A C2v-Symmetric Barbaralane

Reiher, Markus; Kirchner, Barbara

doi:10.1002/1521-3773(20020916)41:18<3429::aid-anie3429>3.0.co;2-y

Cited by 13 publications

(4 citation statements)

References 28 publications

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“…Analogously to 1 , the introduction of heteroatoms in the related fluxional molecule barbaralane has been investigated. Experimentally, 2,4,6,8‐tetraazasubstitution slightly reduces the barrier of the Cope rearrangement,33 and two heterobarbaralenes were shown to have homoaromatic ground states using ab initio computational methods 34, 35…”

Section: Introductionmentioning

confidence: 99%

Homoaromaticity in aza‐ and phosphasemibullvalenes. A computational study

Greve¹

2011

J of Physical Organic Chem

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The enthalpies of activation ΔH‡ for the Cope rearrangement in several aza‐ and phosphasemibullvalenes have been investigated by MP4/cc‐pVDZ//MP2/cc‐pVDZ and CCSD(T)/cc‐pVDZ//MP2/cc‐pVDZ calculations. One tetraazasemibullvalene and several phosphasemibullvalenes were found to have vanishing ΔH‡ values, which together with calculated large negative Nucleus Independent Chemical Shift (NICS) values and geometrical data show that these molecules have delocalized and bishomoaromatic minima. Furthermore, three azasemibullvalenes were found to have small ΔH‡ values (≤2 kcal/mol) combined with large negative NICS, suggesting that they could also have bishomoaromatic minima. Copyright © 2010 John Wiley & Sons, Ltd.

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Section: Introductionmentioning

confidence: 99%

Homoaromaticity in aza‐ and phosphasemibullvalenes. A computational study

Greve¹

2011

J of Physical Organic Chem

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“…Hoffmann and Dewar , suggested substituents for stabilizing the transition structure for the Cope rearrangement of semibullvalene relative to the two degenerate localized structures and proposed that substituents might even reverse the relative energies of the localized and delocalized structures. Substitution of heteroatoms such as nitrogen, , phosphorus, and boron carbonyl for CH groups in semibullvalenes and barbaralanes has also been predicted to favor delocalized structures over localized structures. In addition, annelation strategies, which employ strain to destabilize the localized semibullvalene structures and thus to favor the delocalized geometries, have been proposed. , The relative importance of ground-state strain and bishomoaromatic stabilization in Cope transition structures of fluxional molecules has recently been analyzed .…”

Section: Introductionmentioning

confidence: 99%

Experimental and Theoretical Study of Stabilization of Delocalized Forms of Semibullvalenes and Barbaralanes by Dipolar and Polarizable Solvents. Observation of a Delocalized Structure that Is Lower in Free Energy than the Localized Form

et al. 2005

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[reaction: see text] UV/vis spectra of thermochromic semibullvalenes 1 and barbaralanes 2, which undergo rapid degenerate Cope rearrangements, display temperature-dependent shoulders (1b, 1d, 1e) or absorption maxima (1c, 2c, 2f) at the low-energy side of their strong UV bands. These long-wavelength absorptions are ascribed to Franck-Condon transitions from delocalized structures 1(deloc) and 2(deloc). Gibbs free energy differences, DeltaG*, between delocalized and localized forms were calculated from the temperature dependence of the long-wavelength absorptions. Dipolar and polarizable solvents strongly affect and even may reverse the relative stabilities of the localized and delocalized forms of 1c, 2c, and 2f. For example, DeltaG*(2c) = 8 kJ mol(-)(1) in cyclohexane, 2 kJ mol(-)(1) in dimethylformamide, and -3 kJ mol(-)(1) in N,N'-dimethylpropylene urea (DMPU), so that (2c(deloc))(DMPU) becomes the global minimum. In contrast to the case for 2c, the intensities of the long-wavelength shoulders of the yellow semibullvalenes 1b, 1d, and 1e are only moderately influenced by solvents, and the rates of Cope rearrangements of the nonthermochromic, colorless barbaralanes 2a and 2b, determined by NMR methods, are almost solvent-invariant. In search of the solute properties that are decisive in determining the influence of solvent upon DeltaG*, electrical dipole and quadrupole moments and molecular polarizabilities have been calculated using the B3LYP/6-31G* method and solvation energies have been computed with the conductorlike polarized continuum model (CPCM). The results of these calculations indicate that the solvent effects are due to the greater polarity and polarizability of the delocalized structures relative to the localized structures.

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“…Theoretical analyses by Hoffmann and Dewar suggested that electron-withdrawing groups at C 2,8 and C 4,6 , as well as electron-donating groups at C 1,5 , might stabilize the bishomoaromatic transition state 2 preferentially and eliminate the barrier. In addition to these electronic effects, strain-induced destabilization of the localized structure 1 through small-ring annelations at C 2,8 or/and C 4,6 , ,, as well as stabilization of the delocalized structures through Li + coordination and heteroatom substitutions, have been explored. Despite extensive attempts, experimental realization of a definitive neutral bishomoaromatic semibullvalene is still elusive .…”

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confidence: 99%

Neutral Bishomoaromatic Semibullvalenes

Jiao

Wang

et al. 2003

J. Am. Chem. Soc.

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Isolobal substitution of CH units by boron carbonyl groups (BCO) at C2,6 and C2,8,4,6 in semibullvalene favors the delocalized neutral bishomoaromatic systems substantially. The homoaromaticity is documented by the computed diatropic nucleus independent chemical shifts (NICS). In addition, BCO substitution can result in low-lying triplet and open-shell singlet states. In contrast, substitution at C1,5 more than doubles the related Cope rearrangement barrier. The C2,6 and C2,8,4,6 BCO-substituted barbaralanes, barbaralones, and bullvalenes have substantially reduced barriers.

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A C2v-Symmetric Barbaralane

Abstract: A homoaromatic C2v‐symmetric tetraphosphabarbaralane is the candidate for the first barbaralane derivative with “inverse” barrier to Cope rearrangement according to quantum chemical calculations (see scheme) as its Cs‐symmetric form with localized double bonds is higher in energy.

Cited by 13 publications

References 28 publications

Homoaromaticity in aza‐ and phosphasemibullvalenes. A computational study

Homoaromaticity in aza‐ and phosphasemibullvalenes. A computational study

Experimental and Theoretical Study of Stabilization of Delocalized Forms of Semibullvalenes and Barbaralanes by Dipolar and Polarizable Solvents. Observation of a Delocalized Structure that Is Lower in Free Energy than the Localized Form

Neutral Bishomoaromatic Semibullvalenes

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