Nitrogen-containing compounds dehydrogenation

Dehydrogenations are very easy in six-membered nitrogen-containing compounds that already contain one or two double bonds in the heterocyclic ring. Such dehydrogenations are carried out by nitric acid [455], arsenic oxide (AsjOj) [480], manganese dioxide [825], potassium ferricyanide [919], nitrobenzene, and others (equations 41 and 42). 

Dehydrogenation of nitrogen-containing compounds 10.1.1. Amino acids and peptides... 

Most nitrogen-containing organic compounds are liable to oxidation on reaction with DIB. Simple N-H dehydrogenations constitute a major area of applications, often accompanied by further transformation. 

Zeolites are known to catalyze the formation of various nitrogen-containing aromatic ring systems. Examples include the synthesis of pyridines by dehydrogenation / condensation / cyclization of C -Cg precursors [1], the formation of methylpyridines by high-temperature isomerization of anilines [2], the amination of oxygen-containing heterocyclic compounds [3] and the Fischer indole synthesis [4,5]. The latter synthesis consists (see Scheme 1) of a condensation towards a phenylhydrazone followed by an acid-catalyzed cyclization with elimination of ammonia. The two reaction steps are usually combined in a one-pot procedure. 

The first synthesis and isolation of an a type nitrogen-containing phenalenyl radical, a 2,5,8-tri-ferf-butyl-l,3-diazaphenalenyl 29, was achieved in 2002. The synthesis involved reduction of compound 26 followed by condensation with fBuCHO and dehydrogenation catalyzed by Pd/C. Oxidation with Pb02 and recrystallization led to a green crystal of 29. The crystal exhibited increased stabihty in air due to heteroatomic modification [21] (Scheme 3). 

More recently, chemiluminescence detectors based on redox reactions have made possible the detection of many classes of compounds not detected by flame ionization. In the redox chemiluminescence detector (RCD), the effluent from the column is mixed with nitrogen dioxide and passed across a catalyst containing elemental gold at 200-400°C. Responsive compounds reduce the nitrogen dioxide to nitric oxide. The nitric oxide is reacted with ozone to give the chemiluminescent emission. The RCD yields a response from compounds capable of undergoing dehydrogenation or oxidation and produces sensitive emissions from alcohols, aldehydes, ketones, acids, amines, olifins, aromatic compounds, sulfides, and thiols. 

Certain heterocyclic compounds, particularly those containing a nitrogen atom, have been prepared in high yields by catalytic dehydrogenation. Several catalysts are available including those employed for hydrogenation (cf. method 2). 

Dehydrogenations, which involve the elimination of hydrogen Ifom organic molecules, lead to compounds containing double bonds, multiple bonds, or aromatic rings. For practical reasons, only the formation of carbon-carbon double bonds, of carbon-nitrogen double bonds in cyclic amines, and of aromatic rings (both carbocyclic and heterocyclic) will be discussed in this chapter. The conversion of alcohols into aldehydes and ketones and of amines into imines and nitriles will be discussed in the chapter Oxidations (Chapter 3). 

Not all oxidations of alkaloids can be ascribed to microsomal P-450 systems. Mammalian liver contains a flavoprotein oxidase, first discovered in rabit liver and called quinine oxidase . It is also an aldehyde oxidase, and its mechanism of action on heterocyclic nitrogen compounds probably involves addition of hydroxide ion to a suitable ring position, followed by dehydrogenation. Thus, the introduced oxygen atom comes from water rather than from molecular oxygen, in contrast to the P-450 oxygenases. [2]. 

The reaction time and concentrations used depend on the reactants. Reaction is usually effected at 60°. Formation of condensation products from the ketones is largely repressed by working in inert solvents such as benzene, dioxan, and toluene. Aluminum isobutoxide, as well as the isopropoxide, has proved its value as metal alkoxide usually it is added to the reaction mixture in the proportion of 0.5 mole per mole of alcohol. Compounds containing nitrogen and halogen can also be oxidized by the Oppenauer method. Dehydrogenation of cholesterol to cholestenone will be described as an example 454... 

Another approach to AB regeneration involves digestion of spent material followed by reduction. The dehydrogenation of AB produces a complex mixture of products of general composition BNHx, all of which contain B-N linkages that must be broken. Acidic compounds (HX) can be used to cleave these bonds to release the nitrogen component with the formation of B-X bond-containing... 

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