Thermal isomerizations in the vinylcyclo-propane system



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Bornylene undergoes a retro Diels-Alder reaction to give 1,5,5-trimethylcyclopentadiene and ethylene. The 1,5,5-trimethycyclopentadiene rearranges further to 1.2.3- and 1,2,4-trimethylcyclopentadiene. Two other products identified as 1,2,3-trimethylcyclopentene and 1.2.3-trimethylbicyclo[2.2.l]heptene-2 were also isolated. Thermolysis of two deuterated bornylenes and determination of deuterium distributions in the rearrangement products establishes the intermolecular nature of the methyl migrations. A mechanism for the radical chain process is presented and existing chemical evidence to the contrary is rationalized in terms of this pathway. The cis-trans isomerization of 2-cyano-l-vinylcyclopropane is a unimolecular, first order process. The Arrhenius expression over the temperature range 210.0-258.54[degrees]C is k = 10[raised 14.45[plus-minus].22] exp (-41,060[plus-minus]1,660/RT) From this kinetic study the resonance stabilization of -CH-C[equivalence]N is found to be 7 kcal/mole. The synthesis of 4-exo-methylene spiro[2.4]heptane is outlined. The molecule is unusually sensitive to catalysis and valid kinetic data was not obtained.