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Methods to synthesize diverse collections of sub- stituted piperidines are valuable due to the prevalence of this heterocycle in pharmaceutical compounds. Here, we present a general strategy to access N-(hetero)arylpiperidines using a pyridine ring-opening and ring-closing approach via Zincke imine inter- mediates. This process generates pyridinium salts from a wide variety of substituted pyridines and (heteroaryl)anilines; hydro- genation reactions and nucleophilic additions then access the N- (hetero)arylpiperidine derivatives. We successfully applied high- throughput experimentation (HTE) using pharmaceutically relevant pyridines and (heteroaryl)anilines as inputs and developed a one-pot process using anilines as nucleophiles in the pyridinium salt-forming processes. This strategy is viable for generating piperidine libraries and applications such as the convergent coupling of complex fragments. 

Opening and reclosing pyridines enables selective halogenation at the 3-position of the ring. 

Radical couplings of cyanopyridine radical anions represent a valuable technology for functionalizing pyridines, which are prevalent throughout pharmaceuticals, agrochemicals, and materials. Installing the cyano group, which facilitates the necessary radical anion formation and stabilization, is challenging and limits the use of this chemistry to simple cyanopyridines. We discovered that pyridylphosphonium salts, installed directly and regioselectively from C–H precursors, are useful alternatives to cyanopyridines in radical–radical coupling reactions, expanding the scope of this reaction manifold to complex pyridines. Methods for both alkylation and amination of pyridines mediated by photoredox catalysis are described. Additionally, we demonstrate late-stage functionalization of pharmaceuticals, highlighting an advantage of pyridylphosphonium salts over cyanopyridines. 

Fluoroalkyl groups profoundly affect the physical properties of pharmaceuticals and influence virtually all metrics associated with their pharmacokinetic and pharmacodynamic profiles. Drug candidates increasingly contain CF3 and CF2H groups, and the same trend in agrochemical development shows that the effect of fluoroalkylation translates across human, insect, and plant life. New fluoroalkylation reactions have undoubtedly stimulated this uptake; however, methods that directly convert C–H bonds into C–CF2X (X = F or H) groups in complex drug-like molecules are rare. For pyridine, the most common aromatic heterocycle in pharmaceuticals, only one approach, via fluoroalkyl radicals, is viable for pyridyl C–H fluoroalkylation in the elaborate structures encountered during drug development. Here, we have developed a set of bench-stable fluoroalkylphosphines that directly convert the C–H bonds in pyridine building blocks, drug-like fragments, and pharmaceuticals into fluoroalkyl derivatives. No pre-installed functional groups or directing groups are required; the reaction tolerates a variety of sterically and electronically distinct pyridines and is exclusively selective for the 4-position in most cases. The reaction proceeds via initial phosphonium salt formation followed by sp2-sp3 phosphorus ligand-coupling, an underdeveloped manifold for C–C bond formation. 

Abstract Methods to synthesize alkylated pyridines are valuable because these structures are prevalent in pharmaceuticals and agrochemicals. We have developed a distinct approach to construct 4‐alkylpyridines using dearomatized pyridylphosphonium ylide intermediates in a Wittig olefination‐rearomatization sequence. PyridineN‐activation is key to this strategy, andN‐triazinylpyridinium salts enable coupling between a wide variety of substituted pyridines and aldehydes. The alkylation protocol is viable for late‐stage functionalization, including methylation of pyridine‐containing drugs. This approach represents an alternative to metal‐catalyzedsp²‐sp³cross‐coupling reactions and Minisci‐type processes. 

Abstract A pyridine–pyridine coupling reaction has been developed between pyridyl phosphonium salts and cyanopyridines using B₂pin₂as an electron‐transfer reagent. Complete regio‐ and cross‐selectivity are observed when forming a range of valuable 2,4′‐bipyridines. Phosphonium salts were found to be the only viable radical precursors in this process, and mechanistic studies indicate that the process does not proceed through a Minisci‐type coupling involving a pyridyl radical. Instead, a radical–radical coupling process between a boryl phosphonium pyridyl radical and a boryl‐stabilized cyanopyridine radical explains the C−C bond‐forming step.