| | 1 | Author
| Gernot Frenking, Jochen Schmidt, Helmut Schwarz | Requires cookie* | | | Title
| MNDO-Rechnungen zur unimolekularen CO-Abspaltung aus Isomeren C3H5CO + -Ionen MNDO Calculations on the Unimolecular CO Loss from Isomerie C3HsC0 + Ions  | | | | Abstract
| In agreement with experimental results MNDO calculations indicate that unimolecular CO loss from the isomeric ions CH2 = CHCH2-CO+ (1), C-C3H5-CO+ (2), and CHsCH = CH-CO + (3), gives exclusively allyl cation (4). It is shown that, whereas CO loss from 1 is a continuously endothermic process, the analogous reaction from 2 and 3 follows relatively complicated pathways. For 2, the energetically most feasible reaction involves an isomerization of 2 to 1 via the intermediate a-oxo-and /?-oxocyclobutyl cations (8) and (9). This reaction sequence is 10 kcal • mol -1 lower in energy than direct CO loss from 2 coupled with isomerization of the incipient cyclopropyl cation (7) to allyl cation (4). For 3 it is shown that this ion undergoes isomerization to 2 prior to CO loss. The alternative reaction, i.e. direct CO elimination giving rise to 1-propenyl cation (11) or isomerization to 1 were 35 and 43 kcal • mol -1 higher in energy than the process 3 -* • 2. | | | |
Reference
| Z. Naturforsch. 35b, 1031—1034 (1980); eingegangen am 21. März 1980 | | | |
Published
| 1980 | | | |
Keywords
| Carbocation Chemistry, Quantum Mechanical Calculations, Potential Energy Surfaces | | | |
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| default:Reihe_B/35/ZNB-1980-35b-1031.pdf | | | | Identifier
| ZNB-1980-35b-1031 | | | | Volume
| 35 | |
| 3 | Author
| Z. Naturforsch | Requires cookie* | | | Title
| Herrn | | | | Abstract
| Water loss from protonated gaseous 1-decalon and 8a-methyl-l-decalon under the condition of chemical ionisation mass spectrometry proceeds via at least three distinct pathways, all of which commences with a [1.2]-hydrogen or [1.2]-methyl migration, thus giving rise to the formation of the central intermediate 10. H20 loss from 10 proceeds either directly, involving a hydrogen from the position (C-4 a) (Scheme 3; pathway 10 ->15 ->16), or from the rearranged ions 17 and/or 20. The fact that eis-and trans-8a-methyl-l-decalones and their deuterated isotopomers behave essentially identical is explained by loss of the stereochemical integrity of the ring conjunction. The mechanistic features of this unusual process (Scheme 2) have been explored by semi-empirical MINDO/3 calculations for the model system 23 (Scheme 4). The calculations reveal that there are two competing pathways for CH3 migration, one involving a sequence of [1.2]-shifts (23 -^24 ->25 ->26->27), the other -being energetically less favoured -corresponds to a direct [1.3]-methyl migration (Scheme 6; 23 -^33 ->27), in which the methyl group migrates with retention of configuration. This process is symmetry forbidden. For the corresponding symmetry allowed [1.3]-methyl migration with inversion of configuration of the migrating group no pathway could be detected computationally. Both the experi-mental and theoretical investigations allow one to draw a detailed picture of H20 loss from protonated cyclic ketones in the gas phase. -The syntheses of the labelled substrates are described in detail. | | | |
Reference
| Z. Naturforsch. 37b, 724—735 (1982); eingegangen am 23. Dezember 1981 | | | |
Published
| 1982 | | | |
Keywords
| Protonated Ketones, Wagner-Meerwein Rearrangement, Quantum Mechanical Calculations, Potential Energy Surface Calculations | | | |
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| default:Reihe_B/37/ZNB-1982-37b-0724.pdf | | | | Identifier
| ZNB-1982-37b-0724 | | | | Volume
| 37 | |
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