Chiral pyridine N-oxides: useful ligands for asymmetric catalysis

Lewis Base Catalyzed Enantioselective Allylation of α,β‐Unsaturated Aldehydes

“…[2] In the last decade, considerable attention has been paid to a possible variant of this reaction based on the activation of allyltrichlorosilane or its derivatives by chiral Lewis bases, such as pyridine N-oxides. A variety of bi-or monodentate [2][3][4][5][6] catalysts have been synthesized and applied in the catalytic allylation of aromatic aldehydes, which is used as a benchmark reaction to assess the catalytic activity and the scope of asymmetric induction. Although in some cases high enantioselectivity was observed, this trend was not general and the asymmetric induction was often highly dependent on the presence of electron-accepting or -donating groups.…”

New Pathway to C2‐Symmetric Atropoisomeric Bipyridine N,N′‐Dioxides and Solvent Effect in Enantioselective Allylation of Aldehydes

“…[1] The main reason for its success lies in the high level of enantioselectivity that can usually be achieved by the use of rather simple substances. One such class of compounds are bipyridine N,N'-dioxides [2] that activate various substrates through a Lewis base reaction mechanism. Although a great deal of work has been done in this area by the groups of Nakajima, [3] Hayashi, [4] Kočov-ský, [5] Dennmark, [6] and others, [7] who prepared various catalysts and showed that they can catalyze a number of reactions, there are still issues that have not yet been clarified and are awaiting further exploration such as: i) the relationship between the structure of bipyridine N,N'-dioxides and degree of enantioselectivity, ii) the course of the reaction mechanism(s), and also iii) the development of simple synthetic approaches to new potential catalysts.…”

A Simple Approach to Unsymmetric Atropoisomeric Bipyridine N,N′‐Dioxides and Their Application in Enantioselective Allylation of Aldehydes

“…The electron-donating properties of pyridine N-oxides make them rather strong Lewis bases that are able to activate a number of reactants for a plethora of reactions such as allylation and alkylation of carbonyl compounds, conjugated addition, aldol reaction, desymmetrization of epoxides, epoxidation, etc. [2] Although a number of various pyridine-, [3][4][5][6][7][8] bipyridine-, [9][10][11] and terpyridine-based ligands [12] has been introduced into organic synthesis, there is a considerable synthetic demand for the development of new bipyridine-based organocatalysts.…”