Annulenes, antiaromatic

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 ... 

Twin Excited States of Cyclobutadiene, Cyclooctatetraene, and Antiaromatic Annulenes... 

The above arguments are quite general and applicable to other bond-alternated ground states in aromatic molecules (e.g., the distorted [18]-annulene produced with the bicyclic annelations224 as well as in other antiaromatic annulenes and other species... 

Annulenes are aromatic if they have An + 2 electrons, irrespective of their charge. Those having An electrons are antiaromatic. Annulene radicals are nonaromatic.u... 

Vinylic SN2 reactions are discussed on p. 198. Here, we restrict ourselves to nucleophilic susbstitutions at saturated centers. Aromaticity criteria then strongly disfavor retention of configuration. Indeed, the transition state leading to retention, 43, resembles a four-electron (two from the nucleophile and two from the CY bond) antiaromatic annulene. 

We end this section with a comment on how different theories produce different outlooks and create different predispositions. For example, let us consider the noncontroversial case of cyclopropenyl anion and the controversial case of cyclobutadiene, both 4k-electron "antiaromatic" annulenes. The former molecule is computed to have a triplet ground state in and the latter one is computed to have a singlet ground state in geometry. With respect to HMO theory, the CP system can be called "normal" and the CB system "abnormal". More specifically, the former is said to comply to Hund s rule but the latter is claimed to violate it. As a result, spin selection in CB becomes a topic of controversy By contrast, FC theory paints an entirely different picture in which the interesting thing is not that square CB turns out to be singlet but that CP turns out to be triplet ... 

Antiaromatic annulenes are cyclically conjugated hydrocarbons that contain 4nn electrons. Examples of antiaromatic annulenes containing 4n electrons are cyclopropenyl anion, cyclobutadiene, cyclopentadienyl cation, and benzene dication. At its geometry of highest symmetry an antiaromatic annulene... 

In antiaromatic annulene ions with 4m 1 or 4 2 atoms the two degenerate MOs have atoms in common. Consequently, because these MOs are nondisjoint, at the geometry of highest symmetry the triplet is calculated to lie well below the singlet in energy. Thus, as predicted by Hund s rule, many antiaromatic annulene ions (e.g., cyclopentadienyl cation and hexachlorobenzene dication) have been found experimentally to have triplet ground states. 

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... 

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... 

As we have seen (p. 59), these systems are expected to be not only nonaromatic but actually antiaromatic. The chief criterion for antiaromaticity in annulenes is the presence of a paramagnetic ring current, which causes protons on the outside of the ring to be shifted upfield while any inner protons are shifted downfield, in sharp... 

The fact that many 4 systems are paratropic even though they may be nonplanar and have unequal bond distances indicates that if planarity were enforced, the ring currents might be even greater. That this is true is dramatically illustrated by the NMR spectrum of the dianion of 83 (and its diethyl and dipropyl homologs). We may recall that in 83, the outer protons were found at 8.14-8.67 8 with the methyl protons at —4.25 8. For the dianion, however, which is forced to have approximately the same planar geometry but now has 16 electrons, the outer protons are shifted to about -3 8 while the methyl protons are found at 21 8, a shift of 258 We have already seen where the converse shift was made, when [16]annulenes that were antiaromatic were converted to 18-electron dianions that were aromatic. In these cases, the changes in NMR chemical shifts were almost as dramatic. Heat of combustion measurements also show that [16]annulene is much less stable than its dianion. 

Gygax R, Wirz J, Sprague JT, Allinger NL. Electronic structure and photophysical properties of planar conjugated hydrocarbons with a 4n-membered ring. Part III. Conjugative stabilization in an antiaromatic system The conformational mobility of l,5-bisdehydro[12]annulene. Helv Chim Acta 1977 60 2522-9. 

Results from X-ray studies of three annulenes are presented In Table 8. According to Hiickel s rule [14]annulene (14-ANN) and [18]annulene (18-ANN) should be aromatic and most probably planar molecules, while [16]annulene (16-ANN), as a [4n]annulene, should be antiaromatic. The [14]annulene molecule is nonplanar, with a structure that approaches C2h symmetry. The cause of the nonplanarity is the steric overcrowding in the center of the molecule. While the spread of the individual bond lengths implies possible significant differences, there is no significant pattern to the values obtained. 

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. 

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