The Kinetics and Mechanism of the Cyclisation of Some 2'-Hydroxychalcones to Flavanones in Basic Aqueous Solution

Article abstract of DOI:10.1039/P29820001309

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 (k_obs) which, for the reversible reactions concerned, are the sum of the forward and reverse rate coefficients, were fitted to the kinetic form, k_obs=kf_A+k'f_B+k''a_OH- in which k and k' are the rate coefficients for the unimolecular cyclisation of neutral and of ionised chalcones respectively (f_A and f_B 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 a_OH-).Data analysis gave rate coefficients and pK_a 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 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.

Full text of DOI:10.1039/P29820001309

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