Rhodium carbonyl dimer

From the bulky (l,2,4-triisopropyl-3,5-dimethylcyclopentadienyl)rhodiumbis(ethylene) and (l,2,4-tri-/i / -butyl-cyclopentadienyl)rhodiumbis(ethylene) the corresponding (cyclopentadienyl)rhodium carbonyl dimers 67 and 68 were obtained. 

The less bulky ligand (71) studied by Gladfelter leads to dimeric complexes [Rh2(71)2(CO)2] and even tetramers.222 Transformations of rhodium carbonyl complexes in alkene hydroformylation are discussed from the standpoint of the catalytic system self-control under the action of reaction... 

In many instances, the formation of inactive dimers from active, monomeric catalytic species is observed during catalysis. When weak or unstable ligands are used, even larger rhodium carbonyl clusters like Rh4(CO)i2 and Rh5(CO)i5 can be observed [42-44]. The formation of dimers is often a reversible equilibrium (Scheme 6.2). This only leads to a reduction in the amount of catalyst available and does not kill the catalyst. One of the first examples was the formation of the so-called orange dimer from HRh(PPh3)3CO, already reported by Wilkinson [45]... 

Since the hydroformylation reaction for most substrates shows a first order dependence on the concentration of rhodium hydride, the reaction becomes slower when considerable amounts of rhodium are tied up in dimers. This will occur at low pressures of hydrogen and high rhodium concentrations. Dimer formation has mainly been reported for phosphine ligands [17, 42, 45], but similar dimeric rhodium complexes from monophosphites [47] and diphosphites [33, 39] have been reported. The orange side product obtained from HRh(15)(CO)2 was characterized as the carbonyl bridged, dimeric rhodium species Rh2(15)2(CO)2 [39]. 

Dimerization reactions of 1-azirines with several transition metal complexes have been studied (76TL2589). Reaction of 2-arylazirines (289) with an equimolar amount of a Group VI metal carbonyl gives 2,5-diarylpyrazines (290) in good yield. On the other hand, these compounds are converted to 2-styrylindoles (291) with rhodium carbonyl compounds or with dicobalt octacarbonyl in benzene. 

The first account on the carbonylation of heterocyclic compounds with metallo-dendrimers was recently reported by Lu and Alper using Rh-complexed dendrimers on a resin [207]. The building-block techniques of solid-phase chemistry were used to synthesize dendrimers, followed by phosphonation of the dendrimers with diphenylphosphinomethanol. The resulting phosphonated dendrimers were then reacted with chloro(dicarbonyl)rhodium(I) dimer to give dendritic catalysts A and B (31P NMR, 8 - 25 ppm loading of rhodium A, 0.74 mmol g-1 B, 0.83 mmol g ). As a model study, the reaction of l-ferf-butyl-2-phenylaziridine with carbon monoxide in catalytic presence of A afforded the desired [3-lactam... 

CARBONYLATIONS Bis(triphenylphosphine)palladium dichloride. rhodium(I) dimer. Carbon dioxide. Chloromethyl methyl ether, carbonate. 

In addition to dimerization and clustering to multinuclear rhodium carbonyl complexes, active catalyst can also be removed from the reaction mixture by orthometallation. Several reports have appeared on orthometallation of rhodium triphenyl phosphite complexes in literature but... 

In 1989, the Moody s group from UK reported the constmction of seven-membered cyclic ethers (Fig. 3.26) [9] via Rh (Il)-catalyzed intramolecular O-H insertion reaction. With the catalyst of rhodium acetate dimer, the catenulate a-diazo carbonyl substrate containing secondary hydroxyl reacted to obtain seven-membered cyclic ethers under toluene reflux conditions with the yield of 88 %. 

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