Cycloaddition reactions between 1,3‐butadiene radical cations and ethene in the gas phase

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Abstract
The reaction of [1,3‐butadiene]+· with ethenc 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 I) 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 cycloaddition.