Preparation of [closo-CB11H12]- by Dichlorocarbene Insertion Into [nido-B11H14]-

In strongly basic media, the [nido-B11H14]- anion reacts with the haloforms. Dehydrogenation to [closo-B11H11]2- is the only reaction observed with iodoform. With chloroform and bromoform, the cage is expanded by dihalocarbene insertion. The dominant products are the [closo-CB11H12]- and the [2-Br-closo-CB11H12]- anion, respectively. The chief side product is the [closo-B11H11]2- anion, which results from dehydrogenation of the starting material. It was identified by 11B NMR spectroscopy and isolated after acidic aqueous workup in the form of the [nido-7-OH-B11H13]- anion. Since the starting [nido-B11H14]- anion is available from NaBH4 and BF3·Et2O in 50% yield, its conversion to [closo-CB11H12]- with chloroform and base in a 40% yield represents a useful laboratory route to the numerous known but previously very expensive derivatives of [closo-CB11H12]-, highly prized for their very low nucleophilicity.