Cation arenium

Vigalok, A. Shimon, L. J. W. Milstein, D. Methylene arenium cations via quinone methides and xylylenes stabilized by metal complexation. J. Am. Chem. Soc. 1998, 120, 477 183. 

Arenium cations were the first carbocations investigated by GIAO-MP2 NMR chemical shift calculations. 

Activating group More reactive than benzene add electrons to the ring, stabilize the arenium cation... 

Substituents already bonded to an aromatic ring influence both the rate of electrophilic substitution and the position of any further substitution. The effect of a particular substituent can be predicted by a consideration of the relative stability of the first-formed arenium cation, formation of which constitutes the rate-lintiting step. In general, substituents that are electron releasing activate the ring to further substitution - they help to stabilize the arenium ion. Substituents that are electron withdrawing destabilize the arenium ion, therefore, are deactivating and hinder further substitution. 

The structural data reported from the X-ray analysis are very similar to the ab initio calculated structural data for silyl-substituted arenium cations. The calculations of the p-trimethylsilyl (319) (Figure 9) and p-triethylsilyl toluenium ions (318) [HF/6-31G(d)] and the silylbenzenium ion 53 [MP2(fc)6-31G(d)] all show similar geometries and the characteristic geometric data resemble closely the main features found in the crystal structure. 

Harman and co-workers738 reported the synthesis of arenium cations stabilized by Os complexation (for example, 415) via protonation with triflic acid of [Os(NH3)5(Ar)](OTf)2 complexes (Ar = benzene, toluene, xylenes, naphthalene, anthracene). 

Electrophile, E+, attacks n electrons on the benzene ring, form arenium cation (ring stabilizes positive charge)... 

The caged species may escape geminate recombination and produce various species that can initiate cationic polymerization. Solvent (RH) often participates in these reactions producing protonic acids. As shown in Eq. (44), protonic acids are also formed by reaction of radical cations with aryl radicals or by Friedel-Crafts arylation. Up to 70% of the protonic acid is formed upon photolysis of diaryliodonium salts [205]. In addition to initiation by protons, arenium cations and haloarene radical cations can react directly with monomer. The efficiency of these salts as cationic initiators depends strongly on the counterions. Those with complex anions such as hexafluoroantimonate, hexafluorophosphate, and triflate are the most efficient. 

Although a mechanism involving a radical cation has been proposed for the Scholl reaction, as indicated by the paramagnetic properties of many polycyclic aromatic hydrocarbons (PAHs) when they are treated with Lewis acids or concentrated sulfuric acid, it is assumed that the Scholl reaction occurs in a manner similar to the Friedel-Crafts Alkylation, involving an arenium cation instead of a radical cation. In detail, the Scholl reaction of hexaphenylbenzene involves the complexation between a Lewis acid and aromatic nucleus, electrophilic addition, and deprotonation,as illustrated here. In the presence of NaCl or HCl, chloride is beneficial for the elimination of aryl hydrogen by the formation of hydrogen chloride, as indicated by the bold chloride. 

Ultimately, however, it was admitted that the involvement of a parallel arenium cation mechanism could not be ruled out, owing to the coexistence and interchangeability of both species (radical cation versus arenium cation) under the conditions of the Scholl reaction. This fact clearly illustrates the continuing dichotomy that is characteristic of attempts to assign these processes to discrete mechanistic categories. Apparently, the extent to which one mechanism operates at the expense of the other mechanism depends on the chemical structure of the particular precursor, and the reaction conditions applied. 

Neither the synthetic scope nor mechanism was significantly investigated until 1935, when Baddeley and co-workers provided new insights into the Scholl reaction. Their work provided the first mechanistic interpretation, suggesting that the homocoupling reaction of naphthalene proceeds via an arenium cation intermediate. Based on experiments in... 

The accepted mechanism, which proceeds through the arenium cation intermediate, occurs as follows. First, protonation of the aryl species 2 occurs to afford the electrophilic o-complex 3. What follows is a two-step electrophilic aromatic substitution process whereby the electrophilic intermediate 3 is attacked by the adjacent aromatic ring to form a new carbon-carbon bond in intermediate 4. Deprotonation of 4 regenerates the aromatic species in intermediate 5. Finally, oxidation/aromatization of the product occurs with the formal expulsion of H2, resulting in the formation of 6. 

The arenium cation mechanistic pathway was first proposed in 1935 by Baddeley and was later reinforced by Balaban and Nenitzescu. Computational calculations performed by King and co-workers have produced several important conclusions. First, it was determined that the arenium cation mechanistic pathway was thermodynamically favored in studies under both vacuum and solvated conditions due to lower energy transition states than those found in the radical cation mechanistic pathway. Second, due to the increasingly exergonic nature of the reaction and the observed nonaccumulation of intermediates, C-C bond formation, in the case of hexaphenylbenzene, was found to occur slowest for the first bond and fastest for the last bond. The arenium cation mechanistic pathway is further supported by evidence that the Scholl reaction can proceed in acidie solutions that do not promote radical formation, such as anhydrous HF. The cationic mechanism has been shown to predominate in strongly protic conditions. ESR and EPR studies have concluded that the radieal cations previously observed during the Scholl reaction are not part of the actual reaction, but... 

Based oti that mechanism, the effective anode syntheses of polynitroalkyl arenes [50] and ylides (if heteroarene is taken instead of arene) were developed. The process involved an intermediate arenium cation following path -ea-e-p and -... 

Nitration of the toluene appears to be a three-step process, as demonstrated with DFT calculations of benzene nitration in the gas phase by Olah and coworkers." The acetyl nitrate readily transfers a nitronium -like moiety to the Jt-system of the toluene close to the para or ortho sites. This is a jt-complex with no real bonding interaction. An sp to sp hybridization of the para or ortho carbon must then occur to create a o-complex (aka Wheland intermediate or arenium cation). Finally, this 0-complex must transfer the proton from the para or ortho site to a Brpnsted acid site in the zeolite wall. Upon optimizing the H-beta structure with the acetyl nitrate site and toluene, it became clear that the most likely acid transfer site after creation of the o-complex is the All-02 site in our model, which, as we already... 

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