A Sustainable Borrowing Hydrogen Strategy in Natural Deep Eutectic Solvents for the N-Alkylation of Amines From Alcohols

The hydrophobic natural deep eutectic solvent (NADES) thymol/DL-menthol (1:1 mol/mol) is reported here for the first time as an efficient and environmentally responsible reaction medium for the palladium-catalyzed, ligand-free N-alkylation of primary amines with benzylic alcohols. The transformation proceeds via a hydrogen autotransfer (borrowing hydrogen) mechanism under remarkably mild conditions (90°C, aerobic atmosphere), affording a broad range of secondary amines in good to excellent yields. Notably, the combination of the Pd catalyst and NADES demonstrates outstanding robustness and sustainability, allowing efficient reusing for up to seven consecutive cycles through successive addition of fresh reagents to the same medium. This study highlights the potential of hydrophobic NADESs as truly green reaction media for transition-metal-catalyzed transformations. The sustainability of the protocol is further corroborated by a quantitative evaluation of green chemistry metrics, which confirms its favorable environmental profile and overall process efficiency.