General-Acid-Catalyzed Imidazolidine Ring Opening. Hydrolysis of Symmetrical and Unsymmetrical 1,3-Imidazolidines of p-Dimethylaminocinnamaldehyde

Article abstract of DOI:10.1021/ja00529a032

Rate constants have been obtained for ring opening of a series of symmetrical and unsymmetrical 1,3-imidazolidines of p-dimethylaminocinnamaldehyde in H2O at 30 deg C.Ring opening of the N,N'-diphenyl derivative is catalyzed by hydronium ion (k_H = 2290 M^-1 s^-1), and gives rise to a cationic Schiff base with λ_max 505 nm.The reaction is considerably slower in D2O than in H2O, k_H/k_D = 3.0.At pH greater than 6 ring opening is pH independent (k₀' = 1.8 <*> 10^-2 s^-1).Ring opening of the N,N'-dimethylimidazolidine to a species with λ_max 480 nm is hydronium ion catalyzed (k_H = 2 <*> 10⁹ M^-1 s^-1) and pH independent at pH values above 11.5.The unsymmetrical N-isopropyl-N'-phenyl derivative opens to give a species with λ_max 480 nm and with rate constants that are similar to those for the N,N'-dimethyl substituted compound (k_H = 4 <*> 10⁷ M^-1 s^-1).Consequently, this species must be the N-isopropyl Schiff base resulting from breaking of the C-N phenyl bond.General acid catalysis of ring opening was observed in trimethylamine buffer.Only at pH values less than 6 does C-N isopropyl bond breaking become competitive, giving the N-phenyl Schiff base (λ_max 512 nm).The interconversion of Schiff bases (480 -> 512 nm) is general acid catalyzed by buffer acids or a kinetic equivalent.Thus, ring-opening reactions of imidazolidines have been directly monitored, and general acid catalysis has been observed.It can be concluded that, in reactions of the neutral species and hydronium ion, or a general acid, the imidazolidine ring opens preferentially to give the most stable carbonium ion with expulsion of the least basic nitrogen.In the reaction of the unsymmetrical imidazolidine at low pH when there are two protons in the transition state, either C-N bond may break, and the C-N phenyl Schiff base is the favored product.These results are discussed in relation to reactions of N⁵,N¹⁰-methylenetetrahydrofolic acid.