Anti-aromaticity

Even of electron pairs exchanged Anti-aromatic system... 

Minkin, M. N. Glukhovtsev, and B. Y. Simkin, Aromaticity and Anti-aromaticity, John Wley Sons, New... 

Only a few reactions of benzodithiadiazines have been investigated. In common with dithiatriazines 12.8, the anti-aromatic system 12.12 (R = H) undergoes a reversible 5,5 -cycloaddition with norbornadiene. The reaction of 12.12 (R = F) with triphenylphosphine results in a ring contraction to give the imino 2 -phosphane 12.13. ... 

Stabilities of cyclic conjugated molecules encountered in the laboratory seem to owe much to kinetic rather than thermodynamic properties. Many unstable conjugated molecules including anti-aromatic molecules are now known to be isolated as stable ones when they are protected by bulky substituents from intermolecular reactions. 

Recrystallization of 76 (R=H) from CH2CI2 provided crystals adequate for X-ray structural determination. The molecule was found to be saddle shaped with a phenyl ring at each vertex and nadir. The alkynyl bonds were found to be essentially linear and to possess a mean length of 1.194 A, typical for the length of triple bonds in free butadiyne. Although 76 is a dehydrobenzoannulene possessing a 4n TT-electron circuit, the nonplanarity of the macrocycle alleviated much of the strain associated with a flat structure and thus precluded the possibility of anti-aromatic ring currents. 

The bis-phosphonium salt (156) has been condensed with various dialdehydes to give potentially aromatic and anti-aromatic systems, while fully unsaturated 11- (158), 12- (159), and 13-membered (160) sulphur heterocycles have been prepared as shown. These showed no appreciable ring current and are presumably non-planar. 

Anti-Aromaticity". The Jerusalem Symposia on Quantum Chemistry and Biochemistry IE, Jerusalem, 1971. 

Recently, Reinhoudt and Kouwenhoven 53) have reported, in connection with their successful synthesis of monocyclic thiepin 87, that the relatively high stability of 87 is attributed to the presence of the two methoxycarbonyl groups which cause a decrease in the electron density of the 8n-electron system. As a result, the formal anti-aromatic character is reduced. Traynelis and his coworkers 6) have also reported a slight increase in the thermal stability of benzo[6]thiepin when electron-withdrawing groups are present. These conclusions have subsequently been supported by resonance energy calculations on various thiepin derivatives59). Thus, the re-... 

Schlessinger, R. H. Aromaticity, pseudo-aromaticity, anti-aromaticity, in The Jerusalem Symposia on Quantum Chemistry and Biochemistry (eds.) Bergmann, E. D., Pullman, B., p. 158, The Israel Academy of Sciences and Humanities, 1971... 

It is a wide-spread belief that such reactions could not be relevant, since they are spin-forbidden. This need not be true. It appears that the reduced flavin is a soft molecule, which resists planarity in the singlet state because of an anti-aromatic number of delocalized 7r-electrons. Hence, the planar conformation of Flre(j might have an unusually low-lying triplet state, which favors the thermal spin relaxation in RX —... 

Fluorenylidene dications, such as the dications of p- and m-substituted diphenylmethylidenefluorenes, show appreciable antiaromaticity. Evidence of antiaromaticity is demonstrated through H NMR shifts, nucleus independent chemical shifts (NICS), magnetic susceptibility exaltation, A, and (anti)aromatic (de)stabilization energies, ASE. Extension of the research to indenylidenefluorene dications shows that, contrary to expectation, the indenyl cation in these dications is less antiaromatic than the fluorenyl cation. The magnitude of the antiaromaticity is evaluated through comparison to the aromaticity of related dianions and reveals that the fluorenylidene dications are more antiaromatic than the fluorenylidene dianions are aromatic. 

The role of a-electrons in the stabilization of aromatics has been repeatedly emphasized by Shaik and Hiberty48 and has recently received experimental confirmation after a study of the effects of deuterium substitution of annulenes with internal hydrogens, such as the anti-aromatic [16] annul ene and the aromatic [18]annulene.49 50... 

In carbocyclic chemistry, rather firm dividing lines usually exist between aromatic, non-aromatic, and anti-aromatic compounds, while in heterocyclic chemistry enormous variations in the extent of aromatic character are displayed.52 Furthermore, there is an enormous number of potential heterocycles as compared to carbocycles, as will be detailed in section 3 of this review. The degree of aromaticity has classically been judged qualitatively in connection with the diene character of heterocycles manifested in Diels— Alder reactions or polymerizations. In this regard for instance, furan (42) is less aromatic than benzene (43), as is isoindole (44) compared to indole (45) (Scheme 18). Therefore, a quantitative aromaticity scale would be useful. 

It seems that the substituents play an essential role in the kinetic stabilization of the molecule 201 ab initio calculations for the formally anti-aromatic unsubstituted indacene shows that the minimum energy corresponds to a Dzh symmetry and a completely delocalized 12-jr-electron system.202... 

Anti-aromatic 1,2-dithiins 179 display properties opposite to those of 1,4-dithiins 180, whose dications show aromatic stabilization. Unlike other antiaromatic compounds, the 1,2-dithiin derivatives, with eight jr-electrons (such as 181 and 182), appear in... 

The interesting hydrocarbon (104), having both aromatic and anti-aromatic character, could be isolated in low yield by treatment of a bis-ylide with freshly distilled glyoxal.96 Precursors for the preparation of helicenes, e.g. (105), have been synthesized in one-step reactions. The yield of the double Wittig reaction is better using a di-aldehyde rather than a bisphosphonium salt.97 The [12]-annulene (106) is obtained from the reaction of the bis-ylide derived from (107) and a di-aldehyde.98 The condensation of (107) with two moles of 3-methylpent-2-en-4-yn-l-al (108) was also successful.99... 

Deprotonation of 5-membered ring gives aromatic anion deprotonation of 7-membered ring gives anti-aromatic anion. 

Vaska s complex ([IrCl(CO)(PPh3)2]) also catalyzed the carbonylative coupling of diynes, which provided bicyclic cyclopentadienones (Scheme 11.23) [35]. Due to the instability of the products, the substrates are limited to symmetrical diynes with aromatic groups on their termini nonetheless, this reaction still serves as the catalytic and practical procedure for the synthesis of cyclopentadienones, which are anti-aromatic with a 47t system and serve as active synthetic intermediates. 

Common examples of systems often mistaken as being aromatic (because of their alternating double and single bonds) are cyclobutadiene and cyclo-octatetraene (shown in Figure 6-9). In the case of cyclobutadiene, 4n + 2 = 4, giving n = 0.5, while for cyclooctatetraene, 4n + 2 = 8, so that n = 1.5. In these two compounds, n is not an integer, so these systems are anti-aromatic (nonaromatic). Anti-aromatic systems (non-Hilckel systems) are less stable than aromatic or normal systems. 

B. Aromaticity and Anti aromaticity of Heterocyclic Compounds That Are 7t-Isoelectronic with the Parent Hydrocarbon... 

A. Julg, in Aromaticity, Pseudo-Aromaticity, Anti-Aromaticity (E. D. Bergmann and B. Pullman, eds.), pp. 383-385. Isr. Acad. Sci. Humanities, Jerusalem, 1971. 

Aromaticity, Pseudo-aromaticity, Anti-aromaticity (E. D. Bergmann and B. 

---