Transition-Metal-Promoted Reactions of Boron Hydrides. 11. Catalytic and Stoichiometric Alkyne Insertion Reactions. Synthesis and Structural Characterization of a (Metallovinyl)carborane Containing a Boron-Boron Bridging Carbon Atom: nido-4,5-μ--...

Article abstract of DOI:10.1021/ja00184a029

The use of transition-metal complexes to promote or catalyze the two-carbon insertion of an acetylene into the small carborane nido-2,3-Et2C2B4H6 is reported.It was found that the reaction of Na(1+)<2,3-Et2C2B4H5>(1-) with 2-(CH3)2C2)>(1+)BF4(1-) in THF results in the formation of the first (metallovinyl)carborane complex, nido-4,5-μ-1:μ-trans-MeC=CMe)>-2,3-Et2C2B4H5 (I).The structure of I was confirmed by means of a single-crystal X-ray study, which showed that the carborane is bound to the olefin via a B-C-B three-center, two-electron bond with the cage situated trans with respect to the iron atom.I is thus the first structurally characterized example of a polyhedral boron cage compound containing a boron-boron bridging carbon atom.Crystal data for I: space group Pca2₁; Z=4; a=16.697(3), b=10.887(3), c=18.015(4) Angstroem; V=3274.8 Angstroem³.The structure was refined by full-matrix least-squares to a final R of 0.047 and R_w of 0.052 for the 1767 unique reflections that had F₀² > 3?(F₀²).Subsequent thermolysis of I at 110 deg C produced the two-carbon insertion product nido-4,5-Me2-7,8-Et2C4B4H4 (II) in 80percent yield presumably via a β-hydride-abstraction alkene-elimination mechanism.Catalytic dehydro alkyne insertion reactions resulting in the fromation of nido-4,5-Me2-7,8-Et2C4B4H4 were observed when either trinuclear or mononuclear ruthenium complexes, including Ru3(CO)12, Ru3(CO)9(PPh3)3, and Ru(CO)3(PPh3)2, were used to catalyze the reactions of 2-butyne with nido-2,3-Et2C2B4H6.The hydroboration product 5-<(H)MeC=CMe)>-2,3-Et2C2B4H5 was also observed in reactions promoted by Ru3(CO)12; however, the phosphine-substituted complexes exhibited > 98percent selectivity toward the insertion product.