Carbon insertion into arachno- 6,9-C2B8H14 via acyl chlorides. Skeletal alkylcarbonation (SAC) reactions: A new route for tricarbollides

Article abstract of DOI:10.1021/ic401293g

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 H ₂SO₄ 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-C ₄H₉, 2c; C₆H₅, 2d; 4-Cl-C ₆H₄, 2e; 4-Br-C₆H₄, 2f; 4-I-C ₆H₄, 2g; 1-C₁₀H₇, 2h; and 2-C ₁₀H₇, 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-C ₂B₈H₁₄ (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 (R_F = ～0.3). Deprotonation agents, such as NEt₃, NaOH, NaH, etc., convert tricarbaboranes 2 into the corresponding conjugated anions [8-R-nido-7,8,9-C₃B₈H ₁₀]^- (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-C₃B₈H ₁₀] (where R = C₆H₅, 4-I-C ₆H₄ and 1-C₁₀H₇; 2a^-, 2g^-, and 2h^-) and that of the neutral 8-(1-C ₁₀H₇)-nido-7,8,9-C₃B₈H₁₁ (2h) have been established by X-ray diffraction analyses.