Visible-Light Photocatalyzed Deoxygenation of N-Heterocyclic N-Oxides

A Phosphine-Free Air-Stable Mn(II)-Catalyst for Sustainable Synthesis of Quinazolin-4(3H)-ones, Quinolines, and Quinoxalines in Water

The synthesis, characterization, and catalytic application of a new phosphine-free, well-defined, water-soluble, and air-stable Mn(II)-catalyst [Mn(L)(H2O)2Cl](Cl) ([1]Cl) featuring a 1,10-phenanthroline based tridentate pincer ligand, 2-(1H-pyrazol-1-yl)-1,10-phenanthroline (L), in dehydrogenative functionalization of alcohols to various N-heterocycles such as quinazolin-4(3H)-ones, quinolines, and quinoxalines are reported here. A wide array of multisubstituted quinazolin-4(3H)-ones were prepared in water under air following two pathways via the dehydrogenative coupling of alcohols with 2-aminobenzamides and 2-aminobenzonitriles, respectively. 2-Aminobenzyl alcohol and ketones bearing active methylene group were used as coupling partners for synthesizing quinoline derivatives, and various quinoxaline derivatives were prepared by coupling vicinal diols and 1,2-diamines. In all cases, the reaction proceeded smoothly using our Mn(II)-catalyst [1]Cl in water under air, affording the desired N-heterocycles in satisfactory yields starting from cheap and readily accessible precursors. Gram-scale synthesis of the compounds indicates the industrial relevance of our synthetic strategy. Control experiments were performed to understand and unveil the plausible reaction mechanism.

Metal-Free Visible-Light Catalytic Deoxygenation of Azaaryl N-Oxides: Harnessing Photons for Efficient Greener Synthesis

Regio- and chemo-selective functionalization of electron deficient azaarenes often required their transformations to corresponding N-oxides and subsequent removal of oxygen after functionalization to get back the desired substituted azaarenes. An efficient metal-free visible-light photo-redox catalytic deoxygenation of N-oxides of azaheterocyclic compounds has been developed using acridinium based organo-photocatalyst in blue LED light. High efficiency and mildness of this methodology has been demonstrated through higher deoxygenation yield of wide variety of azaheterocyclic N-oxides with reactive functional groups. Robustness of the photocatalytic reduction has been demonstrated through easy scaling-up of the reaction to gram level without much change in the reaction yield.

Rh(III)-Catalyzed C7-Alkylation of Isatogens with Malonic Acid Diazoesters

Rh(III)-catalyzed C7-alkylation of isatogens (indolin-3-one N-oxides) with malonic acid diazoesters has been developed. This strategy utilizes oxygen anion on the N-oxide group of isatogens as a directing group and successfully achieves the synthesis of a series of C7-alkylated isatogens with moderate to good yields (48-86% yields). Moreover, the N-oxides of isatogens can not only serve as the simple directing group for C7-H bond cleavage but also be deoxidized for easy removal.

Synthesis of 1,2,3-Triazine Derivatives by Deoxygenation of 1,2,3-Triazine 1-Oxides

A convenient, efficient, and inexpensive method has been developed for the synthesis of 1,2,3-triazine derivatives via deoxygenation of 1,2,3-triazine 1-oxide using trialkyl phosphites. Triethyl phosphite is more reactive than trimethyl phosphite, and both phosphites form their corresponding phosphates in these reactions. This procedure provides a range of aromatic and aliphatic substituted 1,2,3-triazine-4-carboxylate derivatives cleanly in high yields. Unexpected 1,2,4-triazine derivatives were also obtained as minor products during deoxygenation of 1,2,3-triazine-4-carboxylate 1-oxides having an aliphatic substituent at the 5-position.

Chromium-Catalyzed Reductive Cleavage of Unactivated Aromatic and Benzylic C–O Bonds

Reductive cleavage of aromatic and benzylic C–O bonds by chromium catalysis is reported. This deoxygenative reaction was promoted by low-cost CrCl2 precatalyst combined with poly(methyl hydrogen siloxane) as the mild reducing agent, providing a strategy in forming reduced motifs by cleavage of unactivated C–O bonds. A range of functional groups such as bromide, chloride, fluoride, hydroxyl, amino, and alkoxycarbonyl can be retained in the reduction.

Photocatalytic deoxygenation of N-O bonds with rhenium complexes: from the reduction of nitrous oxide to pyridine N-oxides

The accumulation of nitrogen oxides in the environment calls for new pathways to interconvert the various oxidation states of nitrogen, and especially their reduction. However, the large spectrum of reduction potentials covered by nitrogen oxides makes it difficult to find general systems capable of efficiently reducing various N-oxides. Here, photocatalysis unlocks high energy species able both to circumvent the inherent low reactivity of the greenhouse gas and oxidant N₂O (E⁰(N₂O/N₂) = +1.77 V vs. SHE), and to reduce pyridine N-oxides (E_1/2(pyridine N-oxide/pyridine) = −1.04 V vs. SHE). The rhenium complex [Re(4,4′-tBu-bpy)(CO)₃Cl] proved to be efficient in performing both reactions under ambient conditions, enabling the deoxygenation of N₂O as well as synthetically relevant and functionalized pyridine N-oxides.

A rhenium-based photocatalyst enables the deoxygenation of several compounds containing N–O bonds, such as N₂O and pyridine N-oxides.

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.

Advances in transition metal-free deborylative transformations of gem-diborylalkanes

Carbanions serve as key intermediates in a variety of chemical transformations. Particularly, α-borylcarbanions have received considerable attention in recent years because of their peculiar properties, including the ability of boron atom resonance to stabilise the adjacent negatively charged carbon atom. This feature article summarises recent progress in the synthetic utilisation of α-borylcarbanions, including carbon-carbon bond formation with alkyl halides, alkenes, N-heteroarenes, and carbonyls. Carbon-boron bond formation in organohalides mediated by α-borylcarbanions is also summarised.

Visible-light-mediated synthesis of quinolines

Visible light promoted synthetic routes of quinolines using different strategies are hereby documented.

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.

Light-Driven Metal-Free Direct Deoxygenation of Alcohols under Mild Conditions

Hydroxyl is widely found in organic molecules as functional group and its deprivation plays an inevitable role in organic synthesis. However, the direct cleavage of Csp³-O bond in alcohols with high selectivity and efficiency, especially without the assistance of metal catalyst, has been a formidable challenge because of its strong bond dissociation energy and unfavorable thermodynamics. Herein, an efficient metal-free strategy that enables direct deoxygenation of alcohols has been developed for the first time, with hydrazine as the reductant induced by light. This protocol features mild reaction conditions, excellent functional group tolerance, and abundant and easily available starting materials, rendering selective deoxygenation of a variety of 1° and 2° alcohols, vicinal diols, and β-1 and even β-O-4 models of natural wood lignin. This strategy is also highlighted by its “traceless” and non-toxic by-products N₂ and H₂, as readily escapable gases. Mechanistic studies demonstrated dimethyl sulfide being a key intermediate in this transformation.

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Metal-free direct deoxygenation of alcohols enabled by light

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Traceless non-toxic N₂ and H₂ as by-products

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Broad substrate scope and wide functional group compatibility

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Converting lignin models into simple aromatics

Ruthenium(ii)-catalyzed [5 + 1] annulation reaction: a facile and efficient approach to construct 6-ethenyl phenanthridines utilizing a primary amine as a directing group

A ruthenium(ii)-catalyzed [5 + 1] annulation reaction between 2-arylanilines and cyclopropenones employing a free amine as a directing group has been developed.

Light-enabled metal-free pinacol coupling by hydrazine

A light-driven pinacol coupling protocol without any metals, but with N₂H₄ as a clean non-metallic hydrogen-atom-transfer (HAT) reductant was described.

Efficient carbon–carbon bond formation is of great importance in modern organic synthetic chemistry. The pinacol coupling discovered over a century ago is still one of the most efficient coupling reactions to build the C–C bond in one step. However, traditional pinacol coupling often requires over-stoichiometric amounts of active metals as reductants, causing long-lasting metal waste issues and sustainability concerns. A great scientific challenge is to design a metal-free approach to the pinacol coupling reaction. Herein, we describe a light-driven pinacol coupling protocol without use of any metals, but with N₂H₄, used as a clean non-metallic hydrogen-atom-transfer (HAT) reductant. In this transformation, only traceless non-toxic N₂ and H₂ gases were produced as by-products with a relatively broad aromatic ketone scope and good functional group tolerance. A combined experimental and computational investigation of the mechanism suggests that this novel pinacol coupling reaction proceeds via a HAT process between photo-excited ketone and N₂H₄, instead of the common single-electron-transfer (SET) process for metal reductants.

Photoredox-catalysed formal [3+2] cycloaddition of N-aryl α-amino acids with isoquinoline N-oxides

This work realizes a new synthetic utility of N-aryl α-amino acids as a 1,2-synthon, a new strategy to achieve dearomatization of isoquinolines, and the synthesis of valuable diazabicyclo[3.2.1]octane-based compounds.