Carbocationic cyclization reactions. I. Rearrangement of 4-allyl-1,2,3,4-tetramethylcyclobutenyl cation to 1,2,3,4-tetramethylbicyclo[4.1.0]hept-2,3,4-enyl cation

It was established by NMR spectroscopy (1H and 13C) that 3-allyl-1,2,3-trimethyl-4-methylenecyclobutene is protonated in the acid system HSO3F-SO2FCl with the formation of the 4-allyl-1,2,3,4-tetramethylcyclobutenyl cation, which rearranges to the 1,2,3,4-tetramethylbicyclo[4.1.0]hept-2,3,4-enyl cation (ΔG-90degreeCne ∼ 54 kJ/mole). By means of labeled atoms (a deuterium label) data were obtained in favor of a two-stage mechanism of rearrangement with the intermediate formation of the 1,5,6,7-tetramethylbicyclo[3.2.0]hept-6-en-3-yl cation, where the controlling stage is the first stage, i.e., the carbocationic cyclization stage. A similar transformation is observed during the action of 95% sulfuric acid or the acid system HCl-AlCl3 in methylene chloride on 3-allyl-1,2,3-trimethyl-4-methylene-cyclobutene