1,3- and 1,2-Zwitterionic Complexes of Donor-Acceptor Cyclopropanes in Reactions with Arylallenes

The reaction of 2-arylcyclopropane-1,1-dicarboxylates (ACDC) with GaCl₃ and other Lewis acids in the presence of 3-arylbuta-1,2-dienes, which acts as a trapping agent for the 1,2- or 1,3-zwitterionic intermediates being generated, has been studied. The reaction of 1,2-zwitterionic gallium complexes at low temperature has been found to give products of annulation at the phenyl substituent in the allene, 2-aryl-1-(inden-2-yl)ethylmalonates, as the main products. Only in the case of naphthylcyclopropanedicarboxylate, the formation of a polycyclic frame by annulation at both aromatic moieties occurred upon an increase in temperature to 0 °C. Trapping of 1,3-zwitterions, the primary intermediates of cyclopropane ring opening, by the indicated allene was most successful if catalytic amounts of GaCl₃ were added directly to the reaction mixture. In this case, the reaction followed the pathway of the usual formal (3 + 2)-cycloaddition of 1,3-zwitterions at each of the double bonds of the allene to give the corresponding exo-methylenecyclopentane-1,1-dicarboxylates in up to 93% yields. This process occurred with the highest efficiency at the unsubstituted double bond of 3-arylbuta-1,2-diene.