Aromatic sextet, resonance stabilization

Acylimidazoles and related amides in which the nitrogen atom is part of an aromatic ring hydrolyze much more rapidly than other amides. A major factor is the decreased resonance stabilization of the carbonyl group, which is opposed by the delocalization of the nitrogen lone pair as part of the aromatic sextet. 

So why can tautomerism occur with a hydroxypyri-dine It is because 2-pyridone and 4-pyridone still retain aromaticity, with the nitrogen atom donating its lone pair electrons to the aromatic sextet. This is more easily seen in the resonance structures, and shonld remind ns of the resonance stabilization in... 

Wheland intermediates, a complexes, or arenium ions In the case of benzenoid systems they are cyclohexadienyl cations. It is easily seen that the great stability associated with an aromatic sextet is no longer present in 1, though the ion is stabilized by resonance of its own. The arenium ion is generally a highly reactive intermediate and must stabilize itself by a further reaction, although it has been isolated (see p. 504). 

Resonance stabilization is important in electrophilic aromatic substitution as well. While each of the canonical forms of the Wheland intermediate has a sextet carbon atom, the charge is distributed over the remaining five atoms of the ring by resonance and is thus greatly stabilized. 

The 6th rank in terms of acylation reactivity that is attributed to the acyl imidazolides in Table 6.1 (entry 10) is also plausible. In the acyl imidazolides, the free electron pair of the acylated N atom is essentially unavailable for stabilization of the C=0 double bond by resonance because it is part of the -electron sextet, which makes the imidazole ring an aromatic compound. This is why acyl imidazolides, in contrast to normal amides (entry 2 in Table 6.1) can act as acylating agents. Nevertheless, acyl imidazolides do not have the same acylation capacity as acylpyridinium salts because the aromatic stabilization of five-mem-bered aromatic compounds—and thus of imidazole—is considerably smaller than that of six-membered aromatic systems (e. g., pyridine). This means that the resonance form of the acyl imidazolides printed red in Table 6.1 contributes to the stabilization of the C=0 double bond. For a similar reason, there is no resonance stabilization of the C=0 double bond in N-acylpyridinium salts in the corresponding resonance form, the aromatic sextet of the pyridine would be destroyed in exchange for a much less stable quinoid structure. 

The regioselectivity of Ar-SE reactions with naphthalene follows from the different stabilities of the Wheland complex intermediate of the 1-attack (Figure 5.10, top) compared with that of the 2-attack (Figure 5.10, bottom). For the Wheland complex with the electrophile at Cl these are five sextet resonance forms. In two of them the aromaticity of one ring is retained. The latter forms are thus considerably more stable than the other three. The Wheland complex with the electrophile at C2 can also be described with five sextet resonance forms. However, only one of them represents an aromatic species. The first Wheland complex is thus more stable than the second. The 1-attack is consequently preferred over the 2-attack. 

Resonance effects also influence the basicity of pyrrole. Pyrrole is a very weak base, with a pKb °f about 15. As we saw in Chapter 15, pyrrole is aromatic because the nonbonding electrons on nitrogen are located in a p orbital, where they contribute to the aromatic sextet. When the pyrrole nitrogen is protonated, pyrrole loses its aromatic stabilization. Therefore, protonation on nitrogen is unfavorable, and pyrrole is a very weak base. 

Scheme 1 The azoles, illustrated by imidazole, can behave as weak acids or bases giving resonance-stabilized ions that retain the aromatic sextet.

This resonance stabilization should be even more pronounced in the cyclic dithiocarbene (274), as indicated by the neutral carbene structure (274) and the ylid structures (275-280), in which the potential aromatic sextet of the 1,3-dithiolium ion is retained. Consequently, H-2 in 269 should be acidic. The analogy with the thiazolium and imidazolium ions, which have been intensively studied by Bres-... 

In structures I, II, and IV, the aromatic sextet is preserved in the ring that is not under attack these structures thus retain the full resonance stabilization of one benzene ring (36 kcal/mole). In structures like III, V, and VI, on the other hand, the aromatic sextet is disrupted in both rings, with a large sacrifice of resonance stabilization. Clearly, structures like I, II, and IV are much the more stable. 

In contrast, it is the resonance-stabilized cation derived from cyclo-heptatriene that possesses the aromatic sextet of 7t-electrons. Tropylium bromide is formed by the addition of bromine to cycloheptatriene and... 

Because the imidazole group on the side chain of histidine contains six tt electrons in a planar, fully conjugated ring, imidazole is classified as a heterocyclic aromatic amine (Section 21.2D). The unshared pair of electrons on one nitrogen is a part of the aromatic sextet, whereas that on the other nitrogen is not. The pair of electrons that is not part of the aromatic sextet is responsible for the basic properties of the imidazole ring. Protonation of this nitrogen produces a resonance-stabilized cation. 

Conversely, positive substituents decrease the stability of transition metal-carbon bonds by lessening the stability of resonance structures such as (XIX). The cycloheptatrienyl group cited above is perhaps the extreme example of a positively substituted group since the carbon atom bonded to the metal atom is a portion of a triply unsaturated seven-membered ring, which tends to take on a full positive charge as in the tropylium ion in order to possess the desired aromatic sextet. 

The different acceptor strengths were rationalized on the basis of the resonance stabilities of the radical anions 45and 48 . Thus, the reduction of 45 to form the radical anion leads to an aromatic sextet. In contrast, the canonical structure of the radical anion of 48 involves tetravalent sulphur. These non-classical Kekule thiophenes are unstable compared with classical Kekule thiophenes [118]. 

In the first step of the actual Ar-SE reaction, a substituted cyclohexadienyl cation is formed from the electrophile and the aromatic compound. This cation and its derivatives are generally referred to as a cr or Wheland complex. Wheland complexes are described in the language of the VB method by superpositioning mentally at least three carbenium ion resonance forms (Figure 5.1). In the following, these resonance forms are referred to briefly as sextet formulas. There is an additional resonance form for each substituent, which can stabilize the positive charge of the Wheland complex by a +M effect (see Section 5.1.3). This resonance form is an all-octet formula. 

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