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.