Substituent Effect on the Formation of Arylindanes by Dimerization of Ferulic Acid and its Related Compounds

Homodimerization and crossed-dimerization reactions for ferulic acid and related compounds in an acidic ethanol solution were investigated via a formal [3 + 2] cycloaddition reaction. Arylindanes, as the major products, were produced from some of the acids with hydroxy group and/or methoxy groups at the para and meta positions of the phenyl ring. The formation of arylindane required the following consecutive reactions: (I) protonation of the first produced ester to afford to a benzylic cation, (II) which reacts with another ester to afford a new benzylic cation, and (III) followed by cyclization to produce the dimer. It is suggested that the para-substituent promoted both step I and step II and the meta-substituent accelerated step III. To rationalize the selectivity of homodimerization, the stability of the benzyl cations and the specific electron distribution on the molecular surface of each ester is discussed based on the results of the molecular orbital calculations.