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Free-radical reactions have been for long time the domain of the physical chemists and the basic chemical industry (polymerization of vinyl monomers, oxidation by molecular oxygen, chlorination of methane etc.), where the use of simple molecules and the possibility of partial conversions without heavy problems of product separations makes less dramatic the aspects of regio and chemoselectivity. As synonym of unselectivity, free radical reactions were considered of poor use in the synthesis of fine chemicals or sophisticated molecules, where a high selectivity is an essential condition for the success, or in the involvement of biological processes. Within the last 15 years, however, an authentic explosion of synthetic applications of free radical reactions occurred; they have gained a remarkable position among the selective methods of synthesis. At the same time the great importance of free radical reactions in fundamental biological processes and in the metabolism of drugs has been recognized. Thus a specialized meeting on these topics was generally felt appropriate. I had the honour and the onus to organize this workshop because for more than 30 years I have been involved in the research of free radical reactions. Thanks to a generous grant from NATO Scientific Affairs Division and the financial support of CNR and chemical industry (Montedison, Enichem, Zambon) such a meeting among almost 50 specialists and 15 observers of sufficient standard to take advantage of the discussion, became possible at Bardolino (Italy).
This book describes the experience over 25 years of the senior author with the chemistry of organic free radicals. It begins with a mechanistic study of industrial importance on the pyrolysis of chlorinated alkanes. It continues with a theory on the biosynthesis of phenolate derived alkaloids involving phenolate radical coupling. There follows 20 years of practical work to prove the theory correct, especially in the case of morphine alkaloids. The book then describes the work on nitrile photolysis (Barton reaction) which involved the invention of new radical chemistry leading to a simple synthesis of the important hormone, aldosterone. There follows a description of the invention of an important new method for the deoxygenation of biologically important molecules, especially sugars and nucleosides, using radical chemistry applied to thiocarbonyl derivatives. Some years later, in a logical extension to carboxylic acids, another new reaction was invented which provides carbon, nitrogen, oxygen and other radicals under mild conditions. A final chapter summarizes recent applications of thiocarbonyl group derived radical reactions by other authors.
This volume describes the recent developments in the free-radical mediated synthesis and elaboration of heterocycles. The first chapter, dealing with radical cascade processes illustrates the power and the beauty of radical chemistry with some striking examples of total synthesis of complex natural heterocycles. As organic chemists strive towards sustainability, radical chemistry has recently seen major advances and efforts in this direction, including C-H activation of arenes and unactivated alkyl groups. Photochemical activation, for a long time the preferred mode of activation in radical chemistry has also seen an unexpected revival with the advent of visible light metal- and organocatalyzed photoredox processes. A survey of these emerging areas is provided along with the concepts at the origin of these developments. The venerable Minisci reaction allows for direct access to functionalized heterocycles. This process has lately seen an interesting renaissance and is discussed in this volume. Addition of heteroatom-centered radicals onto unsaturated systems constitutes another powerful method to construct heterocycles. Examples of such a strategy are proposed along with the formation of various heterocycles relying on homolytic substitution at sulfur, phosphorus and selenium. Additionally free-radical functionalization of reactive functional groups including isonitriles, isothiocyanates and related unsaturated systems which offer a straightforward route towards useful aromatic and non-aromatic heterocycles are discussed. Finally, as metals are able to trigger single electron transfer both in reductive and oxidative modes this provides another possibility for the synthesis of heterocycles. Significant research efforts have focused on the use of samarium, copper and other metals to access a broad variety of heterocycles in a single pot process, starting from readily available raw material. Examples and mechanistic insights are discussed by experts in this area.
The second decade of the 21st century has witnessed a resurgence in the use of radical chemistry in both methodology and total synthesis. Many contemporary advancements can be attributed to the popularization of radical generation via photoredox catalysis, which negates the last few decades' frequent use of stoichiometric or hazardous reagents in radical reactions. With these new techniques comes advanced understanding of the underlying radical redox mechanisms as well as new and unique questions. Due to the complexity inherent in the design of successful radical reactions, it is imperative to develop thorough mechanistic understanding. Herein our group has elucidated the mechanistic behavior of three distinct radical systems. The synthesis of a variety of phenanthridines and amides has been completed via radical addition to isonitriles catalyzed by the dimeric gold photocatalyst [Au2(dppm)2]Cl2. This catalyst has further enabled radical clock studies and has been used to determine the rate constant of primary radical addition to biphenyl isonitriles. The photoredox generation of chlorine atoms for HAT has been undertaken with the iridium polypyridyl complex [Ir(dF(CF3)ppy)2(dtbbpy)]Cl, with results supporting a tuning of HAT selectivity by solvent-chlorine complexation. Finally, mechanistic data suggest that single electron transfer from a base in DMSO catalyzes an unorthodox etherification of phenols.