Antiaromatic, and

Even though resonance tells us that the negative charge m cycloheptatnenyl anion can be shared by all seven of its carbons this delocalization offers little m the way of sta bilization Indeed with eight rr electrons cycloheptatnenyl anion is antiaromatic and rel atively unstable... 

Cyclic conjugation is continuous in o-benzoquinone and discontinuous in p-benzoquinone (Scheme 15, cf. Scheme 4). The donors (the C=C bonds) are on one side of the cyclic chain and the acceptors (the C=0 bonds) are on the other side in o-benzoquinone. In p-benzoquinone the donors and the acceptors are alternatively disposed along the chain. The thermodynamic stability of o-benzo-quinone is under control of the orbital phase property. The continuity conditions are not satisfied. o-Benzoquinone is antiaromatic. The thermodynamic stability of p-benzoquinone is free of the orbital phase (neither aromatic nor antiaromatic) and comes from the delocalization between the four pairs of the neighboring donors and acceptors. In fact, p-benzoquinone, which melts at 116 °C, is more stable than o-benzoquinone, which decomposes at 60-70 °C. 

The alternate approach of Dewar and Zimmerman can be illustrated by an examination of the 1,3,5-hexatriene system.<81,92> The disrotatory closure has no sign discontinuity (Hiickel system) and has 4n + 2 (where n = 1) ir electrons, so that the transition state for the thermal reaction is aromatic and the reaction is thermally allowed. For the conrotatory closure there is one sign discontinuity (Mobius system) and there are 4u + 2 (n = 1) ir electrons, so that the transition state for the thermal reaction is antiaromatic and forbidden but the transition state for the photochemical reaction is aromatic or allowed (see Chapter 8 and Table 9.8). If we reexamine the butadiene... 

Multifold Aromaticity, Multifold Antiaromaticity, and Conflicting Aromaticity Implications for Stability and Reactivity of Clusters... 

Aromaticity/antiaromaticity in cluster systems has certain peculiarities when compared with organic compounds. The striking feature of chemical bonding in cluster systems is the multifold nature of aromaticity, antiaromaticity, and conflicting aromaticity [3-10]. Double aromaticity (the simultaneous presence of [Pg.439]

Anderson and coworkers [59-66] produced boron cluster cations Bj-B in molecular beams using laser vaporization and studied their chemical reactivity and fragmentation properties. The structures of B3 —IBI3 cations have been established computationally (see review [7] for details) represented in Figure 29.1. In this chapter, we are discussing stability and reactivity of Bj — B 3 cations on the basis of their multifold aromaticity, multifold antiaromaticity, and conflicting aromaticity. 

The presented consideration of the family of cationic boron clusters exemplifies that the assessment of stability and reactivity of clusters can be performed at the qualitative level using multifold aromaticity, multifold antiaromaticity, and variety of conflicting aromaticities. 

Four-membered rings should be antiaromatic, and they have not been reported in uncomplexed form. 

Aromatic, antiaromatic, and acyclic polyenes primarily differ in bond lengths, which serves as a basis for the structural indices of aromaticity reflecting the degree of alternation of bond lengths in a ring. 

In Sections III,A,4 and III,B,l,f, examples of ring transformation reactions have been mentioned. Here we shall discuss in detail the interesting case of isomerism between an eight-membered ring with an 8-7r-electron system (thus, in principle, antiaromatic) and a 10-n pentalene derivative (briefly mentioned in Section III,B,4,a). Scheme 14 shows... 

Oxazinyl anions are obtained when AH- 1,3-oxazines (or their tautomers) are treated with strong base in aprotic media. The anions are antiaromatic, and are only stable at low temperature, when they are considered to exist in equilibrium with oxazabicyclo[3.1.0]hexenyl valence tautomers (Scheme 8) (75AG(E)581). 

In terms of Eq. (1), the development of delocalization in a hetero[15]-annulene may simply be described as a situation in which the stabilizing influence of AS0 (rigidity) overcomes the combined destabilization imparted by AH0n (antiaromaticity) and A/fo ([Pg.89]

The importance of the arguments we have outlined lies in the fact that they provide a theoretical foundation both for aromaticity-antiaromaticity and for pericyclic selection rules. They furthermore demonstrate the relationship between the two The topological equivalence between an array of p orbitals in a w system of a carbon chain or ring and a pericyclic transition state, composed of an... 

Another example is l,3,5-cycloheptatriene-7-d, where the equilibrium constant [equatorial]/[axial] is 1.25 favoring the conformer 42 with deuterium equatorial.113 Since deprotonation of either conformer would produce the same cycloheptatrienyW anion, the axial conformer must be more acidic, with a Apifa = -logi01.25. This is a case where the separate acidity constants would not be measurable, because the barrier to conformational equilibration is only 6kcalmol 1 and because the anion is antiaromatic and very unstable. 

Barriers for ring inversion, shown in Table 2, were obtained in this study <1997PCA3371> and other work, as well as nucleus-independent chemical shifts (NICSs) of thiepine 1 <2000MI177>. From NICS values of thiepine, the planar structure appeared to be antiaromatic and the more stable boat-like structure as nonaromatic. The structure and energies were compared with those of 1,2- and 1,4-dithins <2000MI177>. 

Avital Shurki was born in 1970 after two years in the army, she began studying chemistry in 1990 at The Hebrew University, where she finished summa cum laude and was on the Dean s and Rector s Lists. In 1995 she received the Sara Wolf Prize for Graduate students. In 1994 she joined the group of S. Shaik and completed her Ph.D. degree in Quantum Chemistry in 1999. One of the main themes in her dissertation was the aromaticity, antiaromaticity, and -delocalization. In 2000 she received the Rothchild Fellowship and joined the group of Professor A. Warshel at USC. Her research interests involve applications of VB ideas and calculations to chemical reactivity and bonding. 

Let us continue with this assumption that the 77-components of the antiaromatic and the successive aromatic annulene (e.g., C4 vs C6, C8 vs C10, etc.) have the same 77-distortivity. Thus, we can use the 77-dis-tortivity value calculated for cyclobutadiene (see Table 2, from —10.4 to —11.0 kcal/mol) and assume that in antiaromatic annulenes, C,H (n = AN), the 77-distortivity is a linear function of the annulene size, n (its number of 77-bonds is nl2), and that the successive aromatic annulene (n = AN + 2) has precisely the same 77-distortivity, as specified in eqs 8 and 9 ... 

The very delicate ji a-balance poses an opportunity to study delocalized antiaromatic species and to gain new insight into the resonance energy of antiaromatics and its manifestations, e.g., in the magnetic properties. The calculations of van Wiillen and Kut-zelnigg185 show that AU, cyclobutadiene has a positive... 

Cyclobutadiene (26) is antiaromatic and its isolation is not possible. Flowever, it can be stabilized by -coordination of Fp+ to one of the double bonds to give 27, and the uncomplexed double bond in 27 undergoes Diels-Alder reaction with cyclopentadiene to give 28 [4]. As described in Section 9.2, cyclobutadiene (26) can be stabilized as a diene by the )/4-coordination of Fe(CO)3. 

We have introduced the concept of homoheteroaromaticity (the equivalent of homoaromaticity [18] for heterocycles) to describe the structure of some cations obtained by protonation of l//-azepine (4) and 5//-dibenz[/ ,/]azepine (5). B3LYP/6-311++G, GIAO, and NICS calculations together with some NMR experiments lead us to conclude that the neutral molecules are antiaromatic and that the cations 6 and 7 are homoaromatic [22],... 

Chem Rev 105(10) whole issue devoted to aromaticity, antiaromaticity and related topics... 

In 1930, when Htickel first derived his rule, he considered only aromatic annulenes. Antiaromatic and nonaromatic systems are extensions introduced by later authors, in particular by Dewar. 

Hence 9 and 10 are aromatic, 11 is antiaromatic and 8 is nonaromatic. Consequently, Reaction (4.1) will not occur easily because its (nonaromatic) transition state is not stabilized. We can consider it to be less forbidden than Reaction (4.4), but forbidden nonetheless. This shows clearly that there is no distinct boundary between forbidden and allowed reactions they are merely the two limits of a continuum. 

D.L. Cooper, J. Gerratt and M. Raimondi,The spin-coupled description of aromatic, antiaromatic and nonaromatic systems, in Pauling s legacy Modem modelling of the chemical bond, Vol. 6, ed. Z. B. Maksic and W. J. Orville-Thomas (Elsevier, Amsterdam, 1999). 

Because of its nonplanar geometry, cyclooctatetraene is not antiaromatic and its hydrogens appear at 5.75 8, a value typical for alkenes. However, the triple bonds of the compound called benzo-l,5-cyclooctadiene-3,7-diyne force this molecule to assume a nearly planar geometry. The pi system of its eight-membered ring contains eight electrons. (Only two of the electrons of each triple bond are part of the conjugated system.)... 

The question of aromaticity versus antiaromaticity and delocalized versus localized double bonds in pentalene (2) dates back to 1922, when Armit and Robinson compared it with naphthalene and postulated that the former might be similarly aromatic [32, 33]. While the first synthesis of a non-fused hexaphenylpentalene (38) [30] provided only some clues as to the non-aromatic reactivity of the pentalene skeleton, the tri-tert-butyl derivative 39, prepared and studied by Hafner et al. in great detail [31], gave a better insight. The ring-proton signals of this alkyl-substituted pentalene 39 are shifted upfield compared to those of fulvene (27) and other cyclic polyenes. This observation led to the conclusion that the pentalene derivative 39 should be an antiaromatic species. However, the results did not permit a distinction... 

Aromatic, Antiaromatic, and Nonaromatic Compounds 722 16-6 Huckel s Rule 722... 

Use Huckel s rule (and the criteria for its application), rather than resonance, to determine which annulenes and ions are aromatic, antiaromatic, and nonaromatic. 

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