Formation accompanying homolytic

Olefin Formation Accompanying Homolytic Transition Metal-Alkyl Bond Dissociation... 

Processes accompanied by a decrease in volume, such as C—C bond formation, in which the distance between two carbon atoms decreases from the van der Waals distance of ca 3.6 A to the bonding distance of ca 1.5 A, are accelerated by raising the pressure and equilibria are shifted toward the side of products (AV < 0, AV < 0). The reverse reaction, a homolytic bond cleavage, leads to an increase in volume (AV / > 0, AV > 0). Pressure induces a deceleration of such a process and a shift in equilibrium toward the side of reactants. However, in an ionization, such as an ionic dissociation, the attractive interaction between the ions generated and the solvent molecules leads to a contraction... 

Figure 3.9b portrays homolytic bond formation by the recombination of radicals and is accompanied by charge transfer from A to B. The radicals must be singlet coupled. The interaction of triplet-coupled electron pairs is repulsive and does not lead to bond formation. The reverse process describes homolytic bond cleavage and results in singlet-coupled free radicals. 

Products which can be ascribed to the intermediate formation of radicals have long been observed in carbene reactions. In the gas phase these products could arise by homolytic decomposition of excited primary products before collisional deactivation rather than from radicals generated in the course of insertion. This is not so in solution. It is found that, in the thermal decomposition of diphenyldiazomethane (Bethell et al., 1965) or photolysis of diphenylketene (Nozaki et al., 1966) in toluene solution, the product of insertion of diphenylmethylene into the benzylic carbon-hydrogen bonds, 1,1,2-triphenylethane, is accompanied by substantial amounts of 1,1,2,2-tetraphenylethane and bibenzyl. This is a strong indication that discrete diphenylmethyl and benzyl radicals are formed, and, taken in conjunction with EPR (Section IIB) and other evidence (Etter et al., 1959) that diphenylmethylene is a ground-state triplet, would support the view that equation (20) is an adequate representation of triplet insertion. 

In the other two mechanisms (homolytic splitting and insertion) hydride formation is accompanied by formal oxidation of the metal, and reactivity is closely linked to the susceptibility of the latter to oxidation. Thus, the high reactivity of Co(CN).-, toward Ho, compared with that of Co(CNCH i)- reflects the tendency of CN" to stabilize preferentially the higher oxidation state, and CNCH the lower oxidation state, of cobalt. For square planar complexes the expected order of the tendency toward oxidation, and hence of reactivity toward Ho, is (subject to some modification by ligand variation),... 

Homolytic cleavage of oxaziridines by ferrous salts, already described by Emmons, is a useful source of alkyl radicals. From oxaziridine 28, easily accessible from cyclohexanone and Af-chloromethylamine, on the action of ferrous salt, reaction products of radical 29 are obtained In the presence of FeCl2 oj-chlorohexanoic acid methylamide (30) is formed quantitatively in the absence of reaction partners, dimerization occurs to 31. As shown in a very careful investigation by Hawkins in the case of the N-cyclohexyl compound, the dicarboxylic acid derivative is accompanied by appreciable quantities of branched-acid derivatives, formed by a radical rearrangement 29 -> 32. The radical 29 adds to pyridine and gives rise, after rearomatization, to the formation of 70-80% of 2- and 4-substituted pyridine (33). ... 

For metal complexes of alkyls containing 3-hydrogen atoms, metal-alkyl homolytic bond dissociation often is accompanied by olefin formation. An example ( ) Is provided by Equation 30 (Saloph = N,N -bid(salicylidene)-o-phenylenediamine). 

These techniques rely upon high shear to cause bond scissions. Ruptured bonds result in formations of free-radical and ionic species. When this application of shear is carried out in the presence of monomers, block copolymers can form. This approach is exploited fairly extensively. Such cleavages of macromolecules can take place during cold mastication, milling, and extrusion of the polymers in the viscoelastic state. Both homolytic and heterolytic scissions are possible. The first yields free-radical and the second ionic species. Heterolytic scissions require more energy but should not be written off as completely unlikely." Early work was done with natural rubber. It swells when exposed to many monomers and forms a viscoelastic mass. When this swollen mass is subjected to shear and mechanical scission, the resultant radicals initiate polymerizations. The mastication reaction was shown to be accompanied by formation of homopolymers. Later, the technique was applied to many different polymers with many different monomers. ... 

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