Hydrogenation of benzene derivatives

The synthesis of enantiopure cyclohexane derivatives has been investigated by hydrogenating arenes by use of chiral auxiliaries bound either to the support of the catalyst or to the substrate, or by use of chiral phase-transfer reagents [15]. Although significant progress has been reported, enantioselectivity is still moderate (maximum 68 % e. e.). 

In recent years the Asahi Corporation has developed a benzene-to-cyclohexene process involving a liquid-liquid two-phase system (benzene-water) with a solid ruthenium catalyst dispersed in the aqueous phase. The low solubility of cyclohexene in water promotes rapid transfer towards the organic phase. An 80000 t annum plant using this process is in operation. Another way to scavenge the intermediate cyclohexene is to support the metal hydrogenation catalyst on an acidic carrier (e. g. silica-alumina). On such a bifunctional catalyst the cyclohexene enters catalytic alkylation of the benzene (present in excess) to yield cyclohexylbenzene [19], which can be converted, by oxidation and rearrangement reactions, into phenol and cyclohexanone. 

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The total hydrogenation of benzene derivatives represents an important industrial catalytic transformation, in particular with the conversion of benzene into cyclohexane, a key intermediate in adipic acid synthesis, which is used in the production of Nylon-6,6 (Scheme 1). This reaction is still the most important industrial hydrogenation reaction of monocyclic arenes [1]. 

Hydrogenation of aromatic nitro compounds [8,18,29] and hydrogenation of benzene derivatives [2,9,21] have been generally accepted as model reactions to check the heterogeneous nature of catalyst, because homogeneous species are not believed to be active. But at least two well-studied examples show that molecular catalysts can hydrogenate benzene [36,37]. 

These were prepared by tethering Rh and Pt complexes to silica-supported metal catalysts (metal = Pd, Ni, Ru, Au). The catalysts are very active in the hydrogenation of benzene derivatives to the corresponding substituted cyclohexanes under mild conditions. The activities are higher than those of the separate homogeneous complexes, complexes just tethered to silica, or the silica-supported heterogeneous catalysts. When the sol-gel-entrapped [Rh2Co2(CO)12] complex was heat-treated at 100°C, immobilized metallic nanoparticles were formed.425 The catalyst thus prepared efficiently catalyzed substituted benzene derivatives. 

Attempts have also been made to develop biphasic methodologies for the hydrogenation of aromatics. The hydrogenation of benzene derivatives was studied using various Ru complexes.468,469 A trinuclear cluster cationic species was isolated as the tetrafluoroborate salt and showed increased activity in hydrogenation.470 The air/ moisture-stable [bmim][BF4] ionic liquid and water with [Rh r -CgHg).,] [BF4] as the catalyst precursor is an effective system under usual conditions (90°C, 60 atm)471 A system composed of stabilized Rh(0) nanoparticles proved to be an efficient catalyst in the hydrogenation of alkylbenzenes 472... 

Table 11.8 Hydrogenation of benzene derivatives under biphasic conditions. Adapted from Ref. [61].

The ring hydrogens of benzene derivatives absorb downfield in the region between... 

Cyclohexane-based systems are almost invariably prepared by catalytic hydrogenation of benzene derivatives (e.g., 4-alkyl-benzoic acids and 4-alkylphenols, see Scheme 6) but the 4-alkoxy systems are not often used because the alkoxy-oxygen atom is then isolated in a non-polarizable region of the product. Low temperature, low pressure procedures are possible but give mainly the cis-product whereas high pressure, high temperature hydrogenations with Raney nickel catalyst produce predominantly the tra i -4-alkylcyclohexane-1 -carboxylic acid, if basic conditions are used, since the proton a- to the carboxylic acid function is... 

After succeeding in the direct synthesis of allyidichlorosilane hy reacting elemental silicon with a mixture of allyl chloride and hydrogen chloride in 1993," Jung el cil. reinvestigated the Friedel-Crafts reactions of benzene derivatives with allyidichlorosilanes in detail (Eq. (2)). 

Two soluble nanocatalysts have been investigated in partial hydrogenation. The results obtained by Einke or Dupont s catalysts are unsatisfactory but prove that nanoparticles are a potential catalyst for this reaction, hi summary, partial hydrogenation of benzene and its derivatives is still a challenge but will be the focus of future research. 

Addition of molten sulfur to limonene in a 9 kl reactor led to a violent runaway exothermic reaction. Small scale pilot runs had not shown the possibility of this. Heating terpenes strongly with sulfur usually leads to formation of benzene derivatives with evolution of hydrogen sulfide. 

At present, the efficient partial hydrogenation of benzene and its derivatives has been rarely described with well-defined soluble nanoparticles catalysts. Nonetheless, this remains an interesting area for research, with promising future applications. 

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. 

The important derivatives of benzene are shown in Table 8.8. Ethylbenzene is made from ethylene and benzene and then dehydrogenated to styrene, which is polymerized for various plastics applications. Cumene is manufactured from propylene and benzene and then made into phenol and acetone. Cyclohexane, a starting material for some nylon, is made by hydrogenation of benzene. Nitration of benzene followed by reduction gives... 

The hydrogenation of benzene and its alkyl-substituted derivatives takes place stepwise (Scheme 11.5). On the basis of instrumental and isotope exchange studies, the involvement of tt-adsorbed (8, 9) and cr-adsorbed (10) species was suggested 96-98... 

This process competes favorably with benzylic hydrogen abstraction in toluene, less in ethylbenzene, and least in cumene (31). Such reactions do not seem significant in the oxidation of benzene derivatives. However, naphthalene reacts about 20 times as rapidly with phenyl radical as does benzene (16), and radical addition to the naphthalene nucleus may at least partly account for the slow oxidation rate in the methylnapthalenes. Among the minor products from both methylnaphthalene oxidations were compounds of molecular weight 296 ... 

Two excellent reviews <71AHC(13)235, 72IJS(C)(7)6l) have dealt with quantitative aspects of electrophilic substitution on thiophenes. Electrophilic substitution in the thiophene ring appears to proceed in most cases by a mechanism similar to that for the homocyclic benzene substrates. The first step involves the formation of a cr-complex, which is rate determining in most reactions in a few cases the decomposition of this intermediate may be rate determining. Evidence for the similarity of mechanism in the thiophene and benzene series stems from detailed kinetic studies. Thus in protodetritiation of thiophene derivatives in aqueous sulfuric and perchloric acids, a linear correlation between log k and —Ho has been established the slopes are very close to those reported for hydrogen exchanges in benzene derivatives. Likewise, the kinetic profile of the reaction of thiophene derivatives with bromine in acetic acid in the dark is the same as for bromination of benzene derivatives. The activation enthalpies and entropies for bromination of thiophene and mesitylene are very similar. 

In experiments of major importance, first published in 1950, Melander found that in the nitration and bromination of a number of benzene derivatives the tritium isotope effect (kHlkT) is not 10-20 as is to be expected if carbon-hydrogen bond breaking occurs in the rate-determining step, but rather is less than 1.3. The direct displacement mechanism was thus ruled out, and the two-step mechanism of Equation 7.70 with the first step rate-determining was implicated.157... 

Catalytic hydrogenation of benzene is the commercial method for producing cyclohexane and substituted cyclohexane derivatives. The reduction cannot be stopped at an intermediate stage (cyclohexene or cyclohexadiene) because these alkenes are reduced faster than benzene. 

Replacement of the —SO3H group by a hydrogen. With benzene derivatives, this is done by heating with water or steam and acid. (p. 762)... 

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