Carbenium ion center

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

Propagation proceeds by successive additions of monomer molecules to the carbenium ion center... 

Since the solvents used (e.g., chlorinated hydrocarbons, benzene, THF) are only mildly polar, the negative counter-ion will be held near the propagating carbenium ion center. Highly polar solvents are not generally useful since they either react with and destroy the initiator and propagating centers or deactivate them by strong complexation. 

It was found that Cope rearrangement of the structurally rigid tetracyclic molecule 506 is remarkably accelerated by creating a remote (i.e. non-conjugated) carbenium ion center by an ionization of a ketal group (equation 191)249. The possibility of both classical and non-classical ion participation in this Cope rearrangement was revealed by using MNDO calculations. 

A number of nucleophiles X smoothly add to the carbenium ion center of 227 (in acetonitrile)... 

It is difficult to interpret these large deshieldings in any way other than as a direct proof that (i) the hybridization state of the carbon atom at the carbenium ion center is sp2 and (ii) at the same time, the sp1 center carries a substantial positive charge. 

The experimental carbon Lv binding energy difference (3.9 eV) between the carbenium ion center and the remaining three carbon atoms is in the limit of that predicted by ab initio calculation (4.45 eV). Comparable results were obtained for the tert-pentyl cation (AIih, c c = 4 0.2 eV). 

The reactivity of superelectrophile 46 precludes its direct observation by NMR, but as discussed in Chapter 2, stabilization of the carbenium ion center with para -methoxyphenyl groups allowed the dicationic species (5) to be observed at low temperature. 

When nitronium tetrafluoroborate was attempted to react with the trityl cation in CH2CI2 or sulfolane, no nitration occurred due to the deactivating effects of the carbenium ion center in 215. Nitration of deactivated substrates is also readily accomplished by reaction with NO2CI with three mole excess AICI3 suggesting Lewis acidic electrophilic solvation of the nitronium cation (217, eq 62).105... 

In dication 188, the NBO charge at the carbenium ion center is +0.69 and at the acyl carbon is +1.09. The tert-butyl cation has been found to have NBO charge at its carbenium center of +0.67, suggesting a modest superelectrophilic activation of the carbenium ion center in 188, compared with the tert -butyl cation. When charges on the methyl groups are also considered, structure 188 is similar to the protosolvated tert -butyl cation 196. 

Although formally considered a 1,5-dication, 50 possesses a structure in which the carbenium centers are constrained at a distance of separation of 3.11 A. NMR studies show the carbenium ion centers at 513C 207.7, consistent with the carbocationic structure 50. In cyclic voltamographic analysis, the compound 51 shows an especially high oxidation potential (two-electron oxidation peak at 1.10 V), when compared to analogous dications and triarylmethyl monocations.19 It has also been shown that... 

More highly stabilized l,8-bis(diarylmethyl)naphthalene dications have been prepared, including the p -methoxyphenyl derivative 53.20 This dication is generated from ionization of the diol in HBF4 and (CF3CO)2O.20a Dication 53 has been characterized by experimental studies (single crystal X-ray analysis and NMR) and theoretical calculations. The carbenium ion centers are found to be separated by just 3.076 A (X-ray and ab initio results) and show 13C NMR resonances at A3C 191.8. Two electron reduction is also shown to give the acenaphthene derivative 54. 

Dication 56 has been isolated and studied by crystallography, revealing a separation of the carbenium ion centers by 3.66 A. 

The contact ion pair R R2HC LBr , in contrast to a free carbenium ion R R2HC , is chiral. Starting from enantiomerically pure (R)-2-bromooctane, the contact ion pair first produced is also a pure enantiomer. In this ion pair, the bromide ion adjacent to the carbenium ion center partially protects one side of the carbenium ion from the reaction by the nucleophile. Consequently, the nucleophile preferentially reacts from the side that hes opposite the bromide ion. Thus, the solvolysis product in which the configuration at the reacting C atom has been inverted is the major product. To a minor extent the solvolysis product with retention of configuration at the reacting C atom is formed. 

Tab. 2.2 Stabilization of a Trivalent Carbenium Ion Center by Conjugating Substituents Experimental Findings and Their Explanation by Means of Resonance Theory...

Fig. 2.17. MO interactions responsible for the stabilization of trivalent carbenium ion centers by suitably oriented unsaturated substituents ("conjugation").

The very large inherent differences in the stability of carbenium ions are reduced in solution—because of a solvent effect—but they are not eliminated. This solvent effect arises because of the dependence of the free energy of solvation AGhyd(R ) on the structure of the carbenium ions (Table 2.1, row 2). This energy becomes less negative going from Me to Ph— CH2 , as well as in the series Me —> Et —> r Pr —> tert-Bu . The reason for this is hindrance of solvation. It increases with increasing size or number of the substituents at the carbenium ion center. 

Fig. 2.20. C,C bond-length reductions and elongations in the cumyl cation (compared to cunene) confirming the stabilization of the carbenium ion center through conjugation, and H3d -C bond-length reduction (compared to a-methylstyrene) due to the additional stabilization of the carbenium ion center caused by hyperconjugation (cf. Figure 2.19).

The [l,2]-alkyl migration A —> B of Figure 14.7 converts a cation with a well-stabilized tertiary carbenium ion center into a cation with a less stable secondary carbenium ion center. This is possible only because of the driving force that is associated with the reduction of ring strain a cyclobutyl derivative A is converted into a cyclopentyl derivative B. 

The reactions shown in Scheme 4.49 imply an ease of formation for carbocationic intermediates with carbenium ion center(s) at the bridgehead position(s). Studies aimed at the generation and spectral investigation of these cationic species under long-life conditions will most certainly be forthcoming as the peculiarity of the fenestrane framework reveals itself in the appearance of new and unusual structural effects. 

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