Structural Effects in Solvolytic Reactions. 44. Effect of Increasing Electron Demand on the Carbon-13 NMR Shifts in 1-Aryl-l-hydroxyethyl Carbocations. Deviations in the σ < sup > c+ < /sup > -ΔδC+ Plot for These Protonated Acetophenones Containing Strongly Electron-Withdrawing Substituents

Abstract

A series of meta- and para-substituted 1-aryl-l-hydroxyethyl carbocations were prepared by protonation of the corresponding acetophenones in SbFg/FSO3H/SO2ClF at -78 °C and the carbon-13 NMR shifts were measured at -70 °C. A plot of A5C+ shifts against the σc+ constants exhibits deviations from linearity for electron-demanding substituents. The deviation is attributed to more shielding of the cationic carbon center than that estimated by the σc constants. Previously, such shielding effects were observed for side-chain conjugated benzene derivatives containing electron-demanding substituents. For example, the carbonyl carbon atom in substituted acetophenones becomes more shielded on substitution with electron-demanding substituents. This was attributed to inductive π-polarization of the conjugating side chain. Accordingly, the present study establishes that the deviations observed for the 13C shifts of the carbonyl carbon atom in acetophenones with electron-demanding substituents persist in the protonated acetophenones (1-aryl-l-hydroxyethyl carbocations). © 1983, American Chemical Society. All rights reserved.