Pentalene, antiaromaticity

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]. 

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. 

In certain cases a reductive elimination is prevented due to the unfavorable formation of the antiaromatic benzocyclobutadiene (Equation (155)). Annulated fulvenes and pentalenes are accessible by this methodology, in excellent yields (Equation (156)).130 130a-130d... 

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... 

Pentalene, 22, is another antiaromatic molecule known to undergo bond-shift equilibrium with a barrier of approximately 4 kcal/mol.182 A second-order perturbation Hiickel treatment100 by Heilbronner and Shaik183 indicated that linking pentalene into bipen-talene, 23, should cause one of the moieties to undergo delocalization or at least lower the barrier for achievement of the local D2h situation. Many antiaromatic species seem to behave in the same manner.184... 

Hence heptafulvene is non-aromatic but cyclooctatetraene and pentalene (XXI) are antiaromatic (aromatic energy —26/5). 

Bicyclo[3.3.0]octa-l,3,5,7-tetraene (2), trivially called pentalene [26, 27], is the second member in the series of fully unsaturated oligoquinanes. Huckel MO theory predicts that this planar hydrocarbon with its 8 r-electron system should be an antiaromatic species [25]. 2-Methylpentalene (37) has been generated by a retro-Diels-Alder reaction and deposited as a film at —196 °C on a NaCl or quartz plate for its spectroscopic characterization. It rapidly dimerized upon warming the cold plates to temperatures above —140 °C [26]. Only two stable derivatives of pentalene not complexed to a metal [29], the hexaphenyl- (38) [30] and 2,4,7-tri-tert-butylpentalene (39) [31], have hitherto been reported (Figure 4). 

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... 

Pentalene has eight n electrons and is antiaromatic. Pentalene dianion, however, has ten tc electrons and is a stable, aromatic ion. 

The peculiarity of the pentalene molecule resides in its central bond which connects nonalternant atoms (a perturbed [8]annulene), its completely conjugated and alternating w bond periphery, and its apparent antiaromatic nature. The transannular bond conveys planarity to the structure without contributing to its stabilization. Consequently, pentalene is expected to be highly reactive since, in a sense, it has ground state properties customarily found in excited states. The many early attempts to synthesize pentalene have been reviewed224-226 and will not be considered per se here. Suffice it to say that the lability of minimally substituted pentalenes ultimately required the implementation of rather specific reaction conditions for their successful synthesis (vide infra). 

Azulene satisfies all the criteria for aromaticity, and it has a Huckel number of n electrons 10. Both heptalene (12 7i electrons) and pentalene (8 n electrons) are antiaromatic. 

There is much evidence that cyclic conjugated systems of An electrons show no special stability. Cyclobutadiene dimerises at extraordinarily low temperatures (>35K).28 Cyclooctatetraene is not planar, and behaves like an alkene and not at all like benzene.29 When it is forced to be planar, as in pentalene, it becomes unstable to dimerisation even at 0 °C.30 [12]Annulene and [16]annulene are unstable with respect to electrocyclic reactions, which take place below 0 °C.31 In fact, all these systems appear on the whole to be significantly higher in energy and more reactive than might be expected, and there has been much speculation that they are not only lacking in extra stabilisation, but are actually destabilised. They have been called antiaromatic 32 as distinct from nonaromatic. The problem with this concept is what to make the comparisons with. We can see from the arguments above that we can account for the destabilisation... 

The structure analysis of the dilithium pentalenediide 45 reveals a C2-sym-metric ion triplet with the two lithium cations located on opposite sides of the two different rings. The structural parameters were extremely well reproduced by ab initio calculations [35] (see Fig. 3). Both the experimental structural parameters and calculated magnetic susceptibility exaltation classify the 107r-elec-tron species 45 as an aromatic compound. Apparently, the lithium counterions in 45 do not exert any significant effect on the bond lenghts of the dianion 22. On the other hand, the antiaromatic pentalene 2 and its aromatic dication 22+ show the characteristic bond length alternation (Fig. 3) [35]. 

The fully unsaturated dicyclopenta[cd,g/z]pentalene 4 (Fig. 7) remains elusive. Several attempts at generating 4 have been made, but so far all available knowledge about the system rests on computational studies which predict the structure and properties of this highly strained 7r-system [35]. In contrast to computational results for pentalene 2 and acepentalene 3, calculations for 4 predict an aromatic singlet ground state with a delocalized 7r-system. While the dianion of 4 is calculated to also be aromatic, the dication 42+ should be antiaromatic [35]. 

There are four classical nonaltemant jr-electron hydrocarbons which have long been the subject of many studies of their aromatic character fulvene, heptafulvene, azulene, and pentalene. " Eulvene, heptafulvene, and pentalene have been considered for a long time as nonaromatic (or even antiaromatic), both for their chemical Instability and the negative REPE values. More recent studies strongly support this hypothesis. The X-ray determined geometry of heptafulvene" allowed us to estimate the aromaticity the HOMA value for the ring is equal to 0.257. The case of fulvene is more complicated—only geometries for the substituted species are known. The mean HOMA value calculated for 11 exocyclically substituted fulvenes is 0.001. ... 

Both naphthalene (Section 14.8A) and azulene (Section 14.8B) have 10 77 electrons and are aromatic. Pentalene (below) is apparently antiaromatic and is unstable even at —100 °C. Heptalene has been made but it adds bromine, it reacts with acids, and it is not planar. Is Hiickel s rule applicable to these compounds If so, explain their lack of aromaticity. 

Hiickel s rule should apply to both pentalene and heptalene. Pentalene s antiaromaticity can be attributed to its having 8 tt electrons. Heptalene s lack of aromaticity can be attributed to its having 12 tt electrons. Neither 8 nor 12 is a Hiickel number. 

Attempts to prepare the antiaromatic compound pentalene have so far failed. Various transition metal complexes of pentalene are, however, easily prepared [for example, ruthenium carbonyl complexes, Eq. (167) (Brookes et at., 1973)] so that pentalene might be generated in situ from such complexes and trapped with various reagents (Knox and Stone, 1974). 

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