Five-Membered Rings with Two Heteroatoms

As must be expeeted for all these studies of long-range interaetions, large basis sets with diffuse funetions in eombination with eorrelation methods (at least MP2) are mandatory. DFT ealeulations were performed in order to determine the preferred addition site of OH to protonated 4-ethylimidazole [99JPC(A)1283]. As supported by eomputed hyperfine eoupling eonstants, the OH radieal adds 

Deviations of this magnitude are very unusual but can also be found for other sensitive heterocycles, such as 1,2,5-oxadiazole-2-oxides. Although the authors 

However, polar solvents shift the equilibrium toward the Z conformer 12a, which has a significantly larger dipole moment. 

The influence of an ort/io-imidazole substituent on the bond dissociation energy of the O—H bond in phenol was studied by DFT calculations [00IJQ714]. The imidazole ring is twisted with respect to the phenol ring by 59° and causes a decrease of the bond dissociation energy by about -1 kcal/mol with respect to the unsubstituted molecule only. 

The electronic spectrum of imidazole was studied at the CASSCF and CASPT2 levels [96JPC6484]. In the gas phase the first and second excited singlet 

We have looked at the five-membered aromatic heterocycles pyrrole, furan and thiophene in Section 11.5. Introduction of a second heteroatom creates azoles. This name immediately suggests that nitrogen is one of the heteroatoms. As soon as we consider valencies, we discover that in order to draw a five-membered aromatic heterocycle with two heteroatoms, it must contain nitrogen A neutral oxygen or sulfm atom can have only two bonds, and we cannot, therefore, have more than one of these atoms in any aromatic heterocycle. On the other hand, there is potential for having as many nitrogens as we like in an aromatic ring. 

We can visualize these heterocycles as similar to the simpler aromatic systems pyrrole, furan and thiophene. For example, in imidazole, each carbon and nitrogen will be sp hybridized, with p orbitals contributing to the aromatic rt system. The carbon atoms will each donate one electron to the rt system. Then, as in pyrrole, the NH nitrogen supplies two electrons, and, as in pyridine, the =N- supplies one electron and retains a lone pair. Oxygen or sulfur would also supply two electrons, as we saw in furan and thiophene. 

It also follows that a compound like imidazole has one pyridine-like nitrogen, and one pyrrolelike nitrogen. We may thus expect to see imidazole having properties resembling a combination of either pyridine- or pyrrole-like reactivity. The availability and location of lone pair electrons is crucial to our imderstanding of imidazole chemistry, and it often helps to include these in the stmcture. 

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Volumes 3 and 4 cover five-membered rings with two heteroatoms, or more than two heteroatoms, respectively, each with their fused carbocyclic compounds. 

Volume 3 Five-membered Rings with Two Heteroatoms and Fused Carbocyclic Derivatives... 

Synthesis of Five-Membered Rings with Two Heteroatoms 393... 

M. M. Karelson, A. R. Katritzky, M. Szafran, and M. C. Zerner,/. Chem. Soc., Perkin Trans. 2, 195 (1990). A Theoretical Treatment of Solvent Effects on the Tautomeric Equilibria of Five-Membered Rings with Two Heteroatoms. 

IV. Quinones with a Condensed Five-Membered Ring with Two Heteroatoms... 

The 1,3- and 1,2-azoles, five-membered rings with two heteroatoms, present a fascinating combination of heteroatom types - in all cases, one heteroatom must be of the five-membered heterocycle (pyrrole, thiophene, furan) type and one of the imine type, as in pyridine imidazole with two nitrogen atoms illustrates this best. Contributor 39 is a particularly favourable one. 

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