RATE AND EQUILIBRIUM-CONSTANTS FOR OXAZOLIDINE AND THIAZOLIDINE RING-OPENING REACTIONS

Rate and equilibrium constants have been measured for opening and clos ing 2-[(4 dimethylamino)phenyl]-1,3-oxazolidine and -thiazolidine ring s to form iminium ions in aqueous solution at 25 degrees. Comparing ra tes of closure of the oxazolidine ring with attack of external hydroxi de ion on the iminium ion yields a ratio of rate constants of 25 for i ntra- and intermolecular attack of oxyanion nucleophiles in this endo- cyclic ring-closure reaction. This ratio is significantly less than th e factor of 10(3)-10(4) commonly observed for similar exo-cyclic ring closures and is consistent with the presence of ring strain in the end o-cyclic transition state as envisaged in Baldwin's rules. Changes in N-alkyl substituent (Me, Et, Bu, Pr-i, Bu(t) and phenyl) lead to mild (and parallel) changes in reaction rates and equilibrium constants for oxazolidine ring closure. For the corresponding thiazolidine rings, c omparison of the N-H thiazolidine derivative of benzaldehyde with N-bu tyl and N-phenyl derivatives of cinnamaldehyde shows a much larger var iation in rate constants for ring closure with N-substituent (10(8)-fo ld), but still small changes in equilibrium constants and rate constan ts for attack of hydroxide ion upon the iminium ion (<100-fold). This surprising contrast in kinetic and equilibrium behaviour is discussed in terms of non-bonded interactions in the transition state for ring-c losure reactions.