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intermediate: Hexadehydro[18]annulene

intermediate: Tridehydro[18]annulene

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Synthesis of [18]Annulene - Chempedia - LookChem

The preparation of 10-(1-carboalkoxyalkyl)-2,7-methanoaza[10]annulenes by the electrocyclic ring-closure of ketenimine intermediates, which are formed by the reaction of triphenylalkylidenephosphorane and 6-vinylcyclo-hepta-1,3,5-isocyanate, is described.

[18]annulene with six peripheral n ..

Mobius orbital symmetric with respect to (ideal) one of degenerate pair in Huckel MO theory;4,10 can show characteristic intersecting ring motif; susceptible to contamination from ó-orbitals making interpretation more difficult; qualitative measure only; not yet generally evaluated a Schleyer, P. v. R.; Freeman, P.; Jiao, H.; Goldfuss, B. Angew. Chem., Int. Ed. Engl. 1995, 34, 337. b Reference 16. c Reference 8c. Wannere, C. S.; Schleyer, P. v. R. Org. Lett. 2003, 5, 865. Schleyer, P. v. R.; Puhlhofer, F. Org. Lett. 2002, 4, 2873. For a recent review of aromatic stabilization energies, see: Cyranski, M. K. Chem. Rev. 2005, 105, 3773. d Reference 24. e Reference 11f. f Julg, A.; Francois, P. Theor. Chim. Acta 1967, 7, 249. g Reference 3. h References 2 and 23. i References 8c and 13. j For a recent review, see Herges, R. Chem. Rev. 2005, 105, 3758. See also ref 11j. k References 10, 36, and 37.

Synthesis of a [18]annuleno[18]annulene - ScienceDirect

3700 Chemical Reviews, 2005, Vol. 105, No. 10 Rzepa of the more important of these, both computed and experimental, that have been proposed for quantifying aromaticity are summarized in Table 1. These reference points are included here for convenience with the caveats that not all have been evaluated as necessarilyrelevant for Mobius conjugatedrings and thatmanyare relativelyapproximatein theirnature. Reference to these will be made when specific systems are discussed herein.

The smallest annulene for which such a strategy might be conceivablyaccommodatedwithoutprohibitive angular strain is transoid [8]annulene, but calculations15 have revealed little resulting conjugation between the E and neighboring Z double bonds, due to essentiallyorthogonaloverlapsof the adjacent 2p AOs. The result is little evidence of any aromatic character in this molecule. In effect, the rotation between pairs of neighboring p-orbitals was very unevenly distributed around the ring in the fully relaxed geometry. The authors noted that fixing two dihedral angles during geometry optimization to artificially delocalize the rotation had the effect of significantly increasing the ring aromaticity (as quantified using the NICS(0) index in Table 1, and described more fully in section 3.3).

Synthesis of a [14]annuleno[18]annulene - ScienceDirect

-15 to -120 ppmâcgs Mobius annulenes show opposite tendency to Huckel annulenes for magnetic susceptibility;e dependent on reference fragments; positive values indicate anti-aromaticity. NICS(0)h -10 to -15 ppm NICS(0) at the heavy atom ring centroid; perturbed by in-plane local effects and associated effects from ó frameworks 1H NMR chemical shiftsi 7-9 ppm (outer protons) appears typical of both Huckel and Mobius aromatic rings induced current densitiesj little evaluated for nonplanar Mobius systems ð molecular orbitalsk

idealized sp2 angles, 10ð-electron nine-membered ring systems are nevertheless thought to be on the edge of existing as planar species.19 With 2 however, the calculated planar geometry shows strong bond length alternation (maximum ¢rC-C 0.131 Å), normally taken to indicate lack of aromatic delocal- izing stability and being more characteristic of nonor anti-aromatic systems. The force constants calculated for the planar stationary point show two negative values, the more negative of which (î 179 cm-1) reveals vectors corresponding to deplanarization toward a C2 symmetric species (some 20 kcal mol-1 lower in energy). The less negative force constant

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  • Synthesis of 2,7-Methano-aza[10]annulene Derivatives -

    PROBLEM 1.13Represent the electron distribution among the orbitals in (a) [10]-Annulene (b) [12]-Annulene

  • Cyclodecapentaene or [10]annulene is an annulene with ..

    [18]Annulene put into a new ..

  • (benzene, cyclooctadecanonaene or [18] annulene), non ..

    Annulenes may be aromatic (benzene, cyclooctadecanonaene or [18]annulene), non-aromatic ([10] ..

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[18]annulene show only one peak in its proton ..

On the basis of his analysis Hückel proposed that only certain numbers of electrons could lead to aromatic stabilization. Only when the number of electrons is 2, 6, 10, 14, and so on, can a closed-shell electron configuration be realized. These results are summarized in Hückel’s rule: Among planar, monocyclic, fully conjugated polyenes, only those possessing (4n 2) electrons, where nis an integer, will have special aromatic stability.

[18]-annulene the crystal structure shows a D 6h ..

AB - The title compound was synthesized by thermal reaction of 3-phosphoranylidene-amino-1,6-methano[10]annulene with 2-chlorotropone in a single step. The examination of 1H NMR spectrum revealed that there is little contribution of peripheral 18-π electron conjugation, but it is rather composed of 1-azaazulene and methano[10]annulene moieties.

[18]annulene model is shown to be a self-consistent ..

five transoid bonds (¢rC-C ) 0.049 Å, HOMA ) 0.85, Julg ) 0.95). The NICS(0) values for these three annulenes (-14.6, -14.5, and -10.2 ppm, respectively) also fall typically into the aromatic region, as do the magnetic susceptibility exaltations (-36.5, a particularly notable -112.9, and -95.2 cgsâppm, respectively, see Table 1).

Synthesis of 1,6-Didehydro[10]annulene

N2 - The title compound was synthesized by thermal reaction of 3-phosphoranylidene-amino-1,6-methano[10]annulene with 2-chlorotropone in a single step. The examination of 1H NMR spectrum revealed that there is little contribution of peripheral 18-π electron conjugation, but it is rather composed of 1-azaazulene and methano[10]annulene moieties.

Cyclooctadecanonaene - Wikipedia

Asecond isomer of [10]-annulene (the cis, trans, cis, cis, trans stereoisomer) can have bond angles close to 120°but is destabilized by a close contact between two hydrogens directed toward the interior of the ring. In order to minimize the van der Waals strain between these hydrogens, the ring adopts a nonplanar geometry, which limits its ability to be stabilized by electron delocalization. It, too, has been prepared and is not very stable. Similarly, the next higher (4n 2) system, [14]-annulene, is also somewhat destabilized by van der Waals strain and is nonplanar.

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