Two Heads are Better than One：Weak Bonds Joint Effects Catalyze the Cleavage of Strong C-C Bond of Lignin-Inspired Compounds and Lignin in Air by Ionic Liquids.

Oxidation of lignin through selective C-C bond cleavage via metal-free and mild strategies is promising but challenging to value-added aromatics. We discovered that cation of ionic liquids (ILs) could effectively catalyze this kind of strong bond cleavage by forming multiple weak hydrogen bonds, enabling the reaction conducted in air at temperature lower than 373 K without metal-containing catalysts. The cation (1-propylronitrile-3-methylimidazolium) [CPMim] + affords the highest efficiency in C-C bond cleavage, in which high yields (>90%) of oxidative products are achieved. [CPMim] + could form three ipsilateral hydrogen bonds with oxygen of C=O and ether bond at both sides of C-C bond. The weak bonds joint effects could promote adjacent C-H bond cleave to form free radicals, and thereby catalyze the strong C-C bond fragment. This work opens up an eco-friendly and energy-efficient route for direct valorization of lignin by enhanced IL's property via tuning cation.