Ring fusion isomers of dithienoborepins: perturbations of electronic structure, aromaticity, and reactivity in boron-containing polycyclic heteroaromatics

Abstract

Through a combination of rational design and synthetic serendipity, two new structural isomers of the dithienoborepin (DTB) architecture have been realized. Unlike previous members of this family, these boron-containing polycyclic aromatics are unsymmetrical with respect to the fusion orientation of the central borepin and flanking thiophene rings. Characterization of the unsymmetrical dithienoborepins through spectroscopic, crystallographic, electrochemical, and computational methods reveal that the electronics, aromaticity, and chemical reactivity can differ subtly (as for DTB 3) or in dramatic fashion (DTB 4) throughout the series, as dictated by the nature of the unique π-conjugation in each structure. Synthetic elaboration of the three most closely related DTB isomers (1, 2, and 3) into π-extended derivatives by Pd-catalyzed cross-couplings revealed an enhancement in their inherent electronic property differences, demonstrating the feasibility of such an approach for molecular property tuning.