Redox-neutral α-C-H bond functionalization of secondary amines with concurrent C-P bond formation/N-alkylation

Metal-free methylation of a pyridine N-oxide C–H bond by using peroxides

Metal-free methylation of a pyridine N-oxide C–H bond was developed using peroxide as a methyl reagent under neat conditions. Pyridine N-oxide derivatives with various groups (e.g., Cl, NO₂, and OCH₃) were all suitable substrates.

General base-tuned unorthodox synthesis of amides and ketoesters with water

We discovered a highly reactive λ³-hypervalent iodane species using an inorganic/organic base for the unorthodox synthesis of amides and ketoesters through grafting terminal alkynes. In contrast to the metal-catalyzed dehydrative approaches the in situ generated nonmetallic reagent efficiently created C–N/C–O and CO bonds with amines/alkynes and water at rt.

Intramolecular [3 + 2]-cycloadditions of azomethine ylides derived from secondary amines via redox-neutral C-H functionalization

Azomethine ylides are accessed under mild conditions via benzoic acid catalyzed condensations of 1,2,3,4-tetrahydroisoquinolines or tryptolines with aldehydes bearing a pendent dipolarophile. These intermediates undergo intramolecular [3 + 2]-cycloadditions in a highly diastereoselective fashion to form polycyclic amines with four new stereogenic centers. Challenging substrates such as piperidine, morpholine, and thiomorpholine undergo the corresponding reactions at elevated temperatures.

Redox-neutral α-sulfenylation of secondary amines: ring-fused N,S-acetals

Secondary amines react with thiosalicylaldehydes in the presence of catalytic amounts of acetic acid to generate ring-fused N,S-acetals in redox-neutral fashion. A broad range of amines undergo α-sulfenylation, including challenging substrates such morpholine, thiomorpholine, and piperazines. Computational studies employing density functional theory indicate that acetic acid reduces the energy barriers of two separate steps, both of which involve proton transfer.

The redox-Mannich reaction

A complement to the classic three-component Mannich reaction, the redox-Mannich reaction, utilizes the same starting materials but incorporates an isomerization step that enables the facile preparation of ring-substituted β-amino ketones. Reactions occur under relatively mild conditions and are facilitated by benzoic acid.

Dual C-H functionalization of N-aryl amines: synthesis of polycyclic amines via an oxidative Povarov approach

Iminium ions generated in situ via copper(I) bromide catalyzed oxidation of N-aryl amines readily undergo [4 + 2] cycloadditions with a range of dienophiles. This method involves the functionalization of both a C(sp(3))-H and a C(sp(2))-H bond and enables the rapid construction of polycyclic amines under relatively mild conditions.

Redox-neutral α,β-difunctionalization of cyclic amines

In contrast to the continuously growing number of methods that allow for the efficient α-functionalization of amines, few strategies exist that enable the direct functionalization of amines in the β-position. A general redox-neutral strategy is outlined for amine β-functionalization and α,β-difunctionalization that utilizes enamines generated in situ. This concept is demonstrated in the context of preparing polycyclic N,O-acetals from simple 1-(aminomethyl)-β-naphthols and 2-(aminomethyl)-phenols.

Redox-neutral α-oxygenation of amines: reaction development and elucidation of the mechanism

Cyclic secondary amines and 2-hydroxybenzaldehydes or related ketones react to furnish benzo[e][1,3]oxazine structures in generally good yields. This overall redox-neutral amine α-C-H functionalization features a combined reductive N-alkylation/oxidative α-functionalization and is catalyzed by acetic acid. In contrast to previous reports, no external oxidants or metal catalysts are required. Reactions performed under modified conditions lead to an apparent reductive amination and the formation of o-hydroxybenzylamines in a process that involves the oxidation of a second equivalent of amine. A detailed computational study employing density functional theory compares different mechanistic pathways and is used to explain the observed experimental findings. Furthermore, these results also reveal the origin of the catalytic efficiency of acetic acid in these transformations.

Redox-neutral α-arylation of amines

The direct α-arylation/N-alkylation of cyclic amines was achieved in a redox-neutral fashion under mild conditions. Transformations occur in the absence of any additives or are promoted by simple carboxylic acids.

Catalyst-free three-component reaction to synthesize chiral α-amino phosphine oxides

A novel method for high diastereoselective syntheses of chiral α-aminophosphine oxides by a three-component reaction involve a metal-free system was performed.