Diastereoselective hydrogenation of o-Toluic acid (cas 118-90-1) coupled with (S)-proline and (S)-pyroglutamic acid methyl esters on ruthenium catalysts

Add time:08/28/2019    Source:sciencedirect.com

The diastereoselective hydrogenation of two o-Toluic acid (cas 118-90-1) derivatives, N-(2-methyl-benzoyl)-(S)-proline methyl ester here named substrate 1, and N-(2-methyl-benzoyl)-(S)-pyroglutamic acid methyl ester abbreviated as substrate 1′, was studied on metallic ruthenium particles deposited on two oxidic supports, Al2O3 and SiO2 and on different carbon materials, such as an activated carbon, a carbon molecular sieve and two high surface area graphites. The asymmetric induction was dependent on the chiral auxiliary employed: cis isomers were formed preferentially, with configuration (1R, 2S, 2′S) and (1S, 2R, 2′S) for substrates 1 and 1′, respectively. The diastereoselectivity also depended on the nature of the support and higher values of diastereomeric excess (d.e.) were obtained on catalysts supported on metal oxides, 35% for substrate 1 and 82% for 1′, than on catalyst supported on carbon materials, 23% for substrate 1 and 61% for 1′. No relevant effects were found among the various allotropic forms of carbon. The introduction of an additive, ethyldicyclohexylamine (EDCA), in the hydrogenation solution of substrate 1 reduced the d.e. from 21 to 10% for high surface area graphite catalyst, while for alumina supported catalyst the d.e. was inversed from 35% (1R, 2S, 2′S) isomer to 2% in favour of isomer (1S, 2R, 2′S). All these findings confirm the importance of catalytic supports in modifying the adsorption properties of ruthenium particles deposited on them.

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