Field-induced π-polarization in barrelene derivatives: a computational study based on structural variation

Substituent Effect Study for 13Cβ SCS in a Meta-Styrene Series. Reynolds Dual Substituent Parameter Model Investigation

This article investigates the transmission of the meta-substituent, X, effect to the carbon β (C_β) of 17 different styrenes (X-C₆H₄-CH═CYZ) using the literature ¹³C substituent chemical shift (SCS) of C_β. The ¹³C SCSs of C_β were modeled by Reynolds substituent field σ_F and resonance σ_R constants. The model is considered a dual substituent parameter model, SCS = ρ_Fσ_F + ρ_Rσ_R. The coefficients of determination of the modeled ¹³C SCS for the 17 series fall between 0.9878 and 0.9983. The π-polarization concept was used to rationalize the field and resonance effects felt at C_β. The ρ_F values of the meta-series were all lower than the para-series ρ_F values. The substituent field effect transmits to C_β from C_ipso. The substituent resonance effect transmits from C_ipso to C_β initially through the π-bond to C_ortho and C_para, next to the side chain, and then the generated charge will induce the vinyl group via π-polarization. The ρ_F felt at C_β was almost double the ρ_R when compared with the short distance between C_meta and C_β, as opposed to C_ortho and C_para. Stabilization of the π-polarization of the vinyl group will increase, with the field and resonance effects felt at C_β and vice versa.