Annulenes, dications

Diprotonation of (97) with H2SO4 gives a species which is protonated at both carbonyl groups, not the delocalized annulene dication, presumably for steric reasons.140 Oxidation dications of perylenes, e.g. (98), are [4n + 2] species with diamagnetic ring currents, whereas the dibcnzo[ /. // ] derivative [two more benzene rings at the top and... 

Alternatively, the porphyrin ring can be constructed starting from [16]annulene. In the first step, two electrons are added to form the corresponding [16]annulene dianion, which is transformed into porphyrin Cl") by adding bridges and heteroatoms. Unlike the structure derived from [18]annulene, the dianion-based model has a fourfold symmetry, and was considered suitable for the description of metal complexes [23] (see Sect. 2.3.2). In yet another approach [24], based on the so-called perimeter model, the porphyrin macrocycle is derived from the [20]annulene dication (I "). Both the [16]- and [20]annulene models were employed to describe electronic absorption spectra and magnetic circular dichroism of porphyrinoids [24, 25],... 

Scheme 6.8 (a) Synthesis of [8]CPP radical cation and dication and (b) structure of all-c/s-[32]annulene dication. 

The isolation of the dication raises questions about the unusual stability of the dication. Uchiyama proposed that the dication of CPPs possesses in-plane aromaticity with a (4 -I- 2)% electron system [66]. In the case of [8]CPP dication ( = 7 for [8] CPP), the nodal patterns and degeneracy of the frontier orbitals of the dication are exactly the same as those of the all-cis-[32] annulene dication (Scheme 6.8b). The aromatic character of the dication is responsible for the anomalous stability of the dication. Uchiyama also suggested that CPP dianions should also have in-plane aromaticity with a (4 -I- 2)it electron system (n = 8 for [8]CPP), suggesting that the dianion species would also be isolable. 

Aromatic [16]Annulene Dication and Dianion from Antiaromatic [16]Ammlene... 

The homoaromatic interaction in other bridged annulenes has also been examined. The dications of several bridged annulenes were prepared and also studied theoretically and by NMR spectroscopy (Mullen et al., 1987 Wallraff et al., 1988). Once again homoaromatic interactions were deemed to be most important in determining the properties of these systems. Another cationic polycyclic potential homoaromatic system was investigated by Murata and Nakasuji (1980). They concluded, from NMR studies, that homoaromaticity was unimportant in the homophenalenyl cations [66], [67] and [68], (They reached the same conclusion for the corresponding anions.)... 

A [16]annulene with two bridging nitrogen atoms (129) was obtained from the reaction of the dication (130) with the respective anhydrides and triethylamine at 140 °C (78JCR(S)224). 

An X-ray crystal structure of annulene-dione (64) indicates an anti,anti configuration between the methylene and sulfur bridges.102 Diprotonation gives highly localized positive charges in the dication (65), mainly due to unfavourable p-orbital overlap. 

Lammertsma and Cerfontain411 have obtained the cyclopropyldicarbinyl dication 197 by diprotonating the l,6-methano[10]annulene in much stronger Magic Acid at 60°C [Eq. (3.49)]. If the same protonation was carried out at — 120°C, they were able to obtain the previously discussed monocation 106.284... 

The third returns the reader to chemi-ionization of cyclopropane derivatives with excited state electron acceptors. In the particular, it has been observed that oxidation of l,6-dimethylbicyclo[4.1.0]hept-3-ene (46) and [4.4.l]propella-2,6-diene (47) result in 2,6-dimethylcycloheptatriene (48) and l,6-methano[10]annulene (49) (equations 28 and 29 respectively). The former compound is conceptually the deprotonation product of the dication wherein the C-C bond between the bridgehead carbons has been totally depleted of electrons. In this case, however, the dication is not expected to be as stable as in the above pyramidal or cyclobutadiene cases and indeed, the product is seemingly not formed via the dication route. The methanoannulene likewise does not arise from the tetracation of its precursor. Rather, proton and electron transfer reactions involving merely radical cations proceed to remove hydrogens sequentially. 

The reactions of porphyrins arise from the aromatic annulene character and the amphoteric behaviour of the pyrrole units which exist as IH- or 2//-structures. Accordingly, porphyrins form tetradentate chelate complexes 7 with many cations, and as weak bases pKaj 7, pKa2 4) they can be protonated to give dications 8. 

A similar reversal is seen when (16]annulene is converted into its dication, which, having 14 TT-electrons, is a diatropic species [42],... 

Surprisingly, treatment of a solution of [16]annulene with fluoro-sulphonic acid at low temperature results in the formation of the [16]annulenium dication, rather than a protonated species, as had been expected [42]. This structure is indicated by the H-n.m.r. spectrum, which also indicates the presence of a diamagnetic ring current. Formation of this species presumably is a reflection of the stability of the resultant 14 ir-electron system. 

It suggests that it is not the size of the ring but the number of electrons present in it determines whether a molecule would be aromatic or antiaromatic. In fact the molecules with An+ 2) n electrons are aromatic whereas with (An, 0) n electrons are antiaromatic. Thus, benzene, cyclopropenyl cation, cyclobutadiene dication (or dianion), cyclopentadie-nyl anion, tropylium ion, cyclooctatetraene dication (or dianion), etc. possess (4 + 2) ti electrons and hence aromatic whereas cyclobutadiene, cyclopentadienyl cation, cycloheptatrienyl anion, cyclooctatetraene (non-planar) etc. have An n electrons which make them antiaromatic . Systems like [10] annulene are forced to adopt a nonplanar conformation due to transannular interaction between two hydrogen atoms and hence their aromaticity gets reduced even if they have (An + 2)n electrons. On the other hand the steric constraints in systems like cyclooctatetraene force it to adopt a tube-like non-planar conformation which in turn reduces its antiaromaticity. Various derivatives of benzene like phenol, toluene, aniline, nitrobenzene etc. are also aromatic where the benzene ring and the n sextet are preserved. In homoaromatic " systems, like cyclooctatrienyl cation, delocalization does not extend over the whole molecule. 

The cation (26) was obtained by protonation of the corresponding azulene in dichloromethane. A tropylium ion-mediated a-cyanation of amines was described. The key step is a hydride transfer from the amine to the cation, resulting in cycloheptatriene and an iminium ion, the latter then reacting with cyanide to give the aminonitrile. The dehydrofropylium-Co2(CO)6 ion has been prepared as a BF4 salt. Various measures suggest that the ion is weakly aromatic, with about 25% of the aromaticity of the tropylium ion. Computational analysis of a number of annulenes predicts that the Mobius dication (CH)i4+ should be stable under persistent ion conditions. In particular, this dication is stable towards reactions such as cis-trans isomerization and electrocyclic rearrangement that limit the lifetime of other Mobius annulenes. 

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