Cyclopropanediamines. 5. Ring-Opening and Cis-Trans Isomerization of 1,2-Cyclopropanediamines in Aqueous Buffers

Article abstract of DOI:10.1021/jo00118a019

In aqueous phosphate buffer, pH = 5.8, at 25 deg C, the N,N'-dimethyl-1,2-cyclopropanediammonium dibromides 1b-3b react rapidly after an initiation phase of several hours to afford methanamine (ca. 1 mol) and traces of pyrroles besides unidentified products.The onset is delayed, and the reactions are slowed down drastically by hydroxylamine. cis-1b*2HBr partly isomerizes to trans-1b, which decomposes much more slowly. trans-3a*2HBr is converted quantitatively to 4-aminobutan-2-one 12a, and trans-3b*2HBr partly to 12b.Trans-4a*2HBr and trans-5a,b*2HBr are stable for several days. trans-6a*2HBr decomposes, while trans-6b*2HBr and trans-6c*2HBr form the 2-styrylpyrroles 19b and 19c, respectively.The reactions are interpreted in terms of a slow formation of carbonyl compounds during the initiation phase, which react with unchanged cyclopropanediamine to give iminium ions (8, 13, or 16).These open the cyclopropane ring to yield the crucial intermediates (9, 14, or 17) which possess both an azomethine ylide and an iminium moiety.These intermediates undergo ring closure to either a cyclopropane ring (9-->trans-8) or a dihydropyrrole ring (17-->18).Alternatively, the intermediates are hydrolyzed to Mannich bases (9-->10, 14-->12), which in turn react with unchanged cyclopropanediamine, thus completing an autocatalytic cycle.