Hexamethylbenzenium ions

However, methylation of hexamethylbenzenium ion can also occur, leading to the formation of heptamethylbenzenium ion. The latter can split off, this time propylene, by a series of ring contraction and ring expansion reactions [116, 117]. 

The conversion of ethylene on a fresh zeolite HZSM-5 catalyst, which had not been used beforehand for methanol conversion, led to the spectra shown in Fig. 37c. The MAS NMR spectrum consists of signals at 14, 24, and 34 ppm caused by alkyl groups of cyclic compounds. Furthermore, a broad signal in the chemical shift range of alkenic and aromatic compounds appeared at ca. 120 ppm. The UV/Vis spectrum consists of bands similar to those shown in Fig. 37b and an additional weak band at ca. 450 nm. The latter may be attributed to condensed aromatics or trienylic carbenium ions (301). A weak shoulder observed at ca. 400 nm is an indication for the formation of hexamethylbenzenium ions (302). 

The first ion shown to undergo a degenerate 1,2-benzyl shift, the i-benzyl-1,2,3,4,5,6-hexamethylbenzenium ion [279 R = CH2Ph], was studied by Sazonova et al. (1977). That the rearrangements occurred by 1,2-benzyl shifts was confirmed using spin-transfer saturation and the activation energy obtained by dynamic H-nmr spectroscopy was 12.4 ih 0.3 kcal mol =... 

Another interesting migrating group is phenyl which was studied in the benzenium system by Borodkin et al. (1977). The 1-phenyl-1,2,3,4,5,6-hexamethylbenzenium ion [279 R = Ph] was found to be in equilibrium with its methyl shifted 3-phenyl isomer [284]. Of the other two possible isomers the 2-phenylhexamethylbenzenium ion [285] was never observed and the 4-phenyl hexamethylbenzenium ion [286] could only be prepared from the corresponding alcohol precursor at low temperature (—100°C). The structure of... 

The hexamethylbenzene skeleton was also used by Olah et al. (1972b) in the study of 1-nitro- and 1-chloro-benzenium ions under conditions of long life. Less substituted ions decompose by proton elimination and make difficult the study of the intermediates in electrophilic aromatic substitution. Treatment of hexamethylbenzene with NO+BF in FSO H-SOa gave a solution of l-nitro-l,2,3,4,5,6-hexamethylbenzenium ions [3(X)] undergoing degenerate rearrangement via 1,2-nitro shifts with an activation energy of... 

With another member of this family of degenerate benzenium ions, the l-bromo-l,2,3,4,5,6-hexamethylbenzenium ion [303], prepared from in FSO3H-SO2CIF and hexamethylbenzene by Detsina et al. (1976), the spin-... 

The migratory aptitudes of some important migrating groups in the 1-R-hexamethylbenzenium ion system are summarized in Table 2 in the form of activation energies. Attempts have recently been reported by Sutula and Koptyug (1979) to verify the experimental migratory aptitudes of H, CH3, NO2 and Cl using extended Huckel calculations. 

Activation energies (f,) for degenerate 1,2-shifts of migrating groups (R) in the l-R-l,2,3,4,5,6-hexamethylbenzenium ion system... 

This procedure was later used to generate a number of other l-R-1,2,3,4,5,6-hexamethylbenzenium ions (R=C2Hs, CHjClQHj heptaethylben-zenium ion (cf. 4-P-X-ethyl-l,l,2,3,5,6-hexamethylbenzenium ions with... 

The isomeric tramfonnations of arenium ions themselves due to 1,2-shifts of the substituents can alK> be used to obtain new ions. Thus, the 1-methoxy-l,2,3,4,5,6-hexamethylbenzenium ion formed by dissolving 1,4-dimethoxyhexamethylcyclohexa-2,5-diene in HSO3F at —75 °C is wholly converted, as the temperature rises to —30 °C, into the 6-methoxy-l,l,2,3,4,5-hexamethylbenzenium ion which at —10 °C is rearranged into the 4-methoxy-l,l,2,3,5,6-hexamethylbenzenium ion... 

An mter ting example of an ion with a meta-located oxygen function is the std-phonated 3-hydroxy-l,l,2,4,5,6-hexamethylbenzenium ion (49) fonned by the action of SO3 on 2- and 4-hydroxy-isomers. The chemical shifts of its CHj-group protons are as follows ... 

Another example is the 3-sulphonated 1,1,2,4,S,6-hexamethylbenzenium ion In the case of substituents conjugating with the x-system of an ion the changes of die chemical shifts of sp -hybridized carbons relative to aromatic precursor are no longer constant reflecting the participation of the substituent in the positive dhaige delocalization. On 4-X-benzenium ions (X = F, Q, Br, CgHj, OH, OCH3) the values of 2 SfCj) are shown to correlate with Op - of the X-substituents ... 

An interesting example of determining the nature of an added particle is reported in To prove that the interaction of hexamethylbenzene with HNO3 in HSO3F yields the l-nitro-l,2,3,4,5,6-hexamethylbenzenium ion (22a) nitric acid was enriched with the isotope. The accompanying doublet splitting of the Cj atom signal (8.3 cps) has corroborated the Cj—N bond in the ion (see also... 

Vibrational spectra of arenium ions with heteroatomic substituents are very scarce. The protonation of hexamethylcyclohexa-2,5-dienone 71) and -2,4-dienone forming the complex (77) HCl - 2 AlClj in CHjClj shifts the stretching vibration band of the carbon-oxygen bond from 1615 cm (C=0)to 1510 cm (see also The band of the OH group at 3450cm and the before mentioned data are in good agreement with those of the PMR, i.e. the complex is a salt of the 4-hydroxy-1,1,2,3,5,6-hexamethylbenzenium ion (57 a)... 

The 1,2-shifts of X-substituents in the l-X-l,2,3,4,5,6-hexamethylbenzenium ions (22) yielding ions of the same structure correspond with degenerated rearrangements to be discus in Sect. IV.2.B. 

H-l,2,3,4,5,6-hexamethylbenzenium ion does not form isomeric ions either. In this case the equilibrium should be quickly established hence, the 1-H-isomer is thermodynamically more stable than the 3-H-isomer. 

For the ions with the substituents X = CH2CI, CH2CH3, QH, (with CTp x — 0.01, —0.295 and —0.179, respectively the equilibrium mixtures contain 1-X- and 3-X-isomers. Thus, dissolving 4-methylene-l-chloromethyl-l,2,3,5,6-pentamethyl-cyclohexa-2,5-dienein HSO3F at —80- —50 °C yields the l-chloromethyl-1,2,3,4,5,6-hexamethylbenzenium ion which at —20 C turns in about 20% into the 3-chloro-methyl isomer Similarly, about 15% of 1-ethyl-1,2,3,4,5,6-hexamethylben-... 

Benzyl-l,l,2,3,5,6-hexamethylbenzenium ion generated at — 70 °C in HSOjF converts, at —55 °C, completely into the 3-benzyl isomer The further rise of the temperature to —40 °C decomposes the 3-benzyl isomer after its rearrangement into a 1-benzyl-substituted ion the benzyl group splits off and leaves hexamethyl-benzene. 

The l-methoxy-l,2,3,4,5,6-hexamethylbenzenium ion (Pi) generated by dissolving l,4-dimethoxy-l,2,3,4,5,6-hexamethylcyclohexa-2,5-dienein HSOj at —75 °C, quickly rearranges at —30 °C into the 2-methoxy-l,l,3,4,5,6-hexamethylbenzenium ion 94) which at —10 °C is completely converted into the 4-methoxy isomer 95)... 

An attempt to generate the l-hydroxy-l,2,3,4,5,6-hexamethylbenzenium ion (ftj) by dissolving l-methoxy-4-hydroxy-l,2,3,4,5,6-hexamethyl-cycIohexa-2,5-diene in HSO3F at —80 °C resulted in the formation of the ion (93) and the 2-hydroxy-... 

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