Aryl halides radicals

Intramolecular dissociative electron transfer in aryl halide radical-anions involves an interaction between the n-aroinatic orbital and the o-type carbon-halogen bond. These orbitals are orthogonal, but bending of the carbon-halogen bond allows the necessary interaction. Qualitatively, the influence of several factors on the bond cleavage rate can be discerned [18],... 

When aryl halides were applied in catalytic coupling reactions, the mechanistic evidence points to initial SET reduction by low-valent nickel phosphine species (selected investigations in [23, 24]). The competition of cage collapse to ArNi(PR3)2X vs. dissociation of the aryl halide radical anion to a free radical and Ni(I) complexes determines the cross-coupling manifolds. Thus, Ni(0)-Ni(II) and Ni(I)-Ni(III) catalytic cycles can occur interwoven with each other and a distinction may be difficult. Common to both is that the coupling process with aryl halides is likely to occur by a two-electron oxidative addition/reductive elimination pathway. 

The extensive determination of fragmentation rates of aryl halide radical anions, due to Saveant and coworkers15a by electrochemical methods, indicates that they range from values of 10-2s-1 for nitro-substituted phenyl halides up to 1010 s-1 for />-cyanophenyl halides. These values are in agreement with measurements by pulse radiolysis42. The fragmentation rates for unsubstituted phenyl halides are too high to be measured even by electrochemical techniques. Besides, 1-bromo- and 1-iodoanthraquinone radical anions have been shown to dissociate from their photoexcited state (Section V. D). 

Arrhenius activation parameters for cleavage reactions of aryl halide radical anions"... 

Aryl radicals electrochemically generated from the cleavage of aryl halide radical anions have been observed to react with nucleophiles other than iodide (Pinson and Saveant, 1974, 1978 Saveant, 1980), a reaction known as the SrjjI reaction (Bunnett, 1978). The most commonly used nucleophiles are thiophenolate, mercaptides, and cyanide ion. The reactions observed are... 

Even the most reactive complexes can show radical chemistry with less reactive alkyl and aryl halides. Radical pathways are associated with loss of stereochemistry or even with retention at carbon where tight radical ion pairs, (Ln) (RX )], are... 

Takeda N, Poliakov PV, Cook AR, Miller, JR. (2004) Faster dissociation Measured rates and computed effects on barriers in aryl halide radical anions. J Am Chem Soc 126 4301-4309. 

Radical anions of haloaromatic compounds are proposed to be intermediates in different type of reactions. Their fragmentation rates, determined electrochemically [300] or by pulse radiolysis [301] range from lO " s for phenyl halides to 10 s for some halonitrobenzenes. The rate of the reaction for some aryl hahde radical anions is too high to be measured electrochemically, the fragmentation of more stable radical anions such as those of 1-bromo- and 1-iodoanthraquinone [302], p-[303] and m-bromo- [304] and p- [303] and w-chloronitrobenzenes [304] occurs at considerably lower rates and the reaction is favored from their photoexcited state. Aryl halide radical anions may present a-n orbital isomerism depending on the orbital symmetry of their singly occupied molecular orbital [305], a proposal derived from theoretical and experimental evidences [306]. The isomerism is possible... 

The radical clock experiments as well as the stereochemical outcome of the reaction along with the reactivity profiles observed pointed to an ET process as the operating mechanism. Linear-free energy relationships were also consistent with this mechanistic pathway (see succeeding text). ET may proceed in two ways, usually referred to as inner-sphere and outer-sphere ET, which can be contemplated as the two extremes of a continuous mechanism [204]. Both processes are dissociative in nature for alkyl halides and presumably do not involve a discrete radical anion, RX" [205]. The situation may, however, be different for aryl halides. Radical anions do exist, and aryl halides probably undergo a stepwise reaction with an electron donor to give rise to RX [206]. 

The terminal amino group of 2-hydrazino-4-phenylthiazole is also the reactive center in reactions with activated aryl halides such as 288. A solution of the product (289) obtained from this reaction when shaken with PbOj gives a deeply colored radical, whose structure has been studied by ESR (Scheme 173) (532. 533). 

Grignard reagents are a very important class of organometallic compounds. For their preparation an alkyl halide or aryl halide 5 is reacted with magnesium metal. The formation of the organometallic species takes place at the metal surface by transfer of an electron from magnesium to a halide molecule, an alkyl or aryl radical species 6 respectively is formed. Whether the intermediate radical species stays adsorbed at the metal surface (the A-modelf, or desorbs into solution (the D-model), still is in debate ... 

Radical-mediated silyldesulfonylation of various vinyl and (a-fluoro)vinyl sulfones 21 with (TMSlsSiH (Reaction 25) provide access to vinyl and (a-fluoro)vinyl silanes 22. These reactions presumably occur via a radical addition of (TMSlsSi radical followed by /)-scission with the ejection of PhS02 radical. Hydrogen abstraction from (TMSlsSiH by PhS02 radical completes the cycle of these chain reactions. Such silyldesulfonylation provides a flexible alternative to the hydrosilylation of alkynes with (TMSlsSiH (see below). On oxidative treatment with hydrogen peroxide in basic aqueous solution, compound 22 undergoes Pd-catalyzed cross-couplings with aryl halides. 

Examples of the intermolecular C-P bond formation by means of radical phosphonation and phosphination have been achieved by reaction of aryl halides with trialkyl phosphites and chlorodiphenylphosphine, respectively, in the presence of (TMSlsSiH under standard radical conditions. The phosphonation reaction (Reaction 71) worked well either under UV irradiation at room temperature or in refluxing toluene. The radical phosphina-tion (Reaction 72) required pyridine in boiling benzene for 20 h. Phosphinated products were handled as phosphine sulfides. Scheme 15 shows the reaction mechanism for the phosphination procedure that involves in situ formation of tetraphenylbiphosphine. This approach has also been extended to the phosphination of alkyl halides and sequential radical cyclization/phosphination reaction. ... 

In certain cases, Michael reactions can take place under acidic conditions. Michael-type addition of radicals to conjugated carbonyl compounds is also known.Radical addition can be catalyzed by Yb(OTf)3, but radicals add under standard conditions as well, even intramolecularly. Electrochemical-initiated Michael additions are known, and aryl halides add in the presence of NiBr2. Michael reactions are sometimes applied to substrates of the type C=C—Z, where the co-products are conjugated systems of the type C=C—Indeed, because of the greater susceptibility of triple bonds to nucleophilic attack, it is even possible for nonactivated alkynes (e.g., acetylene), to be substrates in this... 

A useful new method of preparing arylphosphonates (123) involves the reaction of trialkyl phosphites with aryl halides in the presence of a nickel catalyst.The suggested mechanism is via the nickel complex (124), and is non-radical. 

Diazonium salts can also be converted to halides by processes involving aryl free radicals. In basic solutions, aryl diazonium ions are converted to radicals via the diazo oxide.102... 

The reaction can be carried out efficiently using aryl diazonium tetrafluoroborates with crown ethers, polyethers, or phase transfer catalysts.103 In solvents that can act as halogen atom donors, the radicals react to give aryl halides. Bromotrichloromethane gives aryl bromides, whereas methyl iodide and diiodomethane give iodides.104 The diazonium ions can also be generated by in situ methods. Under these conditions bromoform and bromotrichloromethane have been used as bromine donors and carbon tetrachloride is the best chlorine donor.105 This method was used successfully for a challenging chlorodeamination in the vancomycin system. 

Extension to carbocyclization of butadiene telomerization using nitromethane as a trapping reagent is reported (Eq. 5.48).72 Palladium-catalyzed carbo-annulation of 1,3-dienes by aryl halides is also reported (Eq. 5.49).73 The nitro group is removed by radical denitration (see Section 7.2), or the nitroalkyl group is transformed into the carbonyl group via the Nef reaction (see Section 6.1). 

Exposure of (Me3Ge)2Hg to visible fight in the presence of alkyl halides results in the rapid formation of Me3GeHal and MesGel IgR104. This reaction is thought to occur via a radical chain mechanism (Scheme 26). When the reaction is instead carried out in the presence of aryl halides, the products are R3GeI Ial and Ar—Hg—Ar this reaction has a... 

An alternative approach to developing new reagents for DNA cleavage was founded in the conversion of aryl diazonium salts to aryl halides by the well-known Sandmeyer reaction. The reaction (Eq. 1) is thought to proceed via aryl radicals [7]. These reactions are high yielding and catalytic with respect... 

In the absence of nucleophile, the aryl halide undergoes a two-electron reductive cleavage according to an ECE-DISP mechanism (Scheme 2.21). The two-electron stoichiometry occurs because the aryl radical produced on the one-electron reductive cleavage is easier to reduce than the substrate. The competition between the ECE and DISP pathways is governed by the parameter... 

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