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Effect of substituents on the 13C-NMR chemical shifts of 3-methylene-4-substituted-1, 4-pentadienes - Part I

Valentić Nataša V. (Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, P. O. Box 3503, YU-11120 Belgrade, Yugoslavia)
Vitnik Željko J. (Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, P. O. Box 158, YU-11001 Belgrade, Yugoslavia)
Kozhushkov Sergei I. (Institut für Organische Chemie der Georg-August-Universität Göttingen, Tammannstrasse 2, D-37077 Göttingen, Germany, Fax: ++49 551 399475)
de Majere Armin (Institut für Organische Chemie der Georg-August-Universität Göttingen, Tammannstrasse 2, D-37077 Göttingen, Germany, Fax: ++49 551 399475)
Ušćumlić Gordana S. (Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, P. O. Box 3503, YU-11120 Belgrade, Yugoslavia)
Juranić Ivan O. (Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, P. O. Box 158, YU-11001 Belgrade, Yugoslavia)

The principles of linear free energy relationships were applied to the 13C substituent chemical shifts (SCS) of the carbon atoms in the unsaturated chain of 3-methylene-4-substituted-1,4-pentadienes. Correlations of the SCS with the substituent parameters of Swain and Lupton provide a mutually consistent picture of the electronic effects in these compounds. The pattern of the electronic effects can be fully rationalized by a model based on the direct transmission of substituent effects through-space (direct through-space field effects), and via conjugative interactions (resonance effects), or by substituent-induced polarization of the ¶-system in the unsaturated chain (¶-polarization effect). Semi-empirical MNDO-PM3 calculations suggest the s-cis conformation of 3-methylene-4-substituted-1,4-pentadienes as the one with minimal heat of formation.

Keywords: [3]dendralenes, unsaturated chain carbon 13C SCS, substituent effects, reverse 13C SCS effect, MNDO-PM3 calculations