Antiaromatic polyenes

Cyclooctatetraene, which has eight 71 electrons, seems to fit the category of antiaromatic polyenes since it has 4 ti electrons ( = 2). Nevertheless, cyclooctatetraene is a stable molecule, and reacts like an alkene. For example, it undergoes addition reactions with bromine and is easily hydrogenated. Cyclooctatetraene is not planar. It is not antiaromatic because it exists in a tub conformation, so its 71 orbitals cannot overlap to form a continuous 7t system, which for 871 electrons would be very unstable. Therefore, cyclooctatetraene does not exhibit the general characteristics of either aromatic or antiaromatic compounds. It is nonaromatic. 

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]. 

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 ... 

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 rectangular structure is calculated to be strongly destabilized (antiaromatic) with respect to a polyene model. With 6-3IG calculations, for example, cyclobutadiene is found to have a negative resonance energy of—54.7 kcal/mol, relative to 1,3-butadiene. In addition, 30.7 kcal of strain is found, giving a total destabilization of 85.4 kcal/mol. G2 and MP4/G-31(d,p) calculations arrive at an antiaromatic destabilization energy of about 42kcal/mol. ... 

All the peaks are somewhat upfield of the aromatic region, suggesting polyene character. This stmcture would also be consistent with the observed reactivity since the polyene has a. quinodimethane structure (see Section 11.3). The implication of a nonaromatic stmcture is that the combination of ring strain and the antiaromaticity associated with the four--nembered ring results in a localized system. ... 

Tlie tautomerism of heteroaromatic compounds is intimately related to the problem of aromaticity and antiaromaticity (87MI1,94MI1), this being especially true for the compounds of this chapter, which are borderline between these showing Fliickel behavior and polyenic compounds (nonaromatic). 

Considering their potential antiaromatic character, thiepins are also of theoretical interest. HMO calculations of -resonance energies, using reference data from 27 acyclic polyenes containing sulfur, predict antiaromatic character for thiepin and 2-benzothiepin, whereas 1- and... 

Is this value of 330 kJmol-1 plausible Were pentalene a normal polyene, we would anticipate an enthalpy of formation of ca 4.52,5 + 5.5 or ca 235 kJmol-1. There is thus ca 100 kJ mol-1 of destabilization. Is this due to antiaromaticity since we recognize pentalene as a derivative of planar [8]annulene We think not, for there are two five-membered rings in pentalene each contributing ca 30 kJmol-1 of strain apiece104. 

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. 

Whereas the benzene molecule possesses a structure of D6A symmetry with equal lengths of the CC bonds, for acyclic polyenes alternation of bond lengths is a characteristic [87JCS(P2)S1]. For antiaromatic molecules, alternation is even more pronounced and unlike the aromatic molecules, a high-symmetry structure of the lowest singlet state of the antiaromatic molecules does not correspond to a minimum on the PES. For... 

More sophisticated calculations indicate that cyclic An systems like cyclobutadiene (where planar cyclooctatetraene, for example, is buckled by steric factors and is simply an ordinary polyene) are actually destabilized by n electronic effects their resonance energy is not just zero, as predicted by the SHM, but less than zero. Such systems are antiaromatic [17, 46]. 

An annulene is said to be aromatic, nonaromatic or antiaromatic if its n system is more stable, equally stable or less stable than the corresponding open chain polyene, respectively. 

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... 

Large-Ring Annulenes Like cyclooctatetraene, larger annulenes with (4A0 systems do not show antiaromaticity because they have the flexibility to adopt nonplanar conformations. Even though [12]annulene, [16]annulene, and [20]annulene are (4A0 systems (with N = 3,4, and 5, respectively), they all react as partially conjugated polyenes. 

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. 

Removal of one electron should make no difference to the relative stabilities of polyene molecule ions or even electron polyene fragments as compared to their neutral counterparts, e.g. butadiene and the allyl radical should have the same relative stabihties as the butadiene molecule ion, and the allyl cation. Removal of one electron will, however, alter the stabihties, and thus the reactivities of cychc polyenes. The molecule ions of aromatic hydrocarbons will be substantially less aromatic then their neutral counterparts. Correspondingly the molecule ions of antiaromatic hydrocarbons will not be as antiaromatic as their neutral analogs, e.g. cyclobutadiene + should be relatively more stable than cyclobutadiene. The largest charge effects in hydrocarbons will be observed in nonaltemant ) monocychc hydrocarbons. The cyclopropenium ion 7 and the tropillium ion 2 are both strongly aromatic as compared to their neutral analogs. Consequently CsHs is a very common ion in the mass spectra of hydrocarbons while cyclopropene is not a common product of hydrocarbon pyrolysis or photo-... 

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