Antiaromatic structure

This pattern is familiar to the theoretical organic chemist since it reflects nothing more than the fact that the B + C union in the all-c/ s geometry resembles an antiaromatic structure while the same union in the all-tarns geometry resembles a non-aromatic structure. We have already encountered such situations in our discussion of the conformational isomerism of 1,3-butadiene. 

Very high level ab initio [CCSD(T)//MCSF] calculations have been applied to singlet and triplet cyclopropenyl anion and cyclopropenyl radical. The anion ground state, a singlet with Cg symmetry, is destabilized relative to cyclopropyl anion as expected for an antiaromatic structure it is stabilized, with respect to its conjugate acid and the corresponding radical, by electron-withdrawing substituents such that 1,2,3-tricyanopropene has a predicted pK of 10-15. ... 

Antiaromatic structure (214) has an energy higher by 18.7 kcal/mol relative to the bent acyclic structure (3 2 state) and by 42.7 kcal/mol compared to the linear structure N3 ( Sg4 state) (88JA7225). 

Modified intermediate neglect of differential overlap (MINDO/3) and modified neglect of differential overlap (MNDO) methods with full geometry optimization calculate triazol[4,5-, [l,2,3]triazole to possess antiaromatic structure with C h symmetry but not 1)2,5. In contrast, the aromatic structure with 1)2,5 symmetry is found advantageous for the 2,5-dioxide derivative <1991IZV1825>. 

The XH NMR spectrum shows bands in the region 4.0-6.0 ppm (See Table 6). Following the discussion of priority of paths of delocalization of aceheptylene dianion 232 and acenaphthylene dianion 82 also 332 and 342 show that specific paths of delocalization are favoured. While in the neutral structure 33 and 34 the competition is between aromatic and nonaromatic structures, in the respective dianions the competition is between nonaromatic and antiaromatic structures (Fig. 9). From the spectroscopic parameters, i.e., chemical shifts and coupling constants of the bridge protons it can be concluded that the neutral systems are best represented by structures with an aromatic skeleton connected to a virtually isolated double bond. In the charged systems, viz. 332 and 342 it seems that a nonaromatic path of conjugation is preferred to an antiaromatic path (Fig. 9). These considerations are also reflected in the carbon chemical shifts and in their HOMO-LUMO gap (AE) (vide infra) 122). It can be concluded from all these observations that there is a tendency of aromatic systems to remain so and to avoid as much as possible paratropic antiaromatic contributions. 

The l,3A 5, 2,4-benzodithiadiazines 11 and 12 are mixed heterocyclic-carbocyclic compounds that combine true aromaticity in the all-carbon part (4n - - 2tt electrons) with a nonaromatic heterocyclic part, to give an overall antiaromatic structure (4n7t electrons) <2001CEJ3592>. 

Theoretical calculations indicate that a Dgj, (planar, antiaromatic) structure is a transition state in the double bond shifting reaction of cyclooctatetraene Hrovat, D. A. Borden, W. T. /. Am. Chem. Soc. 1992, 114, 5879. 

An alternative approach to characterizing the aromaticity of planar compounds is the calculation of a nucleus-independent chemical shift (NICS), which is a computed value of the magnetic shielding a virtual (ghost) nucleus would experience at specific locations near a n system. A NICS(0) value refers to shielding in the center of the n system, while a NICS(l) value reports the shielding expected at a point 1A above the center of the molecular plane. Aromatic compounds have negative NICS(O) values, and antiaromatic structures have positive values. NICS values provide a quantitative measure of aromatic character and have been shown to correlate well with other measures of aromaticity. 

The methods described also account for such antiaromatic structures. 

The calculated values of these indices for several selected reactions are given in Table 9, into which the values corresponding to antiaromatic reference structures, which represent a natural counterpart to ideally aromatic standards, were included for comparison. As can be seen from this Table, the predictions of Dewar classification are indeed confirmed since for allowed reactions the similarity to aromatic reference structures are systematically higher than to the antiaromatic ones. On the other hand, for forbidden reactions the similarity to antiaromatic structures dominates so that these structures can be expected to play the role of transition states in this case. The above conclusions suggesting the important role of antiaromatic reference species as the eventual transition states in forbidden reactions was investigated in the study [158], in which the more detailed specification of the structure of antiaromatic transition states was attempted. The basis of this approach is a straightforward reformulation of the above procedure in terms of second order or pair density matrices. These matrices are generally defined by eq. (105),... 

The solvolytic reactivity of the 9-CFs substituted fluorenyl tosylate 46 was strongly depressed compared to R = H, and a rate factor of 10 due to antiaromatic destabilization was estimated. Solvolytic studies with formation of 9-fluorenyl carbocations with COzR, - CONMez, and CR=NOCH3 substituents have also been reported. On the basis of the calculated geometry it was suggested by Creaiy et al. that such cations avoid antiaromatic structures with cyclopentadienyl cation moieties and resemble bis(dienyl) cations 28a. 

The distance between nitrogen atoms in structures Vni and IX are about 200 pm, matching the typical M—M bond lengths for most transition metals. However, unlike G-chelate I, the 22-electron aromatic system of structure IX ( = 5) cannot achieve either an 18-electron (n = 4) or a 26-electron aromatic macrocycle. All attempts to redraw the electronic structure of IX result in antiaromatic structures. 

In Summary Cyclic conjugated polyenes are aromatic if their tt electron count is 4n + 2. This number corresponds to a completely filled set of bonding molecular orbitals. Conversely, 4n TT systems have open-shell, antiaromatic structures that are unstable, are reactive, and lack aromatic ring-current effects in NMR. Finally, when steric constraints impose nonplanarity, cyclic polyenes behave as nonaromatic alkenes. 

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