Flavanoids. Part 6. The kinetics and mechanism of base-catalysed isomerisation of 3-arylideneflavanones
Base-catalysed Z→E conversion of 3-arylideneflavanones provide a unique system suitable for kinetic studies by 1H n.m.r. spectroscopy. Isomerisation studies of 10 enones in [2H5] pyridine showed that a first-order unimolecular reaction was taking place. The Hammett σ–ρ relationship is not adequate to accommodate the substituent effect on rate in the case of p- and m-nitro-3-benzylidene derivatives. The unusual case of the nitro substituent is discussed.