Solvent effects in the acid-catalysed rearrangements of 1,2-oxazines

Abstract

The rates of acid-catalyzed rearrangements of 6-ethoxy-5,6-dihydro-3-phenyl-4H-1,2-oxazines have been investigated in H2SO4/methanol, MeCN and Me2SO by means of spectrophotometric methods. Pseudo first order rate constants were obtained in each case. Results in methanol supported an A1 type mechanism. Also investigated were the correlation of reaction rates with Hammet acidity function (H0) by application of the Bunnet criterion where a slope of -0.67 was obtained. Further, the solvent effect was considered from two points of mechanistic views: the thermodynamic transfer functions of MeOH to MeCN and Me2SO where the rate was found fast in MeCN and slow in Me2SO and the Kirkwood-Buff preferential solvation with aqueous MeOH, MeCN and Me2SO. The techniques supported the proposed transition state structure.