Crucial aspects in the design of chirally modified noble metal catalysts for asymmetric hydrogenation of activated ketones

Abstract

In view of the importance of optically pure chiral products there is ample reason to develop methods that facilitate their efficient production. Compared to the mostly applied homogeneous catalysts based on transition metals coordinated to suitable chiral ligands, heterogeneous chiral catalysts could offer several features that are beneficial for practical application such as stability, ease of catalyst separation and regeneration as well as straightforward access to continuous process operation. Various strategies have been developed for imparting chirality to catalytic active surfaces, among which the chiral modification of active metal surfaces by adsorption of suitable chiral organic compounds has so far been among the most successful. In this tutorial review lessons learned from research on asymmetric hydrogenation on chirally modified noble metals will be presented. Key aspects for the design of such catalysts will be elucidated using chirally modified platinum catalysts for the asymmetric hydrogenation of α-activated ketones as an example.

The Critical Role of Tilted Cinchona Surface Species for Enantioselective Hydrogenation

“…6,9,[17][18][19][20]29,[34][35][36] The flat-lying surface species, also referred to as -bound species, is the most strongly bound cinchona species on Pt or Pd metal catalysts, according to theoretical calculations 20 and experimental observations [28][29][30] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 27 including this experimental work. Weaker bound tilted cinchona species have also been found in previous studies on Pt 28,29 and Pd, 30 but strong surface anchoring via parallel -bonding of the aromatic moiety has been considered favorable for enantioselection 6,9,29,[34][35][36] and theoretical studies have focused on the most strongly bound species. [17]…”

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 28 observations have to be evaluated in the light of the new mechanistic understanding, taking the high parametric sensitivity and complexity of chirally-modified metals into account. 9,21 Regarding the transferability of these mechanistic implications to explain enantioselectivity on Pt catalysts we need to refer to our previous operando spectroscopic study, which focused on the asymmetric hydrogenation of the activated ketone ketopantolactone on CD-modified Pt catalyst in the solvent toluene. 22 In this study, we were not able to quantify the surface coverage and could not clearly stake out surface regimes II and III.…”

“…[6][7][8] A practically very simple approach to transfer enantiocontrol to a conventional achiral hydrogenation catalyst is to modify the noble metal with a suitable organic chiral molecule, referred to as chiral modifier. 9,10 Amongst various chiral organic compounds the naturally-occurring pseudo-enantiomers cinchonine (CN) and cinchonidine (CD) stand out as efficient chiral modifiers. Their addition in small amounts to the liquid reaction solution can lead to strong stereochemical control in heterogeneous hydrogenations on Pt and Pd catalysts.…”

Facile Synthesis of P25@Pd Core-Shell Catalyst with Ultrathin Pd Shell and Improved Catalytic Performance in Heterogeneous Enantioselective Hydrogenation of Acetophenone

“…With the wide application of optically pure chiral compounds in pesticides, pharmaceuticals and fragrances, it is particularly important to develop effective preparation methods for synthesizing single-enantiomer compounds [1][2][3]. Among the many established methods, enantioselective hydrogenation over heterogeneous catalysts is one of the most ideal strategies owing to its inherent operational and economic advantages, e.g., atom economy, easy separation and recovery of catalysts.…”

“…Among the many established methods, enantioselective hydrogenation over heterogeneous catalysts is one of the most ideal strategies owing to its inherent operational and economic advantages, e.g., atom economy, easy separation and recovery of catalysts. Therefore, it has shown great potential in industrial research and caused extensive concern in academic circles [1][2][3][4].…”