co-workers reported a bimetallic rhodium complex that is an active and selective hydroformylation catalyst that was proposed to operate via a cooperative mechanism. 

Related to this work, Seller and Geissler [109] advocated the use of bimetallic catalysts, one for the isomerization and the other for the hydroformylation. Indeed, with a catalytic system comprising a rhodium complex based on a chelating phosphine-phosphite ligand and Ru3(CO)j2 (0.1-0.5 mol%), almost a reversal of the regioselectivity in the reaction with -2-butene in comparison to the monometallic rhodium catalyst l/b = 42 58 TOP = 700 h ) was achieved. 

Mixtures of homogeneous rhodium and iron catalysts were tested in the hydroformylation of 1-hexene by Trzeciak and Zidtkowski [146]. In the absence of a rhodium complex, Fe(CO)5 did not show any catalytic activity at 80 °C and 10 atm of syngas pressure. Addition of Rh(acac)(CO)2 (acac = acetylacetonate) led to the formation of 2-hexene and eventually 3-hexene, but no aldehyde was formed. Hydroformylation commenced only in the presence of Rh(acac)(CO)(PPh3). The bimetallic catalyst benefited from the presence of additional PPhg. At a ratio of P/Rh = 3, a rate acceleration of 2 times was observed, and eventually an 83% yield of aldehyde was obtained. No change in the Ub ratio was observed as a result of these modifications. 

Bimetallic phase-transfer-catalysis is a process whereby a reaction that occurs using two different metal complexes, does not proceed in the absence of either metal species, or proceeds only at reduced rate. An apparent system of this class has been reported, in which Co2(CO)g and [RhCl(l,5-hexadiene)]2 mutually increased their reactivity when used as catalysts for the conversion of nitrobenzene to aniline in a biphasic system (benzene, aqueous NaOH, dodecyltrimethylammonium chloride) in a carbon monoxide atmosphere [73]. However, another member of the same research group later showed [74] that the apparent bimetallic promotion was due to the fact that the alkylammonium salt used as a phase-transfer agent actually inhibited the activity of the active rhodium complex (apparently a cluster, which is active in the absence of both the alkylammonium salt and the cobalt compound) by rendering it insoluble. The added Co2(CO)g reacts with the alkylammonium salt to generate... 

Lithium or potassium indenyl has been used to generate both iridium complexes and isocyanide rhodium complexes of the form (n indenyl)Rh(CNR)2. Bimetallic complexes of Co,Rh and Ir have been made using thallium fulvanate Tl2(CioH8)- With the linked permethylcyclopentadienyl lithium reagent dirhodium complexes such as (19) can be made. ... 

In addition to palladium catalysts, Co(OAc)2 shows a catalytic activity for the arylation of heterocycles, including thiazole, oxazole, imidazole, benzothiazole, benzoxazole, and benzimidazole.78 As shown in Scheme 6, the catalytic system Co(OAc)2/9/Cs2C03 gives G5 phenylated thiazole, while the bimetallic system Co(OAc)2/CuI/9/Cs2C03 furnishes the G2 phenylated thiazole. The rhodium-catalyzed reaction of heterocycles such as benzimidazoles, benzoxazole, dihydroquinazoline, and oxazoline provides the arylation product with the aid of [RhCl(coe)]2/PCy3 catalyst.79 The intermediacy of an isolable A-heterocyle carbene complex is proposed. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

A broad range of olefins, acetals, epoxides, alcohols, and chlorides were demonstrated effective alternative starting materials. Cobalt and rhodium carbonyls and bimetallic complexes were shown to catalyze the domino hydro-... 

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