-Oxyl radicals (compounds with an N-O
fragment) represent one of the richest families of stable and persistent organic radicals with applications ranging from catalysis of selective oxidation ...processes and mechanistic studies to production of polymers, energy storage, magnetic materials design and spectroscopic studies of biological objects. Compared to other
-oxyl radicals, oxime radicals (or iminoxyl radicals) have been underestimated for a long time as useful intermediates for organic synthesis, despite the fact that their precursors, oximes, are extremely widespread and easily available organic compounds. Furthermore, oxime radicals are structurally exceptional. In these radicals, the N-O
fragment is connected to an organic moiety by a double bond, whereas all other classes of
-oxyl radicals contain an R
N-O
fragment with two single C-N bonds. Although oxime radicals have been known since 1964, their broad synthetic potential was not recognized until the last decade, when numerous selective reactions of oxidative cyclization, functionalization, and coupling mediated by iminoxyl radicals were discovered. This review is focused on the synthetic methods based on iminoxyl radicals developed in the last ten years and also contains some selected data on previous works regarding generation, structure, stability, and spectral properties of these
-oxyl radicals. The reactions of oxime radicals are classified into intermolecular (oxidation by oxime radicals, oxidative C-O coupling) and intramolecular. The majority of works are devoted to intramolecular reactions of oxime radicals. These reactions are classified into cyclizations involving C-H bond cleavage and cyclizations involving a double C=C bond cleavage.
Oximes represent one of the fundamental organic compound classes with a wide range of synthetic applications. In the last decade O‐substituted oximes were recognized as the synthetically available ...and versatile precursors of iminyl radicals via one‐electron oxidation or one‐electron reduction employing visible light photoredox catalysts, salts of abundant metals (such as Cu or Fe), or other convenient reagents. Iminyl radicals are powerful synthons for various processes of cyclization, ring‐opening, CH‐functionalization, and coupling. The present review is focused on the synthetic methods based on oxime‐derived iminyl radicals developed in the last few years excluding ring opening reactions of cyclic iminyl radicals that were summarized in recent publications. The review consists of two main parts: (1) reactions of iminyl radicals involving 1,n‐hydrogen atom transfer (n=5 in most cases) and (2) reactions involving the addition of iminyl radical to the carbon‐carbon π‐bond.
Selective oxidative C-O coupling of hydrazones with diacetyliminoxyl is demonstrated, in which diacetyliminoxyl plays a dual role. It is an oxidant (hydrogen atom acceptor) and an O-partner for the ...oxidative coupling. The reaction is completed within 15-30 min at room temperature, is compatible with a broad scope of hydrazones, provides high yields in most cases, and requires no additives, which makes it robust and practical. The proposed reaction leads to the novel structural family of azo compounds, azo oxime ethers, which were discovered to be highly potent fungicides against a broad spectrum of phytopathogenic fungi (
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In the present work, the study of the unusual interaction between copper hexafluoroacetylacetonate and the diacetyliminoxyl radical resulted in two discoveries from different fields: the ...determination of the oxime radical spatial structure and the introduction of an oxime radical into the field of molecular magnetic material design. Oxime radicals are key plausible intermediates in the processes of oxidative CH-functionalization and in the synthesis of functionalized isoxazolines from oximes. Due to the lack of X-ray diffraction data for oxime radicals, the knowledge about their structure is based mainly on indirect approaches, spectroscopic methods (electron paramagnetic resonance and IR), and quantum chemical calculations. The structure of the oxime radical was determined for the first time by stabilizing the diacetyliminoxyl radical in the form of its complex with copper (II) hexafluoroacetylacetonate (Cu(hfac)2), followed by single-crystal X-ray diffraction analysis. Although oxime radicals are known to undergo oxidative coupling with acetylacetonate ligands in transition-metal complexes, a complex is formed with intact hfac ligands. X-ray diffraction studies have shown that the oxime radical is coordinated with copper ions through the oxygen atoms of the carbonyl groups without the direct involvement of the CN–O• radical moiety. The structure of the coordinated diacetyliminoxyl is in good agreement with the density functional theory (DFT) prediction for free diacetyliminoxyl due to the very weak interaction of the radical molecule with copper ions. Remarkably, both weak ferromagnetic and antiferromagnetic interactions between Cu (II) and oxime radicals have been revealed by modeling the temperature dependence of magnetic susceptibility and confirmed by DFT calculations, rendering diacetyliminoxyl a promising building block for the design of molecular magnets.
Capillary electrophoresis with Laser-Induced Fluorescence (CE-LIF) detection is being applied to new analytical problems which challenge both the power of CE separation and the sensitivity of LIF ...detection. On-capillary LIF detection is much more practical than post-capillary detection in a sheath-flow cell. Therefore, commercial CE instruments utilize solely on-capillary CE-LIF detection with a Limit of Detection (LOD) in the nM range, while there are multiple applications of CE-LIF that require pM or lower LODs. This tutorial analyzes all aspects of on-capillary LIF detection in CE in an attempt to identify means for improving LOD of CE-LIF with on-capillary detection. We consider principles of signal enhancement and noise reduction, as well as relevant areas of fluorophore photochemistry and fluorescent microscopy.
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•Comprehensive tutorial of sensitive fluorescence detection with capillary electrophoresis.•Critical theoretical and experimental milestones during optimization of detection efficiency.•Graphical artworks and helpful mathematical formulas to encourage implementation of the tutorial's aim.•Promising approaches to improvement of sensitivity in capillary electrophoresis techniques with fluorescence detection.
The structure of the amorphous fraction and the tensile‐compressive stresses in amorphous‐crystalline radiation‐damaged zircon ZrSiO:U,Th depending on radiation dose and temperature (8–350 K) are ...investigated according to Raman spectroscopy of Boson peak for the first time. The Boson peak at 60–70 cm−1 associated with localized phonon states in the amorphous fraction (
fa) is recorded at low temperatures (
T<100 K) for samples with
fa<30% and over the entire temperature range 8–350 K for
fa>70%. The wider localized states distribution in the latter case is considered as a sign of the amorphous phase structure evolution with an increase in radiation dose. The estimates of an atomic correlation radius based on the Ioffe–Regel criterion are similar to those in glasses,
Rc2.0−2.3 nm. The monotonic increase in
Rc value during heating of zircon with
fa>70% is governed by thermal expansion of the percolating amorphous fraction. The nonmonotonic variations of the
Rc value in zircon with
fa<30% is determined by the stresses in the amorphous fraction due to the mismatch in thermal expansion coefficient (CTE) and elastic moduli of the amorphous and crystalline phases depending on temperature; a change in the sign of the crystalline fraction CTE at 30 K is assumed. The Boson peak disappearance at 100 K in zircon with
fa<30% during heating conforms to with the violation of the phonon localization as a consequence of amorphous fraction contraction and partial ordering. The data obtained are important for predicting the thermal and mechanical properties of heterogeneous radiation‐damaged materials and nanocomposites.
The structure of the amorphous fraction and the tensile‐compressive stresses in amorphous‐crystalline radiation‐damaged zircon ZrSiO:U,Th depending on radiation dose and temperature are investigated according to Raman spectroscopy of Boson peak for the first time. The Boson peak at 60–70 cm−1 associated with localized phonon states in the amorphous fraction is recorded at low temperatures. The wider localized states distribution in the latter case is considered as a sign of the amorphous phase structure evolution with an increase in radiation dose.
In the last decade, free radicals have found a wide application in functionalization of unsaturated compounds, such as alkenes, alkynes, and arenes, via the free‐radical addition to carbon‐carbon ...π‐bonds. In these processes, intermolecular free‐radical attack on the aromatic substrates represents a challenge due to relatively high resistance of aromatic π‐system to addition reactions in comparison to alkene C=C bonds. The free‐radical functionalization of heterocycles is especially interesting due to the diversity of their structures and chemical properties as well as their importance for medicinal chemistry, agrochemistry, and materials science. Addition of C‐centered radicals to heterocycles is widely known as the Minisci‐type reactions and well‐reviewed. In this paper, we have summarized the main achievements in less explored group of processes: functionalization of heterocycles by intermolecular addition of heteroatom‐centered radicals (O‐, N‐, S‐/Se‐, and P‐radicals) with the emphasis on the papers published after 2010. Literature analysis revealed the strong trend towards the usage of electrochemistry and photoredox‐catalysis for the generation of free radicals in recent years. The remaining fundamental problem in this field is the lack of strong experimental support for the proposed mechanisms and frequent existence of several plausible reaction pathways. The progress in mechanistic studies can significantly improve the prediction of optimal reaction conditions depending on the substrates structure.
Revolver optical fibers (RF) are special type of hollow-core optical fibers with negative curvature of the core-cladding boundary and with cladding that is formed by a one ring layer of capillaries. ...The physical mechanisms contributing to the waveguiding parameters of RFs are discussed. The optical properties and possible applications of RFs are reviewed. Special attention is paid to the mid-IR hydrogen Raman lasers that are based on RFs and generating in the wavelength region from 2.9 to 4.4 μm.
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