Cyclization dehydrogenation

Of some relevance in this connection is a study216 on the structure of the anion radicals formed when diaryl sulphones react with n-butyllithium in hexane-HMPA solution under an argon atmosphere. Apparently, a dehydrogenative cyclization and a further one-electron reduction occurs to produce the anion radicals of substituted dibenzothiophene-S, S-dioxides. These anion radicals were studied by ESR spectroscopy. 

According to this scheme, the catalyst serves primarily to promote dehydrogenation. Cyclization of the hexatriene was shown years ago (JJ.) to occur thermally in the gas phase at temperatures well below these dehydrocyclization conditions. Thus, the overall reaction is projected to be the combination of several catalytic dehydrogenation steps and a non-catalytic cyclization step. This projection implies that the design of the catalytic reactor may be important in order to optimize the ratio of void space for cyclization and catalyst space for dehydrogenation. 

This reaction also proceeds in the presence of hydrogen only thus, it may never predominate since under these conditions more rapid hydrogenation of the double bond competes very efficiently with it. Due to rapid double bond isomerization the product contains predominantly the most stable isomer 1-methylcyclopentene. The dependence of the selectivity of hydro-genative versus dehydrogenative cyclization on the structure of the starting hydrocarbon (Fig. 7) shows that methylcyclopentene (MCPe) is not the product of secondary dehydrogenation (55). 

The Ic pattern for carbazole synthesis arises primarily in the dehydrogenative cyclization of diphenylamines. This cyclization can be accomplished photolytically, with an oxygen sensitive intermediate having been detected (equation 58) (81JA6889). Preparative work with substituted diphenylamines has resulted in rather variable yields, depending upon particular substituents. This variability may be the result of alteration in the dominant photochemical process with substituent changes. 

However, the major source of these hydrocarbons is now petroleum. Although aromatic compounds do occur naturally in petroleum, they are mainly obtained by the process of catalytic reforming, in which aliphatic hydrocarbons are aromatized through dehydrogenation, cyclization and isomerization. The process, which is also known as hydroforming, is carried out under pressure at 480-550 °C in the presence of a catalyst, typically chromium(III) oxide or alumina. Benzene is thus produced from... 

In carrying out such a dehydrogenative cyclization on alkyl hydrazones 280 derived from 6-hydrazinotetrazolo[l, 5-/ ]pyridazine (279) using lead... 

Acidic zeolites are known for their excellent catalytic activity in cracking and isomerization of hydrocarbons (75). In the absence of metal, however, these catalysts rapidly deactivate due to the formation of carbonaceous products, usually referred to as coke. The carbonaceous residues are mainly formed via alkylaromatics and polyaromatics, which are the result of dehydrogenation, cyclization, and further alkylation processes. The coke deposits lower the catalytic activity by site poisoning and eventually also by pore blocking, which inhibits access of hydrocarbon molecules to the acid sites (286). 

Dehydrogenative cyclizations are also accomplished with potassium U rricyanide [920] and with hydrogen peroxide in the presence of horseradish peroxidase [1037] (equation 48). 

Synthesis of heterocycles. The 1,5-diketone (1) on reaction with trityl perchlorate undergoes dehydrogenative cyclization to the pyrylium salt (2), which is transformed by reaction with ammonium acetate into the pyridine derivative (3). ... 

Dehydrogenative cyclization of aldose acenaphtho[l,2-e]l,2,4-triazin-3-yl hydrazones (825) with ethanolic iron(III) chloride provided l-(alditol-l-yl)acenaphtho[l,2-e]l,2,4-triazolo[3,4-c]l,2,4-triazines (826). That cyclization occurred at N4 rather than at N2 of the 1,2,4-triazine ring was based on comparison with similar noncarbohydrate products (94BCJ149) (Scheme 216). 

When the )V-tosylaceta (3.2) is heated with dilute hydrochloric acid and dioxan, a surprisingly facile detosylating-dehydrogenative cyclization occurs to give the isoquinolines in high yields—a valuable variation of the Pomeranz-Fritsch reaction. It has been used in a synthesis of the medicinally important ellipticine [2870]. 

Hydroxychalcone is readily cyclized with hot aqueous alkali this is the basis of a widely applicable synthesis of flavanones [B-4]. In addition to the examples of dehydrogenative cyclization of chalcones described in Part 1 (pp. 191, 192), that produced by DDQ should be mentioned. In boiling dioxan, the reaction takes 20-72 h and produces flavones together with smaller amounts of flavanones and aurones. 

NBS-suIphuric acid at 70-100 °C is an efficient reagent for the dehydrogenative cyclization of a substrate containing a thiolactam and a neighbouring benzene ring. A -Chlorosuccinimide-suIphuric acid may give better results on 6-thioxopyrazolo[3,4-i/]pyrimidin-4-one [3349]. 

Dehydrogenative cyclization of 2-alkyl- or 2-arylsulfanylphenylureas 1 with bromine and sodium methoxide in anhydrous methanol leads to the lA4,2,4-benzothiadiazin-3(4//)-ones 2.122 These compounds are cyclic sulfimines which undergo hydrolysis even on recrystallization. When compounds 2 are heated in dioxane/water (1 1) the S1-N2 double bond is hydrolytically cleaved to afford the ureidophenyl sulfoxide 3. Since compounds 2 possess a free electron pair at the sulfur atom, they can be oxidized in good yields to the 1-oxides 4 by treatment with potassium permanganate. In contrast to compounds 2, oxides 4 are not hydrolyzed in boiling, dilute acids. 

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