Antiaromaticity in Monocyclic Conjugated Carbon Rings

Wiberg, Kenneth B.

doi:10.1021/cr990367q

Cited by 203 publications

(176 citation statements)

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“…The minimum D 4h structure of the 1 3 A 2g state in Table 5 is confirmed by the absence of imaginary frequencies in all ab initio vibrational calculations [89][90][91]. The square triplet structure of cyclobutadiene agrees with the reversal of the (4n + 2)/4n rule [22,101] according to which 4n π-electron rings with bond alternating geometry in the lowest singlet have bond equalized structures in the lowest triplet. Triplet aromaticity of cyclobutadiene and more in general of 4n π-electron annulenes up to the cyclononatetraenyl cation has been quantitatively estimated by high-level ab initio calculations in terms of aromatic stabilization energies and of geometric and magnetic properties [102].…”

Section: Structural Instabilitiessupporting

confidence: 54%

“…Monocycles with 4n π electrons are the second large group of conjugated hydrocarbons, the antiaromatic systems [21,22], characterized by the instability with respect to acyclic polyenes and the tendency toward bond alternating geometries [4]. Cyclobutadiene has been considered the reference antiaromatic system [23,24] though recently criticism has been advanced to this view [25,26].…”

Section: Introductionmentioning

confidence: 99%

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Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

Gellini

Salvi

2010

Symmetry

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The paper introduces general considerations on structural properties of aromatic, antiaromatic and non-aromatic conjugated systems in terms of potential energy along bond length alternation and distortion coordinates, taking as examples benzene, cyclobutadiene and cyclooctatetraene. Pentalene, formally derived from cyclooctatetraene by cross linking, is also considered as a typical antiaromatic system. The main interest is concerned with [n]annulenes and model [n]annulene molecular systems, n ranging from 10 to 18. The rich variety of conformational and configurational isomers and of dynamical processes among them is described. Specific attention is devoted to bridged [10]-and [14]annulenes in the ground and lowest excited states as well as to s-indacene and biphenylene. Experimental data obtained from vibrational and electronic spectroscopies are discussed and compared with ab initio calculation results. Finally, porphyrin, tetraoxaporphyrin dication and diprotonated porphyrin are presented as annulene structures adopting planar/non-planar geometries depending on the steric hindrance in the inner macrocycle ring. Radiative and non-radiative relaxation processes from excited state levels have been observed by means of time-resolved fluorescence and femtosecond transient absorption spectroscopy. A short account is also given of porphycene, the structural isomer of porphyrin, and of porphycene properties.

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Section: Structural Instabilitiessupporting

confidence: 54%

Section: Introductionmentioning

confidence: 99%

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

Gellini

Salvi

2010

Symmetry

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“…Since the inner angle of the planar COT (135°) is larger than the ideal bond angle for sp 2 hybridized carbon (120°), there are some strains in the planar D 8h and D 4h transition states, and hence the antiaromatic destabilizations in both D 8h and D 4h structures are at most several kcal mol -1 . In accord with this explanation, the hydrogen transfer energy for the planar D 4h COT going to cyclooctatriene was reported to be -8 kcal mol -1 (B3LYP/6-311+G*) and -9 kcal mol -1 (MP2/6-311+G*) [9], whereas isodesmic (27.8 kcal mol -1 MP2/6-31G*//HF/3-21G [29]; 28.4 kcal mol -1 MP4SDTQ/6-31G**//MP2/6-31G**+ZPE(HF/6-31G*) [30]) and homodesmic (-28.4 kcal mol -1 [30]) stabilization energies gave rather different results. The other important criteria for the study of aromaticity and antiaromaticity are related to the magnetic properties of compounds.…”

Section: Open Accessmentioning

confidence: 79%

“…Accordingly, cyclic conjugation of 4nπ-electrons is considered to be energetically unfavorable. In fact, some 4nπ-electron systems such as cyclobutadiene [8,9] and cyclopropenyl anion [10,11] led to strong destabilization of the compound in contrast to the stabilization characteristic of aromatic compounds. Breslow proposed the term of antiaromaticity to describe such systems [12,13].…”

Section: Open Accessmentioning

confidence: 99%

Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

2010

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Cyclooctatetraene (COT), the first 4nπ-electron system to be studied, adopts an inherently nonplanar tub-shaped geometry of D 2d symmetry with alternating single and double bonds, and hence behaves as a nonaromatic polyene rather than an antiaromatic compound. Recently, however, considerable 8π-antiaromatic paratropicity has been shown to be generated in planar COT rings even with the bond alternated D 4h structure. In this review, we highlight recent theoretical and experimental studies on the antiaromaticity of hypothetical and actual planar COT. In addition, theoretically predicted triplet aromaticity and stacked aromaticity of planar COT are also briefly described.

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“…Cyclobutadienes and cyclooctatetraenes, both prototypical antiaromatic compounds, escape to some extent from the desta- bilizing energy by adopting a rectangular D 2h geometry and a tub conformation (torsional angle between the double bonds can reach 54°), respectively (25). In both cases, the geometry minimizes the interaction between the double bonds and reduces the delocalization.…”

Section: Discussionmentioning

confidence: 99%

Stable four-π-electron, four-membered heterocyclic cations and carbenes

Ishida

Donnadieu

Bertrand

2006

Proc. Natl. Acad. Sci. U.S.A.

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. Subsequent deprotonation of the B-amino derivative affords the corresponding N-heterocyclic carbene, which has been isolated as yellow crystals. According to singlecrystal x-ray diffraction studies, both the carbene and its conjugate acid precursor have a planar structure, with no pyramidalization of the ring atoms. The structural parameters indicate that the four electrons are mostly distributed over the N-C-N fragment with little involvement of the boron center, and therefore both types of heterocycles escape from antiaromaticity. However, considering the ring strain and the presence of the Lewis acid center, the thermal stability of the carbene (mp 98°C without decomposition) is rather surprising. These results clearly suggest that the backbone of N-heterocyclic carbenes can be modified at will, without preventing their isolation.aromaticity ͉ boron C arbenes are compounds with a neutral, dicoordinate carbon atom with only six electrons in its valence shell. They are fundamentally important species that for a long time were considered as prototypical reactive intermediates, too unstable to isolate under usual experimental conditions (1). According to structural, thermodynamic, and magnetic criteria, from the populations, ionization potentials, and inner shell electron energy loss spectroscopy, it has been recognized that the first isolated N-heterocyclic carbenes (NHCs), the imidazol-2-ylidenes A (2) benefit from their aromatic character (3-5) (Fig. 1). Indeed, in addition to the stabilizing effect of the -donating, -attracting amino substituents, it has been concluded that aromaticity provides an additional stabilization of Ϸ25 kcal͞mol (3). We have shown that six--electron NHCs B (6), isoelectronic with borazenes (7), are highly thermally stable. Even more strikingly, the two--electron cyclopropenylidene C is stable enough to be isolated at room temperature (8), despite the absence of -donor heteroatoms directly bonded to the carbene center, a necessary criterion for previously isolated carbenes (9-14).As recently written by Mo and Schleyer (15): ''Although aromaticity is a virtual (not directly measurable) quantity, its usefulness as a central chemical concept is undiminished after two centuries.'' Clearly, abnormal energies govern the chemical behavior of (4n ϩ 2) and (4n) -electron-containing conjugated ring systems, which are more and less stable thermodynamically, respectively, than ''they ought to be' ' (16-18).At that point, the question was raised: is it possible to isolate formally antiaromatic heterocyclic carbenes? This endeavor requires the preparation of the stable conjugate acid precursors, which would also feature a (4n)--electron system. Following the analogy between borazenes and NHCs of type B, we chose a BNCN skeleton, and here we report the synthesis and single crystal x-ray diffraction study of cationic, planar, cyclic four--electron four-membered heterocycles D and a corresponding NHC E. Results and Discussion Treatment of N,N'-2,6-diisopropylphenyl-N-trimethylsilylfor-mamidine with...

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Antiaromaticity in Monocyclic Conjugated Carbon Rings

Cited by 203 publications

References 115 publications

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

Stable four-π-electron, four-membered heterocyclic cations and carbenes

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