A Proton-Catalyzed Dimerization of a 2-Amino-1,3-diene which Ultimately Yields a 3-Aminobicyclo[4.2.0]octa-2,4-diene

The 2-pyrrolidino-1,3-dienes (E)-5 and (Z)-5 are obtained by the reaction of lithium di(tert-butyl)cuprate with propyne iminium salt 3. Quantitative Z → E isomerization of 5 takes place within 4 hours at 20 °C. In benzenesolution, (E)-5 slowly equilibrates with a [1,5]-H shift to form the 1-amino-1,3-diene derivative 6. In concentrated chloroform solution, however, (E)-5 is transformed into the 3-pyrrolidino-bicyclo[4.2.0]octa-2,4-diene 7 which is identified by X-ray crystal structure analysis. A mechanism for this transformation is proposed.