A STUDY OF THE REACTIONS OF 1-METHYL-2-VINYLCYCLOPROPANES

When thermolyzed, 1-alkyl-2-vinylcyclopropanes undergo two types of rearrangements. The first is a {1,3}-sigmatropic shift of carbon to form a substituted cyclopentene. The second is a homo-{1,5}-sigmatropic shift of hydrogen to form substituted 1,4-hexadienes. Two basic mechanisms have been proposed for the {1,3}-carbon shift. The first is a single-transition state, concerted pathway with overall adherence to the Woodward-Hoffmann rules. The second is a multi-step, nonconcerted pathway which passes through an allyl-stabilized biradical structure. Kinetic and isotope effect studies were performed on racemic and optically active substituted 1-methyl-2-vinylcyclopropanes to determine the relationship of the {1,3}-carbon and homo-{1,5}-hydrogen shifts and to elucidate the mechanistic nature of the {1,3}-carbon shift.;Kinetic deuterium isotope effects at the terminal methylene group of 3,3-dimethyl-2-(trans-2-methylcyclopropyl)-1-butene have been measured for the {1,3}-carbon and homo-{1,5}-hydrogen shift processes and for the loss of optical activity of enantiomerically enriched starting material. For the {1,3}-shift process the isotope effect is less than unity while for the homo-{1,5}-hydrogen shift the effect is greater than unity. Deuterium substitution at the terminal methylene has no effect on the rate of loss of optical activity in starting material. These results are consistent with a nonconcerted mechanism for the {1,3}-carbon shift in which the cyclopentene and 1,4-hexadience products arise from a common intermediate.