Free radicals rearrangement reactions

Wallace, T. J., and R. J. Gritter The Free Radical Chemistry of Cyclic Ethers. III. A Free Radical Rearrangement Reaction. J. org. Chem. 27, 3067 (1962). 

Careful studies by Chen revealed that deoxygenation of baccatin derivatives at C-2 can be accompanied by free-radical rearrangement reactions. As examples the cyclic 1,2-thiocarbonate 3.4.20 gave both a simple A-nor product 3.4.21 and the more complex rearrangement product 3.4.22 (124). Deoxygenation of the 10-acetoxy analog of 3.4.20 only gave A-nor product. 

Scheme 12.6 gives some examples of reactions in which free-radical rearrangements have been observed. 

The radical rearrangement reaction, serving as a timing device, has been called a free radical clock 2 It provides a means of evaluating the rate constant for reactions of this radical with other substrates. The example shows how the radical-chromium(II) rate constant can be determined. A number of other instances have been summarized.13... 

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. 

The trimethylsilyl ethers 212 of four-membered 1-alkenyl-1-cyclobutanols rearrange to the ring-expanded 0-mercuriocyclopentanones 213. These can be converted into the a-methylenecyclopentanones 214 through elimination or further expanded by one-carbon atom into cyclohexanones 215 via the Bu3SnH-mediated free radical chain reactions [116]. A similar radical intermediate is suggested to be involved in the ring expansion of a-bromomethyl-fi-keto esters [117]. (Scheme 84)... 

The kinetic data for the reaction of primary alkyl radicals (RCH2 ) with a variety of silanes are numerous and were obtained by applying the free-radical clock methodology. The term free-radical clock or timing device is used to describe a unimolecular radical reaction in a competitive study [2-4]. Three types of unimolecular reactions are used as clocks for the determination of rate constants for this class of reactions. The neophyl radical rearrangement (Reaction 3.1) has been used for the majority of the kinetic data, but the ring expansion rearrangement (Reaction 3.2) and the cyclization of 5-hexenyl radical (Reaction 3.3) have also been employed. 

Ordinarily, free-radical chain reactions begin by abstraction of a weakly bonded hydrogen atom (34) from the substrate by a radical which is regenerated in a subsequent step in the process. The resultant substrate radical may then rearrange or undergo decomposition reactions similar to the elimination of ions by consecutive electron displacements (48,49). The path to the non-labeled formic acid begins with abstraction... 

Brown, H. C., and G. A. Russell The photochlorination of 2-methyl-propane-2-d and a-d1-toluene the question of free radical rearrangement or exchange in substitution reactions. J. Amer. chem. Soc. 74,3995 (1952). 

Phenylpropanoic, 4-phenylbutanoic and 3,3-diphenylpropanoic acids were transformed with xenon difluoride to the corresponding fluorinated products without indication of free-radical rearrangement or cyclization, while phenylacetic acid in the presence of benzene produced diphenylmethane, indicating the involvement of a trivalent species (Scheme 44). Trifluoromethylbenzene was formed in 42% yield by reaction of trifluo-roacetic acid with benzene35-37. 

Wollowitz, S., and Halpem, J., 1984, Free-radical rearrangement involving the 1,2-migration of a thioester groupoModel for the coenzyme-B12 dependent methylmalonyl-CoA mutase reaction, J. Am. Chem. Soc. 106 831998321. 

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. ... 

---