Boron-containing aromatic compounds: synthesis, characterization and reactivity
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AbstractBoron-containing derivatives of barrelene and benzobarrelene were generated in good to excellent yields by Diels-Alder reactions between Lewis base-stabilized borabenzene and strong dienophiles. The resultant Diels-Alder adducts were more stable when the Lewis base donor atom was nitrogen as opposed to phosphorus. All members of the family were found to be highly Lewis acidic, and the neutral donors which ligated the boron atom could not be removed/exchanged. Most compounds exhibited a high degree of thermal stability and underwent ill-defined decomposition only when heated to temperatures in excess of 200 °C. In selected cases the Diels-Alder reaction was found to be reversible and the free borabenzene, formed transiently through a retro-Diels-Alder reaction, could be trapped if a stronger dienophile was present in solution. When one of the barrelenes was treated with 3,6-di(2-pyridyl)-1,2,4,5-tetrazine, acetylene was removed to give the first 2,3-disubstituted borabenzene pyridine adduct. Additionally, syntheses were developed for boron-doped versions of the larger acenes: anthracene, naphthacene and pentacene. Boron-doped acenes, 9-boraanthracene, 5-boranaphthacene, and 6-borapentacene, were generated upon deprotonation of appropriately constructed precursors in which the boron atom was ligated by an Arduengo carbene (1,3-bis(2,4,6-trimethyl)imidazolidin-2-ylidene ). These molecules were more highly coloured than their corresponding all-carbon counterparts, an observable manifestation of their much smaller HOMO-LUMO gap. This decrease in HOMO-LUMO gap was found to be a result of an increase in the energy of the HOMO, and is attributable to the boron's presence in the acene's n system. Additionally, the smallest member of this family, 9-boraanthracene, was also found to react with strong dienophiles and 0 2 to form Diels-Alder adducts. During the course of this research, two stable spirocyclic boronium ions were also generated, both of which proved to be valuable starting materials for neutral boron-centred radicals.
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