Cyclodehydrogenation reaction

An alternative route to pyranthrone, involving baking 1,6-dibenzoylpyrene (6.96) with aluminium chloride, was also devised by Scholl (Scheme 6.18). The Scholl reaction is a key step in the synthesis of several polycyclic quinones the cyclisation of 1-benzoylnaphthalene to give benzanthrone (6.73) has already been mentioned. The mechanism of this cyclodehydrogenation reaction may involve an initial protonation step, if traces of water are present, or complexation with aluminium chloride. Electrophilic substitution is thereby... 

Some synthetic aspects of the above cyclodehydrogenation reaction are truly remarkable (1) MALDI-TOF spectrometry proves that, indeed, 108 hydrogens... 

If the [ 6] electrocyclization is impeded for steric or electronic reasons, the yields of thermal cyclodehydrogenation reactions normally decrease to a few or even less than one percent [38-41]. Nevertheless, the interesting bowl-shaped fullerene fragments diindeno[l,2,3,4-de/g l, 2, 3, 4 -mz70p]chrysene (6, [38d,e]) and triacenaphtho[3,2,l,8-cdejg 3, 2, l, 8 -zjk/m 3",2",r, 8"-opqra]triphenylene (7, [39]) could be obtained from 4 and decacyclene (8), respectively, for the first time (see Scheme 3). 

Scheme 2. Thermal cyclodehydrogenation reactions of stilbene type aromatics [37,38] a) toluene (p = 7 mbar) as carrier gas and radical scavenger...

Systematic studies carried out at C.F.R.I. pertainining to catalytic vapour phase synthesis of pyridine bases, namely 2 4-picolines through cyclodehydrogenation reaction of acetaldehyde and anunonia have been described. Metal oxide modified amorphous silica-aliunina and crystalline alumino-silicate namely ZSM-5 zeolites were found to be active and selective catalysts towards the formation of the lower pyridines bases. The method of preparation, pretreatment vis-a-vis the acidity have been found to affect the catalytic activity and selectivity. It is interesting to observe that metal modified crystalline alumino-silicate ZSM-5 is more selective than amorphous silica-alumina for the formation of 2 4-picoline. A plausible reaction mechanism based on the findings of this study has been proposed. 

Activity of the catalysts for the cyclodehydrogenation reaction of acetaldehyde and ammonia to produce 2 4-picolines was evaluated in fixed bed continuous flow reactor under atmospheric pressure at various temperature.The method for testing the activity of the catalysts was adopted as described(8). 

Phenolic Cyclization and Coupling Reactions. The oxidation of phenolic compounds which either do not possess benzylic hydrogen atoms, or which have an alternative reaction pathway, can result in a variety of interesting products. Cyclodehydrogenation reactions leading to oxygen heterocycles represent... 

Since, as discussed above, cyclodehydrogenation reactions towards graphene-type molecules are mostly p>erformed in solution, the solubility of the precursor molecules - and their compatibihty with the applied reagents - are crucial aspects. The deposition of monomolecular layers on suitable surfaces, which represents... 

The versatihty of the cyclotrimerization and the easy access to substituted phenylacetylene by modem coupling methods offers a wide range of variously decorated HBCs. An intriguing example is shown in Scheme 20. Permethoxylated hexa-pen-hexabenzocoronene (permethoxylated HBC) 90, which results in a double concave conformation, was easily synthesized via cyclotrimerization of a hexamethoxysubstituted phenylacetylene 88 followed by the usual cyclodehydrogenation reaction [50]. The non-planarity of the system is explained by the steric... 

In 1983, Neuse and Loonat first reported a two-stage synthesis approach for the preparation of various PBIs that involves the low-temperature solution polymerization of aromatic fcis(o-diamines) with aromatic dialdehydes and subsequent conversion of the resulting azomethine-type prepolymers to all-aromatic PBIs [8]. The reactions are illustrated in Figure 8.7. The first-stage polycondensation is conducted in dipolar aprotic media at -18°C to +25 C under anaerobic conditions to yield linear and soluble prepolymers. The prepolymers are converted to the desired PBIs by the second-stage oxidative cyclodehydrogenation reactions in the presence... 

Diels-Alder reaction of 22 and cyclopentadienones 24. The second step is an oxidative cyclodehydrogenation to yield HBCs 27. 

Scheme 29). Cyclodehydrogenation of 82 with stoichiometric amounts of palladium(II) acetate in N,AT-dimethylformamide (DMF) under reflux provides murrayastine 83 [17]. The same sequence of reactions can be applied to the synthesis of murrayaline A [17]. 

Crospovidone is a cross-linked homopolymer of A-vinyl-2-pyrrolidone. Acetylene and formaldehyde react to form butynediol. Hydrogenation and subsequent cyclodehydrogenation gives butyrolactone. The reaction of butyrolactone with ammonia produces pyrrolidone, which is vinylated with acetylene under pressure. The linear polymerization of the vinylpyrrolidone yields polyvinylpyrrolidone, a soluble binder, whereas the popcorn (branched) polymerization yields crospovidone, an insoluble... 

These methods utilize molecules of type B (p. 362) (=104) a. Synthesis of Benzofurans by Thermal Ring Closure and Cyclodehydrogenation (Thermal, Catalytic or Photochemical) of o-Substituted Phenols. The Hansch Reaction. o-Alkylhydroxy-compounds on heating with a suitable catalyst give either a benzofuran or a 2,3-dihydrobenzo-furan with the same number of carbon atoms.285-292 Side products are formed by decomposition of the starting material. The initial phenol can be o-alkylated and ring-closed in one stage. 

The principal source of toluene is catalytic reforming of refinery streams. This source accounts for ca 79% of the total toluene produced. An additional 16% is separated from pyrolysis gasoline produced in steam crackers during the manufacture of ethylene and propylene. The reactions taking place in catalytic reforming to yield aromatics are dehydrogenation or aromatization of cyclohexanes, dehydroisomerization of substituted cyclopentanes, and the cyclodehydrogenation of paraffins. The formation of toluene by these reactions is shown. 

It has been known for some time that irradiation of azobenzene (324) in either 22 N sulfuric acid350 351 or acetic acid with added ferric chloride 352 yields benzo[c]cinnoline (325). This is accompanied by the formation of an almost equal quantity of benzidine (326), undoubtedly arising by rearrangement of hydrazobenzene (327). The mechanism of this reaction differs, therefore, from that of the stilbene cyclodehydrogenation, and azobenzene itself functions as the hydrogen acceptor. Yields of not more than 50% of benzo[c]cinnoline are generally observed. 

A number of related reactions are worthy of mention. Benzanilide (334) is converted into phenanthridone (335) by irradiation in benzene in the presence of iodine.362 Cyclodehydrogenation is also observed in the anilides of indole-2-carboxylic acid and indole-3-carboxylic acid on irradiation in acetone.363 Irradiation of diphenyl-amine304 and certain of its JV-substituted derivatives365 yields the corresponding carbazole. The mechanism of this reaction differs from... 

Catalytic cyclodehydrogenation of 5-amino-2-ethylthiophenol affords 6-aminobenzo[6]thiophene.239,241 6-Acetamido-2,3-di-bromo-,77 6-acetamido-2-bromo-3-methyl-,102 6-acetamido-3-bromo-2-methyl-,102 and 6-acetamido-3-bromobenzo[f>]thiophene107 may be obtained from the corresponding 6-acetyl compound by means of the Schmidt reaction. In some cases the 6-acetamido compound is accompanied by a smaller amount of the amine sulfate.102,107 6-Aminobenzo[6]thiophene may be converted into 6-chloro- or 6-cyanobenzo[6]thiophene by means of the Sandmeyer reaction.241... 

The treatment of 7V-tosyl-/>-anisidine (75) and several of its derivatives with BTIB in the presence of activated alkenes provides an interesting example of an intermolecular cyclodehydrogenation sequence (99H1785). Arylpropenes are converted to 7V-tosylindolines 76 under these conditions, while alkenyl sulfides lead to the 7V-tosylindoles 77 (Scheme 22). The intermediate formation of 7V-tosylnitrenium ions in these reactions was suggested. 

Novel carbon frameworks have been developed from polycyclic hydrocarbons. Thus, Kuck and coworkers143 have recently reported an unexpected tandem reaction, which formally consists of a condensation/cyclodehydrogenation sequence starting from triptidan-9-one 233 leading to the fn/wso-tetracyclic propellane 234 (Scheme 72). The reaction of the tribenzo[3.3.3]propellane ketone 233 with benzylhthium/TMEDA afforded an efficient one-pot peri annulation of a dihydronaphthalene (Scheme 72). The key step of this unexpected tandem reaction was determined to be a nucleophilic cychzation followed by hydride elimination. 

Cyclodehydrogenation chromenes Reaction of o-(3,3-dimethylallyl)phenols with trityl tetrafluoroborate results in dehydrogenation and cyclization to chromenes. DDQ has usually been used for this purpose. 

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