Reductive radical addition

Radical addition to alkenes is strongly influenced by polar effects, and reductive radical additions are nearly always anti-Markovnikov. Resonance stabilisation of the unpaired electron in R can decrease the rate of addition, while the stabilisation of the adduct radical... 

Yang and Burton studied reductive radical additions of iododifluoroacetate 37 to olefins 38 and dienes catalyzed by 6-17 mol% of a catalyst generated from NiCl2 and stoichiometric amounts of zinc in the presence of water (Fig. 8) [90, 91]. Olefins gave the reductive addition products 40a in 60-83% yield, while 1,5-hexadiene or 1,8-nonadiene provided double addition products exclusively in 55% and 73% yield. 1,7-Hexadiene gave an inseparable mixture of the expected acyclic double addition product and a tandem addition/cyclization product, in which the former dominated. The radical nature of the addition is supported by inhibition of the reaction by para-dinitrobenzene. The reaction proceeds probably via initially formed atom transfer product 39, which is subsequently reduced by nickel(0) and zinc. This is supported by deuterium incorporation, when D20 was used instead of water. No deuterium incorporation was observed with THF-dg, thus ruling out hydrogen transfer from the solvent. 

Huang and Chen applied a catalyst generated in situ from 10 mol% of NiCl2, 40 mol% of triphenylphosphine, and stoichiometric amounts of zinc to promote similar reductive radical additions of perfluoroalkyl chlorides 41a to olefins 38 in DMF as the solvent (Fig. 8) [92], The reaction works for electron-rich and electron-... 

Fig. 23 Nickel-catalyzed electrochemically mediated reductive radical addition reactions...

Copper powder was frequently applied in (super)stoichiometric amounts to trigger reductive radical addition reactions. Yang and Burton described the addition of difluoroiodoacetates 37 to olefins 210 in the absence of a solvent in the presence of 10-20 mol% copper powder (Fig. 53) [276, 277]. The method tolerates sensitive... 

Entry Starting Material Reduction Radical addition 1,5-WAtom abstraction... 

Nonaqueous Bases Nonaqueous Nucleophiles Organometallic Catalytic Reduction Acidic Reduction Basic or Neutral Reduction Hydride Reduction Lewis Acids Soft Acids Radical Addition Oxidizing Agents... 

This synthesis is only one example of a wide range of reactions which involve aryl (or alkyl) radical addition to electron-deficient double bonds resulting in reduction.The corresponding oxidative reaction using aryl radicals is the well known Meerwein reaction, which uses copper(II) salts. 

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

In this chapter, we discuss free-radical substitution reactions. Free-radical additions to unsaturated compounds and rearrangements are discussed in Chapters 15 and 18, respectively. In addition, many of the oxidation-reduction reactions considered in Chapter 19 involve free-radical mechanisms. Several important types of free-radical reactions do not usually lead to reasonable yields of pure products and are not generally treated in this book. Among these are polymerizations and high-temperature pyrolyses. 

When double bonds are reduced by lithium in ammonia or amines, the mechanism is similar to that of the Birch reduction (15-14). ° The reduction with trifluoro-acetic acid and EtsSiH has an ionic mechanism, with H coming in from the acid and H from the silane. In accord with this mechanism, the reaction can be applied only to those alkenes that when protonated can form a tertiary carbocation or one stabilized in some other way (e.g., by a OR substitution). It has been shown, by the detection of CIDNP, that reduction of a-methylstyrene by hydridopenta-carbonylmanganese(I) HMn(CO)5 involves free-radical addition. ... 

Reduction — The addition of one electron to the carotenoid molecule would give the radical anion CAR -H e- — CAR". 

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

Epoxides can also be reductively opened to form a radical. An example of an intramolecular cyclization of such a radical has recently been reported <06TL7755>. Treatment of 40 with Cp2TiCl generates an intermediate alkoxy radical, which then adds to the carbonyl of the formate ester. The product, 41, is formed as a 2 1 mixture of isomers at the anomeric carbon. This reaction is one of the first examples of a radical addition to an ester. The major byproduct of this reaction is the exo-methylene compound, 42, arising from a P-hydrogen elimination. 

Iron(II) salts, usually in conjunction with catalytic amounts of copper(II) compounds, have also been used to mediate radical additions to dienes91,92. Radicals are initially generated in these cases by reductive cleavage of peroxyesters of hydroperoxides to yield, after rearrangement, alkyl radicals. Addition to dienes is then followed by oxidation of the allyl radical and trapping by solvent. Hydroperoxide 67, for example, is reduced by ferrous sulfate to acyclic radical 68, which adds to butadiene to form adduct radical 69. Oxidation of 69 by copper(H) and reaction of the resulting allyl cation 70 with methanol yield product 71 in 61% yield (equation 29). 

The consequences of post-irradiation effects in polymer materials are progressive reduction in strength, cracking and embrittlement. Some reduction in these effects can be achieved by annealing of the trapped radicals, addition of appropriate... 

The present volume contains 13 chapters written by experts from 11 countries, and treats topics that were not covered, or that are complementary to topics covered in Volume 1. They include chapters on mass spectra and NMR, two chapters on photochemistry complementing an earlier chapter on synthetic application of the photochemistry of dienes and polyenes. Two chapters deal with intermolecular cyclization and with cycloadditions, and complement a chapter in Volume 1 on intramolecular cyclization, while the chapter on reactions of dienes in water and hydrogen-bonding environments deals partially with cycloaddition in unusual media and complements the earlier chapter on reactions under pressure. The chapters on nucleophiliic and electrophilic additions complements the earlier chapter on radical addition. The chapter on reduction complements the earlier ones on oxidation. Chapters on organometallic complexes, synthetic applications and rearrangement of dienes and polyenes are additional topics discussed. 

Radical addition of dibromodifluoromethane to alkenes followed by sodium borohydride reduction is a convenient two-step method for the introduction of the difluoromethyl group.5 Either one or both carbon-bromine bonds in the intermediate dibromides may be reduced, depending on the reaction conditions. In the case of acyclic dibromodifluoromethane-alkene adducts, the reduction occurs regioselectively to yield the relatively inaccessible bromodifluoromethyl-substituted alkanes. The latter are potential building blocks for other fluorinated compounds. For example, they may be dehydrohalogenated to 1,1-difluoroalkenes an example of this methodology is illustrated in this synthesis of (3,3-difluoroallyl)trimethylsilane. 

Accordingly, many reactions can be performed on the sidewalls of the CNTs, such as halogenation, hydrogenation, radical, electrophilic and nucleophilic additions, and so on [25, 37, 39, 42-44]. Exhaustively explored examples are the nitrene cycloaddition, the 1,3-dipolar cycloaddition reaction (with azomethinylides), radical additions using diazonium salts or radical addition of aromatic/phenyl primary amines. The aryl diazonium reduction can be performed by electrochemical means by forming a phenyl radical (by the extrusion of N2) that couples to a double bond [44]. Similarly, electrochemical oxidation of aromatic or aliphatic primary amines yields an amine radical that can be added to the double bond on the carbon surface. The direct covalent attachment of functional moieties to the sidewalls strongly enhances the solubility of the nanotubes in solvents and can also be tailored for different... 

Stabilization of the (incipient) C 6) carbocation developing in the C-O heterolysis. The rate constants for the heterolysis reaction are a measure of the reducing power of 5,6-dihydro-6-methylpyrimidine-6-yl radicals. On this basis, the cytosine radicals are better reductants than the corresponding uracil radicals, and the radicals derived by hydrogen atom addition to pyrimidines are stronger reductants than those formed by OH radical addition [27]. 

Renaud and co-workers used 78 for the synthesis of (-)-phaseolinic acid (6) and (-)-pertusarinic acid (8) (Scheme 12) [32, 33]. Radical addition of dimethyl phenylselenomalonate to 78 proceeded with rearrangement of the bicyclics to yield the seleno-acetal 79 [34]. After reductive deselenylation and Baeyer-Villiger oxidation treatment of 80 with BU4NI and BBr3 led to a simultaneous cleavage of the ether, the lactone, and the methyl ester func-... 

Cobalamin catalysed reduction of alkyl halides has found use in organic synthesis because, like square planar Ni(o), it allows formation of alkyl radicals in the bulk of the solution away from the electi ode surface. Alkyl radical addition to activated alkenes is achieved in high yields. In the cases of primary alkyl halides,... 

Radical additions to alkenes and aromatic systems are well known reactions. The trapping in this manner of radicals obtained by reduction of the aliphatic carbonyl function has proved to be a versatile electrochemical route for the formation of carbon-carbon bonds. Such reactions are most frequently carried out in protic solvents so that the reactive species is a o-radical formed by protonation of the carbonyl radical-anion. Tlie cyclization step must be fast in order to compete with further reduction of the radical to a carbanion at the electrode surface followed by protonation. Cyclization can be favoured and further reduction disfavoured by a... 

Radical addition to the pyridine ring is another known facile process and coupling of radical intermediates from the reduction of ketones to pyridines in... 

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