Resonance stabilization carbenium ion

In the first step of the actual Ar-SE reaction, a substituted cyclohexadienyl cation is formed from the electrophile and the aromatic compound. This cation and its derivatives are generally referred to as a sigma or Wheland complex. Sigma complexes are described by at least three carbenium ion resonance forms (Figure 5.1). There is an additional resonance form for each substituent, which can stabilize the positive charge of the Wheland complex by a pi electron-donating (+M) effect (see Section 5.1.3). This resonance form is an all-octet formula. 

Aliphatic primary amines also undergo the diazotization reaction in weakly acidic solution however the resulting aliphatic diazonium ions are generally unstable, and easily decompose into nitrogen and highly reactive carbenium ions. The arenediazonium ions are stabilized by resonance with the aromatic ring ... 

The range of structural alternatives explored by valency-deficient carbon species and the subtle interplay of substituents is remarkable. Scheme 7.6 (ORTEP adapted from reference 31) illustrates an example of an X-ray structure clearly describing a localized [C-H C+] carbenium ion (A) where a symmetric bridging structure [C-H-C] + (B) could have been assumed. In this case it is proposed that a charge-transfer interaction between the resonance delocalized cation and the adjacent electron-rich carbazol moiety may be responsible for the stabilization of the localized form over the three-center, two-electron (3c-2e) bridging structure. 

Allenyl cations 1 are a stabilized form of vinyl cations1-3 in which the /1-carbon atom of the vinylic structure is part of the substituent which effects the stabilization of the ion via its electron-donating ability. This leads to a resonance hybrid having formally the alkynyl cation structure 2. Allenyl cations should be distinguished from the allenyl substituted carbenium ions 3 formulated as the mesomeric structures of the vinyl cations 4 (dienyl cations) stabilized by an w-vinyl group (equation 1). 

Arnett and colleagues [219,220] measured the enthalpies of a considerable number of processes where a resonance-stabilized carbenium ion R was reacted with a resonance stabilized carbanion, oxanion, thioanion, or nitroanion, R(T, in mixtures of sulfolane (95%) and 3-methylsulfolane (5%), at 298.15 K ... 

The related reaction of 10 with ferrocenylmethylamine affords, in addition to the C3 adduct (as the minor product), a 2-ferrocenylethyl(dimethylamino)allenyli-dene complex as the major product, formed by migration of the resonance-stabilized [FcCH2] carbenium ion to the terminal carbon atom of the chain (Scheme 3.24) [46], The formation of pyrrolyl- and indolyl-substituted allenylidene complexes by reaction of complex 10 with various pyrroles and N-methylindole [47] has also been rationalized as involving initial attack of the electron-rich heterocyde on C3 of 10 followed by proton migration to the terminal =CH2 entity of the intermediate butenynyl-substituted a-complex (Scheme 3.25). 

However, with triphenylcarbenium-tetrafluoroborate, l.l-dimethoxy-2.6-di-phenyl-4-methyl-X -phosphorin 220 easily splits off a hydride ion to form a resonance-stabilized carbenium-phosphonium-oxonium-ion 221, as Schafer has found The high electron density of the X -phosphorin system, which is in exellent agreement with the theoretical model of Schweig and coworkers (see p. 115), facilitates this interesting reaction. 

The allyl cation (9) is the simplest member of the class of resonance-stabilized cations that includes the alkyl-substituted cyclopentenyl cations. But one could also say that the carbenium ion (CH3) is the simplest member of a class of cations that includes the trityl cation. In each case, 10 or so orders of magnitude of acidity separate the primitive member from its more elaborate derivatives. 

In early 1993, Haw and co-workers (107) reported in situ studies of allyl alcohol-/-13C on HZSM-5 and CsHX. No persistent carbenium ions were observed, but 1,3 label exchange was observed for the alcohol on the weakly acidic zeolite. We interpreted this as support for a transient allyl cation. The low stability of this cation was invoked to explain the failure to observe this species as a persistent species. Downfield signals observed in that study were attributed to the formation of propanal. Later in 1993, Biaglow, Gorte, and White (BGW) (108) reported similar studies conducted at different loadings and assigned a downfield resonance (variously reported at 216 and 218 ppm by BGW) to the allyl cation in HZSM-5. 

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. 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

Scheme 8 The role of formaldehyde in the formation of carbenium-iminium ion 17 and resonance stabilization of the latter...

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

Allyl halides heterolyze just as easily as benzyl halides because they also produce a resonance-stabilized carbenium ion. Even faster heterolyses are possible when the charge of the resulting carbenium ion can be delocalized by more than one unsaturated substituent and can thereby be stabilized especially well. This explains the remarkably high SN1 reactivities of the benzhydryl halides (via the benzhydryl cation) and especially of the triphenylmethyl halides (via the trityl cation) ... 

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