Alkenes anionic radical reactions

A major part of regioselechve conversions comprises addihons to unsymmetrically subshtuted alkenes, subshtuhons in al-lylic posihons or in aromahc compounds, conversions of anions, radicals, or carboca-hons with two reachve sites, and reactions at different CH bonds. 

The electro-synthetic reactions of activated alkenes involve carbon-carbon bond formation, which, after much controversy, is now believed generally to involve radical-anion/radical anion coupling rather than the alternative radical-anion/substrate reaction. The history of this mechanistic debate is well documented168. 

Most technically important polymerizations of alkenes occur by chain mechanisms and may be classed as anion, cation, or radical reactions, depending upon the character of the chain-carrying species. In each case, the key steps involve successive additions to molecules of the alkene, the differences being in the number of electrons that are supplied by the attacking agent for formation of the new carbon-carbon bond. For simplicity, these steps will be illustrated by using ethene, even though it does not polymerize very easily by any of them ... 

The formation of CT complexes between alkenes is considered to be the first step of the cycloaddition reactions, and it may also be the first step of some types of olefin polymerization23. The CT complex obtained from strong electron donors and strong electron acceptors may produce a complete charge separation with formation of an ion-radical pair (cation radical and anion radical pair), as illustrated by Scheme 2. 

In the polar reaction, a proton in HBr first adds to the terminal sp2 carbon in isobutene to produce a stable tert-butyl cation (8), and then it reacts with the counter bromide anion to form tert-butyl bromide. Thus, the proton in HBr adds to the less substituted sp2 carbon in alkene to produce a more stable carbocation. This is based on the Markovnikov rule. In radical reactions, the hydrogen atom of HBr is abstracted first by the initiator, PhCO (or Ph ) derived from (PhC02)2, and the formed bromine atom then adds to the terminal sp2 carbon in isobutene to form the stable (3-bromo tert-butyl radical (9), and then it reacts with HBr to produce /so-butyl bromide and a bromine atom. This bromine atom again... 

The representative examples of reaction with the first type of carbanions, depicted in equations 67183 and 6 8275, show that (functionalized) 2-chloro-2-nitropropane can be utilized as a ketone equivalent. Illumination, followed by elimination of the elements of HN02 (by using a second equivalent of anion as a base)274 or of N02 and C02 (by heating with sodium bromide after the irradiation)275 from the intermediate jft-nitro products, constitutes a one-pot synthesis of alkenes. The method is especially useful to prepare highly substituted alkenes, because the radical reaction step in which the C—C bond is formed is... 

It was reported by Rozhkov and Chaplina130 that under mild conditions perfluorinated r-alkyl bromides (r-RfBr) in nonpolar solvents can be added across the n bond of terminal alkenes, alkynes and butadiene. Slow addition to alkenes at 20 °C is accelerated in proton-donating solvents and is catalyzed by readily oxidizable nucleophiles. Bromination of the it bond and formation of reduction products of t-RfBr, according to Rozhkov and Chaplina, suggest a radical-chain mechanism initiated by electron transfer to the t-RfBr molecule. Based on their results they proposed a scheme invoking nucleophilic catalysis for the addition of r-RfBr across the n bond. The first step of the reaction consists of electron transfer from the nucleophilic anion of the catalyst (Bu4N+Br , Na+N02, K+SCN , Na+N3 ) to r-RfBr with formation of an anion-radical (f-RfBr) Dissociation of this anion radical produces a perfluorocarbanion and Br, and the latter adds to the n bond thereby initiating a radical-chain process (equation 91). 

The cationic site is of the immonium type, and is relatively iinreactive, but the radical site is reactive toward electron deficient alkenes such as acrylonitrile, yielding a 50 50 mixture of the diastereoisomeric pyrrolines, after being neutralized by back electron transfer from the sensitizer anion radical. The net result is an interesting example of a net 1,3-dipolar cycloaddition which, in the Huisgen method of classification of cycloadditions, is of the [3 -l- 2] type. The same general reaction had previously been carried out by Padwa, using direct photochemical excitation, a procedure which, in contrast, was highly diastereoselective (90 10, in favor of the trans isomer) [92]. 

Although alkali metal/liquid ammonia reductions (Birch reductions) of simple alkenes is difficult, presumably as a result of the very high energy of an ethene type LUMO, the corresponding reduction of non-terminal alkynes to trawi -alkenes is an efficient and useful synthetic tool for accessing trans-alkenes [116]. The mechanism for this reaction (Scheme 69), involves the homogeneous reduction of the alkyne to the corresponding anion radical by the solvated electrons present in liquid ammonia solutions of alkali metals. 

An interesting aspect of this reaction is the formation of substantial amounts of cw-2-butene, which would appear to require the intermediacy of the j-cis-1,3-butadiene anion radical, even though butadiene exists almost exclusively in the s-trans conformation (98 %). At —33°C, 13 % of the 2-butene mixture is the cis alkene, and at -78 °C 50 % of the mixture is cw-2-butene. In the case of 1,3-pentadiene, 68 % of the 2-pentene is the cis isomer. The most plausible explanation for these stereochemical results appears to be the reversible reduction of the diene to the diene anion radical at -78 °C by the pool of solvated electrons, which yields an equilibrium mixture of the s-cis and j-tran5-anion radicals (ca. 50 50), which are... 

In the previously described reactions, the basicity of anion radicals and their reactions with proton donors was emphasized. In the absence of viable proton donors or even in their presence, if the anion radical is relatively stable, radical coupling may be the dominant reaction. Thus even in aqueous solution, the anion radicals of alkenes substituted with strongly electron withdrawing moieties may undergo coupling in preference to protonation. The synthesis of adiponitrile from acrylonitrile (Scheme 71) is an outstanding example [118]. 

These cyclizations both involve the reductive intramolecular addition of an electron deficient alkene function to an aldehyde carbonyl function, and both are effected in ca 90 % yields. The mechanism of this latter type of electrochemically induced cyclizations of carbon-carbon double bonds to carbonyl double bonds have been studied rather extensively, with especial attention to the fundamental mechanistic question of whether the cyclization step involves an anion radical, radical, or anionic mechanism [122]. The latter two mechanisms would involve the protonation of the initially formed anion radical intermediate to form a radical, which could then cyclize or, alternatively, be further reduced to an anion, which could then cyclize. Extensive and elegant electrochemical and chemical studies have led to the formulation of these reactions as involving anionic cyclization (Scheme 74). 

Electron transfer between 53b and CO2 is too slow to be important because of the large difference (> 0.4 V) between the reductions potentials of 53b and CO2. The mechanism in Scheme 14 seems to be general for o , S-diactivated alkenes in their reaction with CO2 [161]. The kinetics of the reaction between the radical anions of fumarates, 52a-c, and maleates, 53a-c, with CO2 in DMF (BU4NI) has been studied by RRDE [162]. Reaction between the radical anion and CO2 prior to dimerization was confirmed for 53a by preparative and electroanalytical experiments at different substrate concentrations [165]. The radical anions 53 reacted with CO2 20-50 times faster than did the analogous species 52 , for which the pseudo-first-order rate constants were determined to be in the range 0.35-1.5 s [162]. 

Synthetic polymers can be classified as either chain-growth polymers or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is the most commonly used method, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route. Similarly, monomers such as methyl a-cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic (Michael reaction) pathway. 

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