Pyridiniumsalze als redoxaktive Reagenzien zur Übertragung funktioneller Gruppen

Abstract

In diesem Aufsatz sind die jüngsten Fortschritte beim Verständnis und Design von N‐funktionalisierten Pyridiniumsalzen als redoxaktivierte Einelektronentransfer‐Reagenzien zur Übertragung funktioneller Gruppen zusammengefasst. Anhand einer Auswahl an repräsentativen Methoden werden Reaktivität und grundlegende Fortschritte in diesem sich rasch entwickelnden Bereich diskutiert. Diese Reagenzien weisen zwei grundsätzlich divergierende Reaktionspfade auf: homolytische Fragmentierung zur Freisetzung des N‐gebundenen Substituenten als entsprechendes Radikal oder eine alternative, heterolytische Fragmentierung zur Freisetzung eines radikalischen Pyridiniumkations. Die elementaren Schritte der durch Einelektronentransfer induzierten Fragmentierung werden kritisch diskutiert.

Visible-Light-Mediated Direct Amidation of Arenes and Hetero­arenes with N-Aminopyridinium Salts

“…In the process of exploring suitable amidation radical precursors, we were intrigued by N -aminopyridinium salts, 10 which can undergo photoinitiated N–N bond cleavage to release a series of nitrogen radicals (Scheme 1c ). 11 In 2015, Studer reported the visible-light-induced coupling of N -imidopyridinium salts to arenes and heteroarenes to give a series of imidized products (Scheme 1d ). 12 Considering that N -amidopyridinium salts are easier to prepare, and that amides with free NH groups could be obtained, we developed a photocatalytic method for the synthesis of benzamide derivatives via N -centered radical (Scheme 1e ).…”

Recent Advances in Electron Donor‐Acceptor (EDA)‐Complex Reactions involving Quaternary Pyridinium Derivatives

“…The applications of C‐, N‐, and O‐centered radical intermediates were covered in their review. Later, Dagousset, Carreira, Togni, and co‐workers in 2020, penned an article on the photo‐ and transition metal‐catalyzed transformation of pyridinium salts into C‐, N‐ and O‐centered radical intermediates and its applications in organic synthesis [5a] …”

Recent Advances in Catalytic and Technology-Driven Radical Addition to N,N-Disubstituted Iminium Species

“…Following a similar activation model ( Figure 2 ), several addition sequences of radical species to N- alkoxy and N -amine azaarenium platform molecules (R 1 = OR, NR 2 ) emerged in the literature as powerful functionalization strategies showing that the regioselectivity issue highly depends on the architecture and substitution pattern of the heterocycles. This field of research was covered by very instructive recent reviews [ 17 , 18 , 19 , 20 , 21 ].…”

“…In this review, we intend to cover the recent evolution of the catalytic and technology-driven addition of radical species to iminium and azaarenium compounds having a trisubstituted nitrogen atom ( Figure 2 , R 1 ≠ H). In a complementary point of view of what has been covered in the literature, we will hereby focus on both dipole derivatives (N-R 1 with R 1 = Y − ) and N -alkyl azaarenium salt, which has never been covered in a unified fashion despite some sporadic examples being found in previous reviews on pyridine derivatives [ 17 , 18 , 19 , 20 , 21 , 22 , 23 ]. We essentially cover the literature from 2015, since the Opatz’s review has already discussed this field of research before [ 14 ].…”