Pyridinium Salts as Redox‐Active Functional Group Transfer Reagents

Abstract

In this Review, we highlight recent advances in the understanding and design of N‐functionalized pyridinium scaffolds as redox‐active, single‐electron, functional group transfer reagents. We provide a selection of representative methods that demonstrate reactivity and fundamental advances in this emerging field. The reactivity of these reagents can be divided into two divergent pathways: homolytic fragmentation to liberate the N‐bound substituent as the corresponding radical or an alternative heterolytic fragmentation that liberates an N‐centered pyridinium radical. A short description of the elementary steps involved in fragmentation induced by single‐electron transfer is also critically discussed to guide readers towards fundamental processes thought to occur under these conditions.

Photochemical C–H Silylation and Hydroxymethylation of Pyridines and Related Structures: Synthetic Scope and Mechanisms

“…35 A milder version of this photochemical process was recently disclosed by DiRocco, Krska and co-workers by using 4 [Ir(dF-CF3-ppy)2(dtbpy)]PF6 as the photocatalyst an benzoyl peroxide (PBO) as the terminal oxidant. 39 Based on the dual reactivity of N-alkoxypyridinium ions (NAPs), 40,41,[42][43][44][45] as alkoxy radical precursors that are good hydrogen abstractors 46,47,48 and as electrophilic partners in Minisci reactions, as previously reported by Hong and co-workers [49][50][51][52][53][54][55]56 we hypothesized that NAPs should be good partner with silanes or methanol to accomplish photocatalytic C-H silylation and hydroxymethylation, respectively (Scheme 1).…”

“…It should be noted that Lei et al 57 reported a practically simple hydroxymethylation of a large variety of quinolines and isoquinolines in the presence of substoichiometric amounts of Selectfluor ® . 41 Unfortunately, pyridines were found to be unreactive under their optimized conditions, thus, calling for an alternative approach which would be complementary to Lei's method. With this in mind, we reacted methanol (4) with different N-methoxypyridinium ions (1) under the optimized conditions described above (Figure 2).…”

Heterocyclic Group Transfer Reactions with I(III) N-HVI Reagents: Access to N-Alkyl (Heteroaryl)onium Salts via Olefin Aminolactonization

“…The breadth of available transformations make pyridinium salts valuable templates for accessing functionalized 6-membered aza-heterocyclic scaffolds, which are prevalent in agrochemicals, alkaloid natural products, and are the most commonly encountered heterocyclic motif in FDA approved small molecule drugs 19 . In addition to manipulations of the heterocyclic ring, pyridinium salts can undergo ring openings to produce Zincke aldehydes, which have shown utility as synthetic building blocks 20 , and 2,4,6-triphenylpyridinium salts have emerged as a new and powerful class of radical precursors for deaminative metal-catalyzed cross couplings ( Figure 1b) 21,22 .…”

Investigation of Straightforward, Photoinduced Alkylations of Electron‐Rich Heterocompounds with Electron‐Deficient Alkyl Bromides in the Sole Presence of 2,6‐Lutidine

“…The reactivity of N-substituted pyridinium salts for reductive fragmentation, as postulated in the mechanism, was studied in detail elsewhere and was outlined recently in a review. [30] Next, we conducted a series of optimization and control experiments to prove the necessity of individual reaction components. As already pointed out, in the dark no product formation was observed.…”