Gas-phase reactions of nido-1-methylpentaborane with propyne and 2-pentyne. Formation of B-alkyl nido and closo carbaboranes

The gas-phase reaction between 1-MeB5H8 and propyne at 230°C yields methyl derivatives of nido-2,3-dicarbahexaboranes and closo-2,4-dicarbaheptaboranes. The reaction between 1-MeB5H8 and 2-pentyne at 190°C gives analogous alkyl nido-carbaboranes (but not closo-carbaboranes). Detailed boron and proton NMR data on these products in conjunction with ab initio/GIAO/NMR computations show that the dicarbaboranes 2,4-Me2-2,3-C2B4H6 (3) and 2-Et-3,4-Me2-2,3-C2B4H5 (12) are produced in the gas-phase reactions of B4H10 with ethyne and propyne, respectively. Early work on these two compounds identified them incorrectly as the first tricarbaboranes 2,3-Me2-2,3,4-C3B3H5 and 2-Et-3,4-Me2-2,3,4-C3B3H4, respectively.