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Methyl-camouflaged dicarbaboranes closo-1,2- and 1,10-H2C2B8Me8 have been prepared in high yields either from nido-5,6-H2C2B8H10 or closo-1,2-H2C2B8H8 via electrophilic methylation reactions and cl...

Alkylation of the [nido-5,6-R12C2B8H9]− anions (where R1 = H and Me) with alkyl halides (RX, where R = primary and secondary alkyls) in boiling tetrahydrofuran (THF) proceeds via unusual H2 elimination, followed by cage closure to give a series of the neutral closo-1,2-R12C2B8H7-3-R derivatives in ∼70–80% yields. In contrast, treatment of the unsubstituted [nido-5,6-C2B8H11]− anion with tert-butyl bromide (t-BuBr) led to the formation of the parent closo-1,2-C2B8H10 in >85% yield. The constitution of all compounds isolated has been confirmed unambiguously by multinuclear (11B, 1H, and 13C) nuclear magnetic resonance measurements and α-shift correlation assessments.

The principles of a new α-shift correlation (ASC) NMR method are demonstrated on a model series of substituted derivatives of o-carborane for which reliable NMR data are available. This graphical method revealed an acceptable linear correlation between α(11B) or α(13C) shifts and those induced by substituents in unsubstituted (u) positions of the carborane cluster. The linearity holds for all nuclei involved in skeletal bonding: Δδ(N)u = g × α (where N = 11B, 13C, and 1H). The factor g (slope of the correlation line × 102) becomes an important measure of sensitivity of a given cage position to substituent changes. The β, γ, and δ = A (= antipodal) shifts can be therefore derived from the α-shift, are linearly proportional, and reflect additive character in double substitution. The ASC method appears to be an important tool for quantitative assessment of substituent NMR effects in all exo-substituted boron-cluster systems.

Initial studies on electrophilic halogenation of the dicarbaborane closo-1,2-C2B8H10 (1) have been carried out to reveal that the substitution takes place at B7 and B10 vertexes, which are the most removed from the CH positions. The course of the halogenation is strongly dependent on the nature of the halogenation agent and reaction conditions. Individual reactions led to the isolation of the monosubstituted compounds 1,2-C2B8H9-10-X (2) (where X = F, I) and 1,2-C2B8H9-7-X (3) (where X = Cl, I). Disubstituted carboranes 1,2-C2B8H8-7,10-X2 (4) (where X = Cl, Br, I) were obtained under more forcing conditions. Individual halo derivatives were characterized by mass spectrometry and high-field NMR (11B, 1H,13C) spectroscopy combined with two-dimensional [11B–11B]-COSY, 1H{11B(selective)}, and [11B–1H]-correlation NMR techniques. All of the derivatives bearing a halogen substituent in the B10 position exhibit a remarkable antipodal 13C and 1H NMR shielding at the CH1 vertex, increasing in the order H < I < Br ...

Methyl-camouflaged dicarbaboranes closo-1,2- and 1,10-H

Reported is an unusual tautomeric behavior within the [nido-5,6-C2B8H11]− (1a–) cage that has no precedence in the whole area of carborane chemistry. Isolated were two skeletal tautomers, anions [6-Ph-nido-5,6-C2B8H10-μ8,9]− (2d–) and [5,6-Me2-nido-5,6-C2B8H9-μ9,10]− (3b–), which differ in the positioning of the open-face hydrogen bridge. Their structures have been determined by X-ray diffraction analyses. The 3b–structure is stabilized by intermolecular interaction involving Et3NH+ and B8–B9 and H8 atoms in the solid phase; however, its dissolution in CD3CN causes instant conversion to the more stable [5,6-Me2-nido-5,6-C2B8H9-μ8,9]− (2b–) tautomer. The dynamic electron-correlation-based MP2/6-31G* computations suggest that the parent [nido-5,6-C2B8H11-μ8,9]− (2a–) tautomer is 3.9 kcal·mol–1 more stable than the [nido-5,6-C2B8H11-μ9,10]− (3a–) counterpart and the μ8,9 structure 2– is therefore the most stable tautomeric form in the solution, as was also demonstrated by multinuclear (1H, 11B, and 13C) NMR ...

Initial studies on electrophilic halogenation of the dicarbaborane closo-1,2-C

Reported is an unusual tautomeric behavior within the [nido-5,6-C

Sequential methylation of arachno-6,9-C2B8H14 (1) led to a series of methyl derivatives and finally to the camouflaging of all boron positions by mixed persubstitution. Thus, deprotonation of 1 produced the [arachno-6,9-C2B8H13] anion (1(-)), the methylation of which with MeI in tetrahydrofuran proceeded on the open-face boron vertexes with the formation of 5-Me-arachno-6,9-C2B8H13 (2; yield 28%) and 5,8-Me2-arachno-6,9-C2B8H12 (3; yield 36%). Observed in this reaction was also a side formation of 2-Me-closo-1,6-C2B8H9 (4; yield 6%).The electrophilic AlCl3-catalyzed CH3(+) attack of the neutral 1 in neat MeI at ambient temperature afforded 1,3-Me2-arachno-6,9-C2B8H12 (5), while a 76-h heating at 120 °C generated a mixture of the di- and triiodo derivatives 1,2,3,4,8,10-Me6-5,7-I2-arachno-6,9-C2B8H6 (6) and 1,2,3,4,7-Me5-5,7,10-I3-arachno-6,9-C2B8H6 (7). On the other hand, a HOTf-catalyzed reaction between 1 and MeOTf at reflux resulted in the isolation of 2-TfO-1,3.4,5,7,8,10-Me7-arachno-6,9-C2B8H6 (8; Tf = CF3SO2; yield 65%). The compounds were characterized by multinuclear ((11)B, (1)H, (13)C, and (19)F) NMR spectroscopy, mass spectrometry, and elemental analysis, and the structures of compounds 1, 1(-), 5, and 6 were established by X-ray diffraction analysis.

The reaction between arachno-4-CB 8 H 1 4 and PCl 3 in the presence of PS (PS = proton sponge = 1,8-dimethylamino naphthalene) (dichloromethane, rt, 24 h) produced the neutral phosphacarborane closo-2,1-PCB 8 H 9 (35% yield), while a similar reaction of nido-1-CB 8 H 1 2 gave the isomeric compound closo-6,1-PCB 8 H 9 (27% yield). The structures of both compounds were derived on the basis of the combined ab initio/GIAO/NMR ( 1 H, 1 1 B, 1 3 C) approach. The optimized structures at a correlated level of theory (MP2) with 6-31G* basis set were used as a basis for calculations of the 1 1 B and 1 3 C chemical shifts at GIAO-SCF/II and GIAO-MP2/II, the latter showing excellent agreement with experimental data.