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Quantification of the Push−Pull Effect in Tolanes and a Revaluation of the Factors Affecting the 13 C Chemical Shifts of the Carbon Atoms of the C⋮C Triple Bond

2006; American Chemical Society; Volume: 71; Issue: 10 Linguagem: Inglês

10.1021/jo060199j

1520-6904

Erich Kleinpeter, Anja Schulenburg,

Crystallography and molecular interactions

Variously substituted tolanes were employed to show that the push−pull effect is also active in C⋮C triple bonds by the successful correlation of the occupation quotient π*/π of the π orbital in resonance with the substituted phenyl moieties of tolanes versus the bond length of the C⋮C triple bond. In addition, the influences of the ortho phenyl ring substituents on the 13C chemical shifts of the triple bond carbon atoms, which were estimated by Rubin et al.4 to be "inapplicable for describing triple bond polarization", were re-evaluated, leading to the conclusion that, while anisotropic effects of ortho substituents are negligible, the steric ortho-substituent effects do in fact dominate the deviations obtained. A detailed theoretical NBO/NCS study has been employed to illuminate the facts of this case.