Hydration of the Flavylium Ion. 3. The Effect of 3-Alkyl Substitution

Article abstract of DOI:10.1021/jo00350a046

An investigation is reported of the transformations undergone by four flavylium salts-two 3-methyl-substituted cations, the 3-methylflavylium ion, and the 4'-methoxy-3-methylflavylium ion-and two cations where an ethylene unit bridges the 3-position and the 2'-position of the phenyl ring, the 5,6-dihydrobenzoxanthylium ion, and the 3-methoxy-5,6-dihydrobenzoxanthylium ion.Three transformations have been identified and studied-a rapid and reversible hydration of the cations to produce a pseudobase, the rapid reversible ring opening of this pseudobase to a (Z)-chalcone, and the slow irreversible cis-trans isomerization of this species to an (E)-chalcone.Rate and equilibrium constants have been obtained for these processes and compared with values previously obtained for 3-H-substituted flavylium ions, the parent flavylium ion itself, and the 4'-methoxyflavylium ion.There is little effect of the alkyl substitution on the pseudobase: (Z)-chalcone equilibration.The ethylene-bridged systems undergo cis-trans isomerization at a similar rate to the 3-H systems, while the 3-methyl systems undergo this reaction about 30 times more slowly.The most significant effect, however, is seen on the flavylium ion hydration equilibrium, the bridged cations being about 2 orders of magnitude more stable than their 3-H-substituted counterparts with these in turn 2 orders of magnitude more stable than the 3-methyl analogues.To explain this, it is proposed that there is a steric interaction involving the 3-hydrogen or 3-methyl substituent and an ortho hydrogen on the nearby phenyl ring, resulting in a twisting of this ring so that it is not coplanar with the benzopyrylium portion of the cation.