General-acid-catalyzed imidazolidine ring opening. Hydrolysis of symmetrical and unsymmetrical 1,3-imidazolidines of p-dimethylaminocinnamaldehyde

Abstract

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 "C. Ring opening of the N,N'-diphenyl derivative is catalyzed by hydronium ion ( k~ = 2290 M-' s-I), and gives rise to a cationic Schiff base with A, , , 505 nm. The reaction is considerably slower in D 2 0 than in H20, k H j k D = 3.0. At pH greater than 6 ring opening is pH independent (ko' = 1.8 X s-'). Ring opening of the 1'V.N'-dimethylimidazolidine to a species with A, , , 480 nm is hydronium ion catalyzed ( k~ = 2 X IO9 M-' s-l) and pH independent at pH values above 11.5. The unsymmetrical N-isopropyl-"-phenyl derivative opens to give a species with A, , ,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 (A, , , 512 nm). The interconversion of Schiff bases (480 -51 2 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.480 nm and with rate constants that are similar to those for the N,N'-dimethyl substituted compound ( k~ = 4 X IO7 M-I s-l ).

Segmented Poly(urethane-urea)s Synthesized Directly from Isocyanate-Terminated Prepolymers and Masked Diamines II. Kinetic and Mechanistic Study on Masking and Demasking Reaction

“…5 It should be noted that 820 the rate and catalyst constants with respect to the ketimine formation reactions and the reverse reactions in those equations may be regarded as those of the dehydration reactions and the hydrolysis reactions of carbinolamine intermediates, respectively by ab initio MO method. 6 The rates of decrease in the concentrations of PDA and acetone can be expressed in eq I and 2, respectively, considering the asymmetrical structure of PDA.…”

Fluorinated Pyrimidines and Their Nucleosides

“…Studies with model systems supported this preliminary indication. The kinetics of the ring opening were studied by using unsymmetrically N-substituted 2-(p-dimethylaminostyryl)-1,3-imidazolidines as models for the unsymmetrical imidazolidine ring of 5,10-CH,H4PteClu (125). It was found with these compounds that concerted general acid-catalyzed ring opening occurs to give the most stable iminium cation (corresponding to the N-5 position on the cofactor) by way of a mechanism involvingprotonation on the least basic nitrogen (N-10) and expulsion of the most acidic protonated amine (126).…”

Einfache Pyrrolsynthese durch überraschend leichte 1,3‐Verschiebung in N‐Benzyl‐N‐(2‐benzylamino‐cycloprophyl)‐N‐benzylidenammoniumionen

“…2 HBr rnit 2 mol eines aromatischen Aldehyds (6) in Methanol oder Ethanol in Gegenwart von 3 mol Natriumacetat erhalt man nach 15 h (im Falle von (6d) in 3 d) bei 20-25°C die 2-Arylpyrrole (7) 20-25 oc 2 Nach der Rontgen-Strukturanalyse eines 1,2,4,6-Tetraaryl-1,4-dihydropyrazins liegt der zentrale sechsgliedrige Ring in einer flachen Boot-Konformation vor; allerdings erscheint die cyclische Konjugation durch das Abknicken…”