Carbon atoms aromatic/heteroaromatic compounds

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

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). 

Hydrazides of vicinal acetylene-substituted derivatives of benzoic and azole carboxylic acids are important intermediate compounds because they can be used for cyclization via both a- and /3-carbon atoms of a multiple bond involving both amine and amide nitrogen atoms (Scheme 131). Besides, the hydrazides of aromatic and heteroaromatic acids are convenient substrates for testing the proposed easy formation of a five-membered ring condensed with a benzene nucleus and the six-membered one condensed with five-membered azoles. 

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the... 

In addition to the ligands above, considerable attention is given to more complex ligand systems [4,5] aromatic and heteroaromatic compounds (heteroarenes) (i.e., five- or six-member cyclic structures with delocalized 7i-bonds in the ring containing, besides carbon atoms, either N, P, As, O, S, Se, or Te compounds [6-8]), various chelate-forming compounds, such as macrocyclic crown-ethers, cryptands, porphyrins, and phthalocyanines. 

Most attention has been devoted to an exploration of the chemistry of the borazaromatics. In these compounds, two neighboring carbon atoms of an aromatic system are replaced by the isoelectronic boron-nitrogen groups as illustrated by the 2,1-borazaronaphthalene, 7 2>3>. Their great chemical stability is a distinct feature of heteroaromatic materials and several reviews of their chemistry have been published 4 6>. 

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 first question that needed answering was whether the unsaturated NHC are indeed aromatic. The Hiickel rule states that aromatic systems are monocyclic, homonuclear, planar and possess a delocalised 4n-n2 jr-electron system [96]. Unsaturated NHC fulfil all the criteria except homonuclearity, which would make them heteroaromatic compounds. However, there was doubt as to the delocalisation of the 6 7t-electron system. Althongh there are 6 n-electrons, their distribution is extremely unequal, the backbone carbon atoms C and C contribute one rr-electron each, the nitrogen atoms provide a lone pair each, but the carbene carbon atom C gives none. Can there be a true delocalisation in such circumstances ... 

In this paper reactions of aromatic, heteroaromatic and related diazonium ions with nucleophiles are dia ussed. In such reactions substitution by the diazonium ion of an electrofugic atom or group bonded to carbon takes place. Occasionally reference is made to N- and P-coupling. In Section 4 the respective substitution at nitrogen (formation of diazoamino compounds) is included for comparative purposes. 

Heterocyclic compounds are cyclic structures which contain an atom other than carbon as part of a ring system. Heteroaromatic compounds are a group of these which possess a cyclic conjugated system that can satisfy the (4 + 2)n electron rule and thus have some aromatic stabilization. 

A useful reaction for the synthesis of unsaturated seven-membered heterocycles is the (2 + 2)-cycloaddition of heteroaromatic compounds, e.g., 1 //-pyrrole, furan, or thiophene derivatives, with acetylenes. In combination with a subsequent intramolecular (2 + 2)-cycloreversion (Section IV,B,2) of the annulated cyclobutene moiety, ring enlargement with two carbon atoms can be achieved. 1-Heterocycloheptatrienes, such as benzol6]azepines,26,27,65,66 benzo[fc]oxepins,67,68 benzo[6]-thiepins,69,70 and thiepins,18,71 have been successfully prepared in this way other routes are either nonexistent or laborious.72 In these compounds the reacting carbon-carbon double bond constitutes part of a (4n + 2)7r-electron system and in the (2 + 2)-cycloaddition the resonance energy of the aromatic nucleus is lost. Just like the nonaromatic heterocycles, heteroaromatic compounds have been reported to undergo (2 + 2)-cycloaddition reactions both with electron-deficient and with electron-rich acetylenes. 

There are plenty of examples for the formation of o -adducts derived from the nucleophilic attack by carbanions of C-H-active compounds at unsubstituted carbon atoms of nitroarenes, azines, azinium salts, and other types of aromatic and heteroaromatic compounds [1, 2, 10-18, 45, 114—117]. As a rule, the formed C-adducts can be identified spectroscopically, and in many cases, these... 

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