Nucleophilic additions anionic radical reactions

During the past 15 years since Qo became available in macroscopic quantities in 1990 [1], a wide variety of its derivatives have been synthesized as part of the explosive development of the study of its chemistry [2). Various organic reactions have been reported, most of which are cycloadditions, nucleophilic additions, and radical additions. Fullerenes, as represented by Qo, are now commonly accepted to behave as electron-deficient olefins, hence there have been numerous studies on their anions. This has led to a situation where the other equally important species, the fullerene cations, have been left unexplored for nearly a decade in spite of their significance in both fundamental and application studies. Clearly, a systematic study of this class of species is needed. 

Some of the reactions in this chapter operate by still other mechanisms, among them an addition-elimination mechanism (see 13-15). A new mechanism has been reported in aromatic chemistry, a reductively activated polar nucleophilic aromatic substitution. The reaction of phenoxide with p-dinitrobenzene in DMF shows radical features that cannot be attributed to a radical anion, and it is not Srn2. The new designation was proposed to account for these results. 

As shown in Section 2.2.7, chemical reactions may be triggered by electrons or holes from an electrode as illustrated by SrnI substitutions (Section 2.5.6). Instead of involving the electrode directly, the reaction may be induced indirectly by means of redox catalysis, as illustrated in Scheme 2.15 for an SrnI reaction. An example is given in Figure 2.30, in which cyclic voltammetry allows one to follow the succession of events involved in this redox catalysis of an electrocatalytic process. In the absence of substrate (RX) and of nucleophile (Nu-), the redox catalysis, P, gives rise to a reversible response. A typical catalytic transformation of this wave is observed upon addition of RX, as discussed in Sections 2.2.6 and 2.3.1. The direct reduction wave of RX appears at more negative potentials, followed by the reversible wave of RH, which is the reduction product of RX (see Scheme 2.21). Upon addition of the nucleophile, the radical R is transformed into the anion radical of the substituted product, RNu -. RNu -... 

One way of stabihzing the initial radical or anion radical is therefore the addition of an acid. Expulsion of a base should produce a similar effect. This is indeed the case (Scheme 2.21), and the secondary radical thus formed is similarly easier to reduce than the starting molecule in most cases. RX is a molecule containing a low-lying orbital able to accommodate the incoming electron, thus leading to the primary radical, RX -, before the nucleophile X- is expelled. We consider here the case of a stepwise process in which the reaction pathway involves the intermediacy of the primary radical rather than a... 

Electron transfer sensitization allows either the radical cation or the radical anion of an aromatic alkene to form as desired, which finally results in nucleophile addition with Markovnikov and anti-Markovnikov regiochemistry. In an apolar solvent, the tight radical ion pair undergoes a stereoselective reaction when the electron-accepting sensitizer is chiral (Figure 3.10). ... 

The addition of the nucleophile to the aryl radical is the reverse of the cleavage of substituted aromatic anion radicals that we have discussed in Section 2 in terms of an intramolecular concerted electron-transfer-bondbreaking process and illustrated with the example of aryl halides. The present reaction may thus be viewed conversely as an intramolecular concerted electron-transfer-bond-forming process. The driving force of the reaction can be divided into three terms as in (131). The first of these, the... 

In the course of the reaction, the nitrite ion leaves the primary anion-radical. This produces the cyclohexyl radical in the pyramidal configuration. The vicinal methyl group sterically hinders the conversion of the pyramidal radical into the planar one. With a high concentration of the nucleophile, the rate of addition exceeds the rate of conversion, that is, Then the entering PhS group... 

In addition to nucleophilic capture of alkene or cyclopropane radical cations (see above) radicals may be generated by cleavage of C—X bonds, particularly C—Si bonds. Such cleavage is often assisted by a nucleophile. Because the radical is generated near the radical anion, to which it couples, the resulting C—C bond formation may be considered a reaction of a modified radical (ion) pair. 

Besides this method (1) of adding nucleophiles, anions 133 can also be prepared by (2) reduction of X -phosphorins to radical anions 134 followed by reaction with radicals, and (3) addition of radicals to the P atom, forming phosphorin radicals 135 followed by reduction. 

If the oxidation is carried out with a radical R in the presence of a nucleophile R , an equilibrium reaction forming the neutral radical 135 may take place. 135 could lead to the X -phosphorin 129 via oxidation to 127 and addition of the anion R , or by simple coupling with the radical R. ... 

For example, the distonic anion radical of cyclopentadienylidene trimethylen-emethane reacts under mass spectrometer gaseous-phase conditions with carbon disulfide by sulfur abstraction and with nitric oxide by NO-radical addition. The first reaction characterizes the distonic anion radical mentioned as a nucleophile bearing a negative charged moiety. The second reaction describes the same anion radical as a species having a group with radical unsaturation (Zhao et al. 1996). 

Nucleophilic substitutions in anion radicals and electrophilic substitutions in cation radicals have been considered throughout the book, including the problem of choosing between addition and electron-transfer reactions. Therefore, only some unusual cases will be discussed here. 

Both, strained and unsaturated organic molecules are known to form cation radicals as a result of electron transfer to photoexdted sensitizers (excited-state oxidants). The resulting cation radical-anion radical pairs can undergo a variety of reactions, including back electron transfer, nucleophilic attack on to the cation radical, electrophilic attack on the anion radical, reduction of anion radical, and addition of anion radical to the cation radical. This concept has been nicely demonstrated by Gassman et al. [103, 104], using the photoinduced electron-transfer cydization of y,8-unsatu-rated carboxylic add 232 to y-ladones 233 and 234 as an example (see Scheme 8.65). 

The addition of a nucleophile to a radical to form the radical anion of the substitution product constitutes the main feature of a SRN1 process, although the chain can be short or even nonexistent (reaction 2). For a photoinduced reaction, a quantum yield higher than 1 can be taken as evidence of a chain reaction, although a global quantum yield below 1 cannot be used as a criterion against a chain reaction [9,10]. 

Since 05 is a conjugate base of weak acid, HO2 (p ATa=4.9), and also an anion radical at physiological pH, it can perform both ionic and radical reactions. As an anion 05 has reactivies such as nucleophilic substitution and addition reactions, and as a radical, it performs hydrogen-withdrawal reactions, one-electron reduction or oxidation reactions and the disproportionation reaction. In aqueous medium, the radical reactions are... 

Products with mass equal to the sum of the reagent masses also form, to different extents, in the reactions of 02 with ketones, namely acetone, CF3COCH3 and (CF3)2C0264. These adducts were tentatively assigned the structure of the bound tetrahedral adduct of nucleophilic carbonyl addition. While this reaction is the only one observed with acetone, it competes with H+ abstraction in the case of CF3COCH3 to form the stabilized enolate [CH2=C (CT)CF3] and with ET in the case of (CF3)2CO (electron affinity is ca 33.7 kcal moF1). In this latter case it was concluded that reaction of (CF3)2CO with Of occurs exclusively via ET and that the adduct forms in a secondary process via reaction of the primary product, the radical anion of (CF3)2CO with 302 present in the flow from the preparation of 02 (see Scheme 39). 

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). 

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