Cyclometalated Ruthenium Catalyst Enables Selective Oxidation of N-Substituted Tetrahydroquinolines to Lactams

Reductive alkylation of azoarenes to N-alkylated hydrazines enabled by hexafluoroisopropanol

A one-pot tandem approach to N-alkyl-N,N'-diarylhydrazines was developed using a sequence of reduction of azoarene to hydrazoarene followed by reductive alkylation by an aldehyde. The mild reaction conditions suppress N-N cleaved products and selectively provide trisubstituted hydrazine derivatives. The mechanistic study demonstrates that the iminium formation step is likely to be the rate-limiting step.

Arylboronic Acid Catalyzed Reductive Alkylation of Azoarenes to N-Alkylated Hydrazines

Hydrazine derivatives are generally difficult to prepare despite their appealing structural simplicity. The synthetic challenge may arise from either the similar reactivity or the relatively low activity of the two nitrogen atoms in a hydrazine precursor. The former would lead to severe selectivity issues while the latter would require harsh reaction conditions. Herein we report a facile method for the synthesis of N-alkyl-N, N'-diarylhydrazines by boronic acid-catalyzed one-pot tandem reduction of azoarene to hydrazoarene followed by reductive alkylation with the aldehyde. The mild reaction conditions suppress N-N cleaved products and selectively provide trisubstituted hydrazine derivatives. The reaction is highly chemoselective and tolerates various hydrogenation-sensitive functional groups. The method has been successfully utilized in the late-stage functionalization and diversification of pharmaceuticals. A series of control experiments with boronic acids, boronic ester, and Brønsted acids were interpreted. The study suggests the mode of boronic catalysis involves both Brønsted acid and H-bond donor catalysis. Based on the thermodynamic activation parameters, kinetic Hammett studies, and competitive reaction on the substrate, it has been postulated that the iminium formation step is likely to be the rate-limiting step.

Cyclometalated Ruthenium-Complex-Catalyzed Selective Oxidation of Olefins to Carbonyls

Cyclometalated ruthenium(II)-complex-catalyzed selective oxidative scission of olefins to carbonyls is described. A strong C-donor ligand, paired with a rigid backbone and redox activity of ruthenium, provided high catalytic activity and a long lifetime for olefin oxidation. The catalyst tolerates numerous functional groups and applies to challenging biomass-, natural-product-, sugar-, amino-acid-, and fatty-acid-derived substrates.

Cu(I)-Catalyzed Highly Regioselective C-H Amidation of Quinoline N-Oxides with Dioxazolones

A Cu(I)-catalyzed highly regioselective synthesis of 2-acetamidequinoline N-oxides using dioxazolones with quinoline N-oxides has been reported. The reaction possesses mild reaction conditions and excellent functional group compatibility. Furthermore, the addition of hydrochloric acid promotes the decomposition of copper complexes, which is beneficial for postprocessing.

Ru/O2-Catalyzed Oxidative C-H Activation/Alkyne Annulation Using Quinoline-Functionalized NHC as a Directing and Functionalizable Group

The ruthenium/O2-catalyzed oxidative annulation reaction of imidazo[1,5-a]quinolin-2-ium salts with alkynes via N-heterocyclic carbene-directed C-H activation to obtain π-conjugated fused imidazo[1,5-a]quinolin-2-ium derivatives is reported. Molecular oxygen has been explored as an economic and clean oxidant and an alternative to metal oxidants. The current protocol exhibits a wide range of substrate scope including bioactive (±)-α-tocopherol derivatives. Moreover, most of the annulated products show strong fluorescence properties, indicating their potential for making new light-emitting materials.