The Structures of Alkyl Derivatives of arachno‐1‐CB4H10 from Reactions of B4H10 with Alkynes

New insight into the cleavage of a CC triple bond by the reactive borane intermediate {B4H8} under very mild conditions (70 °C) is provided by the structure of 1, the smallest known arachno-carborane. Comparison of experimental data with the results of ab initio/IGLO/NMR studies prove that 1 is the product isolated in low yields from the gas-phase reaction of B4H10 with MeCCH. Reactions with EtCCH and MeCCMe gave analogues of 1, but the parent compound 2,5-μ-CH2-1-CB4H8 was not accessible.