Halides reaction with radical ions

The reactions of HO and 02 - with alkyl halides exhibit the same general pattern (Scheme 16), with second-order kinetics and inversion of configuration. Radicals are not detected in the reactions with hydroxide ion, which indicates that there probably is not a discrete SET step, but rather that the transfer of the entering and leaving groups is synchronous with a single-electron shift. 

The formation of Grignard reagents takes place at the metal surface. Reaction commences with an electron transfer to the halide and decomposition of the radical ion, followed by rapid combination of the organic group with a magnesium ion.1 It... 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

Although the reaction with thiosulfate and with iodide ions may be a mere reduction of the halide, the reaction with sodium benzoate would appear to be a radical dissociation induced by the attack of a negative ion. The fate of the benzoate ion is unknown. Tris-( -nitrophenyl)-methyl benzoate is a stable substance which does not dissociate into radicals.23... 

In a reducing environment, conditions may allow for the same type of mechanism to occur, but with the radical anion of the spin trap as the intermediate. Actually, the possibility of radical ion-mediated spin trapping was first discussed in a study of a reductive system, namely in the search for radical intermediates in the reaction between alkanethiolates and alkyl halides conducted in the presence of TBN [2] (Crozet et al., 1975). TBN is known to trap primary radicals with formation of nitroxides (attack of R at N), and it was therefore anomalous to find alkoxyaminyl radicals (attack of R at O) in the above reaction. It was suggested that the alkanethiolate or some other reductant reduces TBN to its radical anion, which attacks the alkyl halide via oxygen in an SN2 fashion, as in equations (8) and (9) (see p. 129). 

The relative reactivities of the enolate ions of acetophenone and 2-acetylnaphthalene towards phenyl radicals have been explored in order to determine their suitability as electron donor initiatiors of 5 rnI reactions of enolate ions of 2-acetylthiophene and 2-acetyl fiiran with aryl halides Phl. ... 

The electrochemical oxidation of 2,5-dimethylthiophene in various electrolytes has been investigated (71JOC3673). In non-halide electrolytes such as ammonium nitrate or sodium acetate, the primary anodic process is the oxidation of the thiophene to the cation-radical (159). Loss of a proton, followed by another oxidation and reaction with solvent methanol, leads to the product (160) (Scheme 31). When the electrolyte is methanolic NaCN, however, nuclear cyanation is observed in addition to side-chain methoxylation. Attack by cyanide ion on the cation-radical (159) can take place at either the 2- or the 3-position, leading to the products (161)-(163) (Scheme 32). 

Triarylphosphines were prepared by the reaction between lithium diphenylphosphide in THF and m-and p-iodotoluene (or the corresponding bromo compounds), 4-bromobiphenyl and p-dibromobenzene in yields of 70-80% (isolated after oxidation, as the phosphine oxides).143 The absence of cine substitution products is a synthetic advantage and would have been taken as a prima facie indication that the displacements are examples of the 5rn1 reaction, had the mechanism been recognized at the time. Operation of the radical ion mechanism in DMSO, or liquid ammonia, in which marginally improved yields are obtained, was confirmed by Swartz and Bunnett,48 but no extension to the scope of the reaction was made. Rossi and coworkers have developed a procedure for one-pot preparation of triarylphosphines starting from elemental phosphorus (Scheme 6).146 As an example of the synthesis of a symmetrical tri-arylphosphine, triphenylphosphine (isolated as its oxide) was obtained in 75% yield, with iodobenzene as the aryl halide (ArX in Scheme 6, steps i-iii only). Unsymmetrical phosphines result from the full sequence of reactions in Scheme 6, and p-anisyldiphenylphosphine (isolated as its oxide) was produced in 55% yield, based on the phosphorus used, when chlorobenzene (ArX) and p-methoxyanisole (AiOC) were used. 

From the preceding discussion it may be concluded that the main resonance line at g — 2.0006 in irradiated frozen alkali hydroxide solutions is attributable to the radiation-produced electron trapped around a hydrated O- radical ion. Insofar as the latter is an electron vacancy created by the reaction of the radiation produced holes with the OH ions, the trapped electron may be considered to be analogous to an F center formed in alkali halide crystals, where, however, the electron vacancies exist even prior to irradiation. The term trapped electron (symbolized T ) has been used throughout the present paper. This model will be... 

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