Anti‐Markovnikov Hydroazidation of Alkenes by Visible‐Light Photoredox Catalysis

Iron-Mediated Photochemical Anti-Markovnikov Hydroazidation of Unactivated Olefins

Unactivated olefins are converted to alkyl azides with bench-stable NaN3 in the presence of FeCl3·6H2O under blue-light irradiation. The products are obtained with anti-Markovnikov selectivity, and the reaction can be performed under mild ambient conditions in the presence of air and moisture. The transformation displays broad functional group tolerance, which renders it suitable for functionalization of complex molecules. Mechanistic investigations are conducted to provide insight into the hydroazidation reaction and reveal the role of water from the iron hydrate as the H atom source.

Rational design of metabolically stable HDAC inhibitors: An overhaul of trifluoromethyl ketones

Epigenetic regulation of gene expression using histone deacetylase (HDAC) inhibitors is a promising strategy for developing new anticancer agents. The most common HDAC inhibitors are hydroxamates, which, though highly potent, have limitations due to their poor pharmacokinetic properties and lack of isoform selectivity. Trifluoromethylketones (TFMK) developed as alternatives to hydroxamates are rapidly metabolized to inactive trifluoromethyl alcohols in vivo, which prevented their further development as potential drug candidates. In order to overcome this limitation, we designed trifluoropyruvamides (TFPAs) as TFMK surrogates. The presence of an additional electron withdrawing group next to the ketone carbonyl group made the hydrate form of the ketone more stable, thus preventing its metabolic reduction to alcohol in vivo. In addition, this structural modification reduces the potential of the TFMK group to act as a covalent warhead to eliminate off-target effects. Additional structural changes in the cap group of the inhibitors gave analogues with IC50 values ranging from upper nanomolar to low micromolar in the cytotoxicity assay, and they were more selective for cancer cells over normal cells. Some of the most active analogues inhibited HDAC enzymes with low nanomolar IC50 values and were found to be more selective for HDAC8 over other isoforms. These molecules provide a new class of HDAC inhibitors with a metabolically stable metal-binding group that could be used to develop selective HDAC inhibitors by further structural modification.

Hydroamination of Unactivated Alkenes with Aliphatic Azides

We report here an efficient and highly diastereoselective intermolecular anti-Markovnikov hydroamination of unactivated alkenes with aliphatic azides in the presence of silane. The system tolerates a wide range of azides and alkenes and operates with alkene as limiting reagent. Mechanistic studies suggest a radical chain pathway that involves aminium radical formation, radical addition to alkenes and HAT from silane to β-aminium alkyl radical. The use of sterically bulky silane is proposed to contribute to the excellent diastereoselectivity for HAT. Computational analysis uncovers the reaction pathway of aliphatic azide activation with silyl radical for aminyl radical formation.

Stereoselective Cyclization Cascade of Dihydroquinoxalinones by Visible-Light Photocatalysis: Access to the Polycyclic Quinoxalin-2(1H)-ones

An intriguing stereoselective visible-light photocatalysis of dihydroquinoxalinone derivatives has been realized via cyclization with or without the solvolysis cascade. The reactions provided the polycyclic ring structures with efficient formation of multiple bonds and with high stereoselectivity. X-ray crystallography unequivocally determined the structures of five polycyclic products.

Chemical versatility of azide radical: journey from a transient species to synthetic accessibility in organic transformations

The generation of azide radical (N₃˙) occurs from its precursors primarily via a single electron transfer (SET) process or homolytic cleavage by chemical methods or advanced photoredox/electrochemical methods. This in situ generated transient open-shell species has unique characteristic features that set its reactivity. In the past, the azide radical was widely used for various studies in radiation chemistry as a 1e^- oxidant of biologically important molecules, but now it is being exploited for synthetic applications based on its addition and intermolecular hydrogen atom transfer (HAT) abilities. Due to the significant role of nitrogen-containing molecules in synthesis, drug discovery, biological, and material sciences, the direct addition onto unsaturated bonds for the simultaneous construction of C-N bond with other (C-X) bonds are indeed worth highlighting. Moreover, the ability to generate O- or C-centered radicals by N₃˙ via electron transfer (ET) and intermolecular HAT processes is also well documented. The purpose of controlling the reactivity of this short-lived intermediate in organic transformations drives us to survey: (i) the history of azide radical and its structural properties (thermodynamic, spectroscopic, etc.), (ii) chemical reactivities and kinetics, (iii) methods to produce N₃˙ from various precursors, (iv) several significant azide radical-mediated transformations in the field of functionalization with unsaturated bonds, C-H functionalization via HAT, tandem, and multicomponent reaction with a critical analysis of underlying mechanistic approaches and outcomes, (v) concept of taming the reactivity of azide radicals for potential opportunities, in this review.

Mechanistic Investigation of the Iron-Catalyzed Azidation of Alkyl C(sp3)-H Bonds with Zhdankin's λ3-Azidoiodane

An in-depth study of the mechanism of the azidation of C(sp³)-H bonds with Zhdankin's λ³-azidoiodane reagent catalyzed by iron(II)(pybox) complexes is reported. Previously, it was shown that tertiary and benzylic C(sp³)-H bonds of a range of complex molecules underwent highly site-selective azidation by reaction with a λ³-azidoiodane reagent and an iron(II)(pybox) catalyst under mild conditions. However, the mechanism of this reaction was unclear. Here, a series of mechanistic experiments are presented that reveal critical features responsible for the high selectivity and broad scope of this reaction. These experiments demonstrate the ability of the λ³-azidoiodane reagent to undergo I-N bond homolysis under mild conditions to form λ²-iodanyl and azidyl radicals that undergo highly site-selective and rate-limiting abstraction of a hydrogen atom from the substrate. The resultant alkyl radical then combines rapidly with a resting state iron(III)-azide complex, which is generated by the reaction of the λ³-azidoiodane with the iron(II)(pybox) complex, to form the C(sp³)-N₃ bond. This mechanism is supported by the independent synthesis of well-defined iron complexes characterized by cyclic voltammetry, X-ray diffraction, and EPR spectroscopy, and by the reaction of the iron complexes with alkanes and the λ³-azidoiodane. Reaction monitoring and kinetic studies further reveal an unusual effect of the catalyst on the rate of formation of product and consumption of reactants and suggest a blueprint for the development of new processes leading to late-stage functionalization of C(sp³)-H bonds.

Transition Metal Catalyzed Azidation Reactions

A wide range of methodologies for the preparation of organic azides has been reported in the literature for many decades, due to their interest as building blocks for different transformations and their applications in biology as well as in materials science. More recently, with the spread of the use of transition metal-catalyzed reactions, new perspectives have also materialized in azidation processes, especially concerning the azidation of C–H bonds and direct difunctionalization of multiple carbon-carbon bonds. In this review, special emphasis will be placed on reactions involving substrates bearing a leaving group, hydroazidation reactions and azidation reactions that proceed with the formation of more than one bond. Further reactions for the preparation of allyl and vinyl azides as well as for azidations involving the opening of a ring complete the classification of the material.

Oxidative trimerization of indoles via water-assisted visible-light photoredox catalysis and the study of their anti-cancer activities

Incorporation of water has been revealed to successfully facilitate visible-light photoredox catalysis of indole leading to increased production of C2-quaternary indolinone. The water-promoted photoreaction of indole under catalyst-free conditions by a household compact fluorescence light was also demonstrated. The antiproliferative activity of the synthesized indolinones was evaluated against three human cancer cell lines.

Anti-Markovnikov Hydroazidation of Activated Olefins via Organic Photoredox Catalysis

Organic azides serve as synthetically useful surrogates for primary amines, a functional group which is ubiquitous in bioactive and medicinally relevant molecules. Historically, the formal hydroazidation of simple activated olefins and styrenes has proven difficult due to the inherent propensity of these compounds to oligomerize. Herein is disclosed a method for the anti-Markovnikov hydroazidation of activated olefins, catalyzed by an organic acridinium salt under irradiation from blue LEDs. This method is applicable to a variety of substituted and terminal styrenes and several vinyl ethers, yielding synthetically versatile hydroazidation products in moderate to excellent yield.

Direct Intermolecular Anti-Markovnikov Hydroazidation of Unactivated Olefins

We herein report a direct intermolecular anti-Markovnikov hydroazidation method for unactivated olefins, which is promoted by a catalytic amount of bench-stable benziodoxole at ambient temperature. This method facilitates previously difficult, direct addition of hydrazoic acid across a wide variety of unactivated olefins in both complex molecules and unfunctionalized commodity chemicals. It conveniently fills a synthetic chemistry gap of existing olefin hydroazidation procedures, and thereby provides a valuable tool for azido-group labeling in organic synthesis and chemical biology studies.