Antiaromaticity

Planar cyclic conjugated species less stable than corresponding acyclic unsaturated species are called antiaromatic. They have 4 n electrons. 1,3-Cyclobutadiene ( = 1), for which one can write two equivalent contributing structures, is an extremely unstable antiaromatic molecule. This shows that the ability to write equivalent contributing structures is not sufficient to predict stability. 

Problem 10.7 Cyclooctatetraene (CgHg), unlike benzene, is not aromatic it decolorizes both dil. aq. KMn04 and Br2 in CCI4. Its experimentally determined heat of combustion is —4581 kJ/mol. (a) Use the Hiickel rule to account for the differences in chemical properties of CgHg from those of benzene, (b) Use thermochemical data of Problem 10.4 to calculate the resonance energy, (c) Why is this compound not antiaromatic (d) Styrene, C( H5CH=CH2, with heat of combustion —4393 kJ/mol, is an isomer of cyclooctatetraene. Is styrene aromatic  

The difference —4602 - (—4581) = —21 kJ/mol shows small (negative) resonance energy and no aromaticity, 

Problem 10.8 Deduce the structure and account for the stability of the following substances which are insoluble in nonpolar but soluble in polar solvents, (a) A red compound formed by reaction of 2 mol, of AgBp4 with 1 mol of l,2,3,4-tetraphenyl-3,4-dibromocyclobut-l-ene. (b) A stable compound from the reaction of 2 mol of K with 1 mol of 1,3,5,7-cyclooctatetraene with no liberation of H2. 

The solubility properties suggest that these compounds are salts. The stability of the organic ions formed indicates that they conform to the Hiickel rule and are aromatic. 

Are conjugated systems that contain 4n (4, 8, 12, 16.) rr electrons also aromatic These systems actually are destabilized by delocalization and are said to be antiaromatic. 

The first three examples contain 4tt electrons, whereas the last one contains Sir electrons. All are highly unstable species. On the other hand, cyclooctate-traene, 1-35, an Sir electron system, is much more stable than any of the preceding compounds. This is because the tt electrons in (yclooctatetraene are not delocalized significantly the eight-membered ring is bent into a tublike structure and adjacent tt bonds are not parallel. 

Quinoline, indole, imidazole, purine, and pyrimidine are other examples of heterocyclic aromatic compounds. The heterocyclic compounds discussed in this section will be examined in greater detail in Chapter 20. 

PROBLEM 164 Which of the following compounds could be protonated without destroying its aromaticity  

Refer to the electrostatic potential maps on page 347 to answer the following questions  

An aromatic compound is more stable than a cyclic compound with localized electrons, whereas an antiaromatic compound is less stable than a cyclic compound with localized electrons. Aromaticity is characterized by stability, whereas antiaromaticity is characterized by instability. 

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Stabilizing resonances also occur in other systems. Some well-known ones are the allyl radical and square cyclobutadiene. It has been shown that in these cases, the ground-state wave function is constructed from the out-of-phase combination of the two components [24,30]. In Section HI, it is shown that this is also a necessary result of Pauli s principle and the permutational symmetry of the polyelectronic wave function When the number of electron pairs exchanged in a two-state system is even, the ground state is the out-of-phase combination [28]. Three electrons may be considered as two electron pairs, one of which is half-populated. When both electron pahs are fully populated, an antiaromatic system arises ("Section HI). 

The special case of pericyclic reactions is an appropriate means of introducing the subject These reactions are very common, and were extensively studied experimentally and theoretically. They also provide a direct and straightforward connection with aromaticity and antiaromaticity, concepts that mm out to be quite useful in analyzing phase changes in chemical reactions. 

A more general classification considers the phase of the total electronic wave function [13]. We have treated the case of cyclic polyenes in detail [28,48,49] and showed that for Hiickel systems the ground state may be considered as the combination of two Kekule structures. If the number of electron pairs in the system is odd, the ground state is the in-phase combination, and the system is aromatic. If the number of electron pairs is even (as in cyclobutadiene, pentalene, etc.), the ground state is the out-of-phase combination, and the system is antiaromatic. These ideas are in line with previous work on specific systems [40,50]. 

The results of the derivation (which is reproduced in Appendix A) are summarized in Figure 7. This figure applies to both reactive and resonance stabilized (such as benzene) systems. The compounds A and B are the reactant and product in a pericyclic reaction, or the two equivalent Kekule structures in an aromatic system. The parameter t, is the reaction coordinate in a pericyclic reaction or the coordinate interchanging two Kekule structures in aromatic (and antiaromatic) systems. The avoided crossing model [26-28] predicts that the two eigenfunctions of the two-state system may be fomred by in-phase and out-of-phase combinations of the noninteracting basic states A) and B). State A) differs from B) by the spin-pairing scheme. 

Adopting the view that any theory of aromaticity is also a theory of pericyclic reactions [19], we are now in a position to discuss pericyclic reactions in terms of phase change. Two reaction types are distinguished those that preserve the phase of the total electi onic wave-function - these are phase preserving reactions (p-type), and those in which the phase is inverted - these are phase inverting reactions (i-type). The fomier have an aromatic transition state, and the latter an antiaromatic one. The results of [28] may be applied to these systems. In distinction with the cyclic polyenes, the two basis wave functions need not be equivalent. The wave function of the reactants R) and the products P), respectively, can be used. The electronic wave function of the transition state may be represented by a linear combination of the electronic wave functions of the reactant and the product. Of the two possible combinations, the in-phase one [Eq. (11)] is phase preserving (p-type), while the out-of-phase one [Eq. (12)], is i-type (phase inverting), compare Eqs. (6) and (7). Normalization constants are assumed in both equations ... 

We term the in-phase combination an aromatic transition state (ATS) and the out-of-phase combination an antiaromatic transition state (AATS). An ATS is obtained when an odd number of electron pairs are re-paired in the reaction, and an AATS, when an even number is re-paired. In the context of reactions, a system in which an odd number of electrons (3, 5,...) are exchanged is treated in the same way—one of the electron pairs may contain a single electron. Thus, a three-electron system reacts as a four-electron one, a five-electron system as a six-electron one, and so on. 

The potential surfaces of the ground and excited states in the vicinity of the conical intersection were calculated point by point, along the trajectory leading from the antiaromatic transition state to the benzene and H2 products. In this calculation, the HH distance was varied, and all other coordinates were optimized to obtain the minimum energy of the system in the excited electronic state ( Ai). The energy of the ground state was calculated at the geometry optimized for the excited state. In the calculation of the conical intersection... 

Thus cyclobutadiene like cyclooctatetraene is not aromatic More than this cyclo butadiene is even less stable than its Lewis structure would suggest It belongs to a class of compounds called antiaromatic An antiaromatic compound is one that is destabi lized by cyclic conjugation... 

Huckel proposed his theory before ideas of antiaromaticity emerged We can amplify his generalization by noting that among the hydrocarbons covered by Huckel s rule those with An) tt electrons not only are not aromatic they are antiaromatic... 

Benzene cyclobutadiene and cyclooctatetraene provide clear examples of Huckel s rule Benzene with six tt electrons is a An + 2) system and is predicted to be aromatic by the rule Square cyclobutadiene and planar cyclooctatetraene are An systems with four and eight tt electrons respectively and are antiaromatic... 

As noted earlier planar annulenes with 4n tt electrons are antiaromatic A mem ber of this group [16]annulene has been prepared It is nonplanar and shows a pattern of alternating short (average 134 pm) and long (average 146 pm) bonds typical of a nonaromatic cyclic polyene... 

The five membered cyclopentadienyl system contrasts with cycloheptatrienyl Here the cation has four tt electrons is antiaromatic very unstable and very difficult... 

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... 

Section 11 19 An additional requirement for aromaticity is that the number of rr elec Irons m conjugated planar monocyclic species must be equal to An + 2 where n is an integer This is called Huckel s rule Benzene with six TT electrons satisfies Huckel s rule for n = 1 Square cyclobutadiene (four TT electrons) and planar cyclooctatetraene (eight rr electrons) do not Both are examples of systems with An rr electrons and are antiaromatic... 

FIGURE 13 10 More shielded (red) and less shielded (blue) protons in (a) [18]annulene and (b) [16]annulene The induced magnetic field associated with the aromatic ring current in [18]annulene shields the inside protons and deshields the out side protons The opposite occurs in [16]annulene which is antiaromatic... 

Antiaromatic (Section 11 18) The quality of being destabi hzed by electron delocalization... 

Ring Currents Aromatic and Antiaromatic Magnetic Resonance Imaging Spectra by the Thousands Gas Chromatography GC/MS and MS/MS... 

Ring Currents Aromatic and Antiaromatic Spectra by the Thousands... 

For the antiaromatic three-membered heterocycles, experimental data are available only for thiirenes (and there is some doubt about the true antiaromaticity of thiirenes). Bond lengths have been calculated, however, for these antiaromatic 47r-systems (80PAC1623). In comparison with the corresponding saturated heterocycles, the C—X bond lengths are increased by 0.05 to 0.17 A and the C—C bond length is decreased by 0.2 A. 

The simplest, qualitative, theoretical understanding of the nature of oxirene is provided by Breslow s concept of antiaromaticity. Whatever criticisms may be levelled at this notion (78JA6920), it does correctly predict that oxirene should be unusually unstable. 

Benzimidazolo[l,2-c]thiatri azoles synthesis, 6, 612 Benz[cd]indazole antiaromaticity, 5, 208 Benz[cd]indazole, dihydroaromaticity, 5, 208 Benz[e]indolizine synthesis, 4, 466 Benzipiperylon biological activity, 5, 296 Benz[/]isoindoles synthesis, 4, 266... 

Oxepin, 4-ethoxycarbonyl-2,3,6,7-tetrahydro-synthesis, 7, 578 Oxepin, 2-methyl-enthalpy of isomerization, 7, 555 Oxepin, 2,3,4,5-tetrahydro-reduction, 7, 563 synthesis, 7, 578 Oxepin, 2,3,4,7-tetrahydro-synthesis, 7, 578 Oxepin, 2,3,6,7-tetrahydro-oxidation, 7, 563 reduction, 7, 563 Oxepin-2,6-dicarboxylic acid stability, 7, 565 Oxepinium ions synthesis, 7, 559 Oxepins, 7, 547-592 antiaromaticity, 4, 535 applications, 7, 590-591 aromatization, 7, 566 bond lengths and angles, 7, 550, 551 cycloaddition reactions, 7, 27, 569 deoxygenation, 7, 570 dipole moment, 7, 553 disubstituted synthesis, 7, 584... 

Pyrazine, 2,5-dichloro-3,6-difluoro-synthesis, 3, 190-191 Pyrazine, dihydro-, 3, 177 Pyrazine, 1,2-dihydro-oxidation, 3, 178 reduction, 3, 177 Pyrazine, 1,4-dihydro-antiaromaticity, 3, 177-178 synthesis, 3, 177 Pyrazine, 2,3-dihydro-oxidation, 3, 178 Pyrazine, 2,5-dihydro-synthesis, 3, 178 Pyrazine, 3,6-dihydro-synthesis, 3, 184 Pyrazine, 2,5-dihydroxy-oxidation, 3, 175 Pyrazine, 2,3-dimethyl-1,4-dioxide... 

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