Reactions of AlCl3 and AlBr3 σ-complexes of cyclobutadienes

The useful reactivity of AlCl₃ and AlBr₃ σ-complexes of cyclobutadienes towards various reagents is demonstrated. In reactions with acetylenic derivatives (affording dewarbenzenes), nitriles (affording pyridines in the case of ethylcyanoformate) and sulfur dioxide, cycloadditions to liberated cyclobutadienes are possibly involved; in reactions with isocyanides a nucleophilic attack at the allylic moiety of the σ-complexes appears to take place. The reactivity differences between the tetramethyl- and the 3,4-dimethyl-1,2-tetramethylene-substituted complexes are pointed out. A novel route to dibromocyclobutene derivatives employing AlBr₃ σ-compIexes of cyclobutadienes is presented.