From propenolysis to enyne metathesis: tools for expedited assembly of 4a,8a-azaboranaphthalene and extended polycycles with embedded BN

Abstract

The synthesis of BN-containing molecules, which have an interesting isosteric relationship to their parent all-C cores, has drawn a great deal of attention as an avenue to alter and tune molecular function. Nevertheless, many cores with embedded BN are still hard to synthesize, and thus, further effort is required in this direction. Herein, we present an integrated approach to BN-containing polycycles rooted in an exceptionally clean B–N condensation of amines with a tri-allylborane. Having released propene as the only byproduct, the resulting BN precursors are seamlessly telescoped into BN-containing polycyclic cores via a set of additional methodologies, either developed here ad-hoc or applied for the first time for the synthesis of BN-cycles. As the “sharpening stone” of the process, BN-embedded naphthalene, which has previously only been obtained in low yield, can now be synthesized efficiently through propenolysis, ring-closing metathesis and a new high-yielding aromatization. As a more advanced application, an analogously obtained BN-containing bis-enyne is readily converted to BN-containing non-aromatic tetra-, penta- and hexacyclic structures via ring-closing enyne metathesis, followed by the Diels–Alder cycloaddition. The resulting air-sensitive structures are easily handled by preventive hydration (quaternization) of their B–N bridge; reverting this hydration restores the original B_sp²–N_sp² structure. In the future, these structures may pave the way to BN-anthracenes and other π-extended BN-arenes.