A BN-Isostere of a Porous Organic Polymer Using Azabora-Spirobifluorenes

Porous organic polymers that can be processed using organic solvents are attractive materials for a range of applications. Switching a CC unit for an isoelectronic BN unit is a powerful method to modulate the key properties of organic polymers; however, this approach is underexplored in the field of soluble porous organic polymers. To access isostructural CC/BN porous organic polymers, the synthesis of azabora-spirobifluorenes (BN-SBF) functionalized with methoxy/bromo groups was developed. While the use of 9-bromo-9-borafluorene (1) in pyridine-directed electrophilic borylation enabled the synthesis of a dimethoxy-substituted BN-SBF, this method could not be extended to access a tetramethoxy-BN-SBF derivative. The latter compound, while accessible via a lithiation/transmetalation route, was incompatible with various ether cleavage protocols. A BN-SBF-containing polymer instead was synthesized using a dibromo-functionalized BN-SBF monomer, which also could be synthesized via pyridine-directed electrophilic borylation using compound 1. The BN-SBF-based polymer proved to have comparable porosity to its CC analogue; however, it and the CC analogue did not form robust free-standing membranes. Nevertheless, this work highlights the feasibility of accessing a BN isostere of an established solution-processable porous organic polymer.