Abstract
The asymmetric Henry reaction of 2-acylpyridine N-oxide remains a challenge as N-oxides generally act as competitive catalyst inhibitors or displace activating ligands. A novel variable yield (up to 99%) asymmetric Henry reaction of 2-acypyridine N-oxides catalyzed by a Ni-aminophenol sulfonamide complex with good to excellent enantioselectivity (up to 99%) has been developed. Mechanistic studies suggest that the unique properties of the electron-pairs of N-oxides for complexation with Ni makes the unexpected mononuclear complex, rather than the previously reported dinuclear complex, the active species.
Highlights
The asymmetric construction of chiral quaternary stereocenters represents a considerable challenge in modern organic synthesis [1,2,3,4,5,6]
Considerable effort has been devoted to the development of efficient chiral catalysts for asymmetric Henry reactions of reactive ketones, such as trifluoromethyl ketones, α-keto esters, α-keto amides, α-keto-phosphonates [27,28], and glyoxal hydrates [29]
Pedro and Blay first extended the Henry reactions to 2-acylpyridine N-oxides, which provided a convenient way for synthesizing β-amino tert-alcohols substructure bearing a quaternary stereocenter bonded to a 2-pyridyl moiety [40]
Summary
The asymmetric construction of chiral quaternary stereocenters represents a considerable challenge in modern organic synthesis [1,2,3,4,5,6]. The Henry (nitroaldol) reaction [7,8,9,10,11,12,13] of ketones has become one of the most important and versatile reactions for the construction of quaternary carbons containing hydroxyl and nitro groups. Considerable effort has been devoted to the development of efficient chiral catalysts for asymmetric Henry reactions of reactive ketones, such as trifluoromethyl ketones (for selected examples, see ref [14,15,16,17,18]), α-keto esters (for selected examples, see [19,20,21,22,23,24]), α-keto amides (for selected examples, see [25,26]), α-keto-phosphonates [27,28], and glyoxal hydrates [29].
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