Allene cycloadditions. Part I. Reaction of tetrafluoroethylene with allenic hydrocarbons

Abstract

Thermal cycloaddition of tetrafluoroethylene to propadiene gives a methylenespiroheptane (IV), an octafluorooctalin (VI), and a dispirodecane (VII), in addition to the methylenecyclobutane (I) and the spiroheptane (II) reported previously. The fluoro-olefin reacts with 3-methylbuta-1,2-diene at 150° to give a 61 : 17 : 5 : 2 mixture of isomeric 1 : 1 adducts (IX—XII): the first two arise by [2 + 2] cycloaddition to each of the two allenic π-bonds, the less substituted of which reacts faster; the remaining two adducts arise by prior rearrangement of the allene to isoprene, which reacts mainly at the more substituted double bond to give 2,2,3,3-tetrafluoro-1-methyl-1-vinyl-cyclobutane (XI). Reaction of tetrafluoroethylene with tetramethylallene occurs only after rearrangement of the allene to 2,4-dimethylpenta-1,3-diene, which undergoes cycloaddition only at the less-substituted double bond to give 2,2,3,3-tetrafluoro-1-isobutenyl-1-methylcyclobutane (XVI): the effect of reactant ratio, reaction time, and reactor pretreatment have been investigated. The mechanism of the cycloadditions is discussed.