Pyridine‐Functionalized Covalent Organic Frameworks as Metal‐Free Photocatalysts for Chlorotrifluoromethylation of Alkenes via Inner‐Sphere Mechanism

The photocatalytic chlorotrifluoromethylation of alkenes using trifluoromethanesulfonyl chloride represents an important difunctionalization method. However, the recyclable and stable heterogeneous catalysts have been rarely reported. Herein, two pyridine-functionalized isomeric covalent organic frameworks (denoted as COF-25Th and COF-36Th) with distinct conjugation modes are synthesized and serve as metal-free photocatalysts to achieve chlorotrifluoromethylation of alkenes via an inner-sphere mechanism. The linearly conjugated COF-25Th exhibits enhanced planarity compared to the cross-conjugated COF-36Th, endowing COF-25Th with superior optoelectronic properties. Therefore, COF-25Th displays exceptional photocatalytic chlorotrifluoromethylation activity, achieving a maximum 99% conversion rate for various substrates, along with excellent recyclability. Experimental results and theoretical calculations reveal that the pyridine nitrogen in COF-25Th acts as a binding site for trifluoromethanesulfonyl chloride, mediating the formation of the electron-deficient pyridinium salt that establishes an inner-sphere electron transfer pathway. This work develops a metal-free heterogeneous COF catalyst for efficient chlorotrifluoromethylation of alkenes based on an inner-sphere mechanism.