Cyclopropanediamines. 6. Autocatalytic Conversion of 1-Methyl-r-1,t-2-cyclopropanediamine into 4-Aminobutan-2-one in Aqueous Buffer Solutions of pH 6.5−10. Azomethine Ylides as Intermediates in the Catalysis by Alkyl Ketones

The pK a values of 1,2-cyclopropanediammonium dibromides (1−5·2HBr) were determined by potentiometric titration with KOH. Corrections for overlapping ionizations and for the varying ionic strength were employed in the calculation of the thermodynamic acidities. The equilibrium constant K E = 2.9 of the two monoprotonated forms of trans-3b·H+ was derived from the pK a values and the pH dependence of proton spectra which were interpreted with the help of nuclear Overhauser experiments. The autocatalytic conversion of trans-3a·2HBr into 4-aminobutan-2-one (8) in various buffer solutions of pH = 6.5−10 was monitored by proton spectroscopy. This sequence of reactions is subject to a general acid/base catalysis and also catalyzed by acetone and butanone.