Steven Z. Kan

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 5 H 6 + with ethene is proposed to be direct [4 + 2] Diels–Alder cycloaddition followed by dehydrogenation. The formation of FeC 8 H 10 + from the reaction of FeC 5 H 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 5 H 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 8 H 8 + . A retro-alkyne cycloaddition process is believed to be responsible for the formation of the secondary reaction product FeC 6 H 6 + . For comparison with propyne, the reaction of FeC 5 H 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 8 H 8 ) + . Further kinetics studies on these reactions indicate that they follow linear pseudo-first-order kinetics, suggesting that FeC 5 H 6 + is thermalized and consists of one isomeric structure under our experimental conditions.

Coupling reactions of Fe(benzyne)+ and Fe(naphthyne)+ in the gas phase

The gas phase ion-molecule reactions of Fe(benzyne) + and Fe(naphthyne) + with acetylene, allene, ethene, butadiene, buckminsterfullerene and cyclopropane are studied using Fourier transform ion cyclotron resonance mass spectrometry. In a previous study we proposed that the reaction of Fe(benzyne) + with ethene proceeds by a mechanism in which a multiple carbon-carbon bond couples with a metal-carbon bond of Fe(benzyne) + to form new carbon-carbon and metal-carbon bonds. In this study we include the reactions of Fe(naphthyne) + in order to test the generality of this mechanism and to observe the non-metallic products formed in these reactions. Fe(benzyne) + , for example, reacts with acetylene to yield Fe + and C e H,CH, exclusively. In contrast the reaction of Fe(naphthyne)+ with acetylene produces Fe + , C 10 H 6 C 2 H 2 + and a metallacyclic product Fe-C 10 H 6 -CH 2 H 2 + . This lends support that the neutral product formed in the reaction of Fe(benzyne) + with acetylene is an intact C 6 H 4 C 2 H 2 . Similarly, Fe(naphthyne) + reacts with ethene to form a metallacyclic product Fe-C 10 H 6 -C 2 H 4 + . In the reaction with C 60 , Fe(naphthyne) + is observed to yield the C 60 derivatives C 60 C 10 H 6 + and Fe-C 10 H 6 -C 60 + . These gas phase ion-molecule coupling reactions are analogous to the condensed phase coupling reactions of metal-lobenzynes with molecules containing multiple bonds.