Visible-Light-Induced Photoreduction of Carborane Phosphonium Salts: Efficient Synthesis of Carborane-Oxindole-Pharmaceutical Hybrids

Visible-light-induced photoreaction of carboranes is an effective approach to prepare carborane-containing compounds. While several methods involving boron-centered carboranyl radicals have been established, those for carbon-centered carboranyl radicals are underdeveloped, except for the UV-light-promoted photohomolysis. Herein, we describe a simple but effective approach to access carbon-centered carboranyl radicals by photoreduction of carborane phosphonium salts under blue light irradiation without using transition metals and photocatalysts. The utility of the method was demonstrated by successfully preparing a range of carborane-oxindole-pharmaceutical hybrids by radical cascade reactions. Computational and experimental studies suggest that the carbon-centered carboranyl radicals are generated by single-electron transfer of the photoactive charge-transfer complexes between the salts and the additive potassium acetate.