Nonplanarity in Hueckel 2.pi. aromatic systems. An NMR/IGLO/ab initio proof of the puckered structure of cyclobutadiene dications.

Bremer, Matthias; Schleyer, Paul v. R.; Fleischer, Ulrich

doi:10.1021/ja00185a065

Cited by 76 publications

(38 citation statements)

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“…The anion 3 a forms a solvent-separated ion pair with a lithium ion, which is coordinated by three molecules of diethyl ether. The four-membered ring of the triboracyclobutanide 3 a is strongly (by 598) folded, as expected for an isoster of the 1,3-dihydro-1,3-diborete [2±4] 2 and the cyclobutadienyl dication [1] 1. The substitution pattern of the boron atoms of 3 a does not correspond to the symmetric substitution pattern of its precursor 4.…”

supporting

confidence: 55%

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mentioning

confidence: 99%

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Triboracyclobutanides: Four‐Membered Two‐Electron Aromatic Compounds with Fluctuating Skeletal Bonds

Şahin¹,

Präsang²,

Amseis³

et al. 2003

Angew Chem Int Ed

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Among the series of isoelectronic four-membered, twoelectron aromatic compounds 1±3 (Scheme 1), to date derivatives of the dicationic and uncharged skeletons 1 [1] and 2, [2±4] respectively, have been realized experimentally. The barriers of their ring inversions, which are a measure of the stabilization of the folded over the planar rings, have not been determined experimentally for any example. We report herein the synthesis, crystal structure, and the ring inversion barrier of the anion 3 a (Scheme 2), and show, based on line shape analyses of its temperature-dependent NMR spectra, that its skeletal bonds fluctuate via a distorted triboratetrahedrane anion 6 a (see Scheme 3) as transition state. B3LYP computations [5] on model molecules support this proposal. The unsymmetrically substituted triboracyclobutanide 3 a is obtained by reaction of lithium in diethyl ether with the symmetrically substituted derivative 4, which is accessible from 5 [6] and dichloro(trimethylsilylmethyl)borane (Scheme 2). Attempts to detect the symmetrically substituted compound 3 b by NMR spectroscopic monitoring of the reaction of 4 with lithium naphthalenide in [D 8 ]THF at À80 8C proved unsuccessful. The constitutions of 3 a and 4 are in accord with their NMR data ( Table 1) and are confirmed unambiguously by their crystal structure determinations [7] ( Figure 1).

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confidence: 55%

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mentioning

confidence: 99%

Triboracyclobutanides: Four‐Membered Two‐Electron Aromatic Compounds with Fluctuating Skeletal Bonds

Şahin¹,

Präsang²,

Amseis³

et al. 2003

Angew Chem Int Ed

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“…[11,12] For the phenylenes, reduction of p-antiaromatic overlap, which has been implicated as dominating the electronic interactions within the molecules, [4j] may be an additional driving force for deplanarization. The deviation from planarity of tetrafluorocyclobutadiene [13] and the cyclobutadiene dication [14] is noteworthy in this regard. The preference of the benzene nuclei in the phenylenes to adopt a planar configuration might, on first sight, be expected to oppose the deplanarizing influence of the fused cyclo-butadienes.…”

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confidence: 93%

On the Nature of Nonplanarity in the [N]Phenylenes

et al. 1999

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The [N]phenylenes display marked deformation from planarity in the crystalline state. In order to probe the generality of this phenomenon, several derivatives were synthesized and their single-crystal X-ray structures were obtained. All new compounds displayed some degree of nonplanarity. Thus, for example, the parent triangular [4]phenylene (4 b) has a median bend angle at the ring junction of 1.58 and a range of 0.38 to 3.58, whereas hexakis[triisopropylsilyl(ethynyl)] triangular [4]phenylene (4 c) possesses the bulkiest appendages and the largest median bend angle and range (3.88 and 1.78 ± 5.68, respectively). A detailed analysis of the bending and twisting angles at the ring junctions, however, revealed that the magnitude of deformations were independent of topology, molecular size, and substituent type. In contrast to the phenylenes, a Cambridge Structural Database (CSD) search of unsubstituted and non-peri-substituted naphthalenes and anthracenes shows these molecules to be virtually planar in the solid state. A comparison of the single-point energies (HF/6-31G*) of the phenylenes with the acenes calculated for molecules possessing a fixed bend angle at the ring fusion of 38, 68, 98, and 128 reveals the former to be 26 % to 45 % easier to deform than the latter. Based on these results, the nonplanarity seen for the phenylenes is most likely a consequence of crystalpacking forces deforming particularly flexible molecules.

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“…[8] In contrast, the lithium counterions of the cyclobutadiene dianion 1 described by Sekiguchi et al are located above and below the four-membered ring. [1a] Whereas 1 is planar, the B 4 ring of 4 a is puckered (1218), [9][10][11] as expected for a four-membered, two-electron aromatic compound (Scheme 4). Computations [7] on the dianion 4 c provide a folding angle of 1318.…”

mentioning

confidence: 99%

“…Computations also show that for the dianion of tetramethyltetraborane(4) a fourmembered ring structure is 4.9 kcal mol À1 lower in energy than a distorted tetrahedral geometry. [12] Dianions 4 a and 4 b supplement the known four-membered, two-electron aromatic compounds 7 (C 4 ), [9] 8 (C 2 B 2 ), [10] and 9 (CB 3 ) [11] by the long sought-after [13] first examples with four boron atoms. Such compounds have so far only been known from computations.…”

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confidence: 99%

Dianions of Tetraboranes(4): Puckered Aromatic Four‐Membered Rings and Their Reactions with Conservation of Aromaticity

et al. 2003

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Dedicated to Professor Gernot Boche on the occasion of his 65th birthdaySekiguchi et al. recently described the planar dianion 1, a sixelectron aromatic four-membered ring (Scheme 1).[1a] Upon two-electron oxidation, 1 is transformed into the antiaromatic compound 2, [1b] and double protonation, which requires four electrons for bonding the two protons, produces the nonaromatic compound 3.[1a] Thus, both reactions proceed with loss of aromaticity. Here we present the puckered dianions 4 a and 4 b (Scheme 2), aromatic two-electron four-membered rings of type 4, the corresponding reactions of which result in compounds of types 5 and 6, conserving the aromaticity.Dianion 4 a is obtained by treating 5 a [2,3] with lithium in DME at 25 8C. Treatment with hexachloroethane quantitatively reoxidizes 4 a to 5 a, whereas the reaction of 4 a with Et 2 NBCl 2 results in the formation of amino(triboryl)tetraborane(4) 5 c. Compound 4 b, which is obtained by the reaction of 5 b [4] with lithium in THF at À78 8C, is stable in solution below À30 8C; at higher temperatures a recently described [5] double aromatic B 3 ring is formed by migration of a trimethylsilyl

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Nonplanarity in Hueckel 2.pi. aromatic systems. An NMR/IGLO/ab initio proof of the puckered structure of cyclobutadiene dications.

Cited by 76 publications

References 0 publications

Triboracyclobutanides: Four‐Membered Two‐Electron Aromatic Compounds with Fluctuating Skeletal Bonds

Triboracyclobutanides: Four‐Membered Two‐Electron Aromatic Compounds with Fluctuating Skeletal Bonds

On the Nature of Nonplanarity in the [N]Phenylenes

Dianions of Tetraboranes(4): Puckered Aromatic Four‐Membered Rings and Their Reactions with Conservation of Aromaticity

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