Zn Microenvironment Engineering for Aryl-C(sp2) Cleavage to Phenols and Tertiary Amines.

Abstract

Cleaving inert aryl-C(sp²) bonds present a fundamental catalytic challenge due to their intrinsic robustness, yet it is essential for valorizing lignin into value-added chemicals. Current strategies predominantly convert aromatic rings to phenols at the expense of side-chain carbon utilization, compromising the atom economy. We report zinc coordination microenvironment engineering to achieve efficient aryl-C(sp²) cleavage in phenolic substrates with transformable side chains (e.g., ketones and alkenes), concurrently yielding phenols and tertiary amines. A nitrogen-coordinated zinc catalyst (ZnNC-900) delivered 90.0% phenol and 82.7% N,N-dimethylethylamine from 4-methoxyacetophenone. Significantly, this catalytic system demonstrates broad activity toward multiple native lignin sources, with bamboo lignin processing on a 5.0 g scale affording 1.04 g of alkyl-free phenolics (20.8 wt %) and 0.67 g of N,N-dimethylethylamine (13.4 wt %). Mechanistic studies via spectroscopy and DFT calculations demonstrate that the zinc microenvironment dictates the reaction pathway. Specifically, the ZnNC-900 catalyst, featuring zinc coordinated by three pyrrolic-N atoms and one pyridinic-N atom, reduces the energy barrier by 0.71 eV relative to ZnNC-600 (which exclusively has pyridinic-N coordination). This work provides a catalytic strategy to overcome the limitations of low atom economy, harsh conditions, and narrow substrate scope in lignin valorization.