13C NMR Chemical Shifts and Cationic Reactivity of Linear Conjugated Dienes

The chemical shifts of the C1 and C2 of trans-1-alkyl-1,3-butadienes were correlated with Taft’s σ*, the proportionality constants being −35.9 and 12.3 ppm/σ*, respectively. The values 6.2 and −43.6 ppm/σ* were obtained for the C1 and C2 of 2-alkyl-1,3-butadienes, respectively. The observed chemical shifts of disubstituted butadienes roughly agreed with the values estimated from those of monosubstituted butadienes by means of additivity relationship. The cationic reactivity of linear conjugated dienes should be determined by the magnitude of the steric restriction of substituents and that of the stability of the resulting cation.