Dehydrogenative aromatizations

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. 

When coal or biomass is heated, many reactions including dehydration, cracking, isomerization, dehydrogenation, aromatization, and condensations take place. Products are water, carbon dioxide, hydrogen, other gases, oils, tars, and char. The product yields vary, depending on the particular feedstock composition, particle size, heating rate, solids and gas residence times, and the reactor temperature. 

The use of base catalysis for various reactions, such as olefin isomerization, cyclohexadiene dehydrogenation, aromatic alkylation, and olefin polymerization, has been demonstrated. In some cases, compounds are formed which are difficult to prepare by other means, and some highly selective reactions have been found. 

Crich and Rumthao reported a new synthesis of carbazomycin B using a benzeneselenol-catalyzed, stannane-mediated addition of an aryl radical to the functionalized iodocarbamate 835, followed by cyclization and dehydrogenative aromatization (622). The iodocarbamate 835 required for the key radical reaction was obtained from the nitrophenol 784 (609) (see Scheme 5.85). lodination of 784, followed by acetylation, afforded 3,4-dimethyl-6-iodo-2-methoxy-5-nitrophenyl acetate 834. Reduction of 834 with iron and ferric chloride in acetic acid, followed by reaction with methyl chloroformate, led to the iodocarbamate 835. Reaction of 835 and diphenyl diselenide in refluxing benzene with tributyltin hydride and azobisisobutyronitrile (AIBN) gave the adduct 836 in 40% yield, along with 8% of the recovered substrate and 12% of the deiodinated carbamate 837. Treatment of 836 with phenylselenenyl bromide in dichloromethane afforded the phenylselenenyltetrahydrocarbazole 838. Oxidative... 

Three main types of reactions undergone by these lactim ethers will be discussed in this section. They are (a) hydrolysis, (b) displacement of the alkoxy (or alkylthio) groups by other nucleophiles, and (c) dehydrogenation/aromatization. 

Benzene, naphthalene, toluene, and the xylenes are naturally occurring compounds obtained from coal tar. Industrial synthetic methods, called catalytic reforming, utilize alkanes and cycloalkanes isolated from petroleum. Thus, cyclohexane is dehydrogenated (aromatization), and n-hexane(cycli> zation) and methylcyclopentane(isomerization) are converted to benzene. Aromatization is the reverse of catalytic hydrogenation and, in the laboratory, the same catalysts—Pt, Pd, and Ni—can be used. The stability of the aromatic ring favors dehydrogenation. 

In a related case, Maier and coworker [153] have recently also demonstrated that vinylogous enolates can enter into a cross-coupling step. However, the final products of Pd-catalyzed reactions of the hexahydro naph-thalenone and aryl bromides or iodides in the presence of cesium carbonate and tetraalkylammonium bromide in DMF at room temperature are 7-aryltetralones 208 (Scheme 85). This process can be readily interpreted as a Pd-catalyzed /-selective arylation of the thermodynamic enone enolate with a concomitant dehydrogenation-aromatization of the initial crosscoupling product. 

A radical cyclization approach was developed to synthesize dihydrobenzofurans, which were converted to benzofurans by dehydrogenation (aromatization) <03CC526>. 

The yields of dehydrogenations often leave much to be desired, but in many cases, the purpose of such reactions is not to prepare but, rather, to identify natural products such as terpenes or alkaloids by converting them into known aromatic compounds. With the advent of spectroscopic techniques, dehydrogenative aromatizations have lost much of their im-... 

Dehydrogenation. Aromatization of 3,4-diaryl-2,5-dihydro derivatives of furan, thiophene, and A-arylpyrrole is accomphshed in 80-91% yield by heating with CuBry (3 equiv.) in EtOAc. ... 

In 2013, AstraZeneca researchers reported an application of copper-catalyzed aerobic dehydrogenative aromatization in the 120 g scale synthesis of AZD8926, a drug candidate for the treatment of central nervous system disorders (Scheme 5.6) [29]. This aerobic dehydrogenation process was determined to be superior for large-scale implementation over other oxidation methods, such as a benzoquinone-mediated oxidation and various protocols using peroxide-based... 

Heterocyclic) benzylamines have also been aerobically dehydrogenated by mpg-CN as a photocatalyst, yielding the corresponding Af-benzylidene products [122]. Coordination of the imine to a hole in the VB is thought to facilitate intermediate aminal formation by nucleophilic attack on the C=N double bond. Dehydrogenative aromatizations of several N-heterocycles have also been carried out with mpg-CN [122]. 

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