Zur Kohlenstoff‐Äquilibrierung in cyclischen C₆H‐Kationen in der Gasphase und zum Mechanismus der unimolekularen Ethylen‐Abspaltung

Abstract

Die Untersuchung von sechs doppelt "C-markierten C,H,,X-Derivaten. 6 a -9 , (X = CO,H, Br) und einer [D,] und nicht die von 10. Ein Vergleich der MNDO-und MIND0/3-Rechenverfahren ergibt im Hinblick auf einige relevante strukturelle und energetische Aspekte von pyramidalen Kationen, da0 MNDO generell die Bildungsenthalpie solcher nicht-klassischer lonen zu hoch berechnet, wahrend MIND013 den Cyclopropyl-Kation-Charakter dieser lonen entschieden zu stark bewertet. Die Synthesen der I3C,-markierten Verbindungen werden beschrieben. On the Carbon Equilibration of Cyclic C,H;, Cations in the Gas Phase and the Mechanism of Unimolecular Ethylene EliminationThe investigation of six '3C2-labelled (6a -9) and one [D,]-labelled (18) precursors unambiguOudy demonstrates, that unimolecular ethylene elimination from the metastable gaseous C,HA cations generated from the above mentioned compounds by dissociative ionization is preceded by both complete carbon and hydrogen scrambling. Whereas the scrambling can involve various intermediates, e.g. 3,4, and 5, MNDO calculations indicate that the minimal energy reaction path (MERP) for loss of C,H, from C,HA proceeds via the transition state 15, which is 11 kcal . mol-' lower in energy than the isomeric structure 16. 15, which can be viewed as a partially opened cydopropyl cation ,,solvated" by interaction with C2H,, decomposes to C2H, and C,H; which in agreement with experimental results has the structure of I-methylallyl cation (11) and not that of the isomeric 2-methylallyl cation (10). A brief comparison is made for the MIND013 and MNDO procedures in computing both structural and energetic features of pyramidal carbocations. The results indicate that MNDO overestimates substantially the heats of formations of such nonclassical cations, whereas MIND013 considerably overestimates the cyclopropyl cation ,,character" of the structures investigated. The syntheses of the ''C2-labeIled compounds are described.

Synthetic approaches to regiospecifically mono‐ and dilabelled arenes

“…An additional process with potential for the preparation of specifically dilabelled products is illustrated in Scheme ,209 in which labelled phenylacetonitrile is cycloalkylated at the benzylic centre to give a 1,1‐disubstituted cyclohexane, which is elaborated further. Aromatisation of the cyclohexanes produced in this case can be envisaged.…”

Organic Chemistry and Synthesis Rely More and More upon Catalysts

“…This process would be analogous to the gas phase fragmentation of the corner-protonated cyclopropane into molecular hydrogen and allyl cations. Mechanisms similar to those presented with Equilibrium (44) have been proposed to rationalize the complete hydrogen and carbon atom scrambling in the gaseous cyclohexyl ⇄ methylcyclopentyl cation equilibrium that occurs prior to monomolecular elimination of ethylene [183]. Cation 13 is also formed upon protonation of spiropentane.…”