Remote C-H Bond Functionalizations. Группа авторов

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Chapters 2–5 of this book were focused on discussing various approaches for distal arene C(sp²)–H functionalization based on directing group assisted protocols, Catellani reactions, or via arene cyclo‐ruthenation methods, in Chapter 6 Phipps and coworkers devoted their efforts in summarizing a complementary strategy for remote arene functionalization harnessing the non‐covalent interactions. Although non‐covalent interactions are prevalent in enzymatic reactions but translating such interaction in regioselective functionalization of small molecule in synthetic scale is rare. Despite the several challenges associated in controlling the site selectivity of arene functionalization, in recent years a number of elegant methods were developed by Smith, Kanai, Phipps, Chattopadhyay, and others. Phipps and his co‐authors illuminated about the emergence of non‐covalent interaction in distal arene‐C–H functionalization in Chapter 6.

Although use of directing group, transient mediator or non‐covalent interactions have been popularized in recent years to harness the regioselective transformation of arenes. However, transition metal‐catalyzed functionalization governed by the steric and/or electronic factors was cultivated over the century to mitigate the issues pertaining to the site‐selectivity. Intrinsic biasness derived from the substituted functionality present in arenes or heteroarenes is considered to be the key component in defining the selectivity. While such strategy, precludes preinstallation of directing group or circumvent the complicated catalytic path involved in NBE mediated process, but were largely limited by the nature of substrate as well as usage of excess amount of arenes. However, prudent combination of catalyst, ligand, and reagent designing has been realized in recent years to enable regioselective functionalization of arenes or heteroarene with broad functional group tolerance. An exemplified discussion of such non‐directed distal arene functionalization is made by van Gemmeren et al. in Chapter 7. The prime attention was paid in depicting the recent progresses on non‐directed distal arene functionalization, where arenes were used as limiting reagent.

      In Chapter 8, Dutta and Maiti discussed about the recent progresses in the realm of distal arene para‐C–H functionalizations. Distinction of energetically comparable C

H bonds to achieve regioselective C–H functionalization is one of prime focus of modern synthesis. In this regard, a number of strategies are known in the literature to perpetrate para‐selective functionalization. Although electronic controlled Friedel–Crafts reaction being the early examples to promote para‐C–H functionalization but this strategy is severely restricted with certain substrates and produced ortho‐functionalized product as an unavoidable side product. Thus, the propulsive thrust in establishing strategies exists, which are not dependent on the electronic properties of the targeted substrate. The use of directing group, steric governance, non‐covalent interactions, and radical initiation is cultivated to expand the scope of arene para‐C–H functionalization. Chapter 8 is aimed to provide a comprehensive and exemplified discussion on directing group assisted, steric controlled, and non‐covalent interactions promoted para‐functionalizations to enlighten the scope of para‐selective functionalizations beyond electronic control.

      Chapter 9 deals with heterocycle functionalizations at unusual positions. Heterocycles are prevalent structural core in pharmaceuticals, natural products, and agrochemicals. Regioselective C–H functionalization of heterocycles is of paramount importance as the derivatization of these heterocyclic cores can alter their inherent properties. However, C–H functionalizations of hetero‐arenes are predominantly achieved at electronically biased positions. Therefore, standing against the innate inertness to attain selective C–H functionalization at unusual positions is of paramount importance in order to enrich the repertoire of heterocyclic compounds. The ever‐expanding inquisitive minds have dedicated their efforts in finding and devising suitable methodology to promote site selective C–H functionalization of apparently inert C

H bonds present in heteroarenes. Hirano and Miura have elucidated these recent reports in Chapter 9. Recent progress on C–H functionalization of important heterocycles, namely, indole, (benzo)thiazole, pyrrole, pyridine, quinoline, and others is concisely recapitulated in Chapter 9.

      Unlike arene C(sp²)–H functionalization, aliphatic C(sp³)–H functionalization is relatively challenging due to its inherent inertness, low acidity, and overabundance with flexible long chain. Additionally, control over stereoselectivity is another important aspect to take care. Although functionalization of acidic C

H bonds adjacent to electron‐withdrawing functional group or allylic and benzylic CH bonds was exploited with electrophile, reciprocating such reactivity is impossible for remote C–H functionalization of long chain aliphatic substrates. However, the assistance from directing group enabled the delivery of functional groups at a desired position with uncompromised yield and selectivity. A vivid exemplification about the recent reports on directing group assisted remote functionalization of aliphatic substrates was presented by Li, Zhang, and Shi in Chapter 10.

      Chapter 11 by Li and Zhu articulates the recent progresses on radical initiated distal C(sp³)–H functionalizations. Intramolecular hydrogen atom transfer process has provided a synthetically useful tool to promote regioselective functionalization of aliphatic substrates. Hofmann–Loffler–Freytag (HLF) reaction was considered as the pioneering invention in this realm. Although the potential of this strategy was realized lately in 2010, when a rapid growth was witnessed to promote radical initiated distal aliphatic functionalization via hydrogen atom transfer. In Chapter 11, comprehensive summary on different methods, synthetic applicability, and mechanistic intricacies are discussed from 2010 onwards.

      Chapter 12 is devoted in discussing non‐directed functionalizations of aliphatic compounds, governed by innate reactivity. Although several challenges associated with the site selective functionalization of aliphatic substrates, constant up‐search in finding suitable protocols either by tuning the innate reactivity of particular C

H bond present in the substrate or by controlling the reagent and catalyst has led to revolutionize the modern era of aliphatic C–H functionalization. Sambiagio and Maes have summarized the recent progress on non‐directed aliphatic C–H functionalization at the remote position. Although a major part of aliphatic C–H activation was accomplished by directing group assisted strategy, Chapter 12 includes only non‐directed aspect of aliphatic distal C–H functionalization. Chapter 12 was broadly divided into two parts: (i) the reaction involving distinct formation of metal–carbon bond and (ii) the reactions occurring without the metal–carbon bond formation.

      While the sojourn through transition metal‐catalyzed distal C–H functionalization goes on in Chapters 2–12, in

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