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  • 1
    Electronic Resource
    Electronic Resource
    Benzene Oxidation by ‘Bare’ FeO+ in the Gas Phase (1992)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 75 (1992), S. 1281-1287 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Bare FeO+ reacts in the gas phase with benzene at collision rate (k = 1.3 × 10-9 cm3 molecule-1 s-1), giving rise to the formation of Fe(C6H4)+/H2O(5%), Fe(C5H6)+/CO(37%), Fe(C5H5)+/CO/H. (2%), and Fe+/C6H5OH (56%). Neither the reaction rate nor the product distribution are subject to a significant kinetic isotope effect, thus, ruling out several mechanistic variants described in the literature to the operative for ‘analogous’ arene oxidation processes in solution. A mechanism is suggested which is in keeping with the experimental findings, and which also accounts for some remarkable results obtained, when two [Fe, C6,H6H6O]+ isomers are generated and subjected to a neutralization-re-ionization experiment in the gas phase.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19920750429
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  • 2
    Electronic Resource
    Electronic Resource
    Reaktionen des 4-Bromo-1,6-methano[10]annulen-3-carbaldehyds: Synthese von 3,4-heteroanellierten 1,6-Methano[10]annulenenHerrn Kollegen Ullrich Schöllkopf, Göttingen, in Verbundenheit zum 65. Geburtstag gewidmet (1992)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 75 (1992), S. 825-832 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reactions of 4-Bromo-1,6-methano[10]annulene-3-carbaldehyde: Syntheses of 3,4-Heteroanellated 1,6-Methano[10]annulenesSimilarly to β-chlorovinyl ketones, the title compound 1 reacts with dinucleophiles to give the 3,4-heteroanellated 1,6-methano[10]annulenes 2-4, 6, and 12-16, containing thiophene, pyrazole, pyrimidine, and thiazepine rings (Schemes 2 and 3). On reaction of 1 with tert-butylhydrazin, an unusual Michael-addition-dehydrogenation sequence, resulting in 2,3-anellation of a pyrazole ring (→5), is observed. Synthetic as well as some mechanistic aspects are briefly discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19920750317
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  • 3
    Electronic Resource
    Electronic Resource
    Two-State Reactivity in Organometallic Gas-Phase Ion ChemistryDedicated to Sir Derek H. R. Barton (1995)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 78 (1995), S. 1393-1407 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In contrast to organic reactions, which can almost always be described in terms of a single multiplicity, in organometallic systems, quite often more than one state may be involved. The phenomenon of two states of different multiplicities that determine the minimum-energy pathway of a reaction is classified as two-state reactivity (TSR). As an example, the ion/molecule reactions of ‘bare’ transition-metal-monoxide cations with dihydrogen and hydrocarbons have been analyzed in terms of the corresponding potential-energy hypersurfaces. It turns out that, besides classical factors, such as the barrier heights, the spin-orbit coupling factor is essential, since curve crossing between the high- and low-spin states constitutes a distinct mechanistic step along the reaction coordinates. Thus, TSR may evolve as a new paradigm for describing the chemistry of coordinatively unsaturated transition-metal complexes. This concept may contribute to the understanding of organometallic chemistry in general and for the development of oxidation catalysts in particular.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19950780602
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  • 4
    Electronic Resource
    Electronic Resource
    Dehydration of Alkanones by Bare FeO+ Involves the ω/(ω - 1) Positions: Exclusion of Iron Carbenes as Intermediates. Preliminary Communication (1990)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 73 (1990), S. 380-385 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Fourier-transform ion-cyclotron-resonance (FTICR) mass spectrometry has been used to uncover the mechanisms by which FeO+ dehydrates heptan-4-one (5a) and nonan-5-one (6a) in the gas phase. The study of isotopomeric ketones provides evidence that H2O loss is not due to a 1,1-elimination, thus ruling out the intermediacy of high-valent iron-carbene species. Rather, H2O is generated in a formal 1,2-elimination involving the ω/ω - 1 positions of the alkyl chain (‘remote C—H bond activation’). In the consecutive alkene/H2O elimination, the olefins (ethylene from 5a and propene from 6a) originate from the terminal part of one alkyl chain, and the H-atom is transferred to the FeO+ moiety in the course of this process, builds up together with an H-atom from the ω/ω-1 position of the other alkyl chain the H2O molecule. In either case, the O-atom of H2O is provided by the FeO+ species.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19900730217
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  • 5
    Electronic Resource
    Electronic Resource
    Highly Selective Benzylic C—H Bond Activation of Toluene 1 by ‘Bare’ FeO+ in the Gas Phase. Short communication (1992)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 75 (1992), S. 1792-1797 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of FeO+ with toluene in the gas phase occurs at collision rate (kr = 1.36 × 10-9 cm3 molecule-1 s-1), and labeling experiments demonstrate that the total products due to C—H bond activation involve to 〉 92% the benzylic position. In the ‘hydride’ abstraction process (formation of FeOH and C7H7+), the H-atom originates elusively from the benzylic position to generate a benzyl cation, and an intramolecular kinetic isotope effect kH/kD = 1.75 has been obtained. There is no evidence for the existence of isotopically sensitive branching (‘metabolic switching’) in the system studied.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19920750605
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  • 6
    Electronic Resource
    Electronic Resource
    Intermediacy of Proton-Bound Dimers and Ion/Dipole Complexes in the Unimolecular Decompositions of Dialkyl-Peroxide Radical Cations: Evidence for a coupled proton and hydrogen-atom transfer (1995)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 78 (1995), S. 1999-2010 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The unimolecular fragmentation reactions of the radical cations of diethyl, diisopropyl, dipropyl, isopropyl propyl, and di(tert-butyl) peroxide have been investigated by mass spectrometric and isotopic labeling techniques. Two competing pathways for unimolecular decomposition in the μs time regime (metastable ions) are observed: i) A combination of an α-C—C bond cleavage and a H migration gives rise to proton-bound dimers of two ketone or aldehyde molecules. ii) Ion/dipole complexes of alkyl cations and alkylperoxy radicals are generated by C-O bond cleavage. These complexes either exhibit direct losses of alkylperoxy radicals, or they rearrange via a coupled proton and H-atom transfer, this sequence of unprecedented isomerizations is completed by losses of alkyl radicals. Collisional activation experiments confirm that the ionic products of the latter process correspond to RR′C=OOH+; these ions can be regarded as protonated carbonyl oxides. In addition, we observe the elimination of alkenes leading to hydroperoxide radical cations and the expulsion of HO2⋅ radicals. The latter process implies a C—C bond formation step between the two alkyl fragments leading to higher alkyl cations.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19950780808
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  • 7
    Electronic Resource
    Electronic Resource
    Theory-enforced Re-investigation of the Origin of the Large Metal-Carbon Bond Strength in PdCH2I+ and its reactions with unsaturated hydrocarbonsIn [1], a preliminary account of the reaction of ground-state Pd+ with CH3I is given.Dedicated to Edgar Heilbronner on the occasion of his 75th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1110-1120 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: State-of-the-art ab initio studies demonstrate that the reaction Pd+ + CH3I → PdCH2I+ + H. is endothermic by ca. 20 kcal/mol, which translates into a bond dissociation energy (BDE) of ca. 83 kcal/mol for the Pd+—CH2I bond. This figure is in agreement with an experimental bracket of 68 kcal/mol 〈 BDE(Pd+—CH2I) 〈 92 kcal/mol. Based on these findings, the previously studied Pd+/CH3I system was re-investigated, and double-resonance experiments demonstrate that the formation of PdCH2I+ occurs stepwise via PdCH3+ as a reactive intermediate. Further, ion/molecule reactions of PdCH2I+ with unsaturated hydrocarbons are studied, which reveal the formation of carbon-carbon bonds in the gas phase.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790419
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  • 8
    Electronic Resource
    Electronic Resource
    Generation of ‘Bare’ FeF+ by C—F Bond Activation in the Gas Phase and Evaluation of Thermochemical Data. Short communication (1992)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 75 (1992), S. 2215-2218 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The title compound FeF+ has been generated in a Fourier-transform ion-cyclotron-resonance mass spectrometer via C—F bond activation by reacting hexafluorobenzene with bare FeO+. Bracketing experiments provide a bond-dissociation energy (BDE) for FeF+ of 86 〈 BDE 〈 101 kcal/mol. High-level CAS-MCSCF pseudopotential ab initio calculation, including 501036 electronic configurations, predict a BDF(Fe+—F) of 100.9 kcal/mol.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19920750706
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  • 9
    Electronic Resource
    Electronic Resource
    Unparalleled, Enormous Metal-Carbon Bond Strength in PdCH2I+Dedicated to Albert Eschenmoser on the occasion of his 70th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1-5 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermalized Pd+ cations activate methyl iodide by selective cleavage of a C—H bond under formation of PdCH2I+ and an H-atom. This finding implies that the interaction energy between the metal cation and the CH2I fragment and thus the metal-carbon bond strength exceeds 103 kcal/mol. Theory predicts that the energetically most favorable isomer of this ion exhibits the Pd+—CH2—I structure, which is stabilized by an unprecedented bridging interaction between the two heavy atoms Pd and I.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790102
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  • 10
    Electronic Resource
    Electronic Resource
    Substituent Effects on Fe+-Mediated [4+2] Cycloadditions in the Gas Phase (1997)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 80 (1997), S. 1205-1220 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Fe+-mediated [4+2] cycloaddition of dienes with alkynes has been examined by four-sector ion-beam and ion cyclotron resonance mass spectrometry. Prospects and limitations of this reaction were evaluated by investigating several Me-substituted ligands. Me Substitution at C(2) and C(3) of the diene, i.e., 2-methylbuta-1,3-diene, 2,3-dimethylbuta-1,3-diene, hardly disturbs the cycloaddition. Similarly, variation of the alkyne by use of propyne and but-2-yne does not affect the [4+2] cycloaddition step, but allows for H/D exchange processes prior to cyclization. In contrast, Me substituents in the terminal positions of the diene moiety (e.g., penta-1,3-diene, liexa-2,4-diene) induce side reactions, namely double-bond migration followed by [3+2] and [5+2] cycloadditions, up to almost complete suppression of the [4+2] cycloaddition for 2,4-dimethylhexa-2,4-diene. Similarly, alkynes with larger alkyl substituents (pent-1-yne, 3,3-dimethylbut-1-yne) suppress the [4 + 2] cycloaddition route. Stereochemical effects have been observed for the (E)- and (Z)-penta-1,3-diene ligands as well as for (E,E)- and (E,Z)-hexa-2,4-diene. A mechanistic explanation for the different behavior of the stereoisomers in the cyclization reaction is developed. Further, the regiochemical aspects operative in the systems ethoxyacetylene/pentadiene/Fe+ and ethoxyacetylcne/isoprene/Fe+ indicate that substituents avoid proximity.
    Additional Material: 5 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19970800418
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