Synthesis of Carboranylated Dihydropyrrolo[1,2-a]quinoxalines and Dihydroindolo[1,2-a]quinoxalines by BF₃·OEt₂-Catalyzed Heterocyclization of C-Formyl-o-carboranes and Investigation of Their Oxidation Stability

A BF3·OEt2-catalyzed synthesis of carboranylated dihydropyrrolo[1,2-a]quinoxalines and dihydroindolo[1,2-a]quinoxalines in 30–99% yields is presented through the heterocyclization of various C-modified C-formyl-o-carboranes with 1-(2-aminophenyl)-pyrroles/indoles. A systematic comparative investigation of their oxidation stability in air confirmed that 4-carboranyl-4,5-dihydropyrrolo[1,2-a]quinoxaline had better stability than the 4-phenyl analogue. A cage-deboronation reaction for N-acetyl-substituted carboranylated dihydropyrrolo[1,2-a]quinoxaline produced the corresponding 7,8-nido-carborane cesium salt. A kinetic resolution was also realized to obtain an optically pure carboranylated N-heterocycle scaffold bearing a carborane cage carbon-bonded chiral stereocenter.