Pentadienyl cation cationic species

In Scheme 1, the radical cations of the linear hexadienes and some cyclic isomers are contrasted. The heats of formation, AHr, as determined from the heats of formation of the species involved, as well as the heats of formation of the isomeric radical cations themselves clearly reveal the favourable stability of the cyclic isomers and/or fragment ions. Thus, instead of the linear pentadienyl cation (3), the cyclopenten-3-yl cation (2) is eventually formed during the loss of a methyl radical from ionized 1,3-hexadiene (1). Since 1,2-H+ shifts usually have low energy requirements (5-12 kcalmol-1), interconversion of the linear isomers, e.g., 4, and subsequent formation of the cyclic isomers, in particular of the ionized methylcyclopentenes 5 and 6, can take place easily on the level of the... 

Introduction of a second CF2=CF-group to the cationic center results in a striking increase of stability, and F-pentadienyl cation 31 was observed as a long-lived species even at ambient temperature ... 

Protonation of dienol complexes (244) with strong acids (e.g. HBF4, HPFe, or HCIO4) generates the cisoid ( -pentadienyl)iron cations (248) (Scheme 69). This reaction is believed to proceed via initial formation of the transoid pentadienyl cation, followed by isomerization to the more stable cisoid form of the cation. Computational methods on the unsubstituted cationic complex (248, R = Nu = H) suggest that the cisoid form is 9.2 kcal mol more stable. Protonation of an alkene that is adjacent to a diene complex can also lead to the formation of ( -pentadienyl)iron cations. Additionally, hydride abstraction using the trityl cation also produces (jj -pentadienyl)iron cations the rich chemistry of these species will be described later (Section 7.1). 

A computational study of the effect of substituents on the rate of conrotatory thermal cleavage of aziridine has been reported and while the parent compound has a high free energy of activation, this can be lowered by substituent effects. Anionic species are effective in increasing the calculated reaction rate." Quantum dynamical results of the pericyclic rearrangement of cyclooctatetraene have been compared with the traditional scheme of fluxes of electrons in pericyclic orbitals. Pentadienyl cation electrocyclizations have been reviewed. ... 

Similarly, a-pyridones and 3(2//)-isoquinolones have been synthesized from 3-en-5-ynyl nitrones and o-alkynylphenyl nitrones, respectively (Scheme 40) [153]. The proposed mechanism involves an 8e ir-electrocyclization of the initially formed vinylidene I to form seven-membered heterocyclic species II, which can also be represented as its resonance stmcture, the pentadienyl cation II. Cleavage of the N-O bond leads to species III, bearing both imine and ketene functionalities, which undergoes a 6e- Il-electrocyclization to afford the final heterocyclic products. 

The mechanism of the catalysis in the rearrangements of 4-acetoxyhepta-2,5-dienes was investigated by using the 170-labeled acetate 362 in the presence of Pd° and Pdn catalysts187. It was shown by 170 NMR spectroscopy that the reaction catalyzed by Pd° affords a 1 1 mixture of the dienes 363 and 364 which results from the Pd-coordinated pentadienyl species intermediate and 170-acetate (equation 132). By using two Pdu-catalysts, viz. (RCN PdCL (R = Me, Ph) [Pdn(l)] and (PhjP Pd [i.e. Pdn(2)], two rearrangement products 365 and 366, respectively, were obtained. The heterocyclic 1,3-dioxanium cations 367 and 368 were assumed to be intermediates of these isomerizations (equation 133). 

In 1992, Green reported the unexpected synthesis of an 774-vinylke-tenemolybdenum (II) complex while investigating the reactivity of r 4-buta-dienylmolybdenum (II) species.77 He had previously found that upon deprotonation of the cationic 774-butadienemolybdenum(II) complex 56, the neutral 73-pentadienyl complex 57 was formed.78 He also established that this was in fact a fully reversible process, and that upon treatment with... 

Whereas NMR spectroscopy is generally applied to the diamagnetic allyl, pentadienyl, or cyclopentadienyl cations or anions, this technique is generally not applicable to the analogous radical species, although CIDNP (chemically induced dynamic nuclear polarization) has been observed for allyl140 and pentadienyl radical species139. More useful for the radicals, naturally, is EPR spectroscopy. 

Variations in Zr-B bond distances for complexes of the type (CjHjB-R ZrCfe indicate that the nature of R affects the bonding relationship between zirconium and boratabenzene (Table 1). These data imply that for R = Ph, OEt and Me, the interactions between Zr and B are strong. Boratabenzene with R = NPr2 binds in an r -pentadienyl fashion with little overlap between B and Zr. Unfortunately, these metrical parameters do not give sufficient insight into differences in the electron density at the metal. Furthermore, they are measurements of precatalyst structures and it is unclear how they relate to the putative cationic catalytic species. 

Sketch the molecular orbitals for the pentadienyl system in order of ascending energy (see Figures 14-2 and 14-7). Indicate how many electrons are present, and in which orbitals, for (a) the radical (b) the cation (c) the anion (see Figures 14-3 and 14-7). Draw all reasonable resonance forms for any one of these three species. 

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