Potential Antiaromatic Systems

FIGURE 44. Optimized geometries (dz + d) of silacyclobutadiene (106) in the lowest A singlet (a) and 3A triplet (b) states. Bond lengths are in A142b. 

The singlet 106 is, at 6-31G //3-21G, by 33.3 kcalmol-1 more stable than monosila-tetrahedrane 38a, and by 56.3 kcalmol-1 more stable than its decomposition products, 

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Oxepin and its derivatives have attracted attention for several reasons. Oxepin is closely related to cycloheptatriene and its aza analog azepine and it is a potential antiaromatic system with 871-elcctrons. Oxepin can undergo valence isomerization to benzene oxide, and the isomeric benzene oxide is the first step in the metabolic oxidation of aromatic compounds by the enzyme monooxygenase. 

Minkin VI, Glukhovtsev MN, Simkin B Ya (1994) Aromaticity and antiaromaticity electronic and structural aspects. Wiley, New York, pp 157-161. (b) Krogh-Jespersen K, Schleyer PvR, Pople JA, Cremer D (1978) J Am Chem Soc 100 4301. (c) The cyclobutadiene dianion, another potentially aromatic system, has been prepared Ishii K, Kobayashi N, Matsuo T, Tanaka M, Sekiguchi A (2001) J Am Chem Soc 123 5356... 

Thiabenzene 256 and its benzologues 1-thianaphthalene 257, 2-thianaphthalene 258 and 9-thiaanthracene 259 are also potentially antiaromatic 8ji electron systems provided they are planar. However, they adopt a boat conformation in which the S atom lies above the plane of the C atoms, thereby creating a 671 electron homoaromatic system with ylidic character. Calculations show that the energy barrier to inversion at S increases in the order 259 < 257 < 256 < 258 and the calculated dipole moments indicate the greatest ylide character is found in 259. The relative stability is in the order 259 > 257 > 258 > 256 (Table 19) <2006HAC376>. 

In contrast to the Hiickel aromatic systems with 4n -I- 2 /r-electrons, cyclic systems with 4n n-clcctrons are destabilized by cyclic conjugation, and are termed antiaromatic" systems. The extent of such antiaromatic destabilization is estimated from the p A3 values of the corresponding cyclopropcncs, which arc obtained from the thermodynamic cycle shown below using the empirical data of pAi[, and the reduction potentials (E, j and... 

The results of a single crystal x-ray diffraction analysis of potentially fully conjugated 4,1-benzothiazocine (97) have been reported (Scheme 29). Although if planar (97) could formally represent an Sm-electron, antiaromatic system, it in fact is tub-shaped. Bond lengths do indicate charge delocalization, but only throughout the N—C=C—CO portion of the molecule <82CC1060>. 

The potential surfaces of the ground and excited states in the vicinity of the conical intersection were calculated point by point, along the trajectory leading from the antiaromatic transition state to the benzene and H2 products. In this calculation, the HH distance was varied, and all other coordinates were optimized to obtain the minimum energy of the system in the excited electronic state ( Ai). The energy of the ground state was calculated at the geometry optimized for the excited state. In the calculation of the conical intersection... 

The thiirene oxide system is of particular interest due to it being simultaneously both a potentially nonbenzenoid aromatic (4n + 2)n and antiaromatic 4nn Hiickel system. 

The second mechanism, due to the permutational properties of the electronic wave function is referred to as the permutational mechanism. It was introduced in Section I for the H4 system, and above for pericyclic reactions and is closely related to the aromaticity of the reaction. Following Evans principle, an aromatic transition state is defined in analogy with the hybrid of the two Kekule structures of benzene. A cyclic transition state in pericyclic reactions is defined as aromatic or antiaromatic according to whether it is more stable or less stable than the open chain analogue, respectively. In [32], it was assumed that the in-phase combination in Eq. (14) lies always the on the ground state potential. As discussed above, it can be shown that the ground state of aromatic systems is always represented by the in-phase combination of Eq. (14), and antiaromatic ones—by the out-of-phase combination. 

The most common evaluation of aromaticity via energetic criteria is done using calculations either a type of isodesmic reaction (34) or comparison of two isomers that differ only through the aromaticity of one (3). We were interested in the possibility of evaluating stability experimentally and the electrochemical formation of dications such as 8 was attractive. In this approach, the redox potential for formation of the dication would be compared to the redox potential for formation of dications which could not be antiaromatic. If 8 was antiaromatic, its redox potential should be larger and more positive than that of the reference system. This approach was applied to the evaluation of the antiaromaticity of 9... 

While the redox potentials for formation of 9 and 10 indicates the greater instability of these dications compared to reference systems, is this instability due to antiaromaticity The plot of the redox potentials vs the sum of the NICS values for the fluorenyl system, Figure 2, next page, shows a reasonable relationship between these energetic and magnetic measures of antiaromaticity. 

In the 4q case, 6 is not an important contributor to the ground state description of the properties of either 4 or 5. However, with 4 there are alternative modes of homoconjugation possible that involve the external cyclopropane bonds30,32,37 38. This is shown in Scheme 2 for the bicyclo[3.1.0]hexenyl cation. This alternative mode of conjugation of a cyclopropane in a 4q situation, an option not available to the parent 4q antiaromatic unsaturated ring systems39, leads to a fundamentally different set of properties and reactions of these systems as compared to the potentially homoaromatic 4q + 2 cases. 

Ronald Breslow and his collaborators have given some attention to the problem of estimating the degree of destabilization of cyclobutadiene with respect to nonconjugated models. They have concluded from electrochemical measurements of oxidation-reduction potentials of the system 37 38, of which only the quinone 38 has the cyclobutadiene fragment, that the C4H4 ring is destabilized by some 12-16 kcal mole-1 and so is definitely antiaromatic.15... 

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