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

Abstract

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.

Na 13C supstituentska hemijska pomeranja (SHP) ugljenikovih atoma u nezasićenom lancu 3-metilen-4-supstituisanih-1,4-pentadiena su primenjeni principi linearne korelacije slobodne energije. Korelacije SHP sa supstituentskim parametrima Swain-a i Lupton-a pružaju usaglašenu sliku elektronskih efekata u ovim jedinjenjima. Koncept elektronskih efekata može u potpunosti biti objašnjen modelom koji se zasniva na direktnom prenošenju efekata supstituenata kroz prostor (direktni efekti polja kroz prostor) i preko konjugacionih interakcija (rezonancioni efekti), ili putem supstituentom izazvane polarizacije ¶-sistema u nezasićenom lancu (efekti ¶-polarizacije). MNDO-PM3 semiempirijska izračunavanja upućuju na s-cis konformaciju 3-metilen-4-supstituisanih-1,4-pentadiena kao konformaciju sa minimalnom energijom obrazovanja.