Stereoelectronic effects in ring closure reactions: the 2′-hydroxychalcone – flavanone equilibrium, and related systems

Abstract

The 2′-hydroxychalcone (2-HOC6H4COCH=CHC6H4X)–flavanone equilibrium in trifluoroacetic acid (TFA) has been examined. The influence of substituents X on the rate of attainment of equilibrium shows that the 6-endo-trig mode of ring closure by Michael addition is disallowed, by demonstrating a negative ρ value for the reaction rate when X is varied. Reaction therefore proceeds either on the carbonyl-protonated form, which allows twisting about the 2,3 bond, its double bond character being reduced by resonance, or through direct rate-limiting protonation on the 2,3 double bond. Either pathway permits the allowed 6-exo-trig mode of ring closure to be followed. Alternative mechanisms involving intermolecular Michael addition of trifluoroacetate, followed by intramolecular 6-exo-tet displacement are considered. Such Michael adducts can be detected in the ring closures of 2-crotonyl-4-methylphenol and 4,4-dimethyl-1-(2-hydroxyphenyl)-2-penten-1-one in TFA, but they do not appear to lie on the main pathway, because the reactions proceed with equal facility in methanesulphonic acid/chloroform medium, which does not contain a suitable nucleophile for such a mechanism. Further important mechanistic information is given by studying the reactions in TFA-d, together with measurements on the (E)-2-methyl-3-oxo-5-arylpent-4-en-2-ol and 3′-methoxychalcone systems. These isotope effect studies, which yield kH/kD values of about 3, indicate that the proton is in flight during the rate-limiting step, and provide evidence against the mechanism involving a preequilibrium carbonyl protonation, such as in the Nazarov rearrangement of 3′-methoxychalcones, where kH/kD is ca. 0.7. Some results are also reported for ring closure of the 2-aminochalcones in TFA. Keywords: 2′-hydroxychalcones, flavanones, 3′methoxychalcones, 3-aryl-6-methoxyindanones, 2′-aminochalcones, 2-aryl-1, 2, 3, 4-tetrahydroquinolines, Baldwin's rules.