Non‐Linear Effect of Modifier Composition on Enantioselectivity in Asymmetric Hydrogenation over Platinum Metals

Abstract

Prominent non-linear behavior was observed when mixtures of cinchona alkaloids were applied as chiral modifiers in enantioselective hydrogenations over Pt/Al 2 O 3 and Pd/TiO 2 . The phenomenon is traced to differences in the strength and geometry of alkaloid adsorption on the metal surface. In ethyl pyruvate hydrogenation under close to ambient conditions the weaker adsorbing quinidine (QD) outperformed the generally preferred modifier cinchonidine (CD) and afforded the highest ee (96 ± 98%) at 1 ± 5 bar. In the partial hydrogenation of 4-methoxy-6-methyl-2-pyrone to the dihydro derivative 4 CD gave 73% ee to (S)-4 and QD provided 72% ee to (R)-4, and still the alkaloid mixture containing less than 5 mol % CD afforded 15% ee to (S)-4. This nonlinear behavior may be advantageous in asymmetric synthesis as low purity chiral compounds can be applied as efficient modifiers for Pt or Pd.

Enantioselective hydrogenation of ethyl pyruvate catalysed by cinchonine-modified Pt/Al2O3: tilted adsorption geometry of cinchonine

“…In agreement with studies employing a variety of instrumental methodologies, it appears that in the concentration range allowing high ee to be attained (0.01-0.1 mmol L )1 ) the quinoline skeleton of CD is approximately parallel to the Pt surface [24,[27][28][29], whereas the plane of the quinoline skeletons of CN and QD is tilted relative to the Pt surface. Q may take an intermediate position, i.e.…”

Synthetic Modifiers for Platinum in the Enantioselective Hydrogenation of Ketopantolactone: A Test for the Mechanistic Models of Ketone Hydrogenation

“…It has been shown that the non-linear behaviour of modifier mixtures in Pt-and Pd-catalyzed hydrogenation reactions are basically due to their different adsorption strengths on the metal surface. [5,21,51,52] Apparently, 18 cannot compete with cinchonine for the Pt surface sites and thus can barely influence the enantiodifferentiation during hydrogenation of KPL.…”

Chemistry of quinuclidines as nitrogen bicyclic bridged‐ring structures

“…The two most important asymmetric catalyst types are platinum catalyst modified with cinchona alkaloids and related modifiers, successful for α-functionalized ketones such as hydrogenation of ethyl pyruvate to ethyl acetate over cinchona-modified with ee's up to 98% and palladium catalysts modified with cinchona alkaloids which achieve ee's up to 94% for selected activated C=C bonds such as partial hydrogenation of a 4-methoxy-6-methyl-2-pyrone over cinchona-modified [32,33].…”