Chemo-selective hydrogenation reaction

Hernandez and Kalck495,496 synthesized various Ru/tppts complexes and used them as catalysts in the hydrogenation of various cc,J3-unsaturated aldehydes and ketones in a two phase system. Using Ru(H)2(tppts)4 high conversions of cinnamaldehyde (97%) and high selectivity to cinnamylalcohol (95%) were observed under mild reaction conditions (40°C, 20 bar, 3h) 495 More forcing conditions (80°C, 35 bar, 16h) were needed for the hydrogenation of unsaturated a, 3-ketones such as 4-hexen-3-one or benzylideneacetone and only low chemo-selectivities were observed.495... 

In the same way, ruthenium colloidal systems prepared by reduction of RuCh under dihydrogen in the presence of trioctylamine allowed reduction of various substituted aromatics [27]. They studied the stereo- and chemo-selectivities of the hydrogenation of aromatics in a methanol-water system at 50 bar of H2 and at room temperature. The results obtained are presented in Table 11.5. Reaction time to reach 100% conversion depends on the electronic and steric properties of the substituents on the aromatic ring, more electron-rich substrates giving rise to a favored reaction. 

The industrial importance of homogeneous catalysts in hydrogenation reactions lies mainly in their very high chemo-, regio-, and enantiose-lectivities. Two examples of homogeneous cataljdic hydrogenations of high selectivity are shown by reactions 5.1.1 and 5.1.2. 

A novel Ru precursor and a new reaction system had to be found because the classical Ru complexes and conditions for the hydrogenation of C=C bonds did not work. Besides the enantioselectivity, chemo- and cis-selectivity and activity problems (tetrasubstituted C=C) were solved on a very good level. A broad screening of Ru catalysts (partly in collaboration with Solvias) showed that selected Josiphos ligands and DuPhos satisfied the prerequisites (see Table 37.4). 

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