Allyl halides radical anions

The first reported radical reaction promoted by tellurium reagent was probably the conversion of allylic halides into the coupled 1,5-dienes by treatment with telluride anions. The reaction, which gives the best results when employing the reagent prepared in situ from elemental tellurium and lithium triethylborohydride, proceeds through the intermediacy of the thermally unstable bis-allylic telluride followed by extrusion of tellurium and coupling of the formed allylic radicals. 

Phenyl o-radicals generated by reduction of aryl halides can also interact with an intramolecular alkene bond. Ihe method has been developed for the formation of dihydroindoles by reductive cyclization of N-allyl-2-chloroacetanilides. The results indicate the importance of a time interval between electron addition to give a radical-anion and the fragmentation of this species to give the active a-radical, The time interval allows the radical-anion to diffuse away from the electrode surface so that when the a-radical is foimed, it has time to cyclize before it can be reduced at the surface. 

Substituted allyl halides also give substitution products by a radical chain process. Thus 3-bromo-l-nitrocyclohex-l-ene (19) undergoes S l reaction with the 2-nitropropane anion giving product 21 derived from valence tautomerism 20a = 20b of the radical intermediate (scheme 2)100. 

The carbon atom next to a carbon-carbon double bond. The term is used in naming compounds, such as an allylic halide, or in referring to reactive intermediates, such as an allylic cation, an allylic radical, or an allylic anion, (p. 673)... 

The electron transfer reactivity of Ceo has been compared with that of p-benzoquinone which has a slightly more negative one-electron reduction potential ( °red relative to the SCE = -0.50 V) [44] than Ceo (E°red —0.43 V). The rate constants of electron transfer from Cgo and Ceo to electron acceptors such as allyl halides and manganese(III) dodecaphenylporphyrin [45] correlate well with those from semiquinone radical anions and their derivatives. Linear correlations are obtained between logarithms of rate constants and the oxidation potentials of... 

According to Bard and Merz [108], in MeCN containing TBAP, allyl bromide and allyl iodide interact chemically with a mercury electrode to form allylmercury halides. These allylmercury halides undergo reduction to yield diallylmercury, which is itself electroactive. Allyl bromide and allyl iodide are reduced at platinum in MeCN in a two-electron process to give the allyl anion, and the allyl radical is not involved as an intermediate. Reduction of allyl halides at platinum in DMF containing TEAOTs and in the presence of trimethylchlorosilane results in silylated compounds [100]. 

Based on these results and results for reduction of other combinations of allyl halides and activated alkenes, it has been suggested that when the allyl halide is more easily reduced than the alkene, the allylic anion (2-F reduction) adds to the activated double bond of the alkene, giving predominantly the terminal alkene [Eq. (32)]. In contrast, initial formation of the radical anion of the (di)activated alkene may lead to an S>j2 reaction between the radical anion and the allyl halide followed by further reduction of the intermediate radical and final protonation [Eq. (33)] [190,191]. However, electron transfer between the alkene radical anion and especially allyl iodide followed by coupling of the allyl radical and a radical anion cannot be ruled out. 

This section covers the formation of cyclopropanes via cyclization of reactive allylic intermediates (cations, anions, radicals). Included are those transformations of allylic functional derivatives (e.g. allylic halides, alcohols, aldehydes, ketones, acids, esters, boronates, Grignard reagents) to cyclopropyl derivatives that do not actually proceed via allylic reactive intermediates, but which are not covered by other sections of this volume. Additionally, this section will cover methods for the formation of cyclopropanes by pericyclic reactions. 

The greater Sn2 reactivity of allylic halides results from a combination of two effects steric and electronic. Sterically, a CH2CI group is less crowded and more reactive when it is attached to the 5p -hybridized carbon of an allylic halide compared with the sp -hybridized carbon of an alkyl halide. Electronically, the tt-electron MO approximation doesn t apply because the reactant is allyl chloride, not an allyl cation, radical, or anion. Higher level MO treatments such as seen earlier for the Sn2 mechanism in Section 8.3 are readily adapted to allyl chloride, however. According to that picture, electrons flow from the nucleophile to the LUMO of the alkyl halide. 

Facing these difficulties concerning the syrJanti-selectivity in carbonyl allylations, Marton et al., (1996a) extensively studied the zinc-mediated allylation of aldehydes with allyl halides in cosolvent/HaO (NH4CI) and in cosol-vent/H20 (NH4Cl)/haloorganotin media. The stereochemistry seems to be determined by the structure of the radical anions, which were presumed to be formed through an electron-transfer process. 

The formation of organometallic intermediates, however, can be initiated by an allylic radical anion on the metal surface (Figure 4.3). Noteworthy is the high value of the electron affinity of allylic bromide and, to a lesser extent, chloride (Moyano et al, 1990). As a matter of fact, in tin-mediated reactions, Wurtz coupling products were sometimes detected (Kim et al, 1993), but, unlike the zinc-mediated allylation, no dimerization of the carbonyl compounds was observed in the absence of the halide (Einhom and Luche, 1987). 

When steric hindrance in substrates is increased, and when the leaving anion group in substrates is iodide, SET reaction is much induced (Cl < Br < I). This reason comes from the fact that steric hindrance retards the direct nucleophilic reduction of substrates by a hydride species, and the a energy level of C-I bond in substrates is lower than that of C-Br or C-Cl bond. Therefore, metal hydride reduction of alkyl chlorides, bromides, and tosylates generally proceeds mainly via a polar pathway, i.e. SN2. Since LUMO energy level in aromatic halides is lower than that of aliphatic halides, SET reaction in aromatic halides is induced not only in aromatic iodides but also in aromatic bromides. Eq. 9.2 shows reductive cyclization of o-bromophenyl allyl ether (4) via an sp2 carbon-centered radical with LiAlH4. 

The anionic complexes [ j -CpM(CO)3] (M = Cr, Mo, W) react with alkyl halides to provide the air-sensitive complexes j -CpM(CO)3R . With respect to reaction with Mel, the relative nucleophilicities of these anions are in the order W > Mo > Cr . Anionic complexes in which the Cp ligand bears alkyl substituents as well as [TpMo(CO)3]" [Tp = hydridotris(pyrazolyl)borate] , behave analogously. The course of the reaction of [f7 -CpCr(CO)3] with allyl chlorides has been studied In diglyme, one cleanly obtains [>) -CpCr(CO)3]2 and coupling of allyl radicals . In THE, the reaction is somewhat more complex one obtains as the major product the same dimer, though a small amount of (jj -6-alkenylfulvene)Cr(CO)3 also forms . Presumably, the products are derived via initial formation of j -CpCr(CO)3( Callyl). 

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