Itaconic acid catalysts, rhodium complexes

Itaconic acid was hydrogenated rapidly to a 20% ee, and kinetic and spectrophotometric studies on this system were interpreted in terms of a mechanism involving a standard unsaturate route [cf. Eq. (5)]. The actual catalyst was thought to be HRh(DIOP)(DIOP ), where DIOP represents a monodentate DIOP with one dangling —CH2PPh2 moiety (273, 275). Rhodium(I) carbonyls (276) and ruthenium(II) complexes (90, 275) (Section III,B) containing monodentate DIOP have been isolated. 

The sol-gel entrapment of the metal complexes [Ru(p-cymene)(BINAP)Cl]Cl and the rhodium complexes formed in situ from the reaction of [Rh(COD)Cl]2 with DlOP and BPPM has been reported by Avnir and coworkers [198]. The metal complexes were entrapped by two different methods the first involved addition of tetramethoxysilane to a THF solution of the metal complex and triethylamine, while the second method was a two-step process in which aqueous NH4OH was added to a solution of HCl, tetramethoxysilane and methanol at pH 1.96 followed by a THF solution of the appropriate metal complex. The gel obtained by each method was then dried, crushed, washed with boiling CH2CI2, sonicated in the same solvent and dried in vacuo at room temperature until constant weight was achieved. Hydrogenation of itaconic acid by these entrapped catalysts afforded near-quantitative yields of methylsuccinic acid with up to 78% e.e. In addition, the catalysts were found to be leach-proof in ethanol and other polar solvents, and could be recycled. 

Kollner et al. (29) prepared a Josiphos derivative containing an amine functionality that was reacted with benzene-1,3,5-tricarboxylic acid trichloride (11) and adamantane-l,3,5,7-tetracarboxylic acid tetrachloride (12). The second generation of these two types of dendrimers (13 and 14) were synthesized convergently through esterification of benzene-1,3,5-tricarboxylic acid trichloride and adamantane-1,3,5,7-tetracarboxylic acid with a phenol bearing the Josiphos derivative in the 1,3 positions. The rhodium complexes of the dendrimers were used as chiral dendritic catalysts in the asymmetric hydrogenation of dimethyl itaconate in methanol (1 mol% catalyst, 1 bar H2 partial pressure). The enantioselectivities were only... 

Wilkinson s (I) discovery that the soluble rhodium(I) phosphine complex, [Rh(PPh3)3Cl], was capable of homogeneous catalytic hydrogenation of olefins immediately set off efforts at modifying the system for asymmetric synthesis. This was made possible by the parallel development of synthetic methods for obtaining chiral tertiary phosphines by Horner (2) and Mislow (3,4, 5). Almost simultaneously, Knowles (6) and Horner (7) published their results on the reduction of atropic acid (6), itaconic acid (6), a-ethylstyrene (7) and a-methoxystyrene (7). Both used chiral methylphenyl-n-propyl-phosphine coordinated to rhodium(I) as the catalyst. The optical yields were modest, ranging from 3 to 15%. 

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. 

Sol-gel physically entrapped chiral rhodium and ruthenium complexes are used as recyclable and effective catalysts in enantioselective hydrogenation of itaconic acid up to 78% chiral methylsuccinic acid is obtained [65]. 

The immobilization of homogeneous catalysts is still a challenge in catalysis. This field is not discussed in detail in this article, but a promising result is worth to mention. A PYRPHOS-rhodium(I) complex was embedded at a specific site in a protein (biotin) and used for asymmetric hydrogenation of itaconic acid (243). 

Asymmetric Hydrogenation.—The asymmetric hydrogenation of a-acylamino-acrylates and cinnamates using chiral rhodium(i) diphosphine complexes as catalysts is now established as one of the best methods for obtaining optically pure a-amino-acids (see previous reviews in this series). In the past year, some new chiral diphosphines have been added to the already considerable number of such ligands. A bis(diphenylphosphino)-derivative of pyrrolidine in conjunction with Rh can be used to hydrogenate a-acetamidocinnamates and itaconic acid with chiral inductions of 90%, whereas an Rh -diphos complex derived from natural tartaric acid effects the reduction of some a-acylaminoacrylic acids to natural (5)-a-acylamino-acids with optical yields of between 80 and 100%. ... 

Gelman, F Avnir, D., Schumann, H., and Blum, J. (1999) Sol-gel entrapped chiral rhodium and ruthenium complexes as recyclable catalysts for enantioselective hydrogenation of itaconic acid. J. Mol Catal. A, 146 (1-2), 123-128. 

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