Recent Advances in Copper-Catalyzed N-O Cleavage Strategy

Chan-Lam Reaction and Lewis Acid Promoted 1,3-Rearrangement of N-O Bonds to Prepare N-(2-Hydroxyaryl)pyridin-2-ones

We describe the difunctionalization of arylboronic acids to prepare various N-(2-hydroxyaryl)pyridin-2-ones in good yields using N-hydroxypyridin-2-ones as the oxygen and nitrogen sources through a copper(II)-catalyzed Chan-Lam reaction and subsequent BF₃-promoted selective 1,3-rearrangement of N-O bond in a one-pot procedure. Mechanistic studies reveal that the 1,3-rearrangement selectivity is controlled by the formation of the key aryloxypyridinium salt. The obtained products are easily converted to various useful pyridin-2-one scaffolds.

Divergent copper-catalyzed syntheses of 3-carboxylpyrroles and 3-cyanofurans from O-acetyl oximes and β-ketoesters/nitriles

The reaction between O-acetyl oximes and β-ketoesters/nitriles catalyzed by copper generated polysubstituted pyrroles and furans, respectively, under redox–neutral reaction conditions. Using this protocol, pyrroles or furans could be obtained simply by choosing an appropriate active methylene compound. Although both transformations occur essentially under the same reaction conditions, control experiments indicated that they follow different mechanistic pathways.

A copper-catalyzed heteroannulation of O-acetyl oximes and β-ketoesters/nitriles to afford polysubstituted pyrroles and furans was successfully achieved.

External-oxidant-free amino-benzoyloxylation of unactivated alkenes of unsaturated ketoximes with O-benzoylhydroxylamines

A new copper-catalyzed two-component amino-benzoyloxylation of unactivated alkenes of unsaturated ketoximes with O-benzoylhydroxylamines as the benzoyloxy sources is developed. Chemoselectivity of this method toward amino-benzoyloxylation or oxy-benzoyloxylation of alkenyl ketoximes relies on the position of the tethered olefins, and provides an external-oxidant-free alkene difunctionalization route that directly utilizes O-benzoylhydroxylamines as the benzoyloxy radical precursors and internal oxidants for the divergent synthesis of cyclic nitrones and isoxazolines.

Copper-Catalyzed Aqueous N-O Bond Cleavage of 2-Oxa-3-Azabicyclo Compounds to Cyclic cis-1,4-Amino Alcohols

The N-O bond cleavage of 2-oxa-3-azabicyclo substrates, which are readily prepared by the hetero Diels-Alder reaction between nitroso dienophiles and cyclic 1,3-dienes, was effectively catalyzed by heterogeneous copper-on-carbon (Cu/C) under aqueous conditions to give the corresponding cyclic cis-1,4-amino alcohol derivatives. The present method was applied to the direct incorporation of the hydroxy and amino groups derived from a nitroso substrate into cyclic 1,3-dienes with cis-selectivity by the combination of the in situ formation of 2-oxa-3-azabicyclo compounds and following Cu/C-catalyzed N-O bond cleavage. The obtained cis-4-aminocyclohexenols, derived from cyclohexadiene as a cyclic 1,3-diene, could be selectively oxidized by using the ruthenium-on-carbon (Ru/C) catalyst under oxygen atmosphere to the corresponding 4-aminocyclohexenones at 50-65 °C or para-iminoquinones at 100-110 °C as useful reactive synthetic precursors.

Synthesis of α-aminooxy amides through [3 + 3] cycloaddition and Sc(OTf)3-catalyzed double C-N bond cleavage in a one-pot reaction

Various α-aminooxy amides bearing a quaternary carbon at the α-position were prepared in good to excellent yields under mild reaction conditions from N-vinyl nitrones and α-bromohydroxamates. The N-vinyl nitrones tolerate a wide range of N-vinyl fluorenone nitrones and N-vinyl isatin nitrones. Mechanistic studies show that the reaction initially proceeds through [3 + 3] cycloaddition between N-vinyl nitrones and aza-oxyallyl cations generated from α-bromohydroxamates to afford six-membered N,O-heterocycles, followed by double C-N bond cleavage in the presence of the Sc(OTf)3 catalyst. A selective N-O bond cleavage of the obtained α-aminooxy amides is also realized under Fe/NH4Cl conditions. Furthermore, gram-scalable preparations of α-aminooxy amides are easily achieved.