59346-70-2Relevant articles and documents
Halogenation of cubane under phase-transfer conditions: Single and double C - H-bond substitution with conservation of the cage structure
Fokin,Lauenstein,Gunchenko,Schreiner
, p. 1842 - 1847 (2007/10/03)
The first highly selective C - H chlorination, bromination, and iodination of cubane (1) utilizing polyhalomethanes as halogen sources under phase-transfer (PT) conditions is described. Isomeric dihalocubanes with all possible combinations of chlorine, bromine, and iodine in ortho, meta, and para positions were also prepared by this method; m-dihalo products form preferentially. Ab initio and density functional theory (DFT) computations were used to rationalize the pronounced differences in the reactions of 1 with halogen (Hal·) vs carbon-centered trihalomethyl (Hal3C·) radicals (Hal = Cl, Br), For Hal3C radicals the C - H abstraction pathway is less unfavorable (ΔG?298 = 21.6 kcal/mol for C3C· and 19.4 kcal/mol for Br3C· at B3LYP/6-311+G**//B3LYP/6-31G**) than the fragmentation of the cubane skeleton via SH2-attack on one of the carbon atoms of 1 (ΔG?298 = 33.8 and 35.1 kcal/mol, respectively). In stark contrast, the reaction of 1 with halogen atoms preferentially follows the fragmentation pathway (ΔG?298 = 2.1 and 7.5 kcal/mol) and C - H abstraction is more unfavorable (ΔG?298 = 4.6 and 12.0 kcal/mol). Our computational results nicely agree with the behavior of 1 under PT halogenation conditions (where Hal3C· is involved in the activation step) and under free-radical photohalogenation with Hal2 (Della E. W., et al. J. Am. Chem. Soc. 1992, 114, 10730). The incorporation of a second halogen atom preferentially in the meta position of halocubanes demonstrates the control of the regioselectivity by molecular orbital symmetry.
Crystal and molecular structures of 1,4-dibromo- and 1,4-diiodocubane
Kawai, N. T.,Gilson, D. F. R.,Britten, J. F.,Butler, I. S.,Farrell, P. G.
, p. 910 - 914 (2007/10/02)
The crystal structure of 1,4-dibromocubane has been determined by X-ray diffraction.The crystal is monoclinic, P21/n, a = 6.878(1), b = 6.960(7), c = 8.793(2) Angstroem, β = 113.08(2) deg, Z = 2, R = 0.055.The C-Br bond length, at 1.926 Angstroem, is shorter than usual and the carbon framework is probably distorted by longer bonds along the edges.The direct dipole coupling constants between protons, obtained from the partially oriented NMR spectrum, show that a distortion of the proton positions from the ideal cube occurs in the bromo compound but not in diiodocubane, presumably because of the higher strain energy in the former. Key words: halocubanes, crystal structures, NMR spectroscopy, molecular modelling calculations.
