Cycloadditions of simple allyl cations to conjugated dienes. Optimization and scope of the reaction

Abstract

The cycloaddition of simple allyl cations to conjugated dienes, exemplified by the reaction of the 2-methylallyl cation to cyclohexa-1,3-diene, has been studied in detail as a function of silver counterion, solvent, and temperature. The optimum conditions involved stirring an excess of the conjugated diene with 2-methylallyl iodide and silver trifluoroacetate in isopentane at –78° for ca. 6 h. 3-Methylbicyclo[3,2,2]nona-2,6-diene (2) was isolated in 30% yield. For the first time [4 + 3 → 7] cycloadditions with open chain dienes are described; butadiene gives rise to 1-methylcyclohepta-1,4-diene (3a)(6%). Tentative indications for the formation of cycloadducts from the reactions of the 2-methylallyl cation with isoprene and 2,3-dimethylbutadiene are given. The parent allyl cation has been trapped by cyclopentadiene to give bicyclo[3,2,1]octa-2,6-diene (8). The combination of silver trifluoroacetate and isopentane is shown to be of general utility for generating carbonium ions on a preparative scale from alkyl and vinyl halides. The special problems of adding a reactive species such as an allyl cation without terminal substituents to conjugated dienes, particularly open chain derivatives, are discussed and an interesting parallel between Diels–Alder additions of benzyne at ca. 30° and [4 + 3 → 7] cycloadditions of the 2-methylallyl cation has come to light.