Highly Antiaromatic Naphtho-1,2-Diboretes as Precursors for Tunable, Unsymmetrical 1,4-Diboraanthracenes

The four-electron reduction of doubly IMes- and SIMes-stabilized 2,3-bis(dibromoboryl)naphthalene (IMes = 1,3-dimesitylimidazol-2-ylidene; SIMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene, mesityl = 2,4,6-trimethylphenyl) yields the first N-heterocyclic carbene (NHC)-stabilized naphtho[2,3-<i>c</i>][1,2]diboretes <b>3a</b> and <b>3b</b>, which were isolated as dark-blue and red solids, respectively, and analyzed by nuclear magnetic resonance (NMR) and ultraviolet-visible (UV-vis) spectroscopy as well as X-ray crystallography. The synthesis of <b>3b</b> is systematically accompanied by NHC ring-expanding boron insertion side reactions, absent in that of <b>3a</b>. Both experimental and computational data show that unlike their biradical cyclic alkyl(amino)carbene (CAAC)-stabilized counterparts, the 1,2-diborete rings in <b>3a</b> and <b>3b</b> are closed-shell 4π-antiaromatic compounds. The differences between the IMes and SIMes derivatives lie in the additional delocalization of the π-electron density to one of the SIMes ligands in <b>3b</b>, which results in an unsymmetrical conformation with lower B═B double bond character and a more planar C₂B₂ ring. Reactions of <b>3a</b> and <b>3b</b> with terminal alkynes (HC≡CR; R = H, TMS (= SiMe₃), <i>p</i>EP (= <i>p</i>-(HCC)C₆H₄), Fc (= ferrocenyl)) yield the first examples of aromatic 1,4-diboraanthracenes (1,4-DBAs) (<b>6a/b-R</b>) by insertion of the alkyne CC unit into the strained diborete B═B bond. These compounds are highly colorful (green-purple), quasi-planar, and aromatic. While most internal alkynes do not react with <b>3a</b> or <b>3b</b>, reactions with the highly electron-deficient alkyne DMAD (= dimethyl acetylenedicarboxylate) also yield the corresponding yellow-colored 1,4-DBA. Furthermore, the parent 1,4-DBA <b>6a-H</b> undergoes a Diels-Alder reaction with DMAD to yield a naphtho[2,3-<i>c</i>][1,4]diborabarrelene, while <b>6a/b-</b><i><b>p</b></i><b>EP</b> undergoes a 1,4-addition of the alkynyl C-H functionality of a second equivalent of HC≡C(<i>p</i>EP).