Intermediates in the methanol-to-hydrocarbons (MTH) reaction: a gas phase study of the unimolecular reactivity of multiply methylated benzenium cations

Svelle S, Bjorgen MA, Kolboe S, et al. Intermediates in the methanol-to-hydrocarbons (MTH) reaction: a gas phase study of the unimolecular reactivity of multiply methylated benzenium cations. CATALYSIS LETTERS. 2006;109(1-2):25-35.In order to reach a deeper insight into the reaction mechanism of the zeolite catalyzed methanol to hydrocarbons reaction (MTH), the proposed reaction intermediates, i.e., a series of multiply methyl-substituted benzenium ions has been generated in the gas phase by chemical ionization. The fragmentations of the corresponding long-lived (metastable) ions have been investigated. While expulsion of H-2 dominates for the lower homologues, elimination of methane dominates for the higher homologues, accompanied by increasing amounts CH3 center dot. Loss of larger fragments relevant to the MTH-reaction, in particular ethene, propene and even butene, is also observed in minor amounts. This latter finding is consistent with a proposed reaction cycle in the MTH reaction known as the paring mechanism, and the feasibility of this mechanism has thus been demonstrated. The metastable gas-phase ions studied here are considerably more energetic than those residing in a zeolite catalyst, but they were found to decompose with markedly higher selectivity towards alkenes as compared to those activated by collision-induced dissociation (CID)

Concomitant hydride and proton transfer: an essay on competing and consecutive key reactions occurring in gaseous ion/neutral complexes

Kuck D. Concomitant hydride and proton transfer: an essay on competing and consecutive key reactions occurring in gaseous ion/neutral complexes. European Journal of Mass Spectrometry. 2012;18(2):161-181.The interplay of proton transfer and hydride transfer reactions in alkylbenzenium ions and related protonated di- and oligophenylalkanes is presented and discussed. While intra- and inter-annular proton exchange has been recognised to be a ubiquitous feature in protonated arenes, hydride abstraction is much less obvious but can become a dominating fragmentation channel in metastable ions of tert-butyl-substituted alkylbenzenium ions and related carbocations. In such cases, proton-induced release of the tert-butyl cation gives rise to ion/neutral complexes as reactive intermediates, for example, [(CH3)(3)C+center dot center dot center dot arylCH(2)(alpha)(CH2)(n) CH(2)(omega)aryl'] with n >= 0 and highly regioselective intra-complex hydride transfer occurs from all of the benzylic methylene hydride ion donor groups (alpha-CH2 and (omega)-CH2) to the tert-butyl cation acting as a Lewis acid. Substituent effects on the individual contributions to the overall hydride transfer from different donor sites, including ortho-methyl groups, in particular, and the concomitant intra-complex proton transfer from the tert-butyl cation to the neutral diarylalkane constituent corroborate the view of "bisolvated" complexes as the central intermediates, in which the carbenium ion is coordinated to both of the aromatic pi-electron systems. The role of cyclisation processes, converting the benzylic [M-H](+)-type ions into the isomeric benzenium, [M+H](+)-type ions prior to fragmentation, is demonstrated for several cases. This overall scenario, consisting of consecutive and/or competing intra-complex hydride abstraction and proton transfer, intra-annular proton shifts (H+ ring walk) and inter-annular proton transfer, hydrogen exchange ("scrambling") processes, and cyclisation and other electrophilic substitution reactions, is of general importance in this field of gas-phase ion chemistry and more recent examples concerning protonated ethers, benzylpyridinium and benzylammmonium ions are discussed in which these recurring features play central and concerted mechanistic roles as well