Homolytic displacement with

Several very stable radicals (e.g. those derived from AIBN) react with tetraphenyl-biphosphine in a homolytic displacement reaction.39 Phosphoranyl radicals, e.g. (36), were postulated as being intermediates. The reaction of the triplet state of olefin (37) is thought to occur via a similar mechanism. 

Reactions 68-73 (Table IV) represent the radical interactions with metal complexes radical addition (Reaction 68), electron transfer from a radical to a metal (Reaction 70) or from a metal to a radical (Reaction 69), homolytic displacements at the metal or at a ligand (Reactions 71 and 72), and fi-radical fragmentations (Reaction 73). The final reactions (74-79) concern the reactivity of radicals with organic reagents. 

Homolytic cleavage of a tin carbon bond in a tetraorgan-otin compound is less frequently encountered than is heterolytic cleavage. Such radicals as Bn, , and the suc-cinimidyl radical can take part in Sn2 displacement with tetraorganotins (Scheme 8). The reactivity of ITiSn in MezCO towards the succinimidyl radical is in the sequence of R = Me > Et > Pr > Bn this is the reverse of that for radical reactions of iodine and bromine. 

Two main types of reaction occur when alkylcobaloximes react with cobalox-ime(ii) complexes in methanol first, homolytic displacement of cobaloxime(ii) from the alkylcobaloxime is brought about by the attack of the cobaloxime(ii) reagent on the alkyl group, and this is followed by the rapid additional exchange of equatorial ligands between reagent cobaloxime(ii) and displaced cobaloxime(ii). The rate constants for the reduction of a series of Bia models (25) by [Fe(edta)] " are very sensitive to the nature of the axial ligands L. 

An interesting synthesis of trichloroethyl-benzenes provides the first example of homolytic displacement by an alkyl group at a saturated carbon atom. In this way benzylbis(dimethylglyoximato)pyridine cobalt, with BrCCla in chloroform, leads to PhCHiCCls and PhCH2Br. 

An interesting method for the substitution of a hydrogen atom in rr-electron deficient heterocycles was reported some years ago, in the possibility of homolytic aromatic displacement (74AHC(16)123). The nucleophilic character of radicals and the important role of polar factors in this type of substitution are the essentials for a successful reaction with six-membered nitrogen heterocycles in general. No paper has yet been published describing homolytic substitution reactions of pteridines with nucleophilic radicals such as alkyl, carbamoyl, a-oxyalkyl and a-A-alkyl radicals or with amino radical cations. 

The aryl-thallium bond is thus apparently capable of displacement either by electrophilic or by suitable nucleophilic reagents. Coupled with its propensity for homolytic cleavage (spontaneous in the case of ArTlIj compounds, and otherwise photochemically induced), ArTlXj compounds should be capable of reacting with a wide variety of reagents under a wide variety of conditions. Since the position of initial aromatic thallation can be controlled to a remarkable degree, the above reactions may be only representative of a remarkably versatile route to aromatic substitution reactions in which organothallium compounds play a unique and indispensable role. 

The second, albeit scarcely applied, class of radical domino reactions indudes intramolecular homolytic substitutions, which can also result in the formation of rings. As a consequence, the radical is ejected with the displaced center and does not remain with the main ring hence, this type of reaction is normally featured as terminating step. The most typical complication occurring within substitutions is an undesired branching of a sequence caused by the released radical. 

The use of allylstannanes for homolytic allylation depends on the rapid conjugate displacement of R3Sn- by attack of a radical at the y-position of the allyl group. The rate constants for this reaction by primary alkyl radicals with the allylstannanes 22 and 23 in Scheme 9 are close to the value that was estimated previously for allyltributyltin.285,286... 

The rate of fluorine displacement from fluorotoluenes by H-atoms has been measured in single-pulse shock tubes at 988-114 K.158 The addition of CF3 to CgFsCl has been studied.159 The intermediate adduct radical (CF3C6F5C1) was shown to react with an additional CF3 to give CF3CI and C6F5CF3. A range of fluorinated biphenyls can be produced by the reaction of pentafluorobenzene radicals with both electron-rich and -poor aromatics. The isomeric ratios of biphenyls produced indicated an efficient homolytic chain process.160... 

An alternative method for the substitution of a hydrogen atom in -electron deficient heterocycles is using the nucleophilic character of radicals in homolytic aromatic displacement reactions <74AHC(16)123>. Acylation with acyl radicals derived from aldehydes is an especially important approach since Friedel-Crafts-type reactions are not applicable to pteridines. 

The displaced electron is generally assumed to be the electron with the lowest ionization energy. In order of probability, this will be a nonbonding electron followed by a 7t bond electron and then a a bond electron. Thus El yields, in the first instance, a molecular ion which is a radical cation with an unpaired electron. In principle, any remaining energy will then be dissipated by bond cleavages that result in the formation of the most stable cation with a paired electron (even-electron ion). These even-electron ions may be formed by homolytic or heterolytic cleavages. This whole process happens very rapidly (<10-8s) and is the reason for the close similarity of El spectra produced across all different instruments. It is important to remember that mass spectral reactions in the El source are unimolecular. This is because the pressure in the El source is too low for bimolecular (ion-molecule) reactions to occur. 

In a homolytic cyclization, a side-chain alkene displaces a neighbouring nuclear bromine atom on heating the compound in toluene with tributyltin hydride and AIBN. An optical purity of 39% is observed when an asymmetric A/ -substituent is present. Palladium dia< tate-catalysed cyclization (reviews [B-41,3069,3505] of the bromoester (39.1) gives the oxindole in moderate yield the product may consist of a mixture of stereoisomers for example, when R = Ph, one isomer predominates in the ratio 60 13. 

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