Mechanistic insights into mechanochemical oxidation of 1,1-disubstituted alkenes mediated by polymer-derived mechanoradicals

Abstract

Polymer-derived mechanoradicals generated under solid-state conditions offer a unique platform for driving chemical transformations that are difficult to achieve in solution. Here, we report a mechanoradical-mediated oxidation of 1,1-disubstituted alkenes using a polymer as the radical source and molecular oxygen as the oxidant. Ball milling of polystyrene (PS) in the presence of diarylethene (DAE) derivatives under air resulted in backbone cleavage of PS in addition to oxidative cleavage of the alkene moiety to afford the corresponding diaryl ketone (DAK) derivatives. Electron paramagnetic resonance (EPR) spectroscopy revealed the formation of oxygen-centered radical species, suggesting a reaction pathway involving the addition of mechanoradicals derived from PS to DAE, followed by reaction with molecular oxygen. The experiments using poly(methyl methacrylate) (PMMA) instead of PS gave similar results, indicating that the oxidative cleavage proceeds irrespective of the polymer species. Gel permeation chromatography with a UV detector further supported the addition of PMMA-derived mechanoradicals to DAE derivatives. Substituent effect studies showed that DAK formation occurs for DAE derivatives with both electron-donating and electron-withdrawing substituents, whereas oxirane derivatives (DAO) were observed only for derivatives with electron-withdrawing groups, reflecting substituent-dependent stability of DAO. These findings establish polymer-derived mechanoradicals as effective initiators for alkene oxidation in the solid state and demonstrate the potential of mechanochemistry as a powerful platform for elucidating radical oxidation mechanisms under solvent-free conditions.