B-N axis as a facilitating agent for the synthesis of 3D structures: The paradigmatic case of BN-[4.4.4]propellane

Abstract

Flat aromatic compounds containing a boron–nitrogen (BN) fragment have gained significant attention in the field of organic optoelectronics. Unsurprisingly, therefore, an increasing number of synthetic methodology groups have devoted efforts and creativity to developing new strategies for accessing diverse structures incorporating a B=N unit, which is isosteric with the olefinic C=C bond. In contrast, the potential of BN isosterism to expand the structural diversity of three-dimensional architectures based on sp³-hybridized atoms remains largely underexplored. In this study, we introduce a strategy to construct an alkane-type quaternary–quaternary Bsp³–Nsp³ molecular axis via double addition of a carbon-based nucleophile/electrophile pair to a readily accessible olefin-type B=N moiety. The approach is showcased through the synthesis of a BN-[4,4,4]-propellane, in which the rapidly assembled of a tetrahydro-BN-naphthalene intermediate undergoes a polar double allylation of its embedded B=N bond. Despite the unfavorable trans preference in this addition step, efficient [4,4,4]-propellane formation was achieved through a tandem metathesis-based trans-to-cis isomerization and ring-closing reaction. The resulting BN-propellane exhibits a C₃-symmetric helical arrangement in the solid state and shows fluxional behavior in the 1H NMR spectrum at room temperature due to helicity flip, for which variable-temperature NMR measurements yielded an activation barrier (ΔG‡) of approximately 14.6 kcal·mol⁻¹.