130820-49-4Relevant articles and documents
Stereoelectronic effects in ring closure reactions: the 2'-hydroxychalcone-flavanone equilibrium, and related systems
Brennan, Colin M.,Hunt, Ian,Jarvis, Terence C.,Johnson, C. David,McDonnell, Peter D.
, p. 1780 - 1785 (2007/10/02)
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 double 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 mechanism 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 reaction 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 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.
