Abstract
Gaseous CrC3H4O+ is prepared by reaction of Cr+ with monofluoracetone and investigated by means of Fourier transform ion-cyclotron resonance mass spectrometry. CrC3H4O+ only slowly dehydrogenates alkanes larger than propane, but efficiently reacts with alkenes to afford dehydrogenation, ligand loss, and C−C bond cleavage. These reactions can be explained by assuming the metal carbene structure 1 for CrC3H4O+, which reacts with alkenes by an initial [2+2] cycloaddition. Consecutive cycloreversion results in metathesis products that undergo alkylidene-olefin conversion and elimination of the C−C cleaved fragments. For allene, the corresponding vinylidene-acetylene transformation appears to be particularly favorable. In addition, the experimental findings suggest the occurrence of initial hetero-Diels−Alder reactions that involve the carbonyl group of 1. At elevated energies, such as upon complexation by benzene, 1 can rearrange to form the chromaoxacyclobutene 2 and eliminate propyne.
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