Mechanochemical Synthesis of Amides with Uronium-Based Coupling Reagents: A Method for Hexa-amidation of Biotin[6]uril

Solvent-free, atom-efficient, and mechanochemically activated reactions have emerged as synthetic strategy for sustainable chemistry. Herein we report a new mechanochemical approach for the amide coupling of carboxylic acids and amines, mediated by combination of (1-cyano-2-ethoxy-2-oxoethylidenaminooxy)-dimethylaminomorpholinocarbenium hexafluorophosphate (COMU) or N,N,N',N'-tetramethylchloroformamidinium hexa-fluorophosphate (TCFH) and K2HPO4. The method delivers a range of amides in high yields (70-96%) and fast reaction rates. The reaction protocol is mild, keeps stereochemical integrity of the adjacent to carbonyl stereocenters, and streamlines isolation procedure for solid amide products. Minimal waste is generated due to the absence of bulk solvent. We show that K2HPO4 plays a dual role, acting as a base and a precursor of reactive acyl phosphate species. Amide bonds from hindered carboxylic acids and low-nucleophilic amines can be assembled within 90 minutes by using TCFH in combination with K2HPO4 or N-methylimidazole. The developed mechanochemical liquid-assisted amidation protocols were successfully applied to the challenging couplings of all six carboxylate functions of biotin[6]uril macrocycle with phenylalanine methyl ester, resulting in 80% yield of highly pure hexa-amide-biotin[6]uril. In addition, fast and high-yielding synthesis of peptides and versatile amide compounds can be performed in a safe and environmentally benign manner, as verified by green metrics.