KINETIC STUDY OF THE FORMATION AND RUPTURE OF STABLE TETRAHEDRAL INTERMEDIATES. C-O, C-N and C-S BOND FORMATION

Article abstract of DOI:10.1002/poc.610070205

Measured pseudo-first-order rate constants for intramolecular formation of tetrahedral intermediates from N-2-hydroxyethylphthalimide, N-2-aminoethylphthalimide and N-2-thioethylphthalimide at pH > 6 are reported.The reaction is specific and general base catalysed, with β(Bronsted) values 0.44, 0.52 and 0.52 respectively.From a plot of log k_b (general base rate constants) vs γ' , β'_nuc values of 0.01 (with OH^- as specific base), 0.25 (with imidazole as general base) and 0.27 (with HPO4^2- as general base were obtained).The observed relationships p_xy = υβ'_nuc/ - υpK_a = - υβ(Bronsted)/υγ' = 0.03 is supported by the predictions of an energy contour diagram, which, on extrapolation to a non-stable tetrahedral intermediate, predicts a late and slightly protonated transition state for the cleavage process.At pH < 3, these intermediates cleave to yield only the corresponding diacylimides.These reactions are general base and acid catalysed with β > 0.3 and α < 0.1.A fast equilibrium between the intermediate and its N-protonated (amide) form is reached.The general base rupture of the latter is faster than that of the corresponding non-protonated intermediate by a factor of ca 1E9 - 1E10-fold.