Annulene lOJAnnulene

Use Frosts circle to construct orbital energy diagrams for (a) [lOjannulene and (b) [12]annulene Is either aromatic according to Huckel s rule ... 

A second isomer of [lOJannulene (the cis trans cis cis trans stereoisomer) can have bond angles close to 120° but is destabilized by a close contact between two hydro gens directed toward the interior of the ring To minimize the van der Waals strain between these hydrogens the nng adopts a nonplanar geometry which limits its ability to be stabilized by tt electron delocalization It too has been prepared and is not very stable Similarly the next higher (4n + 2) system [14]annulene is also somewhat desta bilized by van der Waals strain and is nonplanar... 

The general tenn annulene has been coined to apply to completely conjugated mono-cyclic hydrocarbons with more than six carbons. Cyclobutadiene and benzene retain their-nfflTtes, but higher members of the group are nfflned [jcjannulene where x is the number of carbons in the ring. Thus, cyclooctatetraene becomes [Sjannulene, cyclodecapentaene becomes [lOjannulene and so on. 

Oxido[10]annulene closely resembles l,6-methano[lOJannulene in many of its spectral properties, particularly in its proton magnetic resonance, ultraviolet, infrared, and electron spin resonance spectra,1 but is chemically less versatile than the hydrocarbon analog due to its relatively high sensitivity toward proton and Lewis acids. 

Castro and Kamey examined the mechanism for the possible interconversion of [lOJannulene isomers. Masamune s original explanation of the NMR of type I and type II forms of [10]annulene involves interconverting structures, so location of the appropriate transition states might help account for the NMR spectra. 

The energetics of the structures of l,2-didehydro[10]annulene differs markedly from that of [lOJannulene. The lowest energy conformation has one trans double bond and has the heart-like conformation (45). The next lowest energy isomer, lying... 

Extrapolating from the discussion of [lOJannulene, we expect larger 4 + 2 systems to be aromatic if they are planar. Mislow " predicted that [14]annulene (109)... 

When the parent compound of the bridged [lOjannulene series, l,6-methano-[10]annulene 11>120) 28 (see Table 6), was first prepared, it was necessary to distinguish between three possible structures a) the "double norcaradiene structure 20, b) a rapidly fluctuating system (c.f. cyclooctatetraene), possibly proceeding through low equilibrium concentrations of the double norcaradiene, 21, or c) a delocalized 10 -electron system 22. 

The cation possesses a more favourable geometry than 1,6-methano-[10]annulene, being able to incorporate the 10 -system into a more planar structure. It is evidently more stable than the tropylium ion, as it can be formed from its neutral hydrocarbon precursor 57 by a hydride-transfer reaction with tropylium tetrafluoroborate 136>. The anion, on the other hand, involves substantial steric strain and is not as planar as either the cation or the neutral bridged [lOJannulene. Whereas formation of the cation from 57 was seen to be most favourable, the anion precursor 58 is much less acidic than cyclopentadiene, although in the presence of... 

Ab initio calculations on all-ofs [lOJannulene confirm that energy is lowered substantially when distortion from planarity takes place [2]. When a planar geometry is considered, the energy surface in its vicinity is very flat and there is very little difference between a structure with localised double bonds or a delocalised structure [2,3]. Different approaches slightly favour bond alternation [2] and delocalisation [3]. It has been pointed out that with [lOJannulene the limit for equalisation of bonds in the (4n+2)annulenes may be being reached [3] the transition from delocalisation to localisation in the series will be gradual, with small distortions in borderline cases which would be very difficult to detect experimentally [3]. 

Aromaticity in the larger 4 A + 2 annulenes depends on whether the molecule can adopt the necessary planar conformation. In the all-cis [lOjannulene, the planar conformation requires an excessive amount of angle strain. The [lOjannulene isomer with two trans double bonds cannot adopt a planar conformation either, because two hydrogen atoms interfere with each other. Neither of these [lOjannulene isomers is aromatic, even... 

When the parent compound of the bridged [lOJannulene series, l,6-methano-[10]annulene 28 (see Table 6), was first prepared,... 

The other series of bridged 14 it-electron compounds comprises those systems with anthracene perimeter (e.g. 32, a system which, if allowed to attain planarity, is also predicted to have aromatic character). These compounds can be considered as the next highest members in the series of bridged annulenes based on linearly fused hydrocarbons, extending the study of the naphthalene-perimeter bridged [lOjannulenes. There is in the system 35 a possibility of geometrical isomerism, in that the... 

Method of the Pfaffian to a Skew-Symmetric Matrix, Consider [12]annulene (see Chart 2), which is a [4k]—cycle, in contrast to [lOjannulene (Chart 1) being a [4k + 2]-cyde. The Kekule structure count is again K = 2, but the adjacency matrix (as well as its submatrices according to the partitioning shown in Chart 2) appear to be singular. Hence eqn. (13) would give zero. A correction is needed. 

For a recent overview of the [lOjannulene problem see King, R. A., Crawford, D., Stanton, J. F., and Schaefer 111, H. F. "Conformations of [10] Annulene More Bad News for Density Functional Theory and Second-Order Perturbation Theory." /. Am. Chem. Soc., 121,10788 (1999), and references therein. 

Annulene is benzene, which is aromatic. We might also expect [lOjannulene to be aromatic since it is a ring with a continuous system of p orbitals and it has a Hiickel number of it electrons. However, the hydrogen atoms positioned inside the ring (shown in red) experience steric hindrance that forces the compound out of planarity ... 

Annulenes are compounds consisting of a single ring containing a fully conjugated it system. Due to steric hindrance, [lOjannulene and [14]annulene do not meet the first criterion and are nonaromatic. 

We consider further structures as well. We optimize the geometry in 3D for [lOjannulene (cyclodecapentaene = CioHio) and for [14]annulene (cyclotetradecaheptaene = C14H14). Figures 3 and 4 right side show that these molecules are indeed a non-planar structure in their relaxed geometries. It should be mentioned that these annulenes also have local energy minimum configuration where their... 

Many annulenes have been obtained by photochemical electrocyclic processes. Thus [lOJannulene was obtained from c/5-9,10-dihydronaphthaIene, and various heteroannulenes have been prepared by very similar procedures. ... 

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