Mechanochemical Reaction Development for Addition, Elimination, and Isomerization
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It is now well appreciated that coupled mechanical forces can influence the rates and outcomes of covalent chemical reactions in isolated polymers and in bulk polymeric materials. Mechanochemical reactions have been widely used in mechanocatalysis, release of small molecules and protons, biasing and probing reaction pathways, stress reporting, stress strengthening and degradable polymers. Mechanochemical reactions involve much more than simply breaking of bonds, and there exist rich opportunities for new reactions to be developed for a wide range of potential applications. In this dissertation, we report new mechanochemical reactions of three types: addition, elimination, and isomerization.Mechanical forces have been used previously to activate latent catalysts by accelerating dissociation of an inhibiting ligand, but tuning catalytic activity by force remains limited to a single demonstration of force-dependent enantioselectivity of Heck and Trost reactions. Chapter 2 describes how force affects the rate of oxidative addition, often the first step within a catalytic cycle. We study the effect of force applied to the biaryl backbone of a bisphosphine ligand on the rate of oxidative addition of bromobenzene to a ligand-coordinated palladium center. Local compressive and tensile forces on the order of 100 pN are generated using a stiff stilbene force probe. We find that a compressive force increases the rate of oxidative addition, whereas a tensile force decreases the rate, relative to that of the parent complex of strain-free ligand. Rates vary by a factor of ~6 across ~340 pN of force applied to the complexes. The applied forces exert an opposite effect on oxidative addition relative to that for reductive elimination, laying the groundwork for mechanically switchable catalysts that can be optimized for individual steps within a closed catalyst cycle. Chapter 3 demonstrates a new mechanochemical elimination reaction, namely the mechanical release of hydrogen fluoride, and its application to triggered polymer degradation. As a versatile reagent for material remodeling, hydrogen fluoride has applications in self-immolative polymers, remodeled siloxanes, and degradable polymers. The responsive, in situ generation of HF in materials therefore holds promise for new classes of adaptive material systems. We achieve the mechanochemically coupled generation of HF from 2-methoxy-gem-difluorocyclopropane (MeO-gDFC) mechanophores in polymers. Pulsed ultrasonication of a MeO-gDFC containing polymer leads to one equivalent of HF release per MeO-gDFC activation. We further quantify the mechanochemical reactivity of MeO-gDFC by single molecule force spectroscopy, and force-coupled rate constants for ring opening reach ~36 s-1 at a force of ~890 pN, 400 pN lower than is required in dialkyl gDFC mechanophores that lack the methoxy substituent. The SMFS and sonication results suggest that MeO-gDFC is a more efficient mechanophore source of HF than its 2-methoxy-gem-dichlorocyclopropane analog is of HCl, in contrast to expectations based on trends in force-free reactivity. We apply the mechanical release of HF to accelerate the degradation of a copolymer containing both MeO-gDFC (3 mol%) and an HF-cleavable silyl ether (25 mol%). The mechanochemical reaction of MeO-gDFC thus provides a mechanically coupled mechanism of releasing HF for polymer remodeling pathways that complements previous thermally driven mechanisms. Finally, in Chapter 4, we report the mechanically driven isomerization of cubane, a compound of longstanding fascination to chemists due to its structure, symmetry, and strain. The mechanical coupling is explored at three regiochemical dispositions: ortho, meta and para. In contrast to the fact that all compounds can be activated thermally, cubane is mechanically activated only when coupled at ortho positions. Through mechanical activation, cubane reacts to form a thermally inaccessible syn-tricyclooctadiene product, in comparison to cyclooctatetraene, which is observed in thermal rearrangements of cubane. Pulsed ultrasonication of such cubane-containing polymer leads to efficient isomerization (57% activation after 4 h sonication). We further quantify the mechanochemical reactivity of cubane by single molecule force spectroscopy, and force-coupled rate constants for ring opening reach ~33 s-1 at a force of ~1.55 nN, lower than required forces of cyclobutanes which are typically 1.8-2.0 nN.
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