Aromaticity anti-Hiickel systems

Anti-Hiickel aromaticity. The aromaticity rules for anti-Hiickel systems are opposite to those for conventional (Hiickel) systems (Table 9). 

The PMO treatment of anti-Hiickel systems presents no problems. The distinction between Huckel and anti-Hiickel types applies only to cyclic conjugated systems, since in an open-chain system it is always possible to choose the phases of AOs so that there are no phase dislocations. The only problem then is that of aromaticity under what conditions are the cyclic anti-Hiickel systems more or less stable than the open-chain analogs ... 

This simple extension of the rules given above (p. 100) extends the PMO treatment to hydrocarbons of all types, both Hiickel and anti-Hiickel. The distinction between the aromaticity of Hiickel and anti-Hiickel systems is crucial to the understanding of the stereochemistry of pericyclic reactions (Sections 5.6 and 6.16) which prefer to proceed through o-n delocalized aromatic transition states. 

Antiaromatic ground states of nonalternant monocyclic hydrocarbons will be aromatic with reference to the excited singlet states of their open-chain analogs nonaromatic (radical), nonalternant systems will be nonaromatic with reference to the excited singlet states of their open-chain analogs. Complete the proof that the rules for aromaticity in anti-Hiickel systems hold for the excited states of monocyclic Hiickel systems. 

We are dealing here with four-atom conjugated systems containing four electrons, i.e., the two pairs of electrons that form the C=C n bond and the CH2—CH2 (7 bond in (144). The disrotatory transition state (145), being of Huckel type, will then be isoconjugate with normal cyclobutadiene and so will be antiaromatic, whereas the conrotatory transition state will be isoconjugate with an anti-Hiickel analog of cyclobutadiene and so will be aromatic (see Table 4.2). 

This argument can obviously be extended to concerted pericyclic reactions of all kinds. The transition state for any such reaction will be isoconjugate with a normal Hiickel-type cyclic polyene or an anti-Hiickel analog of one. If the transition state is aromatic, the resulting stabilization will lower its energy and so accelerate the reaction. If it is antiaromatic, the converse will be true. Since, moreover, the rules for aromaticity in Huckel-type and anti-Huckel-type systems are diametrically opposite, in each case one will be aromatic and the other antiaromatic. If, then, a reaction can follow one of two alternative pericyclic paths, one involving a Hiickel-type transition state and the other an anti-Hiickel-type transition, the reaction will prefer to follow the path in which the transition state is aromatic. If, on the other hand, only one of the two alternatives is sterically possible, the reaction will take place relatively easily if the corresponding transition state is aromatic and with relative difficulty if it is antiaromatic. In the latter case, the antiaromatic transition state will, if possible, be bypassed by a two-step mechanism in which the transition state is linear instead of cyclic [e.g., equation (5.291)]. 

The cyclization of stilbene occurs through an anti-Hiickel transition state analogous to anti-Hiickel phenanthrene. Photodimerizations of polycyclic aromatic systems are also very common. The dimerization of anthracene derivatives is a classical example ... 

Applying the PMO method to these systems in the same way as to Hiickel systems, it can be readily shown that neutral polymethines with 4n carbon atoms are aromatic and those with (4n + 2) atoms anti-aromatic. This is the opposite result to Hiickel systems. 

The Hiickel anti-aromaticity versus Mobius aromaticity effects for the seven-membered systems 1 and 2 have been studied computationally using Gaussian98 at the closed shell B3LYP/6-31G(d) level <20030BC182>. It was shown that Mobius aromaticity was preferred for the respective perfluorinated derivatives 3 and 4. 

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]. 

In [m] circulenes, a family of polyaromatic hydrocarbons so named in 1975 by Wynsberg, in which m refers to the number of aromatic rings arranged in a circle, the total number of n electrons does not indicate aromaticity or anti-aromaticity according to the Hiickel rule. This rule is strictly only applicable to monocyclic systems. It is adequate, however, to consider the inner and the outer n electrons separately whose numbers obey the An + 2 Hiickel criterion for aromaticity, since both these circuits are monocyclic [49]. Coronene, a flat graphite frag-... 

To explain the increase in the rate of the cyclobutene opening 6.292 —> 6.293, we need to remember that the conrotatory pathway will have a Mobius-like aromatic transition structure, not the anti-aromatic Hiickel cyclobutadiene that we saw in Fig. 1.38. We have not seen the energies for this system expressed in 8 terms, nor can we do it easily here, but the numbers are in Fig. 6.42, where we can see that a... 

In this and similar compounds the acetylene bond is supposed to donate only two jt-electrons to the conjugated system while the other jt-bond is located in the plane of the molecule and does not participate in the conjugation. Consequently, this compound satisfies the Hiickel rule for = 4. It indeed possesses aromatic properties. Anti-aromaticism. When a cyclic polyene system is studied it is important to know whether this system is nonaromatic, i.e.not stabilized by conjugation and sufficiently reactive due to the internal tension and other causes, or destabilized by conjugation, i.e. the cyclic delocalization increases the total energy of the system. In the latter case the molecule is called anti-aromatic. Here are typical examples of anti-aromatic systems cyclobutadiene, a cyclopropenyl anion, a cyclopentadi-enyl cation, and others. 

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... 

If LVMO and HOMO are not compatible, a forbidden or high-energy process would normally be necessary to effect cyclization otherwise, anti cycloaddition (Pig. 10c) might be possible, as will be discussed shortly. It may be even simpler to imagine the reactants as they would appear in the transition state and inquire whether one has an incipient aromatic system, i.e. Hiickel (4n + 2)ir cycle (Dewar, 1966 Fukui, 1965, 1966). If so, the cycloaddition is thermally allowed otherwise, forbidden. The nature of these predicted closures would normally be reversed for reactions of the first excited states. 

Thus, from a consideration of both bridged and unbridged annulenes ranging from 6 to 36 peripheral iv-electrons, it has been seen that, in planar systems, n.m.r. spectroscopy can distinguish clearly between those aromatic (4n +2) it systems and the anti-aromatic Ann cases, until, at very larger ring size where Hiickel s rule becomes invalid, both series merge into non-aromatic behaviour. 

The directly bonded C—coupling constants have been used to give a measure of aromaticity in mono- and polycyclic hydrocarbon ring systems.Cyclo-octatetraene reacts with methylene dichloride and methyl-lithium to afford a mixture of syn and anti-9-chlorobicyclo[6,l,0]nona-2,4,6-triene (40 and 41) which, on treatment with a 30% lithium dispersion in tetrahydrofuran, gives cyclonona-tetraenide, which is isolated as the tetraethylammonium salt. The chemical shift and the coupling constant (7i3c-ih = 137 c./sec.) observed for the lithio-salt (42) are indicative of the aromatic character of the ring, in accord with the predictions of Hiickel s Aromaticity rule. 

So far we have considered only reactions in which the pericyclic ring contains an even number of atoms. Reactions of this kind are, however, known in which an odd-numbered ring is involved. A simple example is the Diels-Alder-like addition of 2-methylallyl cation (148) to cyclopentadiene (149) to form the methylbicyclooctyl cation (150). The transition state for this reaction is easily seen to be of Hiickel type (151) and so isoconjugate with tropylium. Since the allyl cation contains only two n electrons, we are dealing here with a six-electron system isoconjugate with the tropylium cation (147) and hence aromatic. In reactions of this kind, both the reactants and the transition state are odd. The reactions are therefore of 001 type. Since, moreover, the aromaticity or antiaromaticity of the transition state is again unrelated to the structures of the reactants or products, the reactions are of anti-BEP type and are consequently classed as 00 J. 

Benzene is the smallest member of the class of aromatic cyclic polyenes following Hiickel s (4 + 2) rule. Most of the 4n ir systems are relatively reactive anti- or nonaromatic species. Hiickel s rule also extends to aromatic charged systems, such as the cyclopentadienyl anion, cycloheptatrienyl cation, and cyclooctatetraene dianion. 

The 4/2+2 rule solved the mystery of the profound difference between benzene, [10]annulene, [14]-annulene, and [ISjannulene on one side and the 4/2 monocyclic systems, like elusive cyclobutadiene and puckered cyclooctatetraene, on the other side. Attempts were made to extend the 4/2+2 rule to polycyclic systems, for which it was not initially designed. Of numerous attempts in this direction, we will mention only that of Platt,who proposed that the 4/2+2 rule be applied to molecular periphery. It turns out that Platt s generalization of the Hiickel 4/2+2 rule is correct when one restricts attention to benzenoid hydrocarbons. For example, the perimeter rule correctly classifies pyrene (which has 16 Jt-elec-trons), perylene (which has 20 //-electrons), and coronene (which has 24 //-electrons) as aromatic as they have 14 or 18 //-electrons on the perimeter. But the perimeter rule does not give a correct answer for the non-benzenoid systems illustrated in Figure 10. The structure on the left, which has 14 //-electrons on the periphery, instead of being aromatic, as will be seen later, is in fact fully anti-aromatic . On the other hand, the structure on the right (corannulene), which has 15 //-electrons on the periphery, is not... 

---