Rhodium complexes binuclear

Rhodium compounds have also been used as catalysts since the late 1960s and mechanistic studies date from the 1970s.534,578-582 The binuclear rhodium complex [(Ph3P)4Rh2(//-OH)2] was found to be an effective catalyst for the reductive carbonylation of nitrobenzenes to carbamate esters. Electron-withdrawing groups at the para-position enhance the reactivity of the substrate.583... 

Ligand 73 was prepared directly from a single enantiomer of the corresponding naphthol of QUINAP 60, an early intermediate in the original synthesis, and both enantiomers of BINOL. Application in hydroboration found that, in practice, only one of the cationic rhodium complexes of the diastereomeric pair proved effective, (aA, A)-73. While (aA, A)-73 gave 68% ee for the hydroboration of styrene (70% yield), the diastereomer (aA, R)-73 afforded the product alcohol after oxidation with an attenuated 2% ee (55% yield) and the same trend was apparent in the hydroboration of electron-poor vinylarenes. Indeed, even with (aA, A)-73, the asymmetries induced were very modest (31-51% ee). The hydroboration pre-catalyst was examined in the presence of catecholborane 1 at low temperatures and binuclear reactive intermediates were identified. However, when similar experiments were conducted with QUINAP 60, no intermediates of the same structural type were found.100... 

Collman et al. prepared halopropyl rhodium complexes by the reaction of dihalopropanes with a macrocyclic rhodium(I) complex [Eq. (29)] (180). The ratio of mononuclear to binuclear product was found to be solvent dependent, with the variations in product ratios in different solvents being related to the solubility of the Rh(I) macrocycle. 

A tetrahedrane-type core geometry can also be ascribed to the violet rhodium complexes Cpf Rh2(E2fc) (E = S, Se, Te) obtained from the reaction of fc(ELi)2 with CpfRh2Cl4 in THE [267]. The binuclear fragment [CpfRh2] is isoelectronic and isolobal with [Ru2(CO)5]. 

Mo2(C5Hs)2(CO)/ or [Mo( -MeC=CMe)(PhS(0)SPh)( /-C5H5)] produce complexes derived from fission of the cyclopropene C(l)-C(3) bond. However, treatment of the last of these carbocycles with the binuclear rhodium complex [Rh2(ju-CO)2( /-C5Me5)2] results in the cleavage of the carbon-carbon double bond and formation of the bridged carbene complex 313. ... 

Binuclear rhodium complexes in particular have met with great success as catalysts for various transformations, especially remarkable are the Rh (11) examples reported by Doyle et al. [31] (and references therein) and Stanley s system for hydroformylation [32-38]. The latter showed an excellent regioselectivity for the hydroformylation of 1-hexene with remarkable turnover numbers (ca. 12,000 cycles) and frequencies (73 min ). The enhanced activity and... 

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. 

Blnuclear Catalysts Stanley et al. have shown how a rhodium complex that is a poor catalyst in monomeric form becomes very active and selective when connected in a binuclear system with a methylene bridge as shown below. Linear to branched ratios as high as 27 to 1 can be achieved. A rhodium hydride is believed to attack a RhCOR group at the neighboring site in the product forming step. This shows how the proximity of two metals can provide useful chemical effects without their being permanently connected by a metal-metal bond. 

Scheme 2.23 Synthesis of a tetraphosphine able to form a binuclear rhodium complex.

The reaction of the imidazolium salt with [Rh(OMe)(COD)]2 in dichloro-methane produced the supported NHC-rhodium precatalyst. Elemental analysis provided evidence that one rhodium atom is coordinated to each imidazolium unit. In a similar manner, also a binuclear rhodium complex has been synthesized. 

The binuclear NHC-rhodium complex 1 developed catalytic activity in the hydroformylation of styrene (CO/H2 = 80 bar, 80 °C, toluene) [55]. Pull conversion was accompanied by high chemoselectivity and moderate iso-regioselectivity (99% of aldehyde, bll = 87 13). The regioselectivity in favor of the branched aldehyde could be enhanced by lowering the temperature to 40 °C. Under similar conditions, the complex 2 was inferior. 

The water-soluble rhodium complex [Rh(p.-pz)(CO)(TPPTS)]2 (pz = pyrazolate) was used as catalyst precursor during the two-phase catalytic hydroformylation of different olefins at 100 °C, 50 bar (CO H2 = 1 1), 600 rpm, and substrate catalyst ratio of 100 1. A reaction order- 1-hexene > styrene > allylbenzene > 2,3-dimethyl-1-butene > cyclohexene-was found. The experiments also showed that the binuclear catalyst precursor was resistant to possible sulfur poisons [108]. 

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