Dehydrogenation Aromatization

It should be noted that both reactions leading to aromatics (dehydrogenation of naphthenes and dehydrocyclization of paraffins) produce hydrogen and are favored at lower hydrogen partial pressure. 

Electrocyclic rearrangements Organolithium reagents, 209 Elimination reactions to form carbon-carbon multiple bonds (see also Aromatization, Dehydrogenation, Deoxygenation of epoxides)... 

Likewise, upon CIR of electron-deficient (hetero)aryl halides 11 and (hetero)aryl propargyl alcohols 12, and subsequent addition of amidinium salts 32, 2,4,6-trisubstituted pyrimidines 89 can be obtained in a consecutive three-component reaction in good yields (Scheme 48) [241]. Interestingly, in all cases the aromatic products 89 are found and not the expected dihydropyrimidines, regardless whether the reaction has been performed under an anaerobic or an aerobic atmosphere. Therefore, it can be assumed that the presence of the transition metal catalysts is beneficial for a terminal aromatizing dehydrogenation. 

Aromatization. Dehydrogenation of Hantzsch 1,4-dihydropyridines occurs under an oxygen atmosphere in the presence of RuClj. 

The reactions of major importance in the octafining process are isomerization of naphthenes and aromatics, hydrogenation of aromatics, dehydrogenation of naphthenes, disproportionation of aromatics, dealkylation of aromatics, and hydrocracking of saturates (Figure 3). The last three reactions, of course, result in loss of product xylenes. These reactions, like the desired isomerization reactions, are carbonium-ion catalyzed. 

Hydrogenation of olefins Hydrogenation of ethylene Hydrogenolysis Hydrogenation of acetylenes Hydrogenation of aromatics Dehydrogenation... 

Besides stmctural variety, chemical diversity has also increased. Pure silicon fonns of zeolite ZSM-5 and ZSM-11, designated silicalite-l [19] and silicahte-2 [20], have been synthesised. A number of other pure silicon analogues of zeolites, called porosils, are known [21]. Various chemical elements other than silicon or aluminium have been incoriDorated into zeolite lattice stmctures [22, 23]. Most important among those from an applications point of view are the incoriDoration of titanium, cobalt, and iron for oxidation catalysts, boron for acid strength variation, and gallium for dehydrogenation/aromatization reactions. In some cases it remains questionable, however, whether incoriDoration into the zeolite lattice stmcture has really occurred. 

Dehydrogenation (the conversion of alicycllc or hydroaroraatic compounds into their aromatic counterparts by removal of hydrogen and also, in some cases, of other atoms or groups) finds wide appUcation in the determination of structure of natural products of complex hydroaroraatic structure. Dehydrogenation is employed also for the synthesis of polycyclic hydrocarbons and their derivatives from the readily accessible synthetic hydroaroraatic compounds. A very simple example is the formation of p-raethylnaphthalene from a-tetra-lone (which is itself prepared from benzene—see Section IV,143) ... 

This is an example of the Doebner synthesis of quinoline-4-carboxylic acids (cinchoninic acids) the reaction consists in the condensation of an aromatic amine with pyruvic acid and an aldehj de. The mechanism is probably similar to that given for the Doebner-Miller sj nthesis of quinaldiiie (Section V,2), involving the intermediate formation of a dihydroquinoline derivative, which is subsequently dehydrogenated by the Schiff s base derived from the aromatic amine and aldehyde. 

Alkyl groups attached to aromatic rings are oxidized more readily than the ring in alkaline media. Complete oxidation to benzoic acids usually occurs with nonspecific oxidants such as KMnO, but activated tertiary carbon atoms can be oxidized to the corresponding alcohols (R. Stewart, 1965 D. Arndt, 1975). With mercury(ll) acetate, allyiic and benzylic oxidations are aJso possible. It is most widely used in the mild dehydrogenation of tertiary amines to give, enamines or heteroarenes (M. Shamma, 1970 H. Arzoumanian. 1971 A. Friedrich, 1975). 

The stronger directing effects present in the indoline ring can sometimes be used to advantage to prepare C-substituted indoles. The aniline type of nitrogen present in indoline favours 5,7-substitution. After the substituent is introduced the indoline ring can be aromatized by dehydrogenation (see Section 15.2 for further discussion). A procedure for 7-acylation of indoline... 

Arylation of Aromatic Compounds. In contrast to Friedel-Crafts alkylations, arylations of aromatics are not as well known, and usually require drastic conditions. They iaclude (/) dehydrogenating condensation (SchoU reaction) (2) arylation with aryl haUdes and (J) arylation with dia2onium hahdes. 

The condensation of cyclohexanol or cyclohexene is generally carried out in the presence of phosphoric acid, pyrophosphoric acid, or HY 2eohtes the aromatization of intermediate cyclohexyUiydroquinone [4197-75-5] (19) is realized in the presence of a dehydrogenation catalyst. 

The most important process to produce 1-naphthalenol was developed by Union Carbide and subsequently sold to Rhc ne-Poulenc. It is the oxidation of tetralin, l,2,3,4-tetrahydronaphthalene/719-64-2] in the presence of a transition-metal catalyst, presumably to l-tetralol—1-tetralone by way of the 1-hydroperoxide, and dehydrogenation of the intermediate ie, l-tetralol to 1-tetralone and aromatization of 1-tetralone to 1-naphthalenol, using a noble-metal catalyst (58). 1-Naphthol production in the Western world is around 15 x 10 t/yr, with the United States as the largest producer (52). 

Like mthenium, amines coordinated to osmium in higher oxidation states such as Os(IV) ate readily deprotonated, as in [Os(en) (NHCH2CH2NH2)] [111614-75-6], This complex is subject to oxidative dehydrogenation to form an imine complex (105). An unusual Os(IV) hydride, [OsH2(en)2] [57345-94-5] has been isolated and characterized. The complexes of aromatic heterocycHc amines such as pyridine, bipytidine, phenanthroline, and terpyridine ate similar to those of mthenium. Examples include [Os(bipy )3 [23648-06-8], [Os(bipy)2acac] [47691-08-7],... 

Future Developments. The most recent advance in detergent alkylation is the development of a soHd catalyst system. UOP and Compania Espanola de Petroleos SA (CEPSA) have disclosed the joint development of a fixed-bed heterogeneous aromatic alkylation catalyst system for the production of LAB. Petresa, a subsidiary of CEPSA, has announced plans for the constmction of a 75,000 t/yr LAB plant in Quebec, Canada, that will use the UOP / -paraffin dehydrogenation process and the new fixed-bed alkylation process (85). 

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