Radical halides

The efficiency of the halide- radical transformation is reported to be near quantitative. The yield of block or graft is then limited by the efficiency of the halide synthesis. Whether AB or ABA blocks are formed depends on the termination mechanism. Similar halo-compounds have been used to initiate A TRP (Section 9.4). 

Electron affinity or reduction of halide radical to halide anion (A )... 

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],... 

Table 14. Unpaired electron distributions in diatomic halide radical anions. Sa, Pa indicate the spin population of the valence s and p orbitals of atom A, derived from the isotropic and anisotropic hyperfine coupling constants 67)...

Table 3 Calculated A//rxn Values for Halide Radicals Abstracting Hydrogens from Aromatic Ring of Benzene or Toluene Methyl Group...

When generating the di(pseudo)halide radical anions radiolytically, one has to keep in mind that the halide ions do not react with H, but HN3/N3 does. Originally, it has been suggested that N3 and H2 are formed (Alfassi et al. 1986), but it was later shown that it reacts according to reaction (18) (Deeble et al. 1990). 

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 halide radical anion stability has been calculated at the STO-3G level and found to have a low barrier to dissociation when solvated by water molecules16. In solution it cannot be observed (equation 1) because of the small barrier to its dissociation (ca 6 kcal... 

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). 

A theoretical study was performed to determine the stability of bicycloalkyl halide radical anions as well as the dependence of the reduction potential of the corresponding neutral compounds on the angular strain energy of the parent hydrocarbons. It was proposed that an increase in strain at the bridgehead carbon is accompanied by a decrease in the photostimulated outer-sphere ET rate for the parent RX compounds43. 

Incorporation of triethylamine into the reaction medium produced more reduction product presumably due to electron transfer from the triethylamine to the excited alkyl halide. This results in a weakly-bound amine-alkyl halide pair [57]. The alkyl halide radical anion releases X- (Scheme 17). In a related example, it is known that solutions of aliphatic amines in CC14 are unstable to light quickly forming white crystalline precipitates [60]. The initial reaction is formation of a singlet radical pair via excitation of a ground state charge-transfer complex. 

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... 

To gain additional information on these fragmentation reactions, the reactivity of a wide variety of structurally modified nitrobenzyl halide radical anions has been investipted (Table 13) [283, 290, 294],... 

The following mechanism was proposed by Walborsky and AronofFin 1965 for the lithiation reaction (Scheme 9). The stereochemistry of the reaction may be explained by a single electron transfer (SET) to the carbon-halogen bond which results in either the formation of an ion paired halide radical anion on the metal surface (pathway 1) or what is... 

Are ion paired halide radical anions R-X and/or weak radical XM complexes R - X M" also intermediates in such ET reactions (pathway B, Scheme 12) ... 

Undoubtedly, in the case of the cyclopropyl halides 106, 107, 110, 111, 114-Hal and 115-Hal the normal mechanism with the cyclopropyl radical participating in the productdetermining step is observed. The cyclopropyl halides 5(1-52 and 118-Hal, however, seem to react in such a way that cyclopropyl halide radical anions CprX M anri or cyclopropyl radical complexes Cpr X M" are kinetically significant. The question whether species of this type indeed exist is an important issue ... 

Symons critically discussed the results of Garst and coworkers. He points out that although it is very unlikely that an ion paired halide radical anion R-X exists in the case of R = 5-hexenyl, R X complexes possessing weak residual charge-transfer interactions would also be expected to differ kinetically from each other as the halide is varied. Of special significance for the interpretation of the results obtained with the cyclopropyl halides 50-52 and 118-Hal are the following conclusions of Symons ... 

The formation of cyclopropyl halide radical anion pairs as intermediates is also invoked in Sr I type substitution reactions by Rossi and Meijs . It seems that the photostimulated reaction of cyclopropyl bromides like 7-bromonorcarane (120) with Ph2P M to give 121 involves a radical chain, and halogen-containing radical anions as chain carrier. 

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