Metalation radicals sources

Indolines and indoles were prepared by a direct electrochemical reduction of arenediazonium salts. As a result, radical intermediates were generated from which 3,4-disubstituted tetrahydrofuran skeleta were constructed <02OL2735>. A short and stereoselective total synthesis of furano lignans was realized by radical cyclization of epoxides using a transition-metal radical source <02JOC3242>. Other preparations of tetrahydrofurans using radical cyclization include the synthesis of novel amino acids L-bis-... 

Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. r-Butyl hydroperoxide is often used as a radical source. Detailed studies on the mechanism of the decomposition indicate that it is a more complicated process than simple unimolecular decomposition. The alkyl hydroperoxides are also sometimes used in conjunction with a transition-metal salt. Under these conditions, an alkoxy radical is produced, but the hydroxyl portion appears as hydroxide ion as the result of one-electron reduction by the metal ion. ... 

Pyridine has been phenylated with the following free-radical sources benzenediazonium chloride with aluminum trichloride the Gomberg reaction " phenylhydrazine and metal oxides A -nitroso-acetanilide dibenzoyl peroxide phenylazotriphenylmethane di-phenyliodonium hydroxide and electrolysis of benzoic acid. ° Although 2-phenylpyridine usually accounts for over 50% of the total phenylated product, each of the three phenyl derivatives can be obtained from the reaction by fractional recrystallization of the... 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

With radical sources other than acyl peroxides, the rearomatization of the a-complex can take place by various, not always well characterized, reactions, such as oxidation by metal salts, hydrogen abstraction by intermediate radicals, disproportionation, and induced decomposition. 

P. A. Baguley, J. C. Walton, Flight from the Tyranny of Tin The Quest for Practical Radical Sources Free from Metal Encumbrances, Angew. Chem. Int. Ed. Engl. 1998, 37, 3072-3082. 

Laser flash photolysis of [CpM(CO>3]2 (M = W, Mb, and Cr) provides a convenient source of CpM(CO)3, an organometallic free radical with 17 valence electrons. It is a transient and highly reactive species. Depending on the circumstances and the other reagents present, the radical will dimerize, undergo halogen and hydrogen atom abstraction reactions, and electron transfer reactions. With tetramethyl-phenylenediamine, there is a cyclic process of electron transfer steps, the net result of which is the catalyzed disproportionation of the metal radical. 

Fig. 18. Diagram of reactive scattering apparatus for the study of non-metal reactions A, scattering chamber B, source chambers C, liquid nitrogen cooled cold shield D, detector E, source bulkheads G, liquid nitrogen trap H, oil diffusion pumps N, free radical source P, nozzle source Q, skimmer E, ion source H, liquid He trap I, ion lenses P, photomultiplier Q, quadrupole rods R, light baffle S, slide valve T, radial electric field pumps (from C. F. Carter et al. 02 by permission of the Chemical...

Both of these paths can be studied by pulse radiolysis when selective reductants can be used to reduce the metal complex or the radical source. For example, one can use hydrated electrons to reduce RX (present in excess over M P) and C02 radicals to reduce M "P. On the other hand, if COj is the sole reducing radical in the system, since this does not react rapidly with RX, one can prepare M P and follow its subsequent reaction with RX. Of course, R- can be prepared also by reactions of various precursors other than RX. 

Reductions that follow the first path are generally effected by a metal, the source of the electrons, and a proton donor, which may be water, an alcohol or an acid. They can result in the addition of hydrogen atoms to a multiple bond or in fission of a single bond between, usually, carbon and a heteroatom. In these reactions an electron is transferred from the metal surface (or from the metal in solution) to the organic molecule giving, in the case of addition to a multiple bond, a radical anion, which in many cases is immediately protonated. The resulting radical subsequently takes up another electron from the metal to form an anion until work-up. In the absence of a proton source, dimerization or polymerization of the radical anion... 

An alternative approach to initiating an ATRP is to use azobisisobutjTonitrile (AIBN), a radical source used commonly in conventional radical polymerizations. By generating free radicals in the presence of a transition metal hahde/hgand complex in its higher oxidation state, the required dynamic equilihrium can be gaierated Initiation... 

Hydrostannation of alkynes proceeds smoothly to give alkenylstaimanes, which are used for further transformafions. The reaction is induced by transition metal catalysts and free-radical sources, giving different regioisomers 86 and 87 [25]. 

An alternate adhesive application area for the polyester acrylates described in this report was in the general field of two-part structural acrylics. Such systems generally cured at room temperature using transition metal or amine-activator and a hydroperoxide or peroxide radical source. A brief study was carried out to determine if the synthesized compounds had potential use as structural adhesives. As before, the nut/bolt combination was used as an indication of adhesion. The results of these limited tests are given in Table 3. 

The two types of inhibitors used in cyanoacrylate adhesives are radical polymerization inhibitors and anionic polymerization inhibitors. The former type are usually phenolic, for example, hydroquinone, p-methyloxyphenol, or hindered phenolics. They are generally employed in 0.1-1.0% concentrations. The amine- or sulfur-based inhibitors and the metal-chelating inhibitors should be avoided owing to their nucleophilic character. Radical inhibitors are used to protect the adhesive during storage from polymerization induced by heat or light and other adventitious radical sources. These... 

However, VDF-IDT-CRPs always require a free radical source (e.g., butyl peroxide (TBPO)), as direct metal catalyzed initiation from perfluoroiodides or any other halides is not available. This is a serious drawback with respect to the precise synthesis of block or graft copolymers based on FMs, as such systems would inevitably lead to mixtures of homo- and copolymers with the current technology. Therefore, the availability of an initiation method directly from halides, most likely mediated by transition metal catalysis, would be highly valuable. 

This reaction leads to the formation of free radicals. Most free radicals are reactive species that rapidly undergo further reaction. Free radicals are frequently needed to initiate reactions. Compounds with weak covalent bonds, such as the bond between less active metals and carbon, or the peroxide, —O—O—bond, are frequently used as free radical sources, as shown ... 

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