Uncover the strength of polyhedral dicarbaboranes as the next generation of superbases in organocatalysis

Abstract

Although boron compounds appear in many chemical transformations, it is hard to imagine, these usually Lewis acidic, species would act as Lewis bases or even base-catalysts. Here we show that the adduct o-2, formed by the reaction of closo−1,2-dicarbadecaborane(10) with two equivalents of an N-heterocyclic carbene, exhibits superbasic properties with proton affinity surpassing that of the parent carbene as well as commercial superbases. It reacts selectively with C-acids, solvents and water, with its reactivity strongly influenced by substrate pKA. o-2 facilitates various Lewis base-catalyzed transformations—including aldol condensation, the Thorpe reaction, Michael addition, Robinson annulation and β-nitrostyrene polymerization—with exceptional efficiency and selectivity. Moreover, o-2 effectively catalyzes highly diastereoselective conjugate additions of 4-substituted pyrazolones to electron-poor olefins and spirocyclizations of pyrazolones with dibenzylideneacetones. A notable tolerance of catalyst to significant amounts of water and base-sensitive functional groups has also been observed. The authors synthesize a compound derived from a carborane bearing N-heterocyclic carbene, which exhibits superbasic properties with proton affinity surpassing that of the parent carbene as well as commercial superbases. The base facilitates various Lewis base-catalyzed transformations, including aldol condensation, the Thorpe reaction, Michael addition and Robinson annulation, with high efficiency and selectivity.