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.
A juicy tale based on many fruitless efforts: The determination of the structure of the natural product santonin and its transformation product, santonic acid (see formulas), occupied organic chemists for nearly 80 years. The story of these efforts captures the early days in the practice of structure determination, and shows how difficult this exercise was before the availability of the powerful tools of IR and NMR spectroscopy, mass spectrometry, and the ultimate weapon, X‐ray crystallography.
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