Hydrogenation of unsaturated carboxylic acids

Hydrogenation of unsaturated carboxylic acids, such as acrylic, methacryUc, maleic, fumaric, cinnamic etc. acids was studied in aqueous solutions with a RhCU/TPPTS catalyst in the presence of p-CD and permethylated P-cyclodextrin [7]. In general, cyclodextrins caused an acceleration of these reactions. It is hard to make firm conclusions with regard the nature of this effect, since the catalyst itself is rather undefined (probably a phosphine-stabilized colloidal rhodium suspension, see 3.1.2) moreover the interaction of the substrates with the cyclodextrins was not studied separately. 

SCHEME 38. Possible reaction pathway of Ru-catalyzed hydrogenation of unsaturated carboxylic acids. 

The results of deuterium labeling experiments shown in Scheme 37 clearly show the operation of a monohydride mechanism in the BINAP-RuOI) catalyzed hydrogenation of unsaturated carboxylic acids. However, with many olefinic substrates with a neutral, rather than anionic, secondary binding site, the products exhibit a similar degree of isotope incorporation at the two hydrogenated centers (Scheme 39). The out-... 

Ohta, T., Takaya, H., Kitamura, M., Nagai, K., Noyori, R. Asymmetric hydrogenation of unsaturated carboxylic acids catalyzed by BINAP-ruthenium(ll) complexes. J. Org. Chem. 1987, 52, 3174-3176. 

Application of subcritical gaseous CO2 to an organic liquid causes the liquid phase to expand noticeably, due to extensive dissolution of the CO2 into the liquid phase (131). This expansion is accompanied by a reduction in the liquid phase viscosity, an increase in the solubility of H2 in the liquid, and an increase in the mass transfer rates from the gas to liquid phase. There is evidence that this can affect the enantioselectivity of reactions in viscous liquids. The enantioselectivity of asymmetric hydrogenation of unsaturated carboxylic acids in a viscous ionic liquid was shown to be strongly affected by CO2 expansion of the liquid, the enantioselectively being improved for one substrate (atropic acid) and decreased for another (tiglic acid). The results were explained in terms of the solubility and rate of transfer of H2 gas into the expanded ionic liquid (23). The same effect was not observed in expanded methanol. 

Indeed, as it was mentioned above. Smith accomplished the enantioselective hydrogenation of unsaturated carboxylic acids using Pt or Pd catalysts modified with chiral silyl ethers. 

Complex organic Ru cortpounds are catalysts for the enantioselective hydrogenation of unsaturated carboxylic acids, used in pharmaceutical, agrochemical, flavors and line chemicals (Fig. 3.1-338) [1.291]. 

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