Cycloheptatrienes protonated, isomerization

Similar reactions under basic conditions are mechanistically ambiguous. They can either be regarded as norcaradiene to cycloheptatriene valence isomerizations or as cyclopropyl to allyl rearrangements promoted by exocyclic proton abstraction. ... 

Mormann, M. Knck, D. Protonated 1,3,5-Cycloheptatriene and 7-Alkyl-1,3,5-Cycloheptatrienes in the Gas Phase Ring Contraction to the Isomeric Alkylben-zenium Ions. J. Mass Spectrom. 1999, 34, 384-394. 

Cycloheptatriene, Norbomadiene, Methylenecyclohexadienes (Isotoluenes) and Bicy-clof3.2.0/heptadienes. The gas-phase ion chemistry of ionized 1,3,5-cycloheptatriene is closely related to that of ionized toluene, in particular, and to that of norbomadiene and other non-aromatic ( yllx isomers. This extensive body of work will not be discussed here since a detailed review on this topic has been published by one of these authors in the context of the gas-phase chemistry of the alkylbenzene radical cations This chemistry pertains also to the well-known isomerization of the even-electron CvHv ions and to their formation from the respective parents, e.g. CyHs" " . A related, albeit chemically different held concerns protonated cycloheptatriene, i.e. the even-electron C7H9+ ions , and alkylcycloheptatrienes, which are closely related to protonated toluene and higher alkylbenzenium ions. A parallel review by one of these authors on protonated alkylbenzenes has been pubUshed, and recent investigations on protonated alkylcycloheptatrienes have highUghtened the complexity of this gas-phase ion chemistry 42 jp a minor extent, ionized and protonated fulvenes have also been investigated with respect to their interconversion to their (mainly arene-derived) isomers. 

The cation (26) was obtained by protonation of the corresponding azulene in dichloromethane. A tropylium ion-mediated a-cyanation of amines was described. The key step is a hydride transfer from the amine to the cation, resulting in cycloheptatriene and an iminium ion, the latter then reacting with cyanide to give the aminonitrile. The dehydrofropylium-Co2(CO)6 ion has been prepared as a BF4 salt. Various measures suggest that the ion is weakly aromatic, with about 25% of the aromaticity of the tropylium ion. Computational analysis of a number of annulenes predicts that the Mobius dication (CH)i4+ should be stable under persistent ion conditions. In particular, this dication is stable towards reactions such as cis-trans isomerization and electrocyclic rearrangement that limit the lifetime of other Mobius annulenes. 

Valence Isomerism ° Norcaradienes. The dicyano-substituted compound (U5) (see p. 38) exists only as the norcaradiene, and at 110°C it undergoes thermolysis to a benzylfulvene rather than valence isomerization. Protonation of the cycloheptatriene (330) at — 60°C gives an equilibrium in which only the norcaradiene (331) is detectable by n.m.r. spectroscopy. The n.m.r. 

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