Enantioselective hydrogenation of activated ketones in the presence of Pt–cinchona catalysts. Is the proton transfer concept valid?

The Critical Role of Tilted Cinchona Surface Species for Enantioselective Hydrogenation

“…In this work in section the beneficial effect of QN addition shifting the poor enantiodifferentiation of surface regime II to surface regime III was shown for the CN-modified Pd-catalyzed hydrogenation. In the past, particularly Margitfalvi and co-workers ,, have observed the same behavior in several catalytic studies for CD-modified Pt-catalyzed hydrogenations involving alumina- and silica-supported Pt catalyst, as well as the model substrates ethyl pyruvate and methyl benzoylformate. For example, Tálas and Margitfalvi observed a tremendous improvement of more than 50% ee upon addition of 60 μM QN to the asymmetric hydrogenation of ethyl pyruvate in toluene at low CD concentration (6 μM CD), and no improvement was observed at 60 μM CD concentration (Figure 1 in ref ).…”

“…In this work in section 3.5 the beneficial effect of QN addition shifting the poor enantiodifferentiation of surface regime II to surface regime III was shown for the CN-modified Pd-catalyzed hydrogenation. In the past, particularly Margitfalvi and co-workers 41,46,47 have observed the same behavior in several catalytic studies for CD-modified Pt-catalyzed hydrogenations involving alumina-and silica-supported Pt catalyst, as well as the model substrates ethyl pyruvate and methyl benzoylformate. For example, Taĺas and Margitfalvi 46 observed a tremendous improvement of more than 50% ee upon addition of 60 μM QN to the asymmetric hydrogenation of ethyl pyruvate in toluene at low CD concentration (6 μM CD), and no improvement was observed at 60 μM CD concentration (Figure 1…”

“…In light of the molecular-level insight provided by this operando spectroscopic study, the catalytic results by Margitfalvi et al, , by Blaser et al, and by us in this work involving various substrates, solvents, and Pt as well as Pd catalysts serve as compelling experimental evidence for the crucial role of the cinchona surface species III for enantioselective hydrogenation on Pd and Pt catalysts and the tilted orientation of the more weakly bound aromatic ring anchor has to be taken into account in future mechanistic considerations. For future experimental and theoretical studies it could be fruitful to focus more strongly on the tilted cinchona species in Pt- and Pd-catalyzed asymmetric hydrogenation.…”

Monitoring Surface Processes During Heterogeneous Asymmetric Hydrogenation of Ketones on a Chirally Modified Platinum Catalyst by Operando Spectroscopy

“…This limitation leaves behind some uncertainty whether the observed species are actively participating or simply "spectators" in the catalytic cycle. [13] The junction of spectroscopic and kinetic data would help to overcome this current limitation of the in situ techniques and speed up progress in the field. [14] With this in mind, we have made an effort here to monitor the surface processes occurring during the catalytic enantioselective hydrogenation of ketones on a CD-modified Pt catalyst under real working conditions.…”

Monitoring Surface Processes During Heterogeneous Asymmetric Hydrogenation of Ketones on a Chirally Modified Platinum Catalyst by Operando Spectroscopy

“…While these in situ IR studies have been carried out in a similar chemical environment as encountered in the catalytic reaction, the methods applied captured only a snapshot of the surface at quasi‐steady state and neither measurable catalytic turnover nor optical yield could be observed. This limitation leaves behind some uncertainty whether the observed species are actively participating or simply “spectators” in the catalytic cycle 13. The junction of spectroscopic and kinetic data would help to overcome this current limitation of the in situ techniques and speed up progress in the field 14…”