Gas-phase Fe +-assisted cycloaddition reactions of cyclopentadiene with small alkenes and alkynes

Abstract

Gas-phase Fe +-assisted Diels–Alder reactions of cyclopentadiene with small alkenes and alkynes were studied using Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. The reaction of FeC 5H 6 + with ethene is proposed to be direct [4 + 2] Diels–Alder cycloaddition followed by dehydrogenation. The formation of FeC 8H 10 + from the reaction of FeC 5H 6 + with propene can be explained by an allylic C–H insertion and reductive elimination of H 2. It is also possible for the reaction to proceed by [4 + 2] Diels–Alder coupling reaction and further dehydrogenation. The most intriguing reactions are the ones with ethyne and propyne, where Fe(tropylium) + and Fe(cyclooctatetraene) + are the predominant products. Reaction of FeC 5H 6 + with ethyne can proceed either through [4 + 2] addition followed by [1,3]-shift and H loss or from [2 + 2] addition, subsequent retro [2 + 2] reaction and H loss. The reaction with propyne is more likely to undergo a [2 + 2] addition and subsequent retro [2 + 2] followed by dehydrogenation to form major product ion, FeC 8H 8 +. A retro-alkyne cycloaddition process is believed to be responsible for the formation of the secondary reaction product FeC 6H 6 +. For comparison with propyne, the reaction of FeC 5H 6 + with allene was also studied. It turned out to be the same major product, Fe(cyclooctatetraene) +. This reaction is proposed to proceed by either [2 + 2] or [4 + 2] cycloaddition and subsequent dehydrogenation to form Fe(C 8H 8) +. Further kinetics studies on these reactions indicate that they follow linear pseudo-first-order kinetics, suggesting that FeC 5H 6 + is thermalized and consists of one isomeric structure under our experimental conditions.