Stabilization Hiickel-aromatic

In a formal sense, isoindole can be regarde,d as a IOtt- electron system and, as such, complies vith the Hiickel (4w- -2) rule for aromatic stabilization, with the usual implicit assumption that the crossing bond (8, 9 in 1) represents a relatively small perturbation of the monocyclic, conjugated system. The question in more explicit terms is whether isoindole possesses aromatic stabilization in excess of that exhibited by pyrrole. 

Although [34]octaphyrin 80 fulfills Hiickel s rule, the II NMR spectrum indicates by the high-field shift of the methine protons that the system is nonaromatic. The X-ray structure analysis demonstrates clearly the reason for the lack of aromatic stabilization, namely the nonplanar loop conformation in which the whole macrocycle is twisted similarly to the [32]octaphyrin structure and which is also found for [36]octaphyrin and [40]decaphyrin structures (vide infra). 

This too is a Hiickel 4n + 2 p electron system (n = 2, this time) and shows quasi-aromatic stability stabilisation by aromatisation has again taken place, remarkably this time in a doubly charged carbanion (18). 

Our success in super-stabilization of cation 6 led us to the preparation of a higher homologue, that is, cyclooctatetraene (COT), fully annelated with BCO units 9 (9). As compared with a large number of studies on its radical anion or dianions, the studies on the cationic species of COT have been quite limited. There have been only one study by Olah and Paquette on the substituted COT dication (70), which is a typical 6n Hiickel aromatic system, and few sporadic studies on radical cations, which involve indirect spectral observations, such as electronic spectra in Freon matrix at low temperature (77,72) and constant-flow ESR study (13). 

The seven-membered fully unsaturated boron heterocycle is named l//-borepin or, more frequently, borepin. It has six 7r-electrons and is therefore a potential Hiickel aromatic, isoelectronic with the tropylium ion. This similarity has induced several groups to search for a synthesis. Recent theoretical calculations have predicted the -stabilization to be less than that of the tropylium ion. On the other hand, borepins are organoboranes and therefore expected to be sensitive to oxygen. 

On the basis of Hiickel s An +2) n- electron rule, all of these systems can be expected to be aromatic in nature. They do indeed exhibit varying degrees of aromatic stabilization depending on the nature and position of the heteroatom. They cannot all be represented by conventional classical structures. Structures (la)-(lc) can be represented by classical covalent bonded structures whereas those of the (Id) type form the nonclassical structures in the sense that they can be drawn only as charge-separated systems or biradicals in systems wherein X/Y are sulfur or selenium atoms, d- orbital participation in bonding is conceivable, leading to tetravalent sulfur or selenium. 

Anti-aromaticity was predicted by the Hiickel approach for conjugated cyclic planar structures with 4n 7i electrons due to the presence of two electrons in antibonding orbitals, such as in the cydopropenyl anion, cydobutadiene, and the cydopentadienyl cation (n = 1), and in the cydoheptatrienyl anion and cydooctatetraene (n = 2). It has been argued that a simple definition of an anti-aromatic molecule is one for which the 1H NMR shifts reveal a paramagnetic ring current, but the subject is controversial. The power of the Hiickel theory indeed resides not only in the aromatic stabilization of cydic 4n + 2 electron systems, but also in the destabilization of those with An electrons [22, 27, 42]. 

Note the signs of the coefficients. We can conclude from what was said above that the higher or lower stability of a cyclic polyene as compared to an acyclic one depends on the combination of signs of the coefficients at the ends of the demethylized compound. If the signs are identical, the even AS is aromatic due to cyclic stabilization if the signs are different, the system is anti-aromatic due to cyclic destabilization. Hence, the Hiickel aromaticity... 

The reaction of 1,3-butadiene with ethylene to give cyclohexene is an allowed reaction if the ethylene fragment approaches the butadiene fragment from one face, preserving a plane of symmetry as indicated in 4. The Hiickel-type arrangement of the p-orbitals shown in 4 involves six electrons and so should express some aromatic stabilization. 

It is important to examine aromaticity in its wicommon features in addition to planarity and aromatic stability. MO calculations carried out by Hiickel in the 1930s showed that aromatic character is associated with planar cyclic molecules that contained 2, 6, 10, 14 (and so on) 7i-electrons. This series of numbers is represented by the term An + 2, where n is an integer, and gave rise to Hiickel s An + 2 rule that refers to the number of 7i-electrons in the p-orbital system. In the case of benzene, n= and thus the system contains six 7C-electrons that are distributed in MOs as shown above. 

Aromatic stability is found in planar, cyclic conjugated systems that satisfy the Hiickel rule in possessing (4// + 2) Tt-electrons. 

The enhanced stability of the benzene molecule can be attributed to the complete shells of n -electron orbitals, analogous to the way that noble gas electron configurations achieve their stability. Naphthalene, apart from the central C-C bond, can be modeled as a ring containing 10 electrons in the next closed-shell configuration Irr These molecules fulfill Hiickel s AN- -2 rule for aromatic stability. 

The chemistry of aromatic compounds was one of the early testing grounds for the application of quantum mechanics to chemical problems. The reason for this was chiefly the simplicity and success of Hiickel molecular orbital (HMO) theory. It is appropriate to see how well DFT explains aromatic behavior. We already have one example of this in Chapter 2 aromatic stability can be correlated with (I — Af chemical hardness. 

Aromatic The Hiickel MO model predicts that rings will have aromatic stability if they have 4 +2... 

On the basis of his analysis Hiickel proposed that only certain numbers of tt electrons could lead to aromatic stabilization. Only when the number of tt electrons is 2, 6, 10, 14, and so on, can a closed-shell electron configuration be realized. These results are summarized in Hiickel s rule Among planar, monocyclic, fully conjugated polyenes, only those possessing (4 + 2) ti electrons, where n is an integer, will have special aromatic stability. 

It has been shown experimentally (heats of hydrogenation, heats of formation) that there is significant homoaromatic stabilization in cycloheptatrienes such as 1, 7, 8, 9, 12 (in the latter three there is additional classical Hiickel aromaticity), as well as in norcaradienes such as 2, 3, 10, 11. In cases where other energetic factors of the pair of valence tautomers 1 2 are very similar, the relative homoaromaticity determines the equilibrium position.97... 

The annulenes are that series of monocyclic polyolefins (C H ) containing a complete system of contiguous double bonds. While benzene (the best known member of this class of compounds) has been in evidence for some time it is only of late that interest in the higher members has become apparent. This interest has its origins in the LCAO-MO theory of re-elec-tron systems as formulated by E. Hiickel (in particular the "Hiickel rule relating aromatic stability to structure). Although the non-classical chemistry of the benzenoid hydrocarbons had previously been the subject of some conjecture, Httckel s theoretical studies provided the first satisfactory explanation of the peculiar stability of this class of compounds and, incidently, the elusiveness of cyclobutadiene. 

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