A Non-Cope among the Cope Rearrangements of 1,3,4,6-Tetraphenylhexa-1,5-dienes
详细信息 查看全文
- 作者：W. von E. Doering ; Ludmila Birladeanu ; Keshab Sarma ; G. Blaschke ; Ursula Scheidemantel ; R. Boese ; Jordi Benet-Bucholz ; F.-G. Klä ; rner ; Jan-Stephan Gehrke ; Bernd Ulrich Zimny ; R. Sustmann ; and Hans
- 刊名：Journal of the American Chemical Society
- 出版年：2000
- 出版时间：January 19, 2000
- 年：2000
- 卷：122
- 期：2
- 页码：193 - 203
- 全文大小：296K
- 年卷期：v.122,no.2(January 19, 2000)
- ISSN：1520-5126
文摘
The set of 1,3,4,6-tetraphenylhexa-1,5-dienes (1) represents a perturbation of Cope's rearrangementby 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 thermodynamicallyby a steric interaction of +2.6 kcal mol-1 per cis double bond, an equilibration that is stereochemically not ofany 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: 
H
=30.7 ± 0.2 kcal mol-1, S = +2.1 ± 0.4 cal mol-1 K-1, and V = +13.5 ± 0.1 cm-3 mol-1, respectively.Structures have been established by X-ray crystallographic analysis; a possible relationship between dihedralangle and bond lengths in the styrene portions is proposed. The entropy of activation is incompatible with achair 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 dissociationas the rate-determining step. Trapping and a crossover experiment give some but only partial support to theintermediacy of free radicals. At higher temperatures, however, electron spin resonance experiments demonstratean equilibrium with kinetically free (E,E)-1,3-diphenylallyl radicals. These observations are rationalized interms of geometric reorganization within the confines of a 'cage'. Resolution by chiral chromatography ofrac -(E ,E )-1 allows recognition of a fast racemization (40-65 C), of which H (21.3 ± 0.1 kcal mol-1),S (-13.2 ± 0.3 cal mol-1 K-1), and V (-7.4 ± 0.4 cm-3 mol-1) are consistent with a pericyclic Coperearrangement. Enriched (Z)-isomers undergo Cope rearrangements in accord with the known influence ofaxiality and the chair/boat alternative on the energy of the transition region.
C) with the following values of the enthalpy, entropy, and volume of activation: H =30.7 ± 0.2 kcal mol-1, S = +2.1 ± 0.4 cal mol-1 K-1, and V = +13.5 ± 0.1 cm-3 mol-1, respectively.Structures have been established by X-ray crystallographic analysis; a possible relationship between dihedralangle and bond lengths in the styrene portions is proposed. The entropy of activation is incompatible with achair 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 dissociationas the rate-determining step. Trapping and a crossover experiment give some but only partial support to theintermediacy of free radicals. At higher temperatures, however, electron spin resonance experiments demonstratean equilibrium with kinetically free (E,E)-1,3-diphenylallyl radicals. These observations are rationalized interms of geometric reorganization within the confines of a 'cage'. Resolution by chiral chromatography ofC), of which H (21.3 ± 0.1 kcal mol-1),S (-13.2 ± 0.3 cal mol-1 K-1), and V (-7.4 ± 0.4 cm-3 mol-1) are consistent with a pericyclic Coperearrangement. Enriched (Z)-isomers undergo Cope rearrangements in accord with the known influence ofaxiality and the chair/boat alternative on the energy of the transition region.