Organophosphorverbindungen, 51<sup>1)</sup> Phosphatriafulvene – Phosphaalkene mit inverser Elektronendichte<sup>2)</sup>

Fuchs, Evelyn; Breit, Bernhard; Heydt, H.; Regitz, Manfred; Schoeller, Wolfgang W.; Busch, Thilo; Krüger, Carl; Betz, P.

doi:10.1002/cber.19911241228

Cited by 43 publications

(23 citation statements)

References 36 publications

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“…Stable singlet carbenes form G -type adducts with phosphinidene (known examples are I–II , ,− ,,− ,− III–IV , V ,, ), as shown in Figure . The resulting inversely polarized , phosphaalkenes − (note the HOMO in Figure structure c ), ,,− are of recent interest, ,,,,,− as the phosphorus analogues ,,− of the deoxy-Breslow intermediate (Figure ), − which, as a strong nucleophile, has a key role in umpolung reactions. − Moreover, the 31 P NMR chemical shifts of these type of compounds were correlated with the π-polarizations of carbene–phosphorus bond as well …”

Section: Introductionmentioning

confidence: 98%

Stretching the P–C Bond. Variations on Carbenes and Phosphanes

2020

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The stability and the structure of adducts formed between four substituted phosphanes (PX3, X:H, F, Cl, and NMe2) and 11 different carbenes have been investigated by DFT calculations. In most cases, the structure of the adducts depends strongly on the stability of the carbene itself, exhibiting a linear correlation with the increasing dissociation energy of the adduct. Carbenes of low stability form phosphorus ylides (F), which can be described as phosphane → carbene adducts supported with some back-bonding. The most stable carbenes, which have high energy lone pair, do not form stable F-type structures but carbene → phosphane adducts (E-type structure), utilizing the low-lying lowest unoccupied molecular orbital (LUMO) of the phosphane (with electronegative substituents), benefiting also from the carbene–pnictogen interaction. Especially noteworthy is the case of PCl3, which has an extremely low energy LUMO in its T-shaped form. Although this PCl3 structure is a transition state of rather high energy, the large stabilization energy of the complex makes this carbene–phosphane adduct stable. Most interestingly, in case of carbenes with medium stability both F- and E-type structures could be optimized, giving rise to bond-stretch isomerism. Likewise, for phosphorus ylides (F), the stability of the adducts G formed from carbenes with hypovalent phosphorus (PXphosphinidene) is in a linear relationship with the stabilization of the carbene. Adducts of carbenes with hypervalent phosphorus (PX5) are the most stable when X is electronegative, and the carbene is highly nucleophilic.

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Section: Introductionmentioning

confidence: 98%

Stretching the P–C Bond. Variations on Carbenes and Phosphanes

2020

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“…Whether or not triafulvene can be described as somewhat aromatic, however, is still debated in the literature. The potential aromatic character has stimulated a large number of theoretical studies on triafulvene 23,26-36 as well as its 4-hetero-substituted analogues, ,− and triafulvene has variously been characterized as aromatic, − nonaromatic, − and antiaromatic…”

Section: Introductionmentioning

confidence: 99%

Aromatic Character of Tria- and Pentafulvene and Their Exocyclic Si, Ge, and Sn Derivatives. An ab initio Study

et al. 1999

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The structures and dipole moments have been calculated for both methylenecyclopropene (triafulvene) and pentafulvene and their exocyclic Si, Ge, and Sn analogues 1a − d and 2a − d, respectively. Ab initio calculations employing the HF, DFT, and MP2 methods, each using split valence plus polarization and triple split valence with two sets of polarization functions basis sets, have been performed. The results of these six levels of theory on each of the eight molecules were compared, and the aromatic character of these systems was examined. The structures 1a − d exhibited pronounced bond alternation in the ring suggesting the existence of only weak cyclopropenyl cation-like resonance contribution. However, very large dipole moments were predicted in the direction that would be expected from an aromaticity contribution. Since Si, Ge, and Sn are less electronegative than C, the large dipole moments in the direction of the exocyclic heteroatom were surprising. The most striking finding was that, while 1a is planar, 4-sila-, 4-germa-, and 4-stannatriafulvene were nonplanar with trans-bent structures in which the exocyclic double bond is bent slightly out of the cyclopropene ring plane and out of the HXH plane (X = Si, Ge, Sn) by large angles. Thus, the Si, Ge, and Sn atoms exhibit pyramidal geometries. The dipole moment of pentafulvene, 2a, (oriented toward the ring) was substantially smaller than that of its Si, Ge, and Sn analogues, 2b − d, The large dipoles of 2b − d were due to greater charge separation resulting from the lower electronegativities of Si, Ge, and Sn versus that of C. The cyclopentadienyl rings exhibited strong bond alternation indicating only a modest cyclopentadienyl anion-like contribution in 2b − d.

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“…The aromaticity and geometrical structure of triafulvenes have been extensively studied in recent years; − some of these molecules are regarded as aromatic, while others are regarded as nonaromatic or even antiaromatic species, on the basis of different experimental and theoretical arguments. For example, cyclopropenone, the most extensively studied 4-heterosubstituted triafulvene, is usually considered to be an aromatic molecule, ,,,,− although some earlier papers question this finding based on the very small magnetic susceptibility anisotropy or the electron distribution of the molecule.…”

Section: Introductionmentioning

confidence: 99%

Substituent Effects on the Structure and Aromaticity of 4-Silatriafulvene

et al. 1998

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The structure and aromaticity of several 4-silatriafulvene derivatives are studied using correlated ab initio MO calculations. Electronegative substituents on silicon have been found to stabilize a nonplanar structure around the formal SiC double bond, while π-electron acceptors planarize the molecule. To assess the aromaticity, the geometry index ΣCC, the nucleus-independent chemical shift (NICS), and the Bird indices are employed. Appropriate substituents on 4-silatriafulvene to enhance the aromatic character have been shown to cause the significant decrease of the SiC π-bond strength. In cyclopropenylidene-1-silacyclopentadiene derivatives, both the three- and five-membered rings have low aromaticity, irrespective of the pyramidality of the molecular skeleton. Remarkably high aromaticity of the three-membered rings have been found in cyclopropenylidene-2-silaallene and 1-cyclopropenylidene-3-cyclopentadienylidene-2-silaallene.

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Organophosphorverbindungen, 51¹⁾ Phosphatriafulvene – Phosphaalkene mit inverser Elektronendichte²⁾

Cited by 43 publications

References 36 publications

Stretching the P–C Bond. Variations on Carbenes and Phosphanes

Stretching the P–C Bond. Variations on Carbenes and Phosphanes

Aromatic Character of Tria- and Pentafulvene and Their Exocyclic Si, Ge, and Sn Derivatives. An ab initio Study

Substituent Effects on the Structure and Aromaticity of 4-Silatriafulvene

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Organophosphorverbindungen, 511) Phosphatriafulvene – Phosphaalkene mit inverser Elektronendichte2)

Cited by 43 publications

References 36 publications

Stretching the P–C Bond. Variations on Carbenes and Phosphanes

Stretching the P–C Bond. Variations on Carbenes and Phosphanes

Aromatic Character of Tria- and Pentafulvene and Their Exocyclic Si, Ge, and Sn Derivatives. An ab initio Study

Substituent Effects on the Structure and Aromaticity of 4-Silatriafulvene

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Organophosphorverbindungen, 51¹⁾ Phosphatriafulvene – Phosphaalkene mit inverser Elektronendichte²⁾