Palladium-catalyzed coupling of ethynylated p-carborane derivatives: Synthesis and structural characterization of modular ethynylated p-carborane molecules

Methodology leading to a new class of rodlike p-carborane derivatives is described, involving the palladium-catalyzed coupling of B-iodinated p-carboranes with terminal alkynes. The products of these reactions contain an alkyne substituent at a boron vertex of the p-carborane cage. Reaction of closo-2-I-1,12-C2B10H11 (1) with closo-2-(C=CH)-1,12-C2B10H11 (3) in the presence of pyrrolidine and catalytic quantities of bis(triphenylphosphine)palladium dichloride and cupric iodide yields 1,2-(closo-1',12'-C2B10H11-2'-Yl)(2) acetylene (4). Oxidative coupling of 3 in the presence; of cupric chloride in piperidine affords 1,4-(closo-1',12'-C(2)B(10)H(1)1-2'-yl)(2)-1,3-butadiyne (5). Reaction of 2 molar equiv. of closo-2,9-I-2-1,12-C2B10H10 (6) with closo-2,9-(C=CH)(2)-1,12-C2B10H10 (7) in the presence of pyrrolidine and catalytic quantities of bis(triphenylphosphine)palladium dichloride and cupric iodide yields closo-2,9-(closo-2'-I-9'-C=C-1',12'-C2B10H10)(2)-1,12-C2B10H10 (8), a rigid, iodine-terminated carborod trimer in which the p-carborane cages are linked at the 2 and 9 B-vertices by alkyne (C=C) bridges. The molecular structures of 5 and the previously described closo-2,9-(C=CSiMe(3))(2)-,12-C2B10H10 (9) have been determined by X-ray crystallography. Crystallographic data are as follows: for 5, monoclinic, space group P2/a, a = 12.352(6) Angstrom, b = 14.169 (6) Angstrom, c = 12.384(5) Angstrom, beta = 109.69(2)degrees, V = 2041 Angstrom(3), Z = 4, R = 0.098, R(w) = 0.135; for 9, monoclinic, space group C2/m, a = 22.111(4) Angstrom, b = 7.565(2) Angstrom, c = 6.943(2) Angstrom, beta = 107.871(8)degrees, V = 1105 Angstrom(3), Z = 2, R = 0.059, R(w) = 0.090.