Substituent Control of Intramolecular Hydrogen Bonding in Formyl-Protonated o-Anisaldehydes: A Stable Ion and Semiempirical MO Investigation

Article abstract of DOI:10.1021/jo00058a018

o-Anisaldehyde and its 5-Br, 5-F, 5-CF3, 5-CN, 5-NO2, and 5-COMe derivatives are protonated at the formyl group in 1 : 1 SbF5-FSO3H/SO2 (or SO2ClF) to give persistent carboxonium ions as mixtures of Z and E geometrical isomers.The cyano, nitro, and acetyl substituents are also protonated, leading to dications and additional geometrical isomers in the nitro and acetyl cases.The carboxonium ions are predominantly in the Z,syn configuration, but with increased amounts of the E,anti configuration with increased electron withdrawal by the substituents.With 5-NO2H(+), Eisomers become more abundant than Z.The formyl protonated 2-(trifluoromethoxy)benzaldehyde shows a strong preference for the E configuration.The preference for the Z,syn form is attributed to intramolecular hydrogen bonding that becomes less favorable as electron density is withdrawn from the methoxyl oxygen.The log Z/E values correlate with differences in energy content of the isomers predicted in AM1 calculations, and the chemical shift of the hydroxyl proton of the carboxonium group correlates well with predicted charge on the proton.