Free radicals to alkenes

A. L. J. Beckwith, Regio-selectivity and stereo-selectivity in radical reactions, Tetrahedron, 1981, 37, 3073-3100 A. L. J. Beckwith, The pursuit of selectivity in radical reactions, Chem. Soc. Rev., 1993, 22, 143-151 B. Giese, Formation of CC bonds by addition of free radical to alkene, Angew. Chem. Int. Ed. Engl., 1983, 22, 753. 

Direct oxidation of alkenes with molecular oxygen11,255,256 initiated by free radicals to yield epoxides occurs through addition of peroxy radicals to produce the more stable P-peroxy alkyl radicals (28) 257... 

Generally, as the potential energy level of SOMO increases (becomes a more reactive radical), free radicals have nucleophilic character, while as the potential energy level of SOMO decreases (becomes a stable radical), free radicals have electrophilic character. Thus, when effective radical reactions are required, small energy difference in SOMO-HOMO or SOMO-LUMO interactions is necessary. For example, the relative reactivities of radical addition reactions of a nucleophilic cyclohexyl radical to alkenes,... 

Alkanes may be prepared by the hydrogenolysis of functional groups such as alkyl halides, by the reduction of imsaturated systems such as alkenes, alkynes and carbonyl compounds and by coupling reactions. The characteristic reactions of alkanes involve the abstraction of a hydrogen atom by a free radical to form a carbon radical. 

Both intermolecular and intramolecular additions of carbon radicals to alkenes and alkynes continue to be a widely investigated method for carbon-carbon bond formation and has been the subject of a number of review articles. In particular, the inter- and intra-molecular additions of vinyl, heteroatomic and metal-centred radicals to alkynes have been reported and also the factors which influence the addition reactions of carbon radicals to unsaturated carbon-carbon bonds. The stereochemical outcome of such additions continues to attract interest. The generation and use of alkoxy radicals in both asymmetric cyclizations and skeletal rearrangements has been reviewed and the use of fi ee radical reactions in the stereoselective synthesis of a-amino acid derivatives has appeared in two reports." The stereochemical features and synthetic potential of the [1,2]-Wittig rearrangement has also been reviewed. In addition, a review of some recent applications of free radical chain reactions in organic and polymer synthesis has appeared. The effect of solvent upon the reactions of neutral fi ee radicals has also recently been reviewed. ... 

How can we account for the unusual reactivity of conjugated dienes In our discussion of halogenation of the simple alkenes (Sec. 3.27), we found that not only orientation but also relative reactivity was related to the stability of the free radical formed in the first step. On this basis alone, we might expect addition to a conjugated diene, which yields a stable allyl free radical, to occur faster than addition to a simple alkene. 

The fact is that conjugated dienes are more reactive than simple alkenes. In the present case, then—and in most cases involving alkenes and free radicals, or alkenes and carbonium ions—the factors stabilizing the transition state are more important than the factors stabilizing the reactant. However, this. is not always true. (It does not seem to be true, for example, in electrophilic addition to conjugated dienes.)... 

On the basis of the stability of the particle being formed, we might expect addition to a conjugated alkcnylbenzcne, which yields a stable benzyl cation or free radical, to occur faster than addition to a simple alkene. 

One can, for example, measure the enthalpy of protonation of an alkene to a carbocation, or the ionization of a free radical to a carbocation. 

The kinetics of the reactions of several atomic and molecular free radicals with alkenes and alkynes have been studied down to low temperatures in CRESU experiments.22 The results of these experiments have been reviewed and analysed by Smith et Based on semi-empirical arguments, as well as correlations of room temperature rate constants, they suggested which reactions of radicals with unsaturated molecules are likely to be fast at ca. 10 K, that is, the temperatures found in the cold cores of dense interstellar clouds. 

For addition reactions, S 100 therefore, the enthalpy of irreversible addition of a free radical to an alkene should be smaller than 43 kJ mol 1 (at T = 300 K). 

Polymerization reactions. There are two broad types of polymerization reactions, those which involve a termination step and those which do not. An example that involves a termination step is free-radical polymerization of an alkene molecule. The polymerization requires a free radical from an initiator compound such as a peroxide. The initiator breaks down to form a free radical (e.g., CH3 or OH), which attaches to a molecule of alkene and in so doing generates another free radical. Consider the polymerization of vinyl chloride from a free-radical initiator R. An initiation step first occurs ... 

Within the predictive capabilities of the models, reactivity is given by bThe larger r- the more reactive the molecule (or ion or radical). Note that the tenriinal carbon atoms in buta-1,3-diene are predicted by Iltiekcl theoiy to be slightly more reactive than the carbon atoms in ethylene. Qualitative eoirelation with experience is seen fur sume alkenes and free radicals in Fig. 7-3,... 

Under CO pressure in alcohol, the reaction of alkenes and CCI4 proceeds to give branched esters. No carbonylation of CCI4 itself to give triichloroacetate under similar conditions is observed. The ester formation is e.xplained by a free radical mechanism. The carbonylation of l-octene and CCI4 in ethanol affords ethyl 2-(2,2,2-trichloroethyl)decanoate (924) as a main product and the simple addition product 925(774]. ... 

---