The synthesis of functional derivatives of the [1-CB9H10]− anion by Brellochs reaction

Reactions of decaborane with various aldehydes in alkaline media were studied. The reactions with HCOH and 2-MeOC6H4CHO give the corresponding arachno-carboranes [6-R-arachno-CB9H13]− (R = H, C6H4-2-OMe), whereas the reactions with C6H5CHO, 4-BrC6H4CHO, 4-MeCONHC6H4CHO, and 2-SC4H3CHO result in the nido-carboranes [6-R-nido-CB9H11]− (R = C6H5, C6H4-4-Br, C6H4-4-NHCOMe, 2-SC4H3). Both the arachno- and nido-carboranes can be easily oxidized with elemental iodine in an alkaline aqueous solution giving the corresponding closo-derivatives [2-R-closo-2-CB9H9]−. These closo-2-isomers, under heating in solution, undergo rearrangement to more thermodynamically favorable closo-1-isomers [1-R-closo-1-CB9H9]−. The structure of (Bu4N)[1-(4-BrC6H4)-1-CB9H9] was determined using single crystal X-ray diffraction.