Search results for "Reactive intermediate"

Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

2014

Perfluorarylborane Lewis acids catalyse the addition of silicon–hydrogen bonds across C=C, C=N and C=O double bonds. This ‘metal-free’ hydrosilylation has been proposed to occur via borane activation of the silane Si–H bond, rather than through classical Lewis acid/base adducts with the substrate. However, the key borane/silane adduct had not been observed experimentally. Here it is shown that the strongly Lewis acidic, antiaromatic 1,2,3-tris(pentafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene forms an observable, isolable adduct with ​triethylsilane. The equilibrium for adduct formation was studied quantitatively through variable-temperature NMR spectroscopic investigations. The interacti…

ChemInform Abstract: The Chemistry of Unsaturated Nitrogen-Heterocyclic Compounds Containing Carbonyl Groups

2010

Publisher Summary Most hydroxy-substituted azaheteroaromatic compounds exist predominately in the NH/carbonyl tautomeric form, and these compounds have been widely discussed. This chapter provides an overview of azaheteroaromatic compounds and I-azetin-4-ones—these are those azaheterocycles that contain in the ring at least one carbonyl group and one or two nitrogen atoms in the form of C=N or N=N bonds. Many of these compounds are unstable but their importance as reactive intermediates is established, and many of them could be used as synthons in the preparation of pharmacologically active compounds. The chapter describes the free existence of I-azetin-4-one and the nucleophilic substituti…

Spectroscopic and DFT Characterization of a Highly Reactive Nonheme FeV–Oxo Intermediate

2018

The reaction of [(PyNMe3)FeII(CF3SO3)2], 1, with excess peracetic acid at −40 °C generates a highly reactive intermediate, 2b(PAA), that has the fastest rate to date for oxidizing cyclohexane by a nonheme iron species. It exhibits an intense 490 nm chromophore associated with an S = 1/2 EPR signal having g-values at 2.07, 2.01, and 1.94. This species was shown to be in a fast equilibrium with a second S = 1/2 species, 2a(PAA), assigned to a low-spin acylperoxoiron(III) center. Unfortunately, contaminants accompanying the 2(PAA) samples prevented determination of the iron oxidation state by Mossbauer spectroscopy. Use of MeO-PyNMe3 (an electron-enriched version of PyNMe3) and cyclohexyl pero…

Photoiodocarboxylation of Activated C═C Double Bonds with CO2 and Lithium Iodide

2018

The photolysis at 254 nm of lithium iodide and olefins 1 carrying an electron-withdrawing Z-substituent in CO2-saturated (1 bar) anhydrous acetonitrile at room temperature produces the atom efficient and transition metal-free photoiodocarboxylation of the C═C double bond. The reaction proceeds well for terminal olefins 1 to form the new C-I and C-C σ-bonds at the α and β-positions of the Z-substituent, respectively, and is strongly inhibited by polar protic solvents or additives. The experimental results suggest that the reaction channels through the radical anion [CO2•-] in acetonitrile, yet involves different intermediates in aqueous medium. The stabilizing ion-quadrupole and electron don…

Experimental and computational studies of unconventional main group compounds : stable radicals and reactive intermediates

2017

Ever since their discovery, radicals have intrigued the minds of experimental and theoretical chemists alike. While the vast majority of radicals are transient species, a large number of stable and persistent radicals are also known. This has enabled the use of radicals in different applications. For example, radicals are highly useful in chemical synthesis due to their selectivity and functional group tolerance. Detailed knowledge of the electronic structure of synthetic intermediates, both radical and non-radical, enables chemists to improve existing synthesis routes and to design completely new ones. This thesis is divided into two parts. The first part begins with an introduction to the…

Sequestration of biological reactive intermediates by trapping as covalent enzyme-intermediate complex

2001

One important class of biological reactive intermediates arising in the course of human xenobiotic metabolism are arene and alkene oxides. The major safeguard against the potential genotoxic effects of these compounds is the microsomal epoxide hydrolase (mEH). This enzyme has a broad substrate specificity but--on the first sight--seems to be inadequately suited for this protection task due to its low turnover number with most of its substrates. The recent progress in the understanding of the mechanism of enzymatic epoxide hydrolysis has shed new light on this apparent dilemma: Epoxide hydrolases convert their substrates via the intermediate formation of a covalent enzyme-substrate complex, …

A Combined Experimental and Theoretical Approach to the Photogeneration of 5,6-Dihydropyrimidin-5-yl Radicals in Nonaqueous Media

2016

The chemical fate of radical intermediates is relevant to understand the biological effects of radiation and to explain formation of DNA lesions. A direct approach to selectively generate the putative reactive intermediates is based on the irradiation of photolabile precursors. But, to date, radical formation and reactivity have only been studied in aqueous media, which do not completely mimic the micro environment provided by the DNA structure and its complexes with proteins. Thus, it is also important to evaluate the photogeneration of nucleoside-based radicals in nonaqueous media. The attention here is focused on the independent generation of 5,6-dihydropyrimidin-5-yl radicals in organic…

2014

Carbon-centered radicals represent highly useful reactive intermediates in organic synthesis. Their nucleophilic character is reflected by fast additions to electron deficient C=X double bonds as present in iminium ions or cationic heterocycles. This review covers diverse reactions of preformed or in situ-generated cationic substrates with various types of C-radicals, including alkyl, alkoxyalkyl, trifluoromethyl, aryl, acyl, carbamoyl, and alkoxycarbonyl species. Despite its high reactivity, the strong interaction of the radical’s SOMO with the LUMO of the cation frequently results in a high regioselectivity. Intra- and intermolecular processes such as the Minisci reaction, the Porta react…