Carbon Insertion into arachno-6,9‑C2B8H14 via Acyl Chlorides. Skeletal Alkylcarbonation (SAC) Reactions: A New Route for Tricarbollides

Reactions between arachno-6,9-C2B8H14 (1) and selected acyl chlorides, RCOCl, in the presence of PS (PS = “proton sponge”, 1,8-dimethylamino naphthalene) in CH2Cl2 for 24 h at reflux, followed by in situ acidification with concentrated H2SO4 at 0 °C, generate a series of neutral alkyl and aryl tricarbollides 8-R-nido-7,8,9-C3B8H11 (2) (where R = CH3, 2a; C2H5, 2b; n-C4H9, 2c; C6H5, 2d; 4-Cl-C6H4, 2e; 4-Br-C6H4, 2f; 4-I-C6H4, 2g; 1-C10H7, 2h; and 2-C10H7, 2i). The best yields were achieved for aryl derivatives (80–95%) while the yields of the corresponding alkyl substituted compounds are lower (60–70%). These skeletal alkylcarbonation (SAC) reactions are consistent with an aldol-type condensation between the RCO group and open-face hydrogen atoms on the dicarbaborane 1, which is associated with the insertion of the carbonyl carbon atom into the structure of arachno-6,9-C2B8H14 (1) under elimination of three extra hydrogen atoms as H2O and HCl. The reactions thus result in an effective R–tricarbaborane cross-coupling. Individual compounds of structure 2 have been purified by chromatography on a silica gel support, using hexane as the mobile phase (RF = ∼0.3). Deprotonation agents, such as NEt3, NaOH, NaH, etc., convert tricarbaboranes 2 into the corresponding conjugated anions [8-R-nido-7,8,9-C3B8H10]− (2–) which were isolated as salts with suitable countercations (for example, Et3NH+, Tl+, NEt4+, etc.). The compounds have been characterized by multinuclear (11B, 1H, and 13C) NMR spectroscopy, mass spectrometry, and elemental analyses. The structures of anions [8-R-nido-7,8,9-C3B8H10]¯ (where R = C6H5, 4-I-C6H4 and 1-C10H7; 2a–, 2g–, and 2h–) and that of the neutral 8-(1-C10H7)-nido-7,8,9-C3B8H11 (2h) have been established by X-ray diffraction analyses.