Complexes with rhodium compounds

Rhodium-Catalyzed Asymmetric Hydrogenation of Olefins. MiniPHOS (1) can be used in rhodium-catalyzed asymmetric hydrogenation of olefinic compounds. The complexation with rhodium is carried out by treatment of 1 with [Rh(nbd)2]BF4in THF (eq 2). The hydrogenation of a-(acylamino)acrylic derivatives proceeds at room temperature and an initial H2 pressure of 1 or 6 atm in the presence of the 0.2 mol% MiniPHOS-Rh complex 2. The reactions are complete within 24—48 h to afford almost enantiomerically pure a-amino acids (eq 3). Itaconic acids, enamides, and dehydro-3-ami no acids can also be hydrogenated with excellent enantioselectivity (eq 4—6). 

Two different strategies have been developed for C-Cl activation with rhodium compounds (a) oxidative addition of C-Cl bonds to electron-rich Rh(I) complexes and (b) -coordination of coordinatively unsaturated, electron-deficient Rh(III) species with the benzene ring of ArCI, followed by aromatic nucleophilic substitution of chlorine in the thus activated aromatic system. 

Complexes of rhodium compounds with enantiomerically pure phosphines facilitate enantiose-lective hydroboration of prostereogenic olefins with 1,3,2-benzodioxaborole (catecholboranc), an achiral hydroborating agent. The reactions are carried out using a 1 1.1 -1.2 molar ratio of the olefin and catecholboranc in the presence of 1 mol % of the catalyst which is generated in situ111"113. The following catalysts are used ... 

With transition metal complexes (typically rhodium compounds), it is possible to functionalize pyrrole and to obtain active electrodes for the hydrogenation of ketones. An example showing the behaviour of a film of poly(pyiTole-Rh(lll)bipyridyl) is the reduction of cyclohexanone into cyclohexanol with a chemical yield of 79% [110], This molecular electrode is also suitable for the reduction of water in hydrogen, Electrocatalytic hydrogenation of other ketones, unsaturated ketones or aldehydes, has been studied recently [174-176], These hydrogenation reactions are performed in an aqueous medium and the modified electrodes are obtained after electropolymerization of pyrrole substituted 2,2 -bipy-ridine or 1,10-phenanthroline complexes of Pd(Il) or of Rh(Ill). More precisely, with the Pd(ll) complex... 

Poly(acrylic acid) modified with triphenylphosphine groups (PAA-PNH) with a phosphine-to-carboxylic acid ratio 1 5 forms stable water-soluble macromolecular complexes with rhodium, (NBD)RhL2 CH3S03, 44, which are active in hydrogenation of unsaturated compounds, such as 4-pentenoic acid and but-1 -en-4-... 

Derivatives of carbohydrates that have been used to complex with rhodium catalysts to promote asymmetric hydrogenation include the phosphorus derivatives (6) and (7)/ and (8) (12) synthesized by esterification of the hydroxy-compounds with the appropriate phosphonyl chloride. Treatment of 7r-allyl-palladium chloride with the phosphite ester (13) gave a complex that was an enantioselective catalyst for hydrogenation of unsaturated acids and esters. ... 

Rhodium complexes with oxygen ligands, not nearly as numerous as those with amine and phosphine complexes, do, however, exist. A variety of compounds are known, iucluding [Rh(ox)3] [18307-26-1], [Rh(acac)3] [14284-92-5], the hexaaqua ion [Rh(OH2)3] [16920-31 -3], and Schiff base complexes. Soluble rhodium sulfate, Rh2(804 )3-a H2 0, exists iu a yellow form [15274-75-6], which probably coutaius [Rh(H20)3], and a red form [15274-78-9], which contains coordinated sulfate (125). The stmcture of the soluble nitrate [Rh(N03)3 2H20 [10139-58-9] is also complex (126). Another... 

There is also clear evidence of a change from predominantly class-a to class-b metal charactristics (p. 909) in passing down this group. Whereas cobalt(III) forms few complexes with the heavier donor atoms of Groups 15 and 16, rhodium(III), and more especially iridium (III), coordinate readily with P-, As- and S-donor ligands. Compounds with Se- and even Te- are also known. Thus infrared. X-ray and nmr studies show that, in complexes such as [Co(NH3)4(NCS)2]" ", the NCS acts as an A -donor ligand, whereas in [M(SCN)6] (M = Rh, Ir) it is an 5-donor. Likewise in the hexahalogeno complex anions, [MX ] ", cobalt forms only that with fluoride, whereas rhodium forms them with all the halides except iodide, and iridium forms them with all except fluoride. 

Reaction of the cyclopentadienyl rhodium and iridium tris(acetone) complexes with indole leads to the species 118 (M = Rh, Ir) [77JCS(D)1654 79JCS(D)1531]. None of these compounds deprotonates easily in acetone, but the iridium complex loses a proton in reaction with bases (Na2C03 in water, r-BuOK in acetone) to form the ri -indolyl complex 119. This reaction is easily reversed in the presence of small amounts of trifluoroacetic acid. 

M(NO)(OCOCF3)2(PPh3)2. Both these complexes have 5-coordinate geometries with monodentate carboxylates. The rhodium compound has a square pyramidal structure with bent Rh-N-O (122°) but the iridium compound has a tbp structure with straight equatorial Ir-N—O (178°). The position of i/(N—O) reflects this difference (1800 cm-1 (Ir) and 1665 cm-1 (Rh)). 

Episulfides can be converted to alkenes. " However, in this case the elimination is syn, so the mechanism cannot be the same as that for conversion of epoxides. The phosphite attacks sulfur rather than carbon. Among other reagents that convert episulfides to alkenes are Bu3SnH, certain rhodium complexes, LiAlH4 (this compound behaves quite differently with epoxides, see 10-85), and methyl iodide.Episulfoxides can be converted to alkenes and sulfur monoxide... 

The role of the rhodium is probably two-fold. Initially due to its Lewis acidity it reversibly forms a complex with the nitrile nitriles are known to complex to the free axial coordination sites in rhodium(II) carboxylates as evidenced by the change of colour upon addition of a nitrile to a solution of rhodium(II) acetate, and by X-ray crystallography. Secondly the metal catalyses the decomposition of the diazocarbonyl compound to give a transient metallocarbene which reacts with the nitrile to give a nitrile ylide intermediate. Whether the nitrile ylide is metal bound or not is unclear. 

A novel chiral dissymmetric chelating Hgand, the non-stabiUzed phosphonium ylide of (R)-BINAP 44, allowed in presence of [Rh(cod)Cl]2 the synthesis of a new type of eight-membered metallacycle, the stable rhodium(I) complex 45, interesting for its potential catalytic properties (Scheme 19) [81]. In contrast to the reactions of stabihzed ylides with cyclooctadienyl palladium or platinum complexes (see Scheme 20), the cyclooctadiene is not attacked by the carbanionic center. Notice that the reactions of ester-stabilized phosphonium ylides of BINAP with rhodium(I) (and also with palladium(II)) complexes lead to the formation of the corresponding chelated compounds but this time with an equilibrium be-... 

Several rhodium(I) complexes have also been employed as ATRP catalysts, including Wilkinson s catalyst, (177),391 421 422 ancj complex (178).423 However, polymerizations with both compounds are not as well-controlled as the examples discussed above. In conjunction with an alkyl iodide initiator, the rhenium(V) complex (179) has been used to polymerize styrene in a living manner (Mw/Mn< 1.2).389 At 100 °C this catalyst is significantly faster than (160), and remains active even at 30 °C. A rhenium(I) catalyst has also been reported (180) which polymerizes MM A and styrene at 50 °C in 1,2-dichloroethane.424... 

Rhodium complexes with chiral dithiolato and dithiother ligands have been studied in rhodium-catalyzed asymmetric hydroformylation. In all instances, enantioselectivities were low.391-393 Catalysis with compounds containing thiolate ligands has been reviewed.394... 

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