ACTIVATION PARAMETERS FOR 1,5-HYDROGEN TRANSFER AND INTRAMOLECULAR CYCLOADDITION IN A THERMALLY GENERATED CYCLOPENTANE-1,3-DIYL

The biradical 2-methyl-2-(3-phenylpropyl)cyclopentane-1,3-diyl is gene rated reversibly by gas-phase pyrolysis of a stereoisomeric mixture of the corresponding bicyclo[2.1.0]pentane derivatives. Three reactions of the biradical (besides ring closure) are detected. One is ring open ing to 3-methyl-3-(3-phenylpropyl)-1,4-pentadiene. A second is intramo lecular transfer of a benzylic hydrogen to the cyclopentane-1,3-diyl. An activation enthalpy of 8 kcal/mol is estimated for this 1,5-hydroge n transfer. The third reaction is intramolecular cycloaddition of the 1,3-diyl to the phenyl ring. An activation enthalpy of 7 kcal/mol is e stimated for this cycloaddition reaction. Two reactions of the new bir adical generated by 1,5-hydrogen transfer are detected. One is ring cl osure to give stereoisomeric cis-1-methyl-4-phenylbicyclo[3.3.0]octane s; the other is transfer of a second hydrogen in a reaction analogous to the disproportionation of alkyl radicals. The ring closure and seco nd hydrogen transfer exhibit no temperature dependence in the ratio of their rate constants and are thus judged to be activationless process es. These reactions of thermally generated, presumably singlet biradic als are compared with the reactions of related monoradicals. The cyclo pentane-1,3-diyl derivative is judged to have very little in the way o f dipolar character and thus to be near one end of a hypothetical spec trum of intermediates encompassing singlet biradicals and zwitterions.