General-Acid-Catalyzed Ring Opening of Oxazolidines. Hydrolysis of 2--N-phenyl-1,3-oxazolidine

Article abstract of DOI:10.1021/jo01299a014

A cationic Schiff-base intermediate is detectable at pH <10 in the hydrolysis of 2-<4-(dimethylamino)styryl>-N-phenyl-1,3-oxazolidine.Thus, ring opening proceeds with C-O bond breaking, i.e., in the direction which gives the most stable carbonium ion intermediate.Ring opening is hydronium ion catalyzed at low pH and only slightly affected by the protonation state of the p-dimethylamino group.From pH 5 to pH 7.5 ring opening occurs in a pH-independent reaction which probably involves unimolecular C-O bond breaking.At higher pH apparent hydroxide ion catalysis is observed in the reversible ring opening, which reflects reclosure of the ring by attack of the neighboring alkoxide ion on the iminium ion.The value of pK_eq for ring opening determined both spectroscopically and from the kinetic data is 8.05.General-acid catalysis occurs in ring opening.Proton transfer and C-O bond breaking are concerted, as shown by the Broensted coefficient α of 0.53.Concerted general-acid catalysis in these reactions is due to ease of C-O bond breaking brought about by stabilization of the developing carbonium ion in the transition state.The hydrolysis of the intermediate cationic Schiff base is pH independent in the pH range 8-13 and hydronium ion catalyzed at low pH due to protonation of the p-dimethylamino group.