Selective Deoxygenation of PyridineN-Oxides through Photoredox Catalysis of a Dilacunary Silicotungstate

Metal-Free Deoxygenation of Amine N-Oxides: Synthetic and Mechanistic Studies

We report herein an unprecedented combination of light and P(III)/P(V) redox cycling for the efficient deoxygenation of aromatic amine N-oxides. Moreover, we discovered that a large variety of aliphatic amine N-oxides can easily be deoxygenated by using only phenylsilane. These practically simple approaches proceed well under metal-free conditions, tolerate many functionalities and are highly chemoselective. Combined experimental and computational studies enabled a deep understanding of factors controlling the reactivity of both aromatic and aliphatic amine N-oxides.

Synthesis of tetrafluorinated piperidines from nitrones via a visible-light-promoted annelation reaction

A method for the one-step construction of 3,3,4,4-tetrafluorinated piperidines from nitrones and readily accessible tetrafluorinated iodobromobutane is described. The reaction requires an excess amount of ascorbic acid as the terminal reductant and is performed in the presence of an iridium photocatalyst activated by blue light. The annelation is a result of a radical addition at the nitrone, intramolecular nucleophilic substitution, and reduction of the N–O bond.

Highly chemoselective deoxygenation of N-heterocyclic N-oxides under transition metal-free conditions

Because their site-selective C-H functionalizations are now considered one of the most useful tools for synthesizing various N-heterocyclic compounds, the highly chemoselective deoxygenation of densely functionalized N-heterocyclic N-oxides has received much attention from the synthetic chemistry community. Here, we provide a protocol for the highly chemoselective deoxygenation of various functionalized N-oxides under visible light-mediated photoredox conditions with Na2-eosin Y as an organophotocatalyst. Mechanistic studies imply that the excited state of the organophotocatalyst is reductively quenched by Hantzsch esters. This operationally simple technique tolerates a wide range of functional groups and allows high-yield, multigram-scale deoxygenation.

Hexavacant γ-Dawson-type phosphotungstates supporting an edge-sharing bis(square-pyramidal) {O2M(μ3-O)2(μ-OAc)MO2} core (M = Mn2+, Co2+, Ni2+, Cu2+, or Zn2+)

In aqueous media, the introduction of additional metal species into polyoxometalates (POMs) with multiple vacant sites, such as a hexavacant Dawson-type phosphotungstate, which is of interest for the synthesis of novel metal oxide clusters, is generally difficult because they easily undergo self-condensation and/or structural decomposition. In this study, we succeeded in developing a novel synthetic method to obtain metal-substituted γ-Dawson-type phosphotungstate monomers by introducing metal species into an organic solvent-soluble lacunary phosphotungstate, TBA4H10[α-P2W12O48] (I) (TBA = tetra-n-butylammonium), in organic media. The reaction of I, which possessed two types of vacant sites, i.e. middle and edge sites, with divalent metal species such as Mn2+, Co2+, Ni2+, Cu2+, or Zn2+ in acetonitrile afforded a series of isostructural POMs M2 (TBA5[γ-P2W12O44M2(OAc)(CH3CONH)2]·nH2O·mCH3CN; M = Mn2+, Co2+, Ni2+, Cu2+, or Zn2+; OAc = acetate) with an edge-sharing bis(square-pyramidal) {O2M(μ3-O)2(μ-OAc)MO2} core. The bis(square-pyramidal) core was selectively placed at the middle site of the hexavacant lacunary phosphotungstate, and the two metals in the core were bridged by two phosphate units and one acetate species. Meanwhile, the edge sites were capped by acetimidate ligands, which protect the reactive lacunary POM from self-condensation. To the best of our knowledge, this is the first report describing the synthesis and characterization of metal-substituted hexavacant γ-Dawson-type POM monomers.

Polyoxometalate LUMO engineering: a strategy for visible-light-responsive aerobic oxygenation photocatalysts

We report efficient visible-light-responsive oxygenation photocatalysis via the strategy of LUMO engineering of polyoxometalates.

Visible-light-responsive catalysis of a zinc-introduced lacunary disilicoicosatungstate for the deoxygenation of pyridine N-oxides

A mononuclear zinc-introduced lacunary disilicoicosatungstate showed efficient photocatalytic activity in the selective deoxygenation of pyridine N-oxides under visible light irradiation.