From Curiosity to Reliable Methodology – Organic Free Radical Chemistry

About thirty-five years ago, an important period of organic radical chemistry started which provided many exciting results. This “radical” wave was driven by findings in the areas of polymer and physical organic chemistry of radicals. The high value of free radical chemistry for organic synthesis was recognized and radical chemistry began to be applied to the synthesis of complex natural products. The knowledge gathered during that period also allowed the development of radical chemistry in neighboring fields such as biology and materials science. Indeed, radical chemistry has gained many facets! In 2012, together with Prof. C. Chatgilialoglu as coeditor, I published an “Encyclopedia of Radicals in Chemistry, Biology and Materials” (Wiley-VCH). In this four-volume encyclopedia, the wide spectrum of radical chemistry is convincingly presented across approximately 2200 pages. The title underlines that radicals are not only important in chemistry, but also in adjacent research areas, as already highlighted above. During the last decade, in particular driven by studies in the field of redox catalysis, radical chemistry has again become a highly active and worldwide intensively investigated research field (another wave!) with a large number of papers published in top class journals. Electrochemistry has found its way back to the synthetic community, and radical transition metal crossover processes have also been developed during that time. This special issue on radical chemistry in Advanced Synthesis & Catalysis shows twenty-seven top-class articles that document well the current trends in this highly active field. The special issue contains five review articles addressing electrochemistry, boron-based radical chemistry, and C−C and C−N bond forming processes. Photochemical radical reactions are in focus in various contributions. Photochemistry can be used to mediate or initiate radical reactions. When considering photochemical radical reactions, a photocatalyst is often required; this can be, for example, a recyclable perovskite or a homogenous redox catalyst, as discussed in this special issue. Radical redox catalysis can also be conducted with catalysts that operate in the absence of light in their ground states. Cu-based systems play a prominent role along these lines and several contributions of this special issue address that important area. Cu-mediated radical C−C bond formation via radical transition metal crossover processes even allow for enantioselective σ-bond formation. These important findings contributed significantly to the area of asymmetric radical chemistry over the last five years and this interesting field is also covered by this issue. Along with copper-based catalysts, environmentally benign and cheap iron-based systems have been successfully applied as catalysts in radical processes. Moreover, radical chemistry has been found to be highly valuable for the introduction of trifluoromethyl or difluoromethyl groups into π-systems, thanks to the high reactivity of perfluoroalkyl radicals. Numerous papers on radical trifluoromethylation have appeared over the past ten years and this topic is covered herein as well. Radical chemistry is well suited for mild C−X bond formation as will be documented in this issue with elegant novel approaches for C−B, C−Se and C−N bond formation. During the past decade, it was also found that in selected cases classical cross-coupling reactions can be conducted in the absence of a transition metal catalyst. Such couplings in most cases proceed via radical processes and mechanistic understanding is the key for the further development of such transformations. Contributions of this special issue are devoted to this emerging sub-field of synthetic radical chemistry. Along these lines, both direct C−H arylation and C−H heteroarylation can be achieved via radical-based methodology. Considering the latter case, the famous and valuable Minsici reaction has undergone a renaissance. One contribution of the special issue is devoted to this reaction. This special issue is dedicated to the 80th birthday of Prof. Bernd Giese – one of the key figures in the field of organic free radicals. I feel that this special issue will give a good overview on the current activities in the field. I invite everybody to read the many exciting articles. For those not active in the area, I hope that this issue will cause you to consider joining the radical research field, where many further exciting findings will surely appear during the next decade.