Ionic-Liquid-Promoted Decaborane Dehydrogenative Alkyne-Insertion Reactions: A New Route to o-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)(2)NCH2- (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.