Thermolabile Kohlenwasserstoffe, 32. Konkurrierende Cope‐;Umlagerung und Homolyse von <i>meso</i>‐ und <scp>DL</scp>‐3,4‐Di(1‐cyclohexen‐1‐yl)‐2,2,5,5‐tetramethylhexan

Herberg, Clemens; Hans,; Beckhaus, Dieter; Kürtvelyesi, Tamas; Rüchardt, Christoph

doi:10.1002/cber.19931260119

Cited by 7 publications

(1 citation statement)

References 34 publications

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“…This equilibration has already been noted by Vögtle and Goldschmitt at 200 °C, and by Perkins and co-workers at 80 °C after boiling in benzene for 60 h, but without comment from either group. With the exception of the example of Gajewski, Benner, and Hawkins, and that uncovered recently by Rüchardt and co-workers in 3,4-di(1-cyclohexen-1-yl)-2,2,5,5-tetramethylhexa-1,5-diene, the interconversion is almost unprecedented. It cannot be accommodated by any concatenation of the three sets of conventional chair, boat, or twist-boat Cope rearrangements (Scheme ).…”

Section: Resultsmentioning

confidence: 88%

A Non-Cope among the Cope Rearrangements of 1,3,4,6-Tetraphenylhexa-1,5-dienes

et al. 1999

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The set of 1,3,4,6-tetraphenylhexa-1,5-dienes (1) represents a perturbation of Cope's rearrangement by four radical-stabilizing phenyl groups all positioned to drive the transition region toward the homolytic−colligative end of the mechanistic spectrum. The appearance of (Z)-isomers being suppressed thermodynamically by a steric interaction of +2.6 kcal mol-1 per cis double bond, an equilibration that is stereochemically not of any Cope type, emerges as the predominant reaction. It is an interconversion of rac -( E , E )-1 and meso -( E , E )-1 (48:52; 77.3−115.3 °C) with the following values of the enthalpy, entropy, and volume of activation: ΔH ⧧ = 30.7 ± 0.2 kcal mol-1, ΔS ⧧ = +2.1 ± 0.4 cal mol-1 K-1, and ΔV ⧧ = +13.5 ± 0.1 cm-3 mol-1, respectively. Structures have been established by X-ray crystallographic analysis; a possible relationship between dihedral angle and bond lengths in the styrene portions is proposed. The entropy of activation is incompatible with a chair or boat Cope rearrangement; the volume of activation is neither low enough for a pericyclic Cope (“concerted”) mechanism nor high enough for a homolytic−colligative mechanism involving full dissociation as the rate-determining step. Trapping and a crossover experiment give some but only partial support to the intermediacy of free radicals. At higher temperatures, however, electron spin resonance experiments demonstrate an equilibrium with kinetically free (E,E)-1,3-diphenylallyl radicals. These observations are rationalized in terms of geometric reorganization within the confines of a ‘cage'. Resolution by chiral chromatography of rac -( E , E )-1 allows recognition of a fast racemization (40−65 °C), of which ΔH ⧧ (21.3 ± 0.1 kcal mol-1), ΔS ⧧ (−13.2 ± 0.3 cal mol-1 K-1), and ΔV ⧧ (−7.4 ± 0.4 cm-3 mol-1) are consistent with a pericyclic Cope rearrangement. Enriched (Z)-isomers undergo Cope rearrangements in accord with the known influence of axiality and the chair/boat alternative on the energy of the transition region.

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Section: Resultsmentioning

confidence: 88%

A Non-Cope among the Cope Rearrangements of 1,3,4,6-Tetraphenylhexa-1,5-dienes

et al. 1999

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Rearrangements of Dienes and Polyenes

Koblik

2009

Patai's Chemistry of Functional Groups

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Thermolabile Hydrocarbons, 33. Thermochemistry and Thermal Decomposition of 9,9′‐Bifluorenyl and 9,9′‐Dimethyl‐9,9′‐bifluorenyl – The Stabilization Energy of 9‐Fluorenyl Radicals

et al. 1994

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Key Words: Heats of formation / Bond cleavage, C-C, kinetics of / Radicals, stability of / Correlation between heats of vaporization and solvent-accessible surfacesFrom thermochemical measurements the heats of formation AH$' (g) for fluorene (l), 9-methylfluorene (2), 9,9'-bifluorenyl (5), and 9,9'-dimethyl-9,9'-bifluorenyl (6) were determined. The homolytic cleavage of the dimers 5 and 6 to 9-fluorenyl (3) and 9-methyl-9-fluorenyl radicals (4), respectively, was studied in mesitylene with thiophenol as trapping agent and found to follow first-order kinetics. The activation parameters AH*(5) = 212.6 (k2.1) kJ mol-', AS'(5) = 70.3 (f4.2) J mol-' K-I, AH*(6) = 164.9 (k0.7) kJ mol-l, and AS*(6) = 88.2 (k1.9) J mol-I K-' were obtained. For 3 and 4 the radical stabilization energies RSE(3) = 67 (k7) kJ mo1-l (relative to isopropyl) and RSE(4) = 64 (k8) kJ mol-1 (relative to tertbutyl) were determined by a comparison with the activation parameters of the thermolysis of alkanes possessing the same strain enthalpy. The heats of formation for 3 and 4 and the C-H bond dissociation energies for the C-H bond in the 9-position of 1 and 2 were determined to be AH,O(g,3) = 300 kJ mol-l, AH,O(g,4) = 268 kJ mol-', BDE,-,(l) = 343 and BDEC-,(2) = 338 kJ mol-' (estimated errors correspond to those of the RSE values). A good correlation between the heats of vaporization of 37 aromatic hydrocarbons and their calculated solvent-accessible surfaces was demonstrated. Hence, a new method to estimate heats of vaporization was established.In the course of our investigation ['T2] of the thermal dissociation of c -C bonds into alkyl radicals according to eq.(1) we found that the enthalpies of activation AH' are linearly dependent on the release of strain enthalpy D, [Ds = H, (R-R) -2fZy(R.)] in the course of the dissociation process on the one hand and on the stabilization enthalpies of the radicals being generated, e.g. by resonance, on the other hand. In this way it became possible to determine radical stabilization energies RSE from kinetic and thermochemical dataI21. For the series cumyl (RSE = 35.2 kJ mol-'), 1,l-diphenylethyl (RSE = 46.0 kJ mol-'), and triphenylmethyl (RSE = 83.7 kJ m~l -' ) [~,~] it was found that the stabilizing effect of the second and third a-phenyl group is smaller than that of the first one. This was ascribed to the non-planarity of benzhydryl and triphenylmethyl radicals, re~pectively [~~~].(1)In order to test this hypothesis we investigated the thermochemistry of the hydrocarbons fluorene (l), 9-methylfluorene (2), 9,9'-bifluorenyl (5), and 9,9'-dimethyl-9,9'-bifluorenyl (6) and studied the thermolysis of 5 and 6 into 9-fluorenyl-type radicals which cannot deviate from planarity because of their rigid structures.From these data the heats of formation APf(g) and the radical stabilization enthalpies RSE of the radicals fluor- enyl (3) and 9-methylfluorenyl (4) were obtained and the C-H bond dissociation enthalpies in the 9-position of 1 and 2 were calculated and compared with results in the literature. Similar w...

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Thermolabile Kohlenwasserstoffe, 32. Konkurrierende Cope‐;Umlagerung und Homolyse von meso‐ und DL‐3,4‐Di(1‐cyclohexen‐1‐yl)‐2,2,5,5‐tetramethylhexan

Cited by 7 publications

References 34 publications

A Non-Cope among the Cope Rearrangements of 1,3,4,6-Tetraphenylhexa-1,5-dienes

A Non-Cope among the Cope Rearrangements of 1,3,4,6-Tetraphenylhexa-1,5-dienes

Rearrangements of Dienes and Polyenes

Thermolabile Hydrocarbons, 33. Thermochemistry and Thermal Decomposition of 9,9′‐Bifluorenyl and 9,9′‐Dimethyl‐9,9′‐bifluorenyl – The Stabilization Energy of 9‐Fluorenyl Radicals

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