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-c][1,2]diboretes 3a and 3b, 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 3b is systematically accompanied by NHC ring-expanding boron insertion side reactions, absent in that of 3a. Both experimental and computational data show that unlike their biradical cyclic alkyl(amino)carbene (CAAC)-stabilized counterparts, the 1,2-diborete rings in 3a and 3b 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 3b, which results in an unsymmetrical conformation with lower B═B double bond character and a more planar C₂B₂ ring. Reactions of 3a and 3b with terminal alkynes (HC≡CR; R = H, TMS (= SiMe₃), pEP (= p-(HCC)C₆H₄), Fc (= ferrocenyl)) yield the first examples of aromatic 1,4-diboraanthracenes (1,4-DBAs) (6a/b-R) 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 3a or 3b, 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 6a-H undergoes a Diels-Alder reaction with DMAD to yield a naphtho[2,3-c][1,4]diborabarrelene, while 6a/b-pEP undergoes a 1,4-addition of the alkynyl C-H functionality of a second equivalent of HC≡C(pEP).