Protonation and ring closure of stereoisomeric α-substituted cinnamic acids in superacidic media studied by ¹³C NMR spectroscopy and computations

Five α-substituted cinnamic acids [(E)- and (Z)-2,3-diphenyl-, (E)- and (Z)-3-(2-methoxyphenyI)-2-phenyl- and (E)-2-(2-methoxyphenyl)-3-phenyl-propenoic acids] have been protonated in fluorosulfonic acid at -78°C. Protonation of the carboxylic group and a second protonation on the methoxy group at -78°C or the ring bearing the methoxy group at 0°C have been observed by ¹³C NMR spectroscopy. Upon protonation (Z)-α-phenylcinnamic acid is transformed to a protonated indenol derivative. Dehydrative ring closure begins at -78°C and goes to completion at 0°C. Similar transformations of the other studied Z-acid are suppressed by the deactivating effect of the protonated methoxy group. Only protonation has been observed for the E-acids at -78°C as well as 0°lculations at the HF/3-21G level provide the equilibrium structures of the corresponding cations. Results of IGLO/¹³C NMR shift calculations are in good agreement with the experimental findings.