UBC Theses and Dissertations
Asymmetric homogeneous hydrogenation of olefinic compounds using a rhodium catalyst with chiral phosphine ligands Mahajan, Devinder
The hydridorhodium(I)phosphine complex HRh(diop)₂ [(+)-diop = (+)-2,3,-0-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane] has been studied for catalytic asymmetric hydrogenation of a number of olefinic substrates. Certain olefinic carboxylic acids have been reduced to products with up to about 60% enantiomeric excess. Some rate data show the importance of the degree of substitution at the olefinic bond in terms of steric effects and the -COOH function of the substrate also plays a role. Kinetic and spectrophotometry studies on the hydrogenation of itaconic acid in n-butanol-toluene (2:1) solutions are best interpreted in terms of a mechanism involving formation of a metal alkyl via coordination of the olefinic substrate, followed by reaction with H₂ to give saturated product and the starting catalyst complex. The actual catalyst is thought to be HRh(diop)(diop*) where diop* represents a monodentate diop with one dangling -CH₂PPh₂ moiety. The catalyst systems are somewhat complicated by the occurrence of a number of slower side-reactions, the extent of these varying from negligible for the more rapidly reducing substrates, to possibly of prime importance in the slow hydrogenations. The initial hydride catalyst, for example, is slowly decomposed to the cationic [Rh(diop)₂] complex by protons from the olefinic substrate. Under H₂ the cation reversibly forms a dihydride. The cation is also formed from a reaction that appears to involve an intermediate alkyl complex and the butanol.
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