Long-range substituent effects in 13C chemical shifts of arylcarbenium ions. Evidence of large substituent–substituent interactions between remote groups

Abstract

For dinuclear arylcarbenium ions of the type p-XC₆H₄·G·C₆H₄Y-p(protonated 1,1-diphenylethylenes, benzophenones, and trans-chalcones)¹³C chemical shifts were used to examine the transmission of substituent effects across the bridge G. For monosubstituted ions (Y = H), linear relationships are obtained between the long range substituent chemical shifts (s.c.s.) observed at the para′-position of the remote unsubstituted ring and the para-s.c.s. of monosubstituted benzenes. The fact that similar correlations are also observed for the para-position of the X-substituted ring but not for the carbenium site, suggests that the bridge G acts only as a relay in the transmission of electronic effects, with an efficiency increasing with the electron demand of the carbenium centre. For disubstituted ions (Y ≠ H) the X-induced s.c.s. at the para′-position largely depend on the nature of the Y groups. It is established that these s.c.s. are linearly related to the shifts observed in the corresponding monosubstituted ions. These substituent–substituent interactions between the remote X and Y groups can be satisfactorily accounted for by using the proportionality relationship (ΔσC-4′)_X^Y=b_Y(ΔσC-4′)_X^H with the b_Y values previously proposed for cationic para-substituted benzenes. This treatment makes it possible to divide the observed long-range s.c.s. into three independent terms which characterize the magnitude of the structural perturbation, the efficiency of the relay by the bridge G, and the susceptibility of the site of observation of the s.c.s. to its environment.