C-H AND C-C BOND ACTIVATION BY BARE TRANSITION-METAL OXIDE CATIONS IN THE GAS-PHASE

Over the last decade the gas‐phase chemistry of bare transition‐metal oxide cations MO+ has received considerable attention. This interest is primarily due to the particular role of metal oxides in the oxidation of organic compounds in a variety of chemical and biochemical transformations. At a molecular level the simplest model system for these processes deals with reactions of bare metal‐oxide ions in the gas phase. Due to the high oxophilicities of the early transition metals, their monoxide cations MO+ do not mediate O‐atom transfer to any organic compounds at all. In contrast, monoxide cations of late transition metal can oxygenate a variety of hydro‐carbons, and the most reactive ions, MnO+, FeO+, NiO+, OsO+, and PtO+, even activate methane. Insight into the reaction mechanisms of these oxidation processes can be obtained by analysis of reaction kinetics, isotope effects, product distributions etc., and for the reactions of MO+ with alkanes the initial CH bond activation by MO+ is often rate‐determining. Interestingly, the high reactivity of some MO+ ions is not always associated with a decrease in regioselectivity; for example, FeO+ ions induce regiospecific γ‐CH bond activation of dialkylketones in the gas phase. The situation for the epoxidation of olefins in the gas phase turns out to be even more complex than for condensedphase analogues. This is primarily because the metal ion that mediates O‐atom transfer to the olefin also catalyzes the isomerization of the epoxides formed, to afford the energetically more stable aldehydes or ketones. Aromatic compounds can also be hydroxylated by MO+ ions, and particularly the oxidation of benzene by bare FeO+ ions in the gas phase reveals striking parallels to the metabolism of arenes. Furthermore, the storing capabilities of ion cyclotron resonance mass spectrometers even permit the design of catalytic processes in which a single metal ion converts more than one substrate molecule into an oxygenated product in a sequence of strictly bimolecular reactions. The most outstanding examples are the Pt+‐mediated oxidation of methane by molecular oxygen and the Co+‐mediated hydroxylation of benzene by N2O as oxidant. Finally, the key features of the gas‐phase reactions are compared with observations in condensed‐phase systems in which metal oxides are anticipated as central intermediates. The result of this comparison is promising in the sense that, in general, the understanding of transition‐metal‐mediated oxidations in the gas phase may lead to a more uniform description of these processes at a molecular level. Ultimately, it is hoped that gas‐phase studies will serve as one of the building blocks in the evolution of tailor‐made catalysts. Copyright © 1995 by VCH Verlagsgesellschaft mbH, Germany