Mechanochemical Ring-Opening of Allylic Epoxides
Copyright © 2019 American Chemical Society. Recent successes in covalent polymer mechanochemistry might more easily be translated to stress-responsive bulk materials if easily scalable and minimalist mechanophores were identified. Epoxides represent attractive modifications of unsaturated polymer backbones for this purpose, but they suffer from limited mechanical reactivity. Here, we report that placing alkenes adjacent to cis-epoxide mechanophores along a polymer backbone results in ring-opening to carbonyl ylides during sonication, whereas epoxides with an adjacent saturated, linear alkyl chain do not. Upon release, tension-trapped ylides preferentially close to their trans-epoxides in accordance with the Woodward-Hoffmann rules. The reactivity of carbonyl ylides is exploited to tag the activated species with spectroscopic labels and to facilitate force-induced cross-linking through a reaction with pendant alcohols on co-polymers. The alkene effect is attributed to a combination of lower activation energy of the reaction (ΔG‡) and greater force-coupled change in the length as the reaction proceeds from the ground to transition state (Δx‡) relative to epoxides without the alkene. Even with alkene assistance, mechanochemical reactivity remains low; single-molecule force spectroscopy establishes that at ∼2500 pN ring-opening proceeds with a rate constant that is less than approximately 5 s-1 in toluene.
Barbee, MH; Wang, J; Kouznetsova, T; Lu, M; Craig, SL
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