Hydrogenation heteroaromatic compounds

C—H bond alkenylations, 10, 222—223 in cross-coupling polymerization, 11, 687 N-Heteroaromatic compounds, hydrogenation, 1, 792 Heteroaromatic stannanes, preparation, 3, 828 Heteroaryl halides, aminocarbonylatioon, 11, 527 Heteroarylstannanes, preparation, 3, 826 Heteroatoms... 

Autocatalysis may arise when the nucleophilic atom of the reagent is bound to a hydrogen atom which is eventually eliminated during the reaction. This occurs with neutral reagents such as primary or secondary amines, thiols, and alcohols. If the displaced group (usually an anion) is a sufficiently weak base, the proton is effectively transferred to any basic reactant. Hence, the best known examples of autocatalysis involve chloro-A-heteroaromatic compounds as the substrates. 

Nucleophilic aromatic substitution of hydrogen in heteroaromatic compounds, reactivity and reaction mechanisms 94MI2. 

Ab initio electron correlated calculations of the equilibrium geometries, dipole moments, and static dipole polarizabilities were reported for oxadiazoles <1996JPC8752>. The various measures of delocalization in the five-membered heteroaromatic compounds were obtained from MO calculations at the HF/6-31G level and the application of natural bond orbital analysis and natural resonance theory. The hydrogen transfer and aromatic energies of these compounds were also calculated. These were compared to the relative ranking of aromaticity reported by J. P. Bean from a principal component analysis of other measures of aromaticity <1998JOC2497>. 

A trialkyl- or triarylstannyl group can also be added to an aromatic ring. One way this can be accomplished is by treating an aromatic or heteroaromatic compound possessing an active hydrogen(s) with an alkyl lithium and then reacting the lithium salt with a trialkyltin halide (equation 97). Another general method that has been used to attach a trialkyl- or... 

Several heteroaromatic compounds can be hydrogenated by [Rh(COD) (PPh3)2]+ species. Thus, this cationic complex has been reported to be a catalyst precursor for the homogeneous hydrogenation of heteroaromatic compounds such as quinoline [32] or benzothiophene [33]. Detailed mechanistic cycles have been proposed by Sanchez-Delgado and coworkers. The mechanism of hydrogenation of benzothiophene by the cationic rhodium(III) complex, [Rh(C5Me5) (MeCN)3]2+, has been elucidated by Fish and coworkers [34]. 

Finally, polymer 594 has been used as an arene-catalyst to activate nickel from nickel(II) chloride and lithium, in order to perform hydrogenation of different organic substrates such as afkenes, afkynes, carbonyl compounds and their imines, alkyl and aryl halides (chlorides, bromides and iodides), aromatic and heteroaromatic compounds as well as nitrogen-containing systems such as hydrazines, azoxy compounds or Af-amino oxides, giving comparable results to those obtained in the corresponding reaction in solution . 

C Yamagami, M Yokota, N Takao. Hydrogen bond effects of ester and amide groups in heteroaromatic compounds on the relationship between the capacity factor and the octanol-water partition coefficients. J. Chromatogr. A 662 49-60 (1994). 

The enantioselective hydrogenation of olefins, ketones and imines still represents an important topic and various highly enantioselective processes based on chiral Rh, Ru or Ir complexes have been reported. However, most of these catalysts failed to give satisfactory results in the asymmetric hydrogenation of aromatic and heteroaromatic compounds and examples of efficient catalysts are rare. This is especially the case for the partial reduction of quinoline derivatives which provide 1,2,3,4-tetrahydroquinolines, important synthetic intermediates in the preparation of pharmaceutical and agrochemical products. Additionally, many alkaloid natural products consist of this stmctural key element. 

The Brpnsted acid catalyzed hydrogenation of quinolines with Hantzsch dihydropyridine as reducing agent provides a direct access to a variety of substituted tetrahydroquinolines (Table 4.2). The mild reaction conditions of this metal-free reduction of heteroaromatic compounds, high yields, operational simplicity and practicability, broad scope, functional group tolerance and remarkably low catalyst loading render this environment-friendly process an attractive approach to optically active tetrahydroquinolines and their derivatives (Table 4.3) (see page 176). ... 

Basic Isotopic Exchange of Hydrogen in Heteroaromatic Compounds N. N. Zatsepina and I. F. Tupitsyn, Usp. Khim. Geterotsikl., 1976, 32-57. 

Hydrogen Exchange of Heteroaromatic Compounds Y. Kawazoe and M. Maeda, Kagaku no Ryoiki, Zokan, 1970, 92, 39-96. 

The principal tautomeric properties of the fundamental biological pyrimidines—cytosine, uracil, and thymine—are due to the presence in these N-heteroaromatic compounds of electron-donor substituents such as NH2 and OH and of SH in some important analogs. The labile hydrogen may remain attached at the exocyclic 0, N, or S atom or migrate to one of the ring nitrogens, giving rise to three principal types of tautomerism (Scheme 1) ... 

The hydrogenation of heteroaromatic compounds can easily be performed at lead cathodes in sulfuric acid electrolytes. Examples are the formation of piperidine or hexahydrocarbazole [17] ... 

The electrochemical hydrogenation of double bonds can be performed either electrocatalytically at Raney nickel, palladium, or platinum modified electrodes [32] or via electron transfer under Birch conditions to the intermediate anion radical [33]. Examples are given in the dihydrogenation of phthalic acid (Eq. 22.15) and the hydrogenation of heteroaromatic compounds (Eq. 22.14). 

Homolytic substitution of heteroaromatic compounds, 16, 123 Hydantoins, chemistry of, 38, 177 Hydrogen cyanide derivatives, synthesis of heterocycles from, 41, 1 Hydrogen exchange base-catalyzed, 16, 1 one-step (labeling) methods, 15, 137 Hydrogenated porphyrin derivatives hydroporphyrins, 43, 73 Hydroxamic acids, cyclic, 10, 199 1 -Hydroxyindoles, 1 -hydroxytryptophans, and 1-hydroxytryptamines,... 

Because a comprehensive review on the catalytic performance of Josiphos ligands has been published,20 we restrict ourselves to a short overview on the most important fields of applications. Up to now, only the (7 )-(S)-family (and its enantiomers) but not the (R)-(R) diastereoisomers have led to high enantioselectivities (the first descriptor stands for the stereogenic center, and the second stands for the planar chirality). The most important application is undoubtedly the hydrogenation of C = N functions, where the effects of varying R and R1 have been extensively studied (for the most pertinent results see Table 15.5, Entries I—4). Outstanding performances are also observed for tetrasubstituted C = C bonds (Entry 5) and itaconic and dehydroamino acid derivatives (Entries 6 and 7). A rare example of an asymmetric hydrogenation of a heteroaromatic compound 36 with a respectable ee is depicted in Scheme 15.6.10b... 

By replacement reactions is generally meant those reactions in which groups besides hydrogen are substituted by an electrophile, often the proton itself. These reactions are very frequent in the chemistry of five-membered heteroaromatic compounds, and kinetic studies on some of them have been reported. 

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