Escape from Flatland: Stereoselective Synthesis of Hexa-aryl Borazines and their sp²-Based 3D Architectures

Borazine and its derivatives can be considered critical doping units for engineering hybrid C(sp2)-based molecules with tailored optoelectronic properties. Herein, we report the first synthesis of hexaarylborazines that, bearing ortho-substituted aryl moieties, extend three-dimensionally. Using a one-pot protocol, we first form an electrophilic chloroborazole and then react it with an aryl lithium (ArLi). By selecting the appropriate ortho-substituent, we can guide the ArLi to add to the BN-core in a specific way, ultimately controlling the stereochemical outcome of the three-substitution reaction. Rationalization of the stereochemical model through computational analysis allowed us to show that when aryl lithium nucleophiles bearing rigid long-range ortho-substituents are used, i.e., stiff substituents. The ortho-substituent shields its side of the electrophilic B3N3 core, biasing the incoming ArLi to add anti at each addition step, forming the final tri-aryl borazine exclusively as cc-isomer. Leveraging this stereoselective approach, prototypical multichromophoric borazine derivatives were prepared, and we showcased how the stereochemical arrangement of these chromophores distinctly influences their redox behavior. This methodology paves the way for previously inaccessible borazines to serve as privileged precursors to transcend the conventional bidimensionality associated with graphenoid systems and pioneer the construction of new forms of three-dimensional C(sp2)-based architectures.