Free radical rearrangements, applications

I have chosen to discuss some applications of free-radical rearrangements in solution for a number of reasons. In the first place, this Symposium is intended to honour Paul Bartlett and Cheves Walling for their many contributions to free-radical chemistry. Early work on free-radical rearrangements by both Bartlett l) and Walling ( ) played vital roles in the subsequent development of the subject Secondly, this Symposium is... 

Derivatives of trifluoroethanethiol have limited though interesting chemistry. Unfortunately, metallated difluorothioenol chemistry has not been reported, because rapid nucleophilic attack occurs even by hindered bases such as LDA. Nakai et al. exploited this high electrophilicity in a tandem addition/elimina-tion-rearrangement sequence [146], but more recent applications have concerned free radical chemistry (Eq. 46). Chlorination of trifluoroethyl phenyl sulfide followed by exposure to tin hydride in the presence of an allylstannane resulted in C-C bond formation with a reasonable level of stereocontrol [147]. 

For the most part we are concerned with the breaking of carbon-carbon and carbon-hydrogen bonds and subsequent rearrangement of intermediate free radicals to produce ethylene. These bonds are broken by the simple application of temperature and because the bonds of interest are strong, high temperatures are required. 

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. ... 

Thiols may be converted into alkenes with one extra carbon atom by successive chloromethylation, oxidation at sulfur, and Ramberg-BScklund rearrangement. If the original thiol is prepared by free radical addition of H2S to an alkene, the sequence constitutes a one-carbon homologation of alkenes. Although these transformations were at one time important applications of the Ramberg-Backlund reaction, they have hardly been used in recent years. 

The mechanism of this reaction was discussed based on the preliminary results of control experiments and analysis of GC-MS data (Scheme 4.35). The reaction is initiated by the addition of azido free radical and terminated with molecular oxygen to generate the peroxide intermediate 65 [136]. Then this species would undergo continuous multi step rearrangement to give product 56. The details of this rearrangement were disclosed in their ensuing research in which DFT calculations were carried out (Scheme 4.36). This research provides concise and practical access to synthetic important molecules, which may find broad applications in synthesis. 

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