STABLE CARBOCATION CHEMISTRY - PART 304 - PROTONATION AND RING-CLOSURE OF STEREOISOMERIC ALPHA-SUBSTITUTED CINNAMIC-ACIDS IN SUPERACIDIC MEDIA STUDIED BY C-13 NMR-SPECTROSCOPY AND COMPUTATIONS

Five alpha-substituted cinnamic acids [(E)- and (Z)-2,3-diphenyl-, (E) - and (Z)-3-(2-methoxyphenyl)-2-phenyl- and (E)-2-(2-methoxyphenyl)-3- phenyl-propenoic acids] have been protonated in fluorosulfonic acid at -78 degrees C, Protonation of the carboxylic group and a second proto nation on the methoxy group at -78 degrees C or the ring bearing the m ethoxy group at 0 degrees C have been observed by C-13 NMR spectroscop y Upon protonation (Z)-alpha-phenylcinnamic acid is transformed to a p rotonated indenol derivative, Dehydrative ring closure begins at -78 d egrees C and goes to completion at 0 degrees C, Similar transformation s of the other studied Z-acid are suppressed by the deactivating effec t of the protonated methoxy group. Only protonation has been observed for the E-acids at -78 degrees C as well as 0 degrees C, Calculations at the HF/3-21G level provide the equilibrium structures of the corres ponding cations, Results of IGLO/C-13 NMR shift calculations are in go od agreement with the experimental findings.