Fulvenes protonation

Alkadienyl cations such as the 1,3,5-trimethylheptadienyl cation 85 show great tendency to cyclize [Eq. (3.37)] and these reactions have been followed by NMR.240 Several novel fulvenes have been protonated to their corresponding dienyl cations241 (e.g., 86 and 87). 

The remarkable stabilizing effect of the cydopropyl substituents is also illustrated by the fact that the fulvene 18, upon protonation with tetrafluoroboric acid in dichloromethane, yields the fulvenyl tetrafluoroborate 22, which is completely stable in solution at —50 °C. Compound 22 could be characterized by NMR spectroscopy (1H and 13C) and survives at ambient temperature for several days. 

The question of aromaticity versus antiaromaticity and delocalized versus localized double bonds in pentalene (2) dates back to 1922, when Armit and Robinson compared it with naphthalene and postulated that the former might be similarly aromatic [32, 33]. While the first synthesis of a non-fused hexaphenylpentalene (38) [30] provided only some clues as to the non-aromatic reactivity of the pentalene skeleton, the tri-tert-butyl derivative 39, prepared and studied by Hafner et al. in great detail [31], gave a better insight. The ring-proton signals of this alkyl-substituted pentalene 39 are shifted upfield compared to those of fulvene (27) and other cyclic polyenes. This observation led to the conclusion that the pentalene derivative 39 should be an antiaromatic species. However, the results did not permit a distinction... 

So, where is the safety catch The important point about Fmoc is that is has a rather acidic proton (p Ca about 25), shown in black. The proton is the Achilles heel treatment of Fmoc-protected amines with base eliminates a fulvene to reveal the NH2 group. 

The dienylic cations 44 with cyclopropyl and phenyl groups were also prepared and characterized by the protonation of respective fulvenes. Other cyclopropyl substituted allyl cations include acyclic 1,3- and 1,4-disubstituted allyl cations 45 and 46 The charge in these cations is localized mainly on the carbon adjacent to the cyclopropyl group. The rearrangement of these cations at higher temperatures was also studied. ... 

The rate constants for protonation at the metal of (6-dimethylamino-fulvene)M(CO)3 increase in the order Cr < Mo < W quantitative results are given but they appear unreliable. Proton transfer between protonated and unprotonated forms of (arene)Cr(CO)j is fast on the NMR time scale and occurs by rate-determining proton removal by the conjugate base ([FSOj] ) of the acid employed" (FSOjH). 

Alkadienyl cations show great tendency to cyclize and these reactions have been followed by NMR80. More recently several novel fulvenes have been protonated to... 

Fulvene Radical Cations, Protonated Fulvene, and Isomeric and C(,H-i Ions. 

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 data are consistent with polymer formation by way of phenyl radical reaction with benzene or the fulvene produced to give cyclic radical intermediates which then polymerize to yield a polycyclopentene chain with pendant phenyl and/or benzyl groups. The average non-olefinic to olefinic proton ratio of 2.9 indicates that the many possible... 

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