Effects of Mixtures of Modifiers on Optical Yield in Enantioselective Hydrogenation: A Test of the Template Model

The Physico-chemical Properties of Cinchona Alkaloids Responsible for their Unique Performance in Chiral Catalysis

“…It does appear that competitive adsorption is affected by both the absolute adsorption strength of the molecule on the surface and its solubility in the solution above; other reports are by and large consistent with these observations [60,61]. The different adsorption behavior of the different cinchona alkaloids leads to differences in performance in catalysis [16,17], and even to some interesting non-linear effects when mixtures of cinchonas are used as chiral modifiers [62,63]. The implications from our work are that: (1) the catalytic behavior is determined by the dominant cinchona on the surface, which is that with the appropriate balance between surface bonding and solubility; and (2) once adsorbed, the effectiveness of the cinchona is determined by the configuration it adopts on the surface.…”

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

“…Wells and coworkers [28] have already observed small deviations from the expected linear correlation in the hydrogenation of 1 using mixtures of the diastereomers CD and CN, or QN and QD. They attributed the deviations between the observed and calculated ees to differences in the adsorption strength of the alkaloids.…”

Competitive Chemisorption between Pairs of Cinchona Alkaloids and Related Compounds from Solution onto Platinum Surfaces

“…In summary, it was established by using in-situ RAIRS that, in competitive environments with pairs of solutes, the strengths of adsorption of quinoline-based compounds from CCl 4 solutions onto platinum surfaces follow the sequence: QN, QD > CD > CN > 6-MeO-Q > L > Q. This behavior could be explained at least in part by differences in solubility and may account for nonlinear effects in chiral modification of hydrogenation catalysts such as the dominance of CD in CD + CN mixed solutions 3 and the virtually null effect of adding Q to CD-modified catalysts. 19 A stronger adsorption of CD versus Q was also concluded from electrochemical studies, although some dependence on the structure of the surface was observed there.…”

“…In heterogeneous catalysis, it typically controls activity and selectivity. In the specific case of chiral modification of platinum-based hydrogenation catalysts by cinchona alkaloids, , interesting nonlinear effects have been reported where mixtures of modifiers do not lead to a performance average of that of the individual components but rather to the dominance of one over the others. − This can be explained by the preferential adsorption of that modifier on the surface. Here we provide direct evidence for such asymmetric adsorption uptake on the basis of in-situ reflection−absorption infrared spectroscopy (RAIRS) measurements.…”