Delocalized systems cations

The formation of the (X-delocalized norbornyl cation via ionization of 2-norbornyl precusors in low-nucleophilicity, superacidic media, as mentioned, can be considered an analog of an intramolecnlar Friedel-Crafts alkylation in a saturated system. Indeed, deprotonation gives nortricyclane,... 

In order for a substitution to occur, a n-complex must be formed. The term a-complex is used to describe an intermediate in which the carbon at the site of substitution is bonded to both the electrophile and the hydrogen that is displaced. As the term implies, a a bond is formed at the site of substitution. The intermediate is a cyclohexadienyl cation. Its fundamental structural characteristics can be described in simple MO terms. The a-complex is a four-7t-electron delocalized system that is electronically equivalent to a pentadienyl cation (Fig. 10.1). There is no longer cyclic conjugation. The LUMO has nodes at C-2 and C-4 of the pentadienyl structure, and these positions correspond to the positions meta to the site of substitution on the aromatic ring. As a result, the positive chargex)f the cation is located at the positions ortho and para to the site of substitution. 

The origin of cyclopropenone chemistry goes back to the successful preparation of stable derivatives of the cyclopropenium cation <5 3), the first member of a series of Huckel-aromatic monocyclic carbo-cations possessing a delocalized system of (4n + 2)-7r-electrons. This experimental confirmation of LCAO-MO theory stimulated efforts to prepare other species formally related to cyclopropenium cation by a simple resonance description of electron distribution, namely cyclopropenone 7 and methylene cyclopropene (triafulvene) 8 ... 

This acid-base equilibrium involves the interconversion of two distinct delocalized systems, a pyrrole and a dihydrodiazepinium cation. Pyrrole... 

Radical Cations with Delocalized it Systems. - Radical cations derived from delocalized systems are often of substantial persistence. Therefore they can be regarded as the classical examples and, consequently, quite a number of studies by UV-Vis and ESR have been published over the years. 

Stabilization that takes place by delocalization of electrons in a it bonded system. Cations, radicals, and anions are often stabilized by resonance delocalization, (p. 163)... 

The neutrally charged /x-nitrido dimer of TPPFe, [(TPPFe)2N] and its phthalocyanine " " and mixed-macrocycle counterparts, are isoelectronic with the TT-cation radical of the /x-oxo dimers discussed above, but they have a different electron configuration. " NMR shifts are more consistent with their description as delocalized system having two Fe + rather than a delocalized porphyrin TT-cation radical. This complex readily reacts reversibly with dioxygen to produce an EPR-active species that has... 

This is certainly related to the fact that in the (non-radical) reaction involving t-BuCl a localized carbocation (Me3C+) is formed, whereas in the radical system (Eq. 16) the electro-fuge is the delocalized radical cation, Me2C+CH2 . 

An electrophile will attack a diene to produce the delocalized allylic cation. The partial charge on each end of the allylic system will be different if the substitution at each end of the allylic unit is different. Since the attraction of unlike charges contributes greatly to the bringing of nucleophile and electrophile together (hard-hard), the latter the partial plus is on an atom, the greater a n atively charged nucleophile is attracted to it. 

It appears that the intermediate primary norbornadien-7-ylmethyl cation did not undergo ring enlargement leading to product arising from a secondary bicyclo[2.2.2]octa-2,5-dienyl cation but, more likely formation of a delocalized homoallyl cation through the double bonds of the norbornadiene system - is responsible for the formation of the tetracyclooctane derivative. Such homoallyl cations have been considered as intermediates in the biosynthesis of terpenes. 

Figure 1.11. Three-center bonding possibilities for some cationic and neutral hydrocarbon systems, (a) Some o delocalized systems (b) some jt delocalized systems (c) some related electron-precise hydrocarbons.

The stability of the dihydrodiazepines and, even more, of the dihydrodiazepinium salts, is due to their delocalized systems of ir-electrons this is especially marked in the monocations where the system is symmetrical. Calculations based on pK data suggest a resonance energy of about 19 kcal mole-1 for these cations.22 A similar calculation suggests that dihydrodiazepine bases have 8 kcal mole-1 less resonance energy than the cor-... 

The dication of 5-indacene (19), however, provided a 10 7i-electron periphery free optimization gave Z>2h symmetry and bond orders and charges as shown. A quite different electronic structure has been obtained in this case from that in the dianion. Whereas the latter has a peripheral delocalized 7i-electron system, the cation appears to have a 6 n-electron system in the central ring with allyllic 2 7i-electron systems in the outer rings. Charge is largely concentrated on the end atoms of the allyl systems however, there is little interaction between the three delocalized systems <84T4455>. 

The authors found that the in situ electrochemical potentiokinetic reactivation (EPR) data obtained for the monocation I" " showed that the two Mn centers had identical spin densities, and hence the cation showed delocalized mixed valence the near-infrared spectrum supported this postulate. If such a delocalized system existed on the infrared timescale, then two bands would be expected in the carbonyl region of the infrared spectrum of 1+ at frequencies intermediate between those of the Mn(I)/Mn(I) neutral complex, (1861 and 1934 cm ), and those of the fully oxidized Mn(II)/Mn(II) species, which were predicted to be ca. 1966 and 2048 cm on the basis of the bands observed for CpMn(II)(CO)2PPh3 +, (Cp = cyclopentadienyl). In contrast, four intense CO absorptions were observed near 1888, 1931, 1952, and 2003 cm , which were typical of the trapped valent Mn(II)/Mn(I) species. The variation in the response obtained on the EPR timescale, ca. 10 s, to that observed on the infrared timescale, ca. 10 s, was taken by the authors as implying a time-dependent localization process. 

The actual E of a particular system is often not as informative as is the comparison of the delocalized system with a reference system having localized orbitals. Figure 4.11 shows the reference system for allyl a double bond separated by an imaginary barrier from a p orbital that may have 0 (cation), 1 (radical), or 2 (anion) electrons. The molecular orbital description of that system, then, is simply a sum of the HMOs of the double bond and of the p orbital. Again, it does not matter whether we are talking about the cation, radical, or anion in Figure 4.11. The HMOs of the reference system are simply those of ethene (E = a -H /3, = a — /8) superimposed on the one HMO for an isolated p orbital (E = a). 

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