Electrophilic Methylation of Decaborane(14): Selective Synthesis of Tetramethylated and Heptamethylated Decaboranes and Their Conjugated Bases

The paper reports specific syntheses of methylated decaborane(14), nido-B10H14 (1), derivatives. The reaction of 1 with an excess of neat MeI and AlCl3 yields 1,2,3,4-Me-4-nido-B10H10 (2) essentially quantitatively when performed at room temperature. Heating the same mixture to 120 degrees C provides 1-I-2,3,4,5,7,8,10-Me-7-nido-B10H6 (3a). The formation of analogous 1-CF3SO2O-2,3,4,5,7,8,10-Me-7-nido-B10H6 (3b) is achieved by heating 1 or 2 with an excess of MeSO3CF3 in the presence of a catalytic amount of HOSO2CF3 to 120 degrees C. Compounds 2 and 3 can be deprotonated to yield the corresponding anions [1,2,3,4-Me-4-nido-B10H9](-) (2(-)), [1-I-2,3,4,5,7,8,10-Me-7-nido-B10H5](-) (3a(-)), and [1-CF3SO2O-2,3,4,5,7,8,10-Me-7-nido-B10H5](3b(-)). The structure of all the compounds isolated has been unambiguously confirmed by multinuclear (B-11 and H-1) NMR measurements, and the structures of 2(-), 3a, 3a(-), and 3b have been established by X-ray diffraction analyses. The very high volatility of 2 has made it impossible to apply X-ray diffraction in this case; therefore, its structure has been derived computationally using the ab initio/GIAO/NMR tool. DFT-based computational protocols have also outlined the reason why it is impossible to obtain an octamethyl derivative of 1 experimentally.