Organokatalyse und C‐H‐Aktivierung treffen auf Radikal‐ und Elektronentransferreaktionen

Abstract

Radikal(isch)e Perspektive: Kürzlich publizierte „organokatalytische C‐H‐Aktivierungen“ werden neu als Basen‐vermittelte homolytische Substitutionen interpretiert. Eine Sequenz aus Addition eines Arylradikals an ein Aren, Deprotonierung (siehe Schema) und Elektronentransfer ist ein Teil der Kettenreaktion. Die Arbeiten sind damit keine konzeptionellen Durchbrüche mehr, können aber gleichwohl neue Synthesemöglichkeiten unter Nutzung der Radikal(anionen)chemie eröffnen.

1,10‐Phenanthroline‐Catalyzed Tandem Reaction of 2‐Iodoanilines with Isothiocyanates in Water

“…It could be seen that electronic effects played a significant role: 2-iodobenzenamines with electron-donating and weak electron-withdrawing groups such as methyl and chloro on the para position proceeded with good to excellent yields (entries [12][13][14][15][16][17][18][19]. However, only moderate yields could be obtained when a strong electron-withdrawing group such as 4-trifluoromethyl was present (entries 20-22).…”

“…Therefore we perceive that the reaction mechanism was different from the previously reported transition metal-catalyzed mechanism, [10b,d] and the second step might be a radical mechanism, in accordance with the mechanism of organocatalytic C À H arylation presented by Shi, Shirakawa/Hayashi and Studer. [11,13] To test this suggestion, a typical radical scavenger tetramethylpiperidine N-oxide (TEMPO) was added under the same conditions, and no trace of desired product could be obtained.…”

“…It was found that moderate to excellent yields were achieved for phenyl isothiocyanates with both electron-donating and electron-withdrawing substituents ( Table 2, entries 1-9). In general, the presence of electron-donating groups and weak electron-withdrawing groups on phenyl isothiocyanates showed excellent efficiencies (entries 1,5,6,[13][14][15]18). However, when phenyl isothiocyanates substituted by a strong electron-withdrawing group such as trifluoromethyl were used, only 55% yield was obtained (entry 8).…”

Transition Metal‐Mediated Direct CH Arylation of Heteroarenes Involving Aryl Radicals

“…It should be noted that transition metal-free approaches have also shown promising results and readers interested by this field can find excellent recent reviews on this topic. [8] We also excluded from this review any non-transition metal-mediated arylation processes involved in a radical chain mechanism with Bu 3 SnH, (Me 3 Si) 3 SiH as well as SmI 2 . This field of research has been already reviewed by Vaillard and Studer. 2 Generation of Aryl Radicals using a Reductant…”

Synthesis of 6‐Alkylated Phenanthridine Derivatives Using Photoredox Neutral Somophilic Isocyanide Insertion

“…The radical R 1 C adds to 1 to give the imidoyl radical I, which subsequently undergoes intramolecular homolytic aromatic substitution (HAS), [14] oxidation, and deprotonation to afford the phenanthridine 3. Since the intramolecular HAS is faster than intermolecular radical couplings, multiple isocyanide insertions and double alkylation are avoided in this process.…”