Benzene ionic hydrogenation

Under the same reaction conditions, however, protonated acetone does not react with benzene. Dications 112 and 113 have been shown to undergo ionic hydrogenation in the presence of cyclohexane, an exceptionally weak nucleophile.45ef... 

Other diprotonated acyl-pyridines have likewise been studied.61 In studies of 5-, 6-, 7-, and 8-hydroxyquinolines and 5-hydroxyisoquinoline, dicationic intermediates like 185 (Table 4) were found to be involved in superacid catalyzed reactions with benzene and cyclohexane.59 For example, 8-hydroxyquinoline (187) reacts in CF SOsH-SbFs to generate dications (188 and 189) and undergoes ionic hydrogenation in the presence of cyclohexane (eq 64). Compound 187 also reacts with benzene in suspensions of aluminum halides (eq 65). 

Interestingly, the comparable monocation (190) is not reactive towards benzene or cyclohexane. This is an indication of the superelectrophilic character of dication 188. The isomeric hydroxyquinolines and 5-hydroxy-isoquinoline react with 5-7 molar excess of aluminum chloride and cyclohexane at 90°C to give ionic hydrogenation products, and the corresponding distonic superelectrophiles (191-193) are proposed as intermediates. 

In a process that is related to the ionic hydrogenation method discussed above, Wristers has discovered that the unusual combination of H2/isopentane/HF/TaFs reduces thiophene to tetrahydrothiophene in 80% yield. These conditions also reduce dibenzothiophene to hexahydrodibenzothiophene. It has also been reported that Zn/TFA reduces thiophenes to 2,5-dihydrothiophenes (equations 31 and 32). However, both acetic acid and HCl/benzene are ineffective with zinc in this regard. ... 

Deakyne. C.A. Meot-Ner (Mautner). M. Unconventional ionic hydrogen bonds. 2. NH - -k. Complexes of oniura ions with olefins and benzene derivatives. J. Am. Chem. Soc. 1985. 107. 474-479. 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

It is important to note that benzene does not behave like a typical cyclic olefin in that the benzene ring undergoes ionic substitution rather than addition reactions the ring also resists hydrogenation and is chemically more inert. Despite this, it is still a common practice to represent benzene with three double bonds as if it were 2,4,6-cyclohexatriene,... 

In fact, partial hydrogenations are rarely described with soluble nanoparticle catalysts. Two examples are explained in the Uterature, one reported by Finke and coworkers in the hydrogenation of anisole with polyoxoanion-stabihzed Rh(0) nanoclusters [26] and one reported by Dupont and coworkers in the hydrogenation of benzene with nanoscale ruthenium catalysts in room temperature imidazoUiun ionic Uquids [69]. hi these two cases, the yields are very modest. 

Scheme 7 Partial hydrogenation of benzene by Ru(0) nanocatalysts in room temperature imidazolium ionic liquid...

Finally, these particles generated in ionic liquids are efficient nanocatalysts for the hydrogenation of arenes, although the best performances were not obtained in biphasic liquid-liquid conditions. The main importance of this system should be seen in terms of product separation and catalyst recycling. An interesting alternative is proposed by Kou and coworkers [107], who described the synthesis of a rhodium colloidal suspension in BMI BF4 in the presence of the ionic copolymer poly[(N-vinyl-2-pyrrolidone)-co-(l-vinyl-3-butylimidazolium chloride)] as protective agent. The authors reported nanoparticles with a mean diameter of ca. 2.9 nm and a TOF of 250 h-1 in the hydrogenation of benzene at 75 °C and under 40 bar H2. An impressive TTO of 20 000 is claimed after five total recycles. 

Ionic liquids have also been applied in transfer hydrogenation. Ohta et al. [110] examined the transfer hydrogenation of acetophenone derivatives with a formic acid-triethylamine azeotropic mixture in the ionic liquids [BMIM][PF6] and [BMIM][BF4]. These authors compared the TsDPEN-coordinated Ru(II) complexes (9, Fig. 41.11) with the ionic catalyst synthesized with the task-specific ionic liquid (10, Fig. 41.11) as ligand in the presence of [RuCl2(benzene)]2. The enantioselectivities of the catalyst immobilized by the task-specific ionic liquid 10 in [BMIM][PF6] were comparable with those of the TsDPEN-coordinated Ru(II) catalyst 9, and the loss of activities occurred one cycle later than with catalyst 9. 

In the ideal biphasic hydrogenation process, the substrate will be more soluble or partially soluble in the immobilization solvent and the hydrogenation product will be insoluble as this facilitates both reaction and product separation. Mixing problems are sometimes encountered with biphasic processes and much work has been conducted to elucidate exactly where catalysis takes place (see Chapter 2). Clearly, if the substrates are soluble in the catalyst support phase, then mixing is not an issue. The hydrogenation of benzene to cyclohexane in tetrafluoroborate ionic liquids exploits the differing solubilities of the substrate and product. The solubility of benzene and cyclohexane has been measured in... 

Figure 8.6 The effect of stirrer speed on turnover frequency for the hydrogenation of benzene to cyclohexane. (Note that the turnover reaches its maximum at lower stirrer speed for the ionic liquid). , ionic liquid , water...

In kinetic studies, the abstraction of F from CF3CH2OH by ArN2+BF4 was shown to be an ionic process, whereas abstraction of an a-H atom from CH3CH2OH was determined to be a radical process. In other kinetic work, the expansion of cyclopentanones has been used to measure the kinetics of hydrogen-atom abstraction reactions for a range of hydrogen donors in benzene including (Me0)2P(0)H( 1.2 x... 

When an aqueous solution containing chlorobenzene (190 pM) and a nonionic surfactant micelle (Brij 58, a polyoxyethylene cetyl ether) was illuminated by a photoreactor equipped with 253.7-nm monochromatic UV lamps, phenol, hydrogen, and chloride ions formed as major products. It was reported that aromatic aldehydes, organic acids, and carbon dioxide would form from the photoreaction of chlorobenzene in water under similar conditions. A duplicate experiment was conducted using an ionic micelle (triethylamine, 5 mM), which serves as a hydrogen source. Products identified were phenol and benzene (Chu and Jafvert, 1994). 

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