Paratropic Delocalized Ring Currents in Flattened Cyclooctatetraene Systems with Bond Alternation

Fowler, Patrick W.; Havenith, Remco W. A.; Jenneskens, Leonardus W.; Soncini, Alessandro; Steiner, Erich

doi:10.1002/1521-3757(20020503)114:9<1628::aid-ange1628>3.0.co;2-0

Cited by 17 publications

(16 citation statements)

References 29 publications

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“…The color of 14 is red with weak absorption band at 459 nm (log ε = 2.11), indicating a relatively narrow HOMO-LUMO gap for an 8π-electron system. However, the NICS value of 14 (+10.6) at the GIAO/HF/6-31+G**//B3LYP/6-31G* level [80] is considerably reduced in comparison with that of D 4h COT (+27.2) at the same level, although CTOCD-DZ calculations support that 14 sustains a weak paratropic ring current [78]. The large bond alternation in 14 is not the main reason for the considerable decrease in antiaromatic paratropicity, since a much larger NICS value (+22.1) was calculated for a hypothetical planar COT having exactly the same bond lengths as those in 14 [81].…”

Section: Planar Cot Annelated With Plural Bicyclo[211]hexene Ringsmentioning

confidence: 98%

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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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Section: Planar Cot Annelated With Plural Bicyclo[211]hexene Ringsmentioning

confidence: 98%

“…In fact, CTOCD-DZ calculations predicted that 12 as well as 13 have a strong paratropic ring current [78]. We also calculated the HOMO-LUMO gap (B3LYP/6-31G**) and NICS(0) value (GIAO/HF/6-311+G**) and the results were 2.56 eV and +17.7 ppm.…”

Section: Planar Cot Annelated With Four Cyclobutene Ringsmentioning

confidence: 99%

Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

2010

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“…As shown in Figure 1, the planar COT rings have a b 2u HOMO and b 1u LUMO. The b 2u →b 1u HOMO–LUMO transition has the symmetry of rotation with respect to the direction of the magnetic field and therefore produces a paratropic contribution in the antiaromatic π system 13a. A recent theoretical study showed that this HOMO–LUMO transition dominates the total π ring current in the planar COT and determines its paratropic nature 26.…”

Section: Resultsmentioning

confidence: 99%

“…Unfortunately, 1 lacks protons, which are useful for the detection of a paratropic ring current by 1 H NMR spectroscopic analysis. A recent theoretical study, which employed the continuous transformation of origin of current density (CTOCD) method,12 predicted that a considerable paratropic ring current should be present in 1 ,13a even though the apparent bond alternation in the COT ring was shown by X‐ray analysis2b and theoretical calculations 9a. 14 …”

Section: Introductionmentioning

confidence: 99%

“…Such a small difference may seem to be attributed to a substantial decrease in both the paratropic and diatropic ring currents in 2 and 3 , respectively, as there is a large bond alternation in 2 (Δ R = R endo − R exo =1.500–1.331=0.169 Å)6 and 3 (Δ R =1.438–1.349=0.089 Å)17 caused by the annelation with highly strained bicyclo[2.1.1]hexane units. However, calculations of both CTOCD and nucleus‐independent chemical shift (NICS)18 demonstrated the presence of considerable amount of antiaromaticity in 2 6, 13a and aromaticity in 3 6. 13b On the basis of these findings, we decided to examine the antiaromaticity in 2 in detail using observed 1 H NMR chemical shifts and theoretical calculations.…”

Section: Introductionmentioning

confidence: 99%

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Antiaromaticity and Reactivity of a Planar Cyclooctatetraene Fully Annelated with Bicyclo[2.1.1]hexane Units

Nishinaga

Uto

Inoue

et al. 2008

Chemistry A European J

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A detailed investigation has been made into the antiaromaticity and chemical reactivity of a planar cyclooctatetraene (COT) molecule fully annelated with bicyclo[2.1.1]hexane units 2. In spite of its planar 8pi-electronic structure, theoretical calculations have indicated that the antiaromaticity of COT 2 is considerably decreased in comparison with a planar COT 16 with D 4h symmetry. This behavior appears to be related to the wider HOMO-LUMO gap of 2 relative to 16, which is caused by the raised LUMO level as a result of the effective sigma-pi* orbital interaction between the strained bicyclic framework and the COT pi system. The two-electron reduction of 2 required the use of potassium mirror or a combination of lithium/corannulene in highly dried [D8]THF at -78 degrees C under vacuum. In contrast, the [4+2] cycloaddition of 2 with tetracyanoethylene (TCNE) proceeded quite smoothly owing to the high-lying HOMO. Reaction of 2 with meta-chloroperbenzoic acid gave all-trans tetraepoxide 23 in the same way as the corresponding benzene derivative 3. While the Simons-Smith-type cyclopropanation of benzene 3 gave tricyclopropanated derivative 21, the reaction of 2 only afforded isomers of dicyclopropanated derivatives 25 and 26. Yet, the reactivity of 2 is higher than the parent COT, which does not show any reactivity under the same conditions. On the basis of homodesmic reactions, it was concluded that release of strain is also an important factor for such relatively high reactivity in the epoxidation and cyclopropanation of bicycloannelated COT 2 as well as benzene 3.

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Experimental and Theoretical Investigations into the Paratropic Ring Current of a Porphyrin Sheet

Nakamura

Aratani

Osuka

2007

Chemistry An Asian Journal

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Anomalous induced magnetic effects were observed in a directly fused square-planar porphyrin sheet 1, in that the protons above the center of the tetraporphyrin core were characteristically shifted downfield in the 1H NMR spectrum. These observations suggest a rare paratropic ring-current effect around the planar cyclooctatetraene (COT) core of 1. To examine the spatial distribution of the induced magnetic effect, face-to-face dimeric complexes of porphyrin sheet 1 with bipyridyl-type guest molecules (G1-G3) were prepared, which provided complexation-induced shifts (CISs) of the guest molecules as a neat experimental guide to the distance dependence of the induced magnetic effects in 1. Nucleus-independent chemical shift (NICS) values of 1 were calculated by varying the distance of the probe from the plane of 1. Whereas a simple bell-type profile was estimated for the complex (1)2-(G1)4, the distance profiles of the CIS became increasingly flat for (1)2-(G2)4 and (1)2-(G3)4. Finally, we investigated the paratropic ring-current effect just above the COT core of the complex 1-(G4)2, which agrees well with the theoretically estimated distance-dependent induced magnetic effect. Consequently, both experimental and theoretical studies on the complexes of porphyrin sheets with guest molecules revealed for the first time a unique distance dependence of the paratropic ring current.

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Paratropic Delocalized Ring Currents in Flattened Cyclooctatetraene Systems with Bond Alternation

Cited by 17 publications

References 29 publications

Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

Antiaromaticity and Reactivity of a Planar Cyclooctatetraene Fully Annelated with Bicyclo[2.1.1]hexane Units

Experimental and Theoretical Investigations into the Paratropic Ring Current of a Porphyrin Sheet

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