Rotational Isomerism Involving an Acetylenic Carbon II : Effect of 1-Halogen Substituents on Rotamer Population and Rotational Barrier around C(sp)-C(sp3) Bonds in Bis(9-triptycyl)ethynes

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  • Shinji Toyota
    Department of Chemistry, Faculty of Science, Okayama University of Science Ridaicho, Okayama, 700-0005
  • Takao Yamamori
    Department of Chemistry, Faculty of Science, Okayama University of Science Ridaicho, Okayama, 700-0005
  • Toshiaki Makino
    Department of Chemistry, Faculty of Science, Okayama University of Science Ridaicho, Okayama, 700-0005
  • Michinori Oki
    Department of Chemistry, Faculty of Science, Okayama University of Science Ridaicho, Okayama, 700-0005

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<jats:title>Abstract</jats:title> <jats:p>A series of (1,4-dimethyl-9-triptycyl)(1-halo-9-triptycyl)ethynes were synthesized to examine the effect of halogen substituents on the rotational isomerism about C(sp)-C(sp3) bonds. The kinetic and thermodynamic data of the isomerization between the ap and sc rotamers were determined by the total lineshape analysis and the intensity of the 1H NMR signals, respectively. The rotational barriers are enhanced with the steric size of halogen atoms from 11.6 (F) to 17.3 (I) kcal mol-1, and there is a linear correlation between the barrier height and van der Waals radius of the 1-substituent. Although large halogen atoms decrease the ratio of the sterically congested sc form, its population tends to be larger than that expected from the steric effect. This finding is attributable to the C-H…X(halogen) interactions between the 1-Me and X groups in the sc form. The possibility of the weak interactions is discussed on the basis of molecular structures optimized by the MM2 calculations.</jats:p>

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