Rhodium pyridines

CO, and methanol react in the first step in the presence of cobalt carbonyl catalyst and pyridine [110-86-1] to produce methyl pentenoates. A similar second step, but at lower pressure and higher temperature with rhodium catalyst, produces dimethyl adipate [627-93-0]. This is then hydrolyzed to give adipic acid and methanol (135), which is recovered for recycle. Many variations to this basic process exist. Examples are ARCO s palladium/copper-catalyzed oxycarbonylation process (136—138), and Monsanto s palladium and quinone [106-51-4] process, which uses oxygen to reoxidize the by-product... 

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

Rapoport s findings have been confirmed in the authors laboratory where the actions of carbon-supported catalysts (5% metal) derived from ruthenium, rhodium, palladium, osmium, iridium, and platinum, on pyridine, have been examined. At atmospheric pressure, at the boiling point of pyridine, and at a pyridine-to-catalyst ratio of 8 1, only palladium was active in bringing about the formation of 2,2 -bipyridine. It w as also found that different preparations of palladium-on-carbon varied widely in efficiency (yield 0.05-0.39 gm of 2,2 -bipyridine per gram of catalyst), but the factors responsible for this variation are not knowm. Palladium-on-alumina was found to be inferior to the carbon-supported preparations and gave only traces of bipyridine,... 

Rhodium-on-carbon has also been found to bring about the formation of 2,2 -biquinoline from quinoline, the yield and the percentage conversion being similar to that obtained with palladium-on-carbon. On the other hand, rhodium-on-carbon failed to produce 2,2 -bipyridine from pyridine, and it has not yet been tried with other bases. Experiments with carbon-supported catalysts prepared from ruthenium, osmium, iridium, and platinum have shown that none of these metals is capable of bringing about the formation of 2,2 -biquinoline from quinoline under the conditions used with palladium and rhodium. ... 

Both amine oxides related to pyridines and aliphatic amine oxides (/25) are easily reduced, the former the more so. Pyridine N-oxide has been reduced over palladium, platinum, rhodium, and ruthenium. The most active was rhodium, but it was nonselective, reducing the ring as well. Palladium is usually the preferred catalyst for this type of reduction and is used by most workers 16,23,84 158) platinum is also effective 100,166,169). Katritzky and Monrol - ) examined carefully the selectivity of reduction over palladium of a... 

Pyridine complexes of Pd- and Pt-pincer ligands are also suitable substrates for olefin metathesis [116]. The first-generation catalyst 9 efficiently mediates the RCM of diallylphosphines and diallyl sulfide when the heteroatom is com-plexed by a cationic [C5H5(NO)(PPh3)Re] moiety [117]. This principle has been exploited in the same study for tungsten, rhodium, and platinum complexes. 

In conclusion, many chiral pyridine-based ligands have been prepared from the chiral pool and have been successfully tested as ligands for the copper- or rhodium-catalyzed cyclopropanation of olefins. Alfhough efficient systems have been described, sometimes leading interestingly to the major cis isomer, the enantioselectivities usually remained lower than those obtained with the copper-bis(oxazoline) system. 

A four-component self-assembling system was described by Kuroda (46). Two rhodium porphyrins are coordinated by the terminal pyridine groups of an extended ligand constructed from a tartrate derivative. 

Ligand self-assembly through coordinative bonding has been used to increase the bulkiness of a monodentate tris-3-pyridyl phosphine ligand employing the zinc porphyrin/pyridine interaction (Scheme 33) [95-97]. The corresponding rhodium catalyst allowed for regioselective hydroformylation of2-octene [95]. 

Quite in contrast to, e.g., [CoCp2]+, borabenzene metal cations show a pronounced affinity toward hard nucleophiles such as amines, OH-, and to some extent even F- and H20. Qualitatively this affinity increases in the order CoCp2]+ 36 < 1 < 61 (69). [CoCp(C5H5BPh)]+ (1) adds tertiary amines at boron. With pyridine, the pyridinioboratacyclohexadienyl complex 70 is formed (K = 174 5 liters mol-1, in MeCN, 20°C), which can be isolated from CH2C12 as PF6- salt (69). The similar rhodium and iridium cations 36 and 37 form the stable cyanide adducts 71 and 72 (69). 

Li-tetrafluorobenzenedicarboxylatobis [trans-carbonylbis(triphenylphos-phine)rhodium(I)] complexes in pyridine resulted in hemidecarboxylation in each case [Eq. (98)] (79). Prolonged heating in an attempt to... 

The interesting complex chemistry of rhodium has been rather neglected this is probably because most of the synthetic methods for obtaining complexes have been tedious. In general, substitutions of chlorine atoms bonded to rhodium by other ligands are slow, and products have usually been mixtures. The situation is now changing, since novel catalytic approaches to rhodium complexes have been developed.1 The catalysis in the present synthesis involves the rapid further reaction of the hydrido complex formed from l,2,6-trichIorotri(pyridine)rho-dium(III) in the presence of hypophosphite ion. 

Many compounds of rhodium (III) may readily be obtained by this general catalytic procedure. Rhodium(III) bromide may be substituted for the rhodium trichloride, and weakly basic nitrogenous ligands (2,2 -bipyridine, o- (or l,10-)phen-anthroline, or dioximes) for the pyridine. The products all have the halogen atom in trans positions. 

The sp2 nitrogen in a pyridine ring can activate a C-H bond of 2-arylpyridines (Equation (13)). The alkylation of the 2-arylpyridine is satisfactorily catalyzed by rhodium catalysts such as [RhCl(coe)2]2/PCy3 and [RhCl(coe)2]2/PPh3.13 13a... 

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