Fully Conjugated Rings Aromaticity

Aromaticity has been long recognized as one of the most useful theoretical concepts in organic chemistry. It is essential in understanding the reactivity, structure and many physico-chemical characteristics of heterocyclic compounds. Aromaticity can be defined as a measure of the basic state of cyclic conjugated TT-electron systems, which is manifested in increased thermodynamic stability, planar geometry with non-localized cyclic bonds, and the ability to sustain an induced ring current. In contrast to aromatic compounds there exist nonaromatic and antiaromatic systems. Thus, pyrazine (69) 

Though it is usually not difficult to classify a given compound as aromatic, nonaromatic or antiaromatic from a qualitative point of view, much more complex problems arise in attempting to describe the aromaticity in quantitative terms. Until now, three main groups of quantitative criteria of aromaticity have been elaborated energetical, structural and magnetic. 

Heats of combustion can give useful comparative data on the thermodynamic stabilities of heterocyclic compounds (74PMH(6)199). The heats of formation of the isomeric diazines pyridazine, pyrimidine and pyrazine are respectively 4397.8, 4480.2 and 4480.6 kJ mol-1 (62ACS916) pyridazine is almost 83 kJ mol -1 less stable than the other two. 

In this way, pyridin-2-one and pyridin-4-one are calculated to be ca. - 30 kJ mor1 less aromatic than pyridine. In the bicyclic quinolone, the difference in aromaticity between the two forms is less. The precise degree of aromatic character possessed by 2- and 4-pyranone is not settled various methods of estimation give divergent values. 

---

As shown in Figure 1, the fully conjugated ring systems that appear in this chapter include the aromatic oxazinium ion 1, which has six ring 7t-electrons, but of which there is no experimental knowledge. The fully conjugated noncharged... 

The most important reaction is the aromatization of partially conjugated indolo[2,3-a]quinolizine compounds, a very useful synthetic route to the fully conjugated ring system. Catalytic dehydro-... 

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 9.2). The unshared pair of electrons on one nitrogen is a part of the aromatic sextet, whereas that on the other nitrogen is not. It is the pair of electrons that is not part... 

These observations indicate that the presence of a fully conjugated ring of it electrons is not the sole requirement for aromaticity the number of it electrons in the ring is also important. Specifically, we have seen that an odd number of electron pairs is required for aromaticity. 

The presence of a fully conjugated ring of % electrons is not the sole requirement for aromaticity. 

The Hiickel description of aromaticity was based in part on benzene, a cyclic fully conjugated hydrocarbon having (4n -l- 2) -electrons (ff = I) in the closed shell (ring). 

There are no fully conjugated spirophosphorus compounds. Novel classes of spiroaromatic ring systems having a common phosphorus atom in which each ring can exhibit either Mobius or Htickel aromaticity have been proposed, such as 132 and 133 <2002J(P2)1499>. 

Fully conjugated aromatic tetrazole rings have structures (1) or (2) and are numbered as shown. These are discussed in Section 4.17.5. Dihydrotetrazoles are dealt with as nonconjugated systems in Section 4.17.6. The most common forms of these are the structures (3) and (4). The A -tetrazolines (3) may give diaziridines when heated or if aromatic substituents are present at C-5 cycloreversion to azides and imines may occur <87BSF525, 88CB1213>. The 1,4-dihydrotetrazoline structure (4) is common for Y = S, O, NR and CR2. 

Fully conjugated 1,2-thiazines have been prepared with both S(ll) and S(vi) oxidation states. While the zwitterionic compound 11 has been known for some time <1984CHECI(3)995, 1996CHEC-II(6)349>, thiazinylium salts 12 have only recently been prepared. Both fully unsaturated 1,2-thiazine derivatives are considered to be nonaromatic due to poor p-d Jt-bonding. Eurthermore, the six-membered ring of l-alkyl-l,2-thiazine 1-oxide 11 is not planar, but instead exists in a puckered, half-boat conformation thereby precluding aromaticity <1978CC197>. 

In this chapter we summarize the reactivity of the six-membered heterocyclic compounds. We describe first the aromatic compounds, where aromatic is defined as fully conjugated round the ring, and then the partially and fully saturated compounds. Within each of these sections we discuss first the reactivity at the ring atoms, then the reactivity of substituent groups. 

Partially unsaturated azepines, like the fully conjugated systems, tend to be unstable in acid solution and undergo either rearrangement or ring contraction, generally to an aromatic system. Protonation takes place at nitrogen or, as in those hydroazepines in which delocalization of the nitrogen lone pair is possible, at the /3 -carbon of the enamine system. 

---