Domino Diels-Alder reactions. 4. C16-Hexaquinacene

Paquette, Leo A.; Snow, Robert A.; Muthard, Jean L.; Cynkowski, Tadeusz

doi:10.1021/ja00473a046

Cited by 44 publications

(14 citation statements)

References 2 publications

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“…21 The structure in Figure 3 was obtained without any geometrical assumptions, although Cs symmetry22 was imposed. In the first case (8) we find a Ar value of 0.031 A (2.1 % increase in cyclopropane bond length), with a corresponding figure of 0.070 A (4.6%) for the full optimization (1).…”

Section: Methodsmentioning

confidence: 81%

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Molecular orbital theory of homoaromatic character

Haddon¹

1979

J. Org. Chem.

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Ab initio STO-2G geometry optimizations have been carried out for the homotropenylium cation (1) and the cyclooctatrienylium cation [planar ( C, ) and nonplanar (C8)]. The homoconjugate W a g e for the homotropenylium cation is found to be 1.578 A at this theoretical level. This brings the ab initio calculations for 1 into line with previous MIND0/3 studies, frontier molecular orbital theories (FMOs), and the Winstein picture of an open cyclopropane unit in the homotropenylium cation. Both the homotropenylium cation and the cyclooctatrienylium cation are found to be markedly nonplanar when fully optimized. STO-BG, STO-3G, and 4-31G calculations on the STO-2G potential minima indicate that the cyclooctatrienylium cation probably lies no more than 10 kcal/mol higher in energy than the homotropenylium cation. This finding requires a revision of the concept of the homoaromatization energy for 1 and implicates the cyclooctatrienylium cation as an intermediate in the bridge-flipping process which occurs in the homotropenylium cation. MINDO/3 calculations are reported for the homotropenylium and cyclooctatrienylium cations together with results for other ions of interest in the area of homoaromatic chemistry. Evidence for homoaromatic character is found for the homotropenylium cation, the homocyclopropenylium cation, and the homocyclooctatetraene dication. The final frontier-dihomoaromatic character-is illustrated with the aid of calculations on the dihomotropenylium cation which is also found to be nonplanar. The homoaromatic cations show a number of well-defined trends. In particular it is shown that nonplanarity in the basal plane, shortened bond lengths between polyenyl and bridge fragments, bond equalization in the polyenyl segment, and lengthening of the homoconjugate linkage operate in concert to confer homoaromatic character. The bicyclo[5.2.0]nonadienylium cation, in which the cyclopropane group of the homotropenylium cation is replaced by a cyclobutane ring, does not exhibit the above characteristics and is concluded to be nonhomoaromatic. These results are in excellent agreement with previous FMO theories of homoaromatic character.The calculations for the homocyclopentadienyl anion and the .homocycloodatetraene dianion provide some evidence for homoaromatic character, but in contrast to the cations, the results for the anions do not lend themselves to a straightforward interpretation. The full sets of canonical molecular orbitals from the cyclooctatrienylium cation and the homotropenylium cation have been transformed to localized molecular orbitals (LMOs). The three *-type LMOs for the cyclooctatrienylium cation accord well with the notion of a heptatrienylium cation conjugation scheme. The *-type LMOs for 1 are of an entirely different character and most clearly evoke the conjugation of a cyclopropane unit with a pentadienyl cation in which each component has essentially retained its integrity. On this basis it is argued that the homotropenylium cation should be regarded as a resonance hybrid between the ful...

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Section: Methodsmentioning

confidence: 81%

“…Lett., 32, 537 (1975). the 7-MR of 1 constrained to be planar (8), as in the previous ab initio study. 21 The structure in Figure 3 was obtained without any geometrical assumptions, although Cs symmetry22 was imposed.…”

Section: Methodsmentioning

confidence: 96%

Molecular orbital theory of homoaromatic character

Haddon¹

1979

J. Org. Chem.

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“…The first report of oxidative enolate coupling with Br 2 was disclosed by Ivanoff and Spassoff in 1935, yet very little follow-up work appears in the literature . Pioneering work in the late 1970s by Rathke, Heiba, Kobayashi, Saegusa, , Mislow, and others − demonstrated the utility of the reaction in both inter- and intramolecular coupling using various V, Cu, and Fe salts.…”

Section: Oxidative Couplingmentioning

confidence: 99%

Methods Utilizing First-Row Transition Metals in Natural Product Total Synthesis

2017

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First-row transition-metal-mediated reactions constitute an important and growing area of research due to the low cost, low toxicity, and exceptional synthetic versatility of these metals. Currently, there is considerable effort to replace existing precious-metal-catalyzed reactions with first-row analogs. More importantly, there are a plethora of unique transformations mediated by first-row metals, which have no classical second- or third-row counterpart. Herein, the application of first-row metal-mediated methods to the total synthesis of natural products is discussed. This Review is intended to highlight strategic uses of these metals to realize efficient syntheses and highlight the future potential of these reagents and catalysts in organic synthesis.

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“…Different strategies to synthesize dodeca-hedrane are currently under investigation (Woodward et al, 1964;Eaton and Mueller, 1972;Paquette et al, 1976Paquette et al, , 1978Balogh et al, 1979). In a previous paper Scarbrough and Nowacki (1974) reported on the structure of peristilane.…”

Section: Introductionmentioning

confidence: 99%

Towards the synthesis of dodecahedrane

Engel¹

1980

Zeitschrift Für Kristallographie - Crystalline Materials

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The crystal und molecular structure of tetrahydro-Ci 6 -hexaquinacene-l,9-dione has been determined by X-ray diffraction techniques. The substance crystallizes in the space group Pnal t -Cf" having cell parameters a = 11.576(1), b = 8.358(1), c = 11.768(1) A and four molecules per unit cell.The crystal structure was determined by a combination of Patterson methods and phase refinement with the tangent formula. The refinement by a block-diagonal matrix least-squares method led to a R value of 0.062 for 1,010 observed reflexions.The molecule consists of six polyfused five membered rings representing half of the dodecahedrane. Three rings are almost planar, the other three rings have half-chair conformations. The molecule does not show C s symmetry because of a distortion due to repulsive forces between close hydrogen atoms. The molecules are held together by van der Waals forces only.

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Domino Diels-Alder reactions. 4. C16-Hexaquinacene

Cited by 44 publications

References 2 publications

Molecular orbital theory of homoaromatic character

Molecular orbital theory of homoaromatic character

Methods Utilizing First-Row Transition Metals in Natural Product Total Synthesis

Towards the synthesis of dodecahedrane

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