Double bonds heteroaromatic compounds

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

Apart from the A-methyl group, three double-bond equivalents and three multiplets remain in the chemical shift range appropriate for electron rich heteroaromatics, Sh = 6.2 to 6.9. A-Methyl-pyrrole is such a compound. Since in the multiplets at Sh = 6.25 and 6.80 the Jhh coupling of 4.0 Hz is appropriate for pyrrole protons in the 3- and 4-positions, the pyrrole ring is deduced to be substituted in the 2-position. 

The X-ray investigation of lH-3,5-dimethylpyrazole-4-diazonium chloride (4.7) demonstrates that heteroaromatic diazonium ions are very similar to those of the aromatic compounds (NN distance 111.3 pm, NC 136.9 pm, indicating a greater degree of NC double bond character (Brint et al., 1985). 

Thiadiazole is classified as a xc-excessive sulfur-containing heteroaromatic compound <84CHEC-I(6)463>. The 1,2,4-thiadiazole nucleus is numbered as m (1). The double bonds in the partially reduced rings are designated A, A, respectively (Figure 1) and these compounds are... 

By virtue of the nature of the atoms in its ring system and the number and position of its double bonds, 1,2,4-thiadiazole possesses aromatic character and is classified, according to Albert,3 as a n-excessive sulfur-containing heteroaromatic compound. 

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). 

Nucleophilic Reactions of Aromatic Heterocyclic Bases Heterocyclic aromatic compounds containing a formal imine group (pyridine, quinoline, isoquinoline, and acridine) also react readily with nucleophilic reagents. A dihydro-derivative results, which is readily dehydrogenated to a new heteroaromatic system. Since the nucleophile always attacks the a-carbon atom, the reaction formally constitutes an addition to the C=N double bond. An actual localization of the C=N double bond in aromatic heterocyclic compounds is incompatible with molecular orbital theory. The attack of the nucleophilic reagent occurs at a site of low 77-electron density, which is not... 

The presence of an exocyclic double bond on aromatic [52] or heteroaromatic [53] compounds dictates the locoselectivity of the cycloaddition occurring preferentially at the alkene terminus, and sometimes with retention of stereochemistry [52b], Thus, the [4+2] photo-oxygenation of a vinylbenzene derivative, such as 60, leads to dihydrobenzo-1,2-dioxin in the primary step, which is followed by rapid oxygenation of the cyclohexadiene chromophore to give diendoperoxides as 61 and 62 (Sch. 34) [52c],... 

The electrochemical behavior of azomethine derivatives, e.g., oximes, of heteroaromatic carbonyl compounds is much like that of the corresponding benzene derivatives.91 Pyridine aldoximes271-274 and ketoximes275 are reduced in acid solution by a four-electron reaction to the amine. The reaction mechanism is probably, as in other oximes,01 a reduction of the protonated compound with cleavage of the N-0 bond, followed by saturation of the C=N double bond. The amine is often further reducible at a more negative potential (Section VI, E). 

Disubstituted 3,6-pyridazinediones show properties different from those of other pyridazinones which possess a more highly developed heteroaromatic character. The olefinic nature of the 4,5 double bond is evident from the ease of hydrogenation in the presence of palladium, the addition of bromine, and additions of the Diels-Alder type. Compounds of this type are also liable to acids or... 

Addition can also occur to the formal ring double bonds of a nitrogen heteroaromatic compound. The usual preference is for addition adjacent to the nitrogen (equation 41), but this may be modified by substitution, cryptand complexation, quatemization, or copper salt catalysis. Pyridine N-oxides may yield deoxygenated product. 

Figure 12.6 Dependence of polymer deposition rate on molecular weight of monomer. Group I (O)- triple-bond-containing, aromatic, and heteroaromatic compounds group II ( ) double-bond-containing and cyclic compounds group III (A) compounds without aforementioned structures group IV (x) oxygen-containing compounds.

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