The microenvironment at electrochemical interfaces plays a crucial role in governing electrode-mediated electron transfer processes. However, elucidating the complex effects of the microenvironment remains challenging. The Butler-Volmer equation has been used in deducing reaction mechanisms and identifying rate-determining steps, but its empirical nature makes it challenging to deduce the molecular-level picture of interfacial electron transfer processes. By contrast, the application of the Marcus-Hush-Chidsey (MHC) electron transfer theory has been constrained by its tenuous connection to experimentally measurable parameters beyond reaction rates. Here we develop a mechanistic framework based on the MHC theory to systematically analyse the cation effect on the Au-catalysed CO2 reduction reaction using experimentally accessible variables. Our analysis reveals consistent trends for both inorganic and organic cations through thermodynamic and kinetic parameters derived from the MHC theory, with potential applications for probing ionomer-electrode interface microenvironments. This study establishes a universal strategy for investigating interfacial microenvironments in electron transfer processes by bridging theoretical parameters with experimental descriptors.
Piperidine and pyridine are nitrogen heterocyclic motifs prominently used in pharmaceuticals and consequently of great importance. The direct reduction of planar pyridines into piperidines with high sp3-carbon content is highly attractive as this offers a route to producing high-value products and broadening the structural space. However, direct hydrogenation of pyridines with homogeneous catalysts is challenging due to their aromatic stability and catalyst-poisoning abilities. Here we describe a robust and selective iridium(III)-catalysed ionic hydrogenation of pyridines to corresponding functionalized piperidines. Important highly reduction-sensitive groups, including nitro, azido, bromo, alkenyl and alkynyl, are inert, enabling access to a broad range of multisubstituted piperidines in high yields, substantially expanding the available chemical space for this relevant scaffold. The method requires low catalyst loadings, is scalable to decagrams and delivers the most synthetically valuable free secondary amines as easily isolable and stable piperidinium salts. Applied in a complex late-stage setting, the pyridine motif in several FDA-approved drugs was successfully and selectively hydrogenated.
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