Hiickel ring

The word aromaticity usually implies that a given molecule is stable, compared to the corresponding open chain hydrocarbon. For a detailed account on aromaticity, see, e.g., Reference [95], The aromaticity rules are based on the Hiickel-Mobius concept. A cyclic polyene is called a Hiickel system if its constituent p orbitals overlap everywhere in phase, i.e., the p orbitals all have the same sign above and below the nodal plane (Figure 7-23). According to HiickeTs rule [96], if such a system has 4n + 2 electrons, the molecule will be aromatic and stable. On the other hand, a Hiickel ring with 4n electrons will be antiaromatic. 

Figure 7-25. Comparison of the disrotatory ring closure and the V2S + V2S reaction with the Hiickel ring.

Figure 7-23. Illustration of a Hiickel ring. Reproduced by permission from Ref. [7-49].

Huckel s rule [7-47], if such a system has 4n -I- 2 electrons, the molecule will be aromatic and stable. On the other hand, a Hiickel ring with An electrons will be antiaromatic. 

If the Hiickel ring is twisted once, as shown in Figure 7-24a, the situation is reversed [7-48]. Therefore, Dewar [7-45] referred to this twisted ring as an anti-Hiickel system. It is also called a Mobius system [7-44, 7-49], an appropriate name indeed. A Mobius strip is a continuous, one-sided surface which is formed by twisting the strip by 180° around its own axis and then joining its two ends. There is a phase inversion at the point where the two ends meet, as seen in Figures 7-24a and b. Figure 7-24c and d depict yet other Mobius strips. 

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). 

Electi ocyclic reactions are examples of cases where ic-electiDn bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are fomred along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

In the case of benzene, Hiickel treatment of the six 2p orbitals on the carbon atoms and perpendicular to the plane of the ring leads to the secular determinant... 

A theoretical, comparative study of the tautomerism of 56 five-membered heterocyclic rings announced in (76AHC(Sl)l) has appeared (81MI40402). The stabilities of the three forms for 5-pyrazolones, 5-pyrazolethiones and 5-aminopyrazoles have been calculated by a simple Hiickel o) iterative method. The relative energies and the substituent and solvent effects are in agreement with the experimental results. 

Heterocyclics with seven and more ring members display an enormous variety of shapes. Bond lengths are often close to those of open chain counterparts, but bond angles can be greatly different. Aromaticity is possible where the conditions of planarity and Hiickel s rule are met, but the majority of fully unsaturated large heterocycles are not aromatic (see below). 

The tropylium and the cyclopropenyl cations are stabilized aromatic systems. These ions are arumatic according to Hiickel s rule, with the cyclopropeniiun ion having two n electrons and the tropyliiun ion six (see Section 9.3). Both ring systems are planar and possess cyclic conjugation, as is required for aromaticity. 

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... 

One criterion of aromaticity is the ring current, which is indicated by a chemical shift difference between protons, in the plane of the conjugated system and those above or below the plane. The chemical shifts of two isomeric hydrocarbons are given below. In qualitative terms, which appears to be more aromatic (Because the chemical shift depends on the geometric relationship to the ring current, a quantitative calculation would be necessary to confirm the correctness of this qualitative impression.) Does Hiickel MO theory predict a difference in the aromaticity of these two compounds ... 

Hiickel models of molecular electronic structure enjoyed many years of popularity, particularly the r-electron variants. Authors sought to extract the last possible amount of information from these models, perhaps because nothing more refined was technically feasible at the time. Thus, for example, the inductive effect was studied. The inductive effect is a key concept in organic chemistry a group R should show a - -1 or a —I effect (according to the nature of the group R) when it is substituted into a benzene ring. 

Previous extended Hiickel calculations on the bis(trifluoromethyl) derivative 6 show the favored isomer to be 6b, while the CNDO/2 method favors 6a [75JCS(P2)559]. More recent ab initio calculations for different R substituents show that electron-releasing substituents favor the ring-opened dithione (6b), whereas electron-withdrawing substituents favor the cyclic structure (6a) (80JA6687). These conclusions are supported by electron diffraction (ED) (76JA899) and photoelectron (PE) spectral data... 

The presence of a ring-current is characteristic of all Hiickel aromatic molecules and is a good test of aromaticity. For example, benzene, a six-7r-clectron aromatic molecule, absorbs at 7.37 5, but cyclooctatetraene, an eight--iT-electxon nonaromatic molecule, absorbs at 5.78 8. 

Thiepin, as a seven-membered conjugated system with sulfur as heteroatom, is a member of the 8 7t-electron heteroannulenes which are antiaroinatic according to Hiickel s rule. In contrast to oxepin, thiepin is not stable at room temperature and no valence isomerism with an arene sulfide has been observed. Stable thiepins are obtained only when two bulky substituents, e.g. /ert-butyl, are introduced into positions 2 and 7. In benzothiepins the annellation effect of the aromatic rings contributes decisively to the stability of these compounds stability increases with an increasing number of fused benzene rings. 

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