Rhodium DIOP complex

The solvated and unsolvated rhodium diop complexes are air-stable, yellow, crystalline solids solutions of the complexes, however, rapidly decompose on exposure to air. HRh[( + )-diop]2 is soluble in benzene, toluene, and dichloromethane it is slightly soluble in petroleum ether and n-hexane, and insoluble in ethanol. 

The occurrence of multinuclear catalysts in hydrogenations catalyzed by rhodium-DIOP systems seems unlikely, although the trans-RhCl(CO)(DIOP) complex 43 is dimeric (276), and in basic methanolic solution the 1 1 diphos complex exists as Rh3(diphos)3(OMe)2 + (138a, Section II, B, 1). 

Use of benzene suspensions containing a neutral rhodium(I)-DIOP complex supported on a cross-linked polystyrene (50) (cf. 13 in Section III,A) for hydrogenation of a-ethylstyrene (to 1.5% ee) and methyl atro-pate (2.5% ee) was less effective than the homogeneous system, as the ethanol cosolvent required for substrate solubility caused a collapse of the resin (296). 

In the reaction shown the (R) product was obtained in 14.8% enantiomeric excess (ee) (68). The optically active ligand, (-)-diop (58), has been used in asymmetric catalytic hydrogenation. For example, the use of the rhodium(I) complex (61) converts the alkene (59) to (R)-N-ac-etylphenylalanine (60) with an optical yield of 72% and a chemical yield of 95% (69). 

Only very low catalyst concentrations down to 5 x 10-5 kmol/m3 are consumed that keeps also the catalyst inventory very small [266], Only 0.08 mg of Rh and about 0.2 mg-13 pg of the very expensive chiral ligands (about 300-1000 /g), depending on their molecular weight, are consumed. Finally, a performance comparison for three different reactors was made for the substrate methylacetamidocinnamate and the two rhodium diphosphine complexes Rh/Josiphos and Rh/Diop (see Figure 4.57). The first reactor was a commercial Caroussel reactor (Radleys... 

Typical square-planar rhodium-olefin complexes such as acetylacetonates (48) have a stoichiometry of two coordinated olefins per metal-atom. Since chelating olefins are bidentate in their cationic rhodium biphosphine complexes, it would be surprising if bis-olefin complexes were never found under hydrogenation conditions. It seems clear, in fact, that they can be the major coordinated species under certain conditions. Thus examples of 2 1 rhodium enamide complexes with biz-diphenyl-phosphinopropane have been observed (49), although the majority of cases involve a8-unsaturated acids co-complexed with DIOP. 

Double asymmetric reduction of (-)menthyl pyruvate and (-)menthyl phenyl-glyoxylate uses rhodium(I) complexes with (-I-)DIOP or ( —)DIOP as cataJysP . Although only a slight effect of (— )menthyl group on the asymmetric induction is observed in the case of (— )menthyl pyruvate, i.e., (+ )DIOP, 85.6% d.e., S (— )DIOP, 82.8%... 

Achiwa et al. [46,47,48,49] further developed the approach to access the monobutyrolactone skeleton by creating a very efficient catalytic asymmetric method. Aryldensuccinic acid mono-methyl esters (23), obtained by Stobbe condensation of dimethyl succinate and the corresponding substituted aldehydes, were enantioselectively hydrogenated using a neutral rhodium (I) complex of (4[Pg.550]

Linear polystyrene has also been used to support asymmetric hydrogenation catalysts containing chiral diphosphine rhodium(I) complexes (50). Asymmetric hydrogenations of itaconic acid were carried out, forming (R)-2-raethylbutanedioic acid with e.e. s ranging from 20-37%. None of the polymer-bound catalysts were more effective than (-)-DIOP-RhCl and the observed e.e. s were found to be dependent on the molecular weight of the polymer chain, its raicrostructure and solubility. 

There are only two controlled kinetic studies of asymmetric hydrogenation, one of which was carried out using neutral diop complexes before the significance of ionization in polar solvents was fully appreciated, and, hence, the pathway studied is not necessarily the most efficient one. In the second, Halpern and Chan demonstrate that the kinetic form of the hydrogenation of methyl z-a-acetamidocinnamate catalyzed by the 1,2- /5(diphenylphosphino)-ethane rhodium cation in methanol is ... 

In Section 4, it is described that chlorotris(triphenylphosphine)rhodium(I) (7) is quite an effective catalyst for the hydrosilylation of carbonyl compounds. For this reason, extensive studies on asymmetric hydrosilylation of prochiral ketones to date have been based on employing rhodium(I) complexes with chiral phosphine ligands. The catalysts all prepared in situ are rhodium(I) complexes of the type, (BMPP>2Rh(S)a (8) [40] and (DIOP)Rh(S)Cl (6) [41], and a cationic rhodium(III) complex, [(BMPP)2lUiH2(S)2] Q04 (5) [42], where S represents a solvent molecule. An interesting polymer-supported rhodium complex (V) [41], and several chiral ferrocenylphosphines [43], recently developed as chiral ligands, have also been employed for asymmetric hydrosilylation of ketones. Included in this section also are selective asymmetric hydrosilylation of a,0-unsaturated carbonyl compounds and of certain keto esters. 

Somewhat different but still distinct effects of hydrosilanes on the stereoselectivity were observed when a rhodium(I) complex with DIOP was employed. As is seen from Table 8, monohydrosilanes such as phenyldimethylsilane reacted with ketones under rather forced conditions to afford, after hydrolysis, the corresponding alcohols in low optical yield. However, both chemical and optical yields were remark-... 

More recently, an alternative mechanism has been proposed [45] for asymmetric hydrosilylation of prochiral ketones using ( f)-DIOP-rhodium(I) complex (6) and a-naphthylpenylsilane, the latter undergoing concomitant conversion into an optically active, bifunctional alkoxysilane, which will be discussed separately (see Section 7.1). According to this proposed mechanism, diastereomeric silylhydrido-rhodium(III) complexes having trigonal bipyramidal structure are assumed as intermediates, which distinguish enantiotopic faces of a prochiral ketone in terms of steric approach control . 

Comparable results have been obtained independently [45] using the DIOP-rhodium(I) complex (6). The addition of an asymmetric organosilane, (/ )-a-NpPhMeSiH (XII), to acetone in the presence of (Ph3P)3FliCl (7) was found to proceed with retention of configuration at the silicon center, equation (30). 

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