Abstract
An increasing interest in the synthesis and use of optically active pyridine N-oxides as chiral controllers for asymmetric reactions has been observed in the last few years. Chiral heteroaromatic N-oxides can work as powerful electron-pair donors, providing suitable electronic environments in the transition state formed within the reaction. The nucleophilicity of the oxygen atom in N-oxides, coupled with a high affinity of silicon to oxygen, represent ideal properties for the development of synthetic methodology based on nucleophilic activation of organosilicon reagents. The application of chiral N-oxides as efficient organocatalysts in allylation, propargylation, allenylation, and ring-opening of meso-epoxides, as well as chiral ligands for metal complexes catalyzing Michael addition or nitroaldol reaction, can also be found in the literature. This review deals with stereoselective applications of N-oxides, and how the differentiating properties are correlated with their structure. It contains more recent results, covering approximately the last ten years. All the reported examples have been divided into five classes, according to the chirality elements present in their basic molecular frameworks.
Highlights
Enantioselective organocatalysis is one of the more rapidly growing fields of research in modern organic chemistry
Among various Lewis base catalysts, those having the pyridine oxide moiety situated within a chiral environment, constitute a distinct class of highly active catalysts that are capable of high asymmetric induction, usually under mild reaction conditions
Considering the fact, that electron-rich Lewis base catalysts are usually more reactive in reaction with halosilane compounds, they decided to prepare compound 8g, having two phenyl rings double substituted with CF3 groups, suitable for aldehydes with halogen groups [36]
Summary
Enantioselective organocatalysis is one of the more rapidly growing fields of research in modern organic chemistry. Since Nakajima’s report in 1998 [4] showing that axially chiral 2,20 bipyridine N,N’-dioxides are effective as catalysts for the asymmetric allylation, the application of N-oxides has attracted considerable attention. Molecules 2019, 24, x they represent, defined as axial, central, helical and planar chirality. It is easier to divide them by type of chirality they represent, defined axial, will central, helical and briefly described in this work, along with the examples of catalytic applications in various and planar chirality. All these classes will be shown and briefly described in this work, along with the enantioselective processes.
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