Cyclopropenyl cation, aromaticity

The results of this charge calculation are summarized in Fig. 4.24 the negative charge on the exocyclic carbon and the positive charges on the ring carbons are in accord with the resonance picture (Fig. 4.24), which invokes a contribution from the aromatic cyclopropenyl cation [50]. Note that the charges sum to (essentially) zero, as they must for a neutral molecule (the hydrogens, which actually also carry... 

Figure 5.50 shows three related molecules, the 7-methyl substituted (the visual orbital progression explained here is not quite as smooth for the unsubstituted molecules) derivatives of the 7-norbomyl cation (a), the neutral alkene norbomene (b), and the 7-norbomenyl cation (c). For each species an orbital is shown as a 3D region of space, rather than mapping it onto a surface as was done in Fig. 5.49. In (a) we see the LUMO, which is as expected essentially an empty p atomic orbital on C7, and in (b) the HOMO, which is, as expected, largely the n molecular orbital of the double bond. The interesting conclusion from (c) is that in this ion the HOMO of the double bond has donated electron density into the vacant orbital on C7 forming a three-center, two-electron bond. Two n electrons may be cyclically delocalized, making the cation a bishomo (meaning expansion by two carbons) analogue of the aromatic cyclopropenyl cation [326], This delocalized bishomocyclopropenyl structure for 7-norbomenyl cations has been controversial, but is supported by NMR studies [327].

As seen from these data the DE of the aromatic cyclopropenyl cation is far larger than that of the acyclic allyl cation because for the cyclopropenyl ion with one split a-bond the 1,3-resonance interaction (pi) is still significant. Such particles are called homoaromatic. The a-skeleton of an aromatic particle can be broken at one, two or three sides, and such ions are called, respectively, mono-, bis- and tris-homoaromatic. The prefixes mono- , bis and tris -stand here for the number of the sides where the a-bond is broken or stretched, but not for the number of CH groups introduced in place of one a-bond. 

The formation of a substituted tropylium ion is typical for alkyl-substituted benzenes, hi the mass spectrum of isopropylbenzene (Fig. 4.24), a strong peak appears at miz = 105. This peak corresponds to loss of a methyl group to form a methyl-substituted tropyhum ion. The tropylium ion has characteristic fragmentations of its own. The tropylium ion can fragment to form the aromatic cyclopentadienyl cation (m/z = 65) plus ethyne (acetylene). The cyclopentadienyl cation in turn can fragment to form another equivalent of ethyne and the aromatic cyclopropenyl cation (m/z = 39) (Fig. 4.25). 

Figure 12.7 shows the energy levels for the stable, aromatic cyclopropenyl cation, and Figure 12.8 shows the energy levels for the unstable, antiaromaric cyclobutadiene molecule. 

Other aromatic ions include cyclopropenyl cation (two rr electrons) and cycloocta tetraene dianion (ten tt electrons)... 

The tropylium and the cyclopropenyl cations are stabilized aromatic systems. These ions are arumatic according to Hiickel s rule, with the cyclopropeniiun ion having two n electrons and the tropyliiun ion six (see Section 9.3). Both ring systems are planar and possess cyclic conjugation, as is required for aromaticity. 

It is strong evidence for Hiickel s rule that 59 and 60 are not aromatic while the cyclopropenyl cation (55) and the cyclopentadienyl anion (39) are, since simple resonance theory predicts no difference between 59 and 55 or 60 and 39 (the same number of equivalent canonical forms can be drawn for 59 as for 55 and for 60 as for 39). 

Quasi-aromatic structures are also known in which the stabilised cyclic species is an ion, e.g. the cycloheptatrienyl (tropylium) cation (15, cf. p. 106), the cyclopentadienyl anion (16, cf. p. 275), both of which have 67te (n = 1), and even more surprisingly the cyclopropenyl cation (17, cf. p. 106) which has 2ne (n = 0) ... 

Structures that are also aromatic are the cyclopropenyl cation (2 jt electrons n = 0) and the cyclopentadienyl anion (6 n electrons n = 1). Although we do not wish to pursue these examples further, they are representative of systems where the number of jr electrons is not the same as the number of carbon atoms in the ring. 

Problem 10.6 Account for aromaticity observed in (o) 1,3-cyclopentadienyl anion but not 1,3-cyclopen-tadiene (b) 1,3,5-cycloheptatrienyl cation but not 1,3,5-cycloheptatriene (c) cyclopropenyl cation (d) the heterocycles pyrrole, furan and pyridine. 

Three-membered ring systems have also provided examples of aromatic and antiaromatic behavior. Despite the very substantial angle strain, Breslow and his collaborators have succeeded in preparing a number of cyclopropenyl cations (51).30 Cyclopropenone (52) has been isolated and is stable in pure form below... 

Comparable stabilization is not possible in structure C because neither a cyclopropenyl system nor a cycloheptatrienyl system is aromatic in its anionic form. Both are aromatic as cations. 

Hiickel s rule has been abundantly verified [17] notwithstanding the fact that the SHM, when applied without regard to considerations like the Jahn-Teller effect (see above) incorrectly predicts An species like cyclobutadiene to be triplet diradicals. The Hiickel rule also applies to ions for example, the cyclopropenyl system two n electrons, the cyclopropenyl cation, corresponds to n 0. and is strongly aromatic. Other aromatic species are the cyclopentadienyl anion (six n electrons, n = 1 Hiickel predicted the enhanced acidity of cyclopentadiene) and the cyclohep-tatrienyl cation. Only reasonably planar species can be expected to provide the AO overlap need for cyclic electron delocalization and aromaticity, and care is needed in applying the rule. Electron delocalization and aromaticity within the SHM have recently been revisited [43]. 

Fig. 4.23 Hiickel s rule says that cyclic n systems with An + 2 n electrons ( = 0, 1, 2,. .. An + 2 = 2, 6, 10,. ..) should be especially stable, since they have all bonding levels full and all antibonding levels empty. The special stability is usually equated with aromaticity. Shown here are the cyclopropenyl cation, the cyclobutadiene dication, the cyclopentadienyl anion, and benzene formal structures are given for these species - the actual molecules do not have single and double bonds, but rather electron delocalization makes all C/C bonds the same...

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