Hexamethylbenzene, protonation

In a related example, the [D, A] complex of hexamethylbenzene and maleic anhydride reaches a photostationary state with no productive reaction (Scheme 17). However, if the photoirradiation is carried out in the presence of an acid, the anion radical in the resulting contact ion pair14 is readily protonated, and the redox equilibrium is driven toward the coupling (in competition with the back electron transfer) to yield the photoadduct.81... 

Since the substituted hydroquinones and quinone dioximes are better electron donors than hexamethylbenzene (as established by cyclic voltammetric studies), donor-induced disproportionation (to generate NO+ NOf) is even more favored. Furthermore, either two successive one-electron oxidations of hydro-quinone (or quinone dioxime) by NO + followed by the loss of two protons from the dication or two sequential oxidation/deprotonation steps complete the oxidative transformation in equation (97). Importantly, the ready aerial oxidation of NO to NO provides the basis for the nitrogen oxide catalysis of hydroquinone (or quinone dioxime) autoxidation as summarized in Scheme 26. 

Fig. 4. PMR-spectrum of the proton addition complex of hexamethylbenzene. (a) HF + BFa at 188°K (Brouwer et al., 1965a), (b) at 243°K, rapid exchange (MacLean and Mackor, 1962). (CH multiplet, CHa doublet).

Methylbenzenes lose a proton from a methyl group to form a benzyl radical. In aqueous M-percbloric acid this reaction is fast with a rate constant in the range 10 lO s and the process is not reversible [24]. The process becomes slower as the number of methyl substituents increases, Hexaethylbenzene radical cation is relatively stable. When the benzyl radical is formed, further reactions lead to the development of a complex esr spectrum. Anodic oxidation of hexamethylbenzene in trifluoroacetic acid at concentrations greater than 1 O M yields the radical-cation I by the process shown in Scheme 6.1 [14], Preparative scale, anodic oxidation of methylbenzenes leads to the benzyl carbonium ion by oxidation of the benzyl radicals formed from the substrate radical-cation. Products isolated result from further reactions of this carbonium ion. 

In dimethylsulfoxide, the two starting cation radicals of Scheme 1-35 have pKa values of -20 and -25, respectively (Bordwell Cheng 1989). It is clear that both species give rise to the stabilized carboradicals after deprotonation. Electron-donating substituents increase the stability of the arene cation radical and render the odd-electron species less acidic for example, the cation radical of hexamethylbenzene has a pKa value of only 2 in AN (Ama-tore Kochi 1991). The cation radical of tris(bicyclopentyl)annelated benzene is not prone to proton loss, due entirely to the spin-charge location more or less in the aromatic (nodal) phase (Rathore Lindeman et al. 1998), Scheme 1-36. 

In the aromatic ring of hexamethylbenzene, the electron density is so much higher than in toluene, that when it is dissolved in liquid HF (Kilpatrick and Luborsky, 1953), a proton becomes attached to the hexamethylbenzene molecule and this hydrocarbon is ionized as a relatively strong base (the ionization constants for hexamethylbenzene and toluene in HF at 20° are 5 x 10-2 and 2 x 10-7 respectively). 

Toluene, durene, hexamethylbenzene, 1- and 2-methylnaphthalenes are oxidized to the corresponding benzaldehydes by irradiation in oxygen-equilibrated acetonitrile sensitized by 1,4-dicyanonaphthalene, 9-cyano-, 9,10-dicyano-, and 3,7,9,10-tetracyanoanthracene. The reaction involves proton transfer from the radical cation of the donor to the sensitizer radical anion or the superoxide anion, to yield the benzyl radical which is trapped by oxygen. In the case of durene, some tetramethylphthalide is also formed with this hydrocarbon it is noteworthy that the same photosensitization, when carried out in an nonpolar medium, yields the well-known singlet oxygen adduct, not the aldehyde [227,228] (Sch. 20). 

The cationic hexamethylbenzene ruthenium complex 254 can be depro-tonated by treatment with t-BuOK to give the ruthenium complexes 255 and 256 in which the r 4-o-xylylene ligand is coordinated via its exocyclic diene (Scheme 23). Protonation of 255 gives the dication 15c, whereas protonation of 256 produces the fluxional tj3-benzyl derivative 257 which is stabilized by an agostic C—H bond (156,157). 

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... 

On the basis of the evaluation of the proton affinity (860.6 kJmoP for hexa-methylbenzene and 845.6 kJmoP for tetramethylbenzene 148)), the possibility of obtaining hexamethylbenzene and tetramethylbenzene as carbocations in the pores of a zeolite had been excluded. However, Haw and co-workers 146) recently demonstrated by means of NMR spectroscopy that H-heptamethylbenzene may be formed in the cavities of a H(3 zeolite. H-hexamethylbenzene and H-tetramethyl-benzene ions have been observed in zeolite H(3 by a combination of IR and UV-visible spectroscopies 149,150). DRS UV-Vis- and FTIR spectroscopy proved to be techniques well suited to verify, under reaction conditions, the existence of stable H-hexamethylbenzene and H-tetramethylbenzene in the zeolite. Owing to the symmetry properties of H-hexamethylbenzene and H-tetramethylbenzene, characteristic changes of their vibrational features were observed when the aromatic system was perturbed upon protonation. In the same study it was found that the lower polymethylbenzene homologues, such as 1,3,5-trimethylbenzene (PA = 836.2 kJmol ), did not undergo appreciable protonation in H(3 zeolite. On the basis of these results, a proton affinity limit for hydrocarbons that form stable... 

An intermediate case occurs in another Ru complex, which is formed (equation 23) by endo protonation of the i -Me4T in the i -hexamethylbenzene complex (r] -Me4T)Ru()] -C6Me6). The C(sp )-S distance (1.91(1)A) in (10) is considerably longer than a normal C(sp )-S bond... 

Complex formation results in a downfield change in the chemical shift of the methyl protons and a decrease in the double-bond infrared stretching frequency from 1680 to 1532 cm h In the presence of a small excess of PdCl2, the complex is rapidly converted to hexamethylbenzene and palladium chloride. 

The acetamidation of alkylaromatics has been partly discussed [Eq. (11)]. Methyl-substituted benzenes are particularly good substrates for side-chain acetamidation, and they have featured in numerous product and mechanistic studies [9-12,21-23,160-162] ethylbenzene and isopropylbenzene give other products predominantly [163]. Hexamethylbenzene has been a favored substrate for mechanistic investigations of acetamidation [160,164], and in this case there is evidence [165] that on the time scale of conventional cyclic voltammetry, proton loss is rapid from hexamethylbenzene radical cation but relatively slow from hexaethylbenzene. 

These are similar to those of the corresponding bis(ethylene) complex. The 100-MHz H nmr spectrum in C6D6 (internal TMS) shows a pair of multiplets at 6 2.41 and 4.41 due to the outer and inner coordinated diene protons, respectively, a multiplet at 5 1.71 due to the methylene protons, and a singlet at 5 1.95 due to the methyl protons of hexamethylbenzene. 

Proton exchange among the three possible protonation sites in mesitylene (the —C—H sites) appeared to occur without intervention of an acid molecule to carry the proton. The activation energy of this process has been measured as 10 kcal mole-1. In hexamethylbenzene the C-methyl positions are protonated and the proton jump process has the same activation energy. In pentamethylbenzene the proton is always located on the —C—H site. Later results of MacLean and Mackor (1962) are more comprehensive. Intermolecular proton transfer via a solvent molecule has been established for mesitylene, anisole and m-xylene, but in hexamethylbenzene the transfer is always intramolecular. High activation energies of at least 8 kcal mole-1 were measured for proton transfers from the carbonium ion and this was associated with a weak interaction between... 

The relative strengths of weakly basic solvents are evaluated from the extent of protonation of hexamethylbenzene by trifluoro-methanesulfonic acid (TFMSA) in those solvents or from the effect of added base on the same protonation in solution in trifluoroacetic acid (TFA), the weakest base investigated. The basicity TFA < di-fluoroacetic acid < dichloroacetic acid (DCA) < chloroacetic acid < acetic acid parallels the nucleophilicity. 2-Nitropropane appears to be a significantly stronger base than DC A by the first approach, although in the second type of measurement, the two have essentially equal basicity. The discrepancy is due to an interaction, possible for hydroxylic solvents such as DC A, with the anion of TFMSA. This anion stabilization is a determining factor of carbocationic reactivity in chemical reactions, including solvolysis. A distinction is made between carbocation stability, determined by structure, and persistence (existence at equilibrium, e.g., in superacids), determined by environment, that is, by anion stabilization. 

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