Heteroaromatic compounds resonance energy

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>. 

Pyrrole is a typical 7t-excessive heteroaromatic compound, with a resonance energy of 103 kJ mokk While thiophene possesses rather more aromatic character, there is less stabilization associated with furan. 

In our view, the explanation for this high reactivity is not to be ascribed to a lack of aromaticity these compounds have resonance energies in excess of that of benzene (Section IV,A). Rather, the reactivity is thought to be due to the ease with which this rc-excessive heteroaromatic system can undergo one-electron oxidation or 1,3-addition to generate another aromatic (benzenoid) system, as shown at structures 67 and 68, respectively. In terms of more familiar systems, the reactivity of the 1,3-positions of isoindole may be thought of as a compounding of the reactivity at the a-positions of pyrrole with that at the meso positions of anthracene. 

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. 

Application of the above-mentioned basicity method is limited, and it can be applied only to systems whose aromaticities are destroyed on protonation. Heteroaromatics such as pyridine and quinoline conserve their aromaticity during protonation therefore a different method is applied, that of considering the basicities of pseudo-bases derived from /V-meth-ylheteroaromatic cations and the corresponding nonaromatic analogues. The principle is shown for isoquinoline in Scheme 14. The model compound used for comparison is 2-methyl-3,4-dihydroisoquinolinium cation. The pAa values for the equilibrium 1 — 2 and 3 — 4 were measured they correspond to an enthalpy difference A/A of 9.2 kcal/mol (X and Y are the differences in the resonance energies between 1 and benzene and 3 and benzene, respectively). 

Resonance Energies of Some Heteroaromatic Compounds and Benzene... 

Although heteroaromatic compounds generally have lower resonance energies than benzene (Table 14.1, p. 629), they also undergo electrophilic aromatic substitution rather than addition. 

As a general trend, six-membered mononuclear N-heteroaromatics such as pyridine and derivatives are much less prone to undergo hydrogenation than bi-and trinuclear N-ring compounds (e.g., quinolines, benzoquinolines, acridines) due to their higher resonance stabilization energy. 

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