Routes to 3-bromoflavanone from erythro-2′-hydroxychalcone dibromide: spectral and kinetic evidence for the dominating role of elimination–addition sequences via(E)- and (Z)-α-bromo-2′-hydroxychalcones

Abstract

Rate coefficients for the formation from erythro-2′-hydroxychalcone dibromide of (E)- and (Z)-α-bromo-2′-hydroxychalcone and for their cyclisation to 3-bromoflavanone in 4 : 1 water–ethanol at pH 7.88 are established by a combination of kinetic and spectrophotometric measurements. The E-isomer is formed in a yield (%) of 35 ± 2 as opposed to 63 ± 8 for the Z-isomer. The Z-isomer cyclised over 20 times faster. Direct cyclisation of the dibromide, if any, is only a very minor route to 3-bromoflavanone. The implications for the elimination mechanism of the preference for syn- over anti-elimination, of the independence of rate of buffer (N-ethylmorpholine) concentration, and of the effect of pH change are briefly considered. Mechanisms discounted are E2 with N-ethylmorpholine or solvent molecules as base, and E1. No firm assignment is possible amongst a number of other mechanisms.