Aryl Amination with Soluble Weak Base Enabled by a Water-Assisted Mechanism
Amination of aryl halides has become one of the most commonly practiced C–N bond-forming reactions in pharmaceutical and laboratory synthesis. The widespread use of strong or poorly soluble inorganic bases for amine activation nevertheless complicates the compatibility of this important reaction class with sensitive substrates as well as applications in flow and automated synthesis, to name a few. We report a palladium-catalyzed C–N coupling using Et<sub>3</sub>N as a weak, soluble base, which allows a broad substrate scope that includes bromo- and chloro(hetero)arenes, primary anilines, secondary amines, and amide type nucleophiles together with tolerance for a range of base-sensitive functional groups. Mechanistic data have established a unique pathway for these reactions in which water serves multiple beneficial roles. In particular, ionization of a neutral catalytic intermediate via halide displacement by H<sub>2</sub>O generates, after proton loss, a coordinatively-unsaturated Pd–OH species that can bind amine substrate triggering intramolecular N–H heterolysis. This water-assisted pathway operates efficiently with even weak terminal bases, such as Et<sub>3</sub>N. The use of a simple, commercially available ligand, PAd<sub>3</sub>, is key to this water-assisted mechanism by promoting coordinative unsaturation in catalytic intermediates responsible for the heterolytic activation of strong element-hydrogen bonds, which enables broad compatibility of carbon-heteroatom cross-coupling reactions with sensitive substrates and functionality.
