Remote Controlled Exchange Rates by Photoswitchable Internal Catalysis of Boronic Ester-Diol Exchange

Barsoum, David

doi:https://doi.org/10.21985/n2-bvaq-pk46

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Remote Controlled Exchange Rates by Photoswitchable Internal Catalysis of Boronic Ester-Diol Exchange

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Dynamic covalent chemistry (DCC) combines the strength and directionality of covalent bonds with the reversibility of supramolecular interactions. The formation and stability of these bonds are typically regulated by parameters such as temperature, pH, concentration, catalyst loading and light. Light is an exceptionally powerful stimulus because it can be applied non-invasively with superior spatial and temporal control. Molecules that can be reversibly switched between two states using different wavelengths of light offer a unique opportunity to remotely control DCC. Previously, photoswitches have been employed to govern the reactivity of dynamic covalent bonds via two principal strategies: (i) by rendering the dynamic bond active or inactive through light-driven valence bond tautomerization; and (ii) by tuning the reactivity of the dynamic bond with an adjacent photoswitch. Here, we present a strategy to remotely control the kinetics of dynamic covalent reactions without affecting their thermodynamics, by designing a photoswitch that modulates the reactivity of an internal catalyst. We have termed this approach photoswitchable internal catalysis (PIC). Here we report a PIC that is capable of tuning the exchange rate between boronic ester and free diol over at least 4 orders of magnitude. The design of our photoswitch consists of two key components: first, the internal catalytic nitrogen in 8-quinoline boronic ester (8-QBE, Figure 1A), which assists in the rapid exchange between boronic ester and free diol; second, an acyl-hydrazone photoswitch that bears an acidic N–H, which forms an intramolecular H-bond when positioned ortho to a basic heterocycle (Figure 1B). Unification of these two moieties yields the PIC system in Figure 1C. In its thermodynamically favored isomer, (E)-PIC, the photoswitch is in the “ON” state, in which internal catalysis from the proximal quinoline mediates rapid boronic ester exchange. Photoisomerization of the acyl-hydrazone to the “OFF” state, (Z)-PIC, results in the formation of an intramolecular H-bond between the quinoline nitrogen lone pair and the acyl-hydrazone N–H, deactivating internal catalysis, resulting in a drastically slower exchange (Figure 1C).

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