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

摘要

The principles of linear free energy relationships were applied to the ~(13)C substi-tuent chemical shifts (SCS) of the carbon atoms in the unsaturated chain of 3-methyle-ne-4-substitiited-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 π-system in the unsatiirated chain (π-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.