Rearrangements involving free radicals

In this chapter, we discuss free-radical substitution reactions. Free-radical additions to unsaturated compounds and rearrangements are discussed in Chapters 15 and 18, respectively. In addition, many of the oxidation-reduction reactions considered in Chapter 19 involve free-radical mechanisms. Several important types of free-radical reactions do not usually lead to reasonable yields of pure products and are not generally treated in this book. Among these are polymerizations and high-temperature pyrolyses. 

Treatment of double-bond compounds with selenium dioxide introduces an OH group into the allylic position (see also 19-14). This reaction also produces conjugated aldehydes in some cases.Allylic rearrangements are common. There is evidence that the mechanism does not involve free radicals but includes two pericyclic steps (a and... 

When dicobalt octacarbonyl, [Co(CO)4]2, is the catalyst, the species that actually adds to the double bond is tricarbonylhydrocobalt, HCo(CO)3. Carbonylation, RCo(CO)3- -CO—>RCo(CO)4, takes place, followed by a rearrangement and a reduction of the C—Co bond, similar to steps 4 and 5 of the nickel carbonyl mechanism shown in 15-30. The reducing agent in the reduction step is tetra-carbonylhydrocobalt HCo(CO)4, ° or, under some conditions, H2. When HCo(CO)4 was the agent used to hydroformylate styrene, the observation of CIDNP indicated that the mechanism is different, and involves free radicals. Alcohols can be obtained by allowing the reduction to continue after all the carbon monoxide is... 

AH the existing evidence indicates quite strongly that, although rearrangement of free radicals occasionally happens, it is not very common and does not involve simple alkyl radicals. 

The 5-hexenyl system has been used by Garst to demonstrate that the intermolecular portion of the Wittig rearrangement of aralkyl alkyl ethers and the ketyl-alkyl iodide reaction involve free radical intermediates. " ... 

Additional metal-catalyzed alkane rearrangements, such as the photochemically initiated mercury-catalyzed dimerization of alkanes, are also known (Equation 6.14), and it is again argued that the process involves free radicals (as well as dehydrogenation). 

Rearrangements of iV-chloroacetanilides induced by ultraviolet radiation or benzoyl peroxide in carbon tetrachloride involve free-radical intermolecular chlorination , as probably do similar reactions of iV-chloro-iV-alkylarylamines and JV-chloroarylsulphanilides . 

It was found that protonation facilitates the 1,2-shifts in several reactions involving free radicals. Based on molecular-orbital calculations, Golding and Radom ° proposed the following general scheme for the intermolecular rearrangement catalyzed by adenosine cobalamin (eqs. 7.2.5 to 7.2.8) ... 

Because many organic peroxides undergo thermolysis to form useful free radicals, they are used commercially as initiators for free-radical reactions. Many organic peroxides also undergo reactions in which free radicals are not involved, eg, heterolyses, hydrolyses, reductions, and rearrangements. Numerous reviews of the chemistry and appHcations of organic peroxides have been pubHshed (11,13—41). 

Adenosylcobalamin (coenzyme B 2) is required in a number of rearrangement reactions that occurring in humans is the methylmalonyl-Co A mutase-mediated conversion of (R)-methylmalonyl-Co A (6) to succinjl-CoA (7) (eq. 1). The mechanism of this reaction is poorly understood, although probably free radical in nature (29). The reaction is involved in the cataboHsm of valine and isoleucine. In bacterial systems, adenosylcobalamin drives many 1,2-migrations of the type exemplified by equation 1 (30). 

A useful synthesis of allylstannanes from primary alcohols involves conversion of the alcohols into their O-substituted 5-methyl carbonodithioates, thermolysis to effect [3,3] rearrangement to the corresponding 5-substituted 5-methyl carbonodithioates, and treatment with a trialkyl-tin hydride under free-radical conditions to form the allylstannane21. This procedure has been applied to the synthesis of functionalized allylstannanes including (5)-( )-4-(benzyloxy)-2-pen-tenyl(tributyl)stannane22. 

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