Synthesis of Boroxine and Dioxaborole Covalent Organic Frameworks via Transesterification and Metathesis of Pinacol Boronates

Boroxine and dioxaborole are the first and some of the most studied synthons of Covalent Organic Frameworks (COFs). Despite their wide application in the design of functional COFs over the last 15 years, their synthesis still relies on the original Yaghi’s condensation of boronic acids (with itself or with polyfunctional catechols), some of which are difficult to prepare, poorly soluble, or unstable in the presence of water. Here we propose a new synthetic approach to boroxine COFs (based on transesterification of pinacol aryl boronates (ArBpin) with methyl boronic acid (MBA) and dioxaborole COFs (through the metathesis of pinacol boronates with methylboryl-protected catechols). The ArBpin and MBA-protected catechols are easy to purify, highly soluble, and bench-stable. Furthermore, kinetic analysis of the two model reactions reveals high reversibility (Keq~1) and facile control over the equilibrium. Unlike the conventional condensation which eliminates water by-products, the by-product of the metathesis (MBA pinacolate) allows for easy kinetic measurements of the COF formation by conventional 1 H NMR. We show the generality of this approach by synthesis of seven known boroxine/dioxaborole COFs whose crystallinity is better or equal to those reported by conventional condensation. We also apply metathesis polymerization to obtain two new COFs, Py4THB and B2HHTP, whose synthesis was previously precluded by their insolubility and hydrolytic instability, respectively, of the boronic acid precursors.