The kinetics and mechanism of the cyclisation of some 2′-hydroxychalcones to flavanones in basic aqueous solution

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Abstract
Rate coefficients for the chalcone–flavanone equilibration reaction have been established for some 2′-hydroxychalcones over the pH range (ca. 8–11) in which their 2′-hydroxy-groups undergo ionisation. The chalcones studied were the parent 2′-hydroxychalcone (I) and its following derivatives : 4′-OMe (II), 6′-OMe (III), 4′-OH (IV), 4′,6′-Me2(V), and 5′,6′-benzo (VI). Pseudo-first-order rate coefficients (kobs) which, for the reversible reactions concerned, are the sum of the forward and reverse rate coefficients, were fitted to the kinetic form Kobs=kfA+k′fB+k″aOH– in which k and k′ are the rate coefficients for the unimolecular cyclisation of neutral and of ionised chalcones respectively (fA and fB are the fractions of total chalcone present in the neutral and ionised forms at the pH concerned) and where k″ is the second-order rate coefficient for the reverse reaction of flavanone which involves hydroxide ion (activity aOH–). Data analysis gave rate coefficients and pKa values for all but chalcone (IV) which has a different kinetic form. A conjugate addition–elimination mechanism is proposed to account for the pH–rate profiles, one of which [chalcone (I)] differs from that previously reported. Possible effects on rate coefficients of non-bonded interactions between 6′-substituents and carbonyl oxygen are briefly considered but the differences between chalcones with and without 6′-substituents are sufficiently small to deter prolonged discussion. There seems to be little serious hindrance of cyclisation for any of the 6′-substituted chalcone anions.

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