CYCLOADDITION REACTIONS BETWEEN 1,3-BUTADIENE RADICAL CATIONS AND ETHENE IN THE GAS-PHASE

The reaction of [1,3-butadiene]+. with ethene in the gas phase has been studied under low-pressure conditions. Experiments were conducted in a Fourier transform ion cyclotron resonance spectrometer equipped with an external ion source. Two competitive reaction paths were identified: [CH2=CH-CH=CH2]+. + CH2=CH2 --> [C5H7]+ + CH3. (Reaction I) and [CH2=CH-CH=CH2]+. + CH2=CH2 --> [C4H6]+. + C2H4 (Reaction II). Deuterium labelling reveals that the methyl loss (Reaction 1) is preceded by extensive H/D scrambling inside the transient collision complex and that the ethene molecule eliminated during Reaction II contains specifically one methylene group from the terminal positions of the 1,3-butadiene radical cation and the other from the initial neutral ethene used as reactant. Measurements of deprotonation enthalpies indicate that the [C5H7]+ ions produced via Channel I have the cyclopentadienyl structure. A reaction mechanism, corroborated by molecular orbital calculations (3-21G), is proposed. It involves the initial formation of a distonic species [CH2CHCHCH2CH2CH2]+. as a key step and competitive processes such as [1 + 2] cycloadditon, 1,n-hydrogen transfers and nonsynchronous Diels-Alder c cloaddition.