A General Strategy for N-(Hetero)arylpiperidine Synthesis Using Zincke Imine Intermediates.

Methods to synthesize diverse collections of substituted 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 intermediates. This process generates pyridinium salts from a wide variety of substituted pyridines and (heteroaryl)anilines; hydrogenation 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.

A Pyridine-Pyridine Cross-Coupling Reaction via Dearomatized Radical Intermediates.

A pyridine-pyridine coupling reaction has been developed between pyridyl phosphonium salts and cyanopyridines using B2 pin2 as 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.

Pyridylphosphonium salts as alternatives to cyanopyridines in radical-radical coupling reactions.

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.