2,5-Bridged 1-Carba-arachno-pentaborane(10) Derivatives – Intermediates on the Way to Pentaalkyl-1,5-dicarba-closo-pentaboranes(5)

Stimulated by a report from (R. Koster, G. Benedikt, M. A. Grassberger, Liebigs Ann. Chem. 1968, 719, 187.) on a carbaborane, for which the structure of 2,3,4,5,-tetraethlyl-1-methyl-2,5-mu-(ethyl-1', 1'-diyl)-1-carbaarachno- pentaborane(10) 1 was proposed, an independent synthesis was developed. Diethyl(ethynyl) borane was generated in situ in the presence of a large excess of "Et2B-H" (hydride bath) by ethynyl/H exchange, using ethynyl(trioctyl) tin to react in a mixture with diethyl(vinyl) borane at low temperature. Hydroboration and Et2B-H- catalysed condensation afforded 1, as shown by B-11, C-13 NMR spectroscopy and DFT calculations. Thermal degradation of 1 gave 2,3,4-triethyl-1,5-dimethyl-1,5-dicarba- closo-pentaborane(5), whereas the pentaethyl-1,5- dicarba-closo-pentaborane(5) resulted from thermally induced decomposition of other suitably substituted arachno carbaboranes.