Facile Electrophilic Iodination of Icosahedral Carboranes. Synthesis of Carborane Derivatives with Boron-Carbon Bonds via the Palladium-Catalyzed Reaction of Diiodocarboranes with Grignard Reagents

Electrophilic diiodination reactions of icosahedral closo-1,2-C₂B₁₀H₁₂ and closo-1,7-C₂B₁₀H₁₂ using 2 molar equiv of iodine monochloride in the presence of catalytic amounts of aluminum chloride yielded the corresponding closo-9,12-I₂-1,2-C₂B₁₀H₁₀ and closo-9,10-I₂-1,7-C₂B₁₀H₁₀ complexes in excellent yields. Palladium-mediated cross-coupling reactions of these diiodocarboranes with a variety of alkyl, aralkyl, and aryl Grignard reagents were reinvestigated, and it was demonstrated that the addition of copper(I) iodide as a cocatalyst is crucial to the success of this reaction. A reaction mechanism involving the initial formation of binary organocopper species (RCu; R = alkyl, aralkyl, aryl) followed by the reaction of (σ-carboranyl)palladium iodides (L₂Pd(CbI)I and L₂Pd(CbR)I; L = PPh₃, Cb = closo-1,2-C₂B₁₀H₁₀, closo-1,7-C₂B₁₀H₁₀) via a four-centered transition state is proposed. The molecular structures of closo-9,10–12-1,7-C₂B₁₀H₁₀, 1, and closo-9,10-(C₆H₅)₂-1,7-C₂B₁₀H₁₀, 10, have been determined. Crystallographic data are as follows: for 1, monoclinic, space group P2₁\c, a = 13.2719-(6) Å, b = 14.9718(6) Å, c = 13.4734(6) Å, β = 114.211(1)°, V = 2441.7(3) ų, Z = 8, ϱ_calcd = 2.15 g cm⁻³, R = 0.043, R_w = 0.064; for 10, tetragonal, space group I4₁\a, a = 9.446(2) Å, c = 39.949(8) Å, V = 3564.1(1) ų, Z = 8, ϱ_calcd = 1.10 g cm⁻³, R = 0.056, R_w = 0.063.