Ionic-Liquid-Promoted Decaborane Dehydrogenative Alkyne-Insertion Reactions: A New Route to <i>o</i>-Carboranes

Unlike in conventional organic solvents, where Lewis base catalysts are required, decaborane dehydrogenative alkyne-insertion reactions proceed rapidly in biphasic ionic-liquid/toluene mixtures with a wide variety of terminal and internal alkynes, thus providing efficient, one-step routes to functional o-carborane 1-R-1,2-C2B10H11 and 1-R-2-R′-1,2-C2B10H10 derivatives, including R = C6H5− (1), C6H13− (2), HC≡C−(CH2)5− (3), (1-C2B10H11)−(CH2)5− (4), CH3CH2C(O)OCH2− (5), (C2H5)2NCH2− (6), NC−(CH2)3− (7), 3-HC≡C−C6H4− (8), (1-C2B10H11)−1,3-C6H4− (9), HC≡C−CH2−O−CH2− (10); R,R′ = C2H5− (11); R = HOCH2−, R′ = CH3− (12); R = BrCH2−; R′ = CH3− (13); R = H2CC(CH3)−, R′ = C2H5− (14). The best results were obtained from reactions with only catalytic amounts of bmimCl (1-butyl-3-methylimidazolium chloride), where in many cases reaction times of less than 20 min were required. The experimental data for these reactions, the results observed for the reactions of B10H13 salts with alkynes, and the computational studies reported in the third paper in this series all support a reaction sequence involving (1) the initial ionic liquid promoted formation of the B10H13 anion, (2) addition of B10H13 to the alkyne to form an arachno-R,R′-C2B10H13 anion, and (3) protonation of arachno-R,R′-C2B10H13 to form the final neutral 1-R-2-R′-1,2-C2B10H10 product with loss of hydrogen.