Arenium cation stability

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). 

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. 

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. 

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

The carbocation formed m this step is a cyclohexadienyl cation Other commonly used terms include arenium ion and a complex It is an allylic carbocation and is stabilized by electron delocalization which can be represented by resonance... 

An important contribution to silylium ion chemistry has been made by the group of Muller, who very recently published a series of papers describing the synthesis of intramolecularly stabilized silylium ions as well as silyl-substituted vinyl cations and arenium ions by the classical hydride transfer reactions with PhjC TPEPB in benzene. Thus, the transient 7-silanorbornadien-7-ylium ion 8 was stabilized and isolated in the form of its nitrile complex [8(N=C-CD3)]+ TPFPB (Scheme 2.15), whereas the free 8 was unstable and possibly rearranged at room temperature into the highly reactive [PhSi /tetraphenylnaphthalene] complex. ... 

The results indicate that the formation of long-lived trimethyl substituted silyl cations, in the presence of aromatic solvents, as claimed by Lambert et al.95 is not feasible under these conditions. Persistent silicenium ions require sterically more shielding substituents at silicon or hypercoordinative stabilization.96 98 13C and 29Si NMR chemical shifts were calculated for a series of disilylated arenium ions 85 using density functional theory (DFT). The calculations predict consistently the unsaturated carbon atoms to be too deshielded by 8-15 ppm. Applying an empirical correction, the deviation between experiment and theory was reduced to -0.4 to 9 ppm, and the 13C NMR chemical shift of the highly diagnostic cipso is reproduced by the calculations (Ad = -3.8 to 2.7 ppm).99... 

Silylium ions, which are not protected sterically or are not stabilized either electronically or by intramolecular interaction with a remote substituent do interact strongly with the solvent and/or the counteranion. The reaction of the transient silylium ion with solvents like ethers, nitriles and even aromatic hydrocarbons lead to oxonium, nitrilium and arenium ions with a tetrahedral environment for the silicon atom. These new cationic species can be clearly identified by their characteristic Si NMR chemical shifts. That is, the oxonium salt [Me3SiOEt2] TFPB is characterized by S Si = 66.9 in CD2CI2 solution at —70°C. " Similar chemical shifts are found for related silylated oxonium ions. Nitrilium ions formed by the reaction of intermediate trialkyl silylium ions with nitriles are identified by Si NMR chemical shifts S Si = 30—40 (see also Table VI for some examples). Trialkyl-substituted silylium ions generated in benzene solution yield silylated benzenium ions, which can be easily detected by a silicon NMR resonance at 8 Si = 90—100 (see Table VI). ... 

The intramolecular stabilization of germyl cation 34 by a remote aryl substituent was demonstrated by the NMR chemical shifts of the coordinated aryl ring. The chemical shift pattern found for the coordinated arene ring of 34 is characteristic for arenium ions and it closely resembles that found bissilylated arenium ions 78. 

Owing to the high Lewis acidity the group 14 organometallic cations are polymerization catalysts par excellence. so Silanorbonyl cations and triethylsilyl arenium have been shown to be efficient catalysts for metal-free hydrosilylation reactions. Chiral silyl cation complexes with acetonitrile have been applied as cata -lysts in Diels Alder-type cyclization reactions °792 intramolecularly stabilized tetracoordinated silyl cations have been successfully used as efficient catalysts in Mukaiyama-type aldol reactions. 

Energies for various possible arenium ions and regioisomeric benzylic cations were computed at the DFT B3LYP/6-31G(d) level or by AMI for comparison with the experimental results. These findings provided further evidence in support of the stability sequence 1-pyrenyl > 4-pyrenyl > 2-pyrenyl in a-pyrene-substituted carbocations as models for the intermediates arising from BaP-epoxide ring opening (Fig. 10). 

Wheland intermediates, a complexes, or arenium ions In the case of benzenoid systems they are cyclohexadienyl cations. It is easily seen that the great stability associated with an aromatic sextet is no longer present in 1, though the ion is stabilized by resonance of its own. The arenium ion is generally a highly reactive intermediate and must stabilize itself by a further reaction, although it has been isolated (see p. 504). 

The novel arenium ion 95 was synthesized266 by one-electron oxidation of the triphenylene-based starting compound to form a radical cation which abstracted a chlorine atom with a concomitant rearrangement to yield the hexachloroantimonate salt. The arenium ion character is apparent from the 13C spectrum (three signals at 813C 212.9, 187.6, and 173.6) and from the bond distances, which are very close to those shown for ion 91. Cation 95 can be stored at room temperature for months. This exceptional stability was attributed to the annelation to the two bicyclo[2.2.2]octane units and the spiroconjugation effect of the fluorenyl moiety.267... 

Product formation was interpreted in terms of transalkylation of substituted triphenylmethanes. Protonation at the ipso position of the substituted phenyl ring to form arenium ion 64 followed by the C—C bond breaking yields the diphenylmethyl cation, which alkylates benzene or is stabilized by hydride transfer (Scheme 5.30). The protonated intermediate 64 is highly unstable when the ring has an electron-withdrawing substituent. Consequently, its transformation is extremely slow and the primary product triphenylmethane can be isolated. 

This particular carbocation is a resonance-stabilized one of the allylic type. It is a cyclo-hexadienyl cation (often referred to as an arenium ion). 

---