Dehydrogenation agents

In certain cases, selem um dioxide may act solely as a dehydrogenating agent, as in the examples ... 

The principal dehydrogenating agents are (i) sulphur, (ii) selenium, and (iii) catalytic metals. 

It is also prepared by direct cyanation of l,4-diamino-2,3-dichloroanthraquinone (35) or 1,4-diaminoanthraquinone (33) in an aprotic organic solvent. In the latter case, the presence of an ammonium compound and a dehydrogenating agent is necessary (55). 

The direction of ring closure can often be influenced by the nature of the dehydrogenation agent. The substituted thiosemicarbazone (223) with AI2O3 in chloroform formed the... 

Dehydration and dehydrogenation combined utihzes dehydration agents together with mild dehydrogenation agents. Included in this class are phosphoric acid, sihca-magnesia, silica-alumina, alumina derived from aluminum chloride, and various metal oxides. 

The most frequently used synthetic route to cycl[3.2.2]azines involves the reaction of an indolizine with a dienophile, for example, DMAD, in the presence of a dehydrogenating agent such as palladium-on-carbon (Scheme 85), although the scope of the reaction is limited by the presence of substituents in one or both of the reactants, and/or the reaction conditions. If C-3 and C-5 of the indolizine are unsubstituted, the cyclazine is the main product a 3,4-dihydrocyclazine may sometimes be isolated as a by-product (see below). 

Sensitization by acetophenone and benzophenone could be demonstrated for the cyclization of 74, R=4—C(CH3)3 to 20, R = 6—C(CH3)s. With the latter sensitizer, enhanced yields of N-hydroxy-2-indoUnone formation have been observed relative to direct irradiation runs. Triplet benzophenone does not act as dehydrogenating agent in the step 19 20, since the pinacolization of benzo-... 

Other dehydrogenation agents include sulfur, selenium dioxide, palladium on charcoal, and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). Some reactions proceed spontaneously. Dehydrogenation also occurred... 

A large rtumber of carbazole syntheses involve the preparation and dehydrogenation of hydrocarbazoles, mainly 1,2,3,4-tetrahydrocarbazoles. They are usually prepared by either the Fischer-Borsche synthesis or the Japp-Klingemann reaction. The most commonly used dehydrogenating agents are palladium on charcoal or chloranil (495,496). 

The cyclohexanone phenylhydrazone (512), obtained by reacting cyclohexanone (510) with phenylhydrazine (511), on indolization, furnished tetrahydrocarbazole 513 which, on dehydrogenation, afforded carbazole 514. The success of the reaction is dependent on the reagent used for indolization and the dehydrogenating agent. The mechanism for the formation of the tetrahydrocarbazole involves a tautomeric equilibrium and the formation of a new C-C bond via a [3,3]-sigmatropic rearrangement followed by elimination of ammonia (495,496,498) (Scheme 5.7). 

The electrocyclic reaction of divinylindoles to functionalized carbazole derivatives has emerged as an efficient method for the synthesis of a diverse range of carbazole derivatives. This reaction has been achieved at high temperatures in the presence of palladium on charcoal or DDQ. The palladium catalyst or DDQ act as dehydrogenating agent (534,535) (Scheme 5.19). 

CH2)2L0) were prepared. - " Such compounds can be oxidized with lead dioxide to N-picryl-9-aminocarbazyls (133). These black species are dehydrogenating agents though stable in dry chloroform the transformation of violet-black color into orange in benzene solution suggests that they... 

Electrocyclic closure of triene 353 in the presence of palladium-charcoal as a dehydrogenating agent produced l-phenyl-2-methyl-3-methoxycarba-zole, and an analogous process converted 354 to 355. ... 

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