Perturbation of cope's rearrangement: 1,3,5-triphenylhexa-1,5-diene. Chameleonic or centauric transition region?

Article abstract of DOI:10.1021/ja9908568

Two types of perturbations of Cope's rearrangement are distinguished by their occupancy of sets of four "active" and two "nodal" positions. A "chameleonic" model of a continuum of chair-like transition regions is defined as extending from two noninteracti

Full text of DOI:10.1021/ja9908568

10112
J. Am. Chem. Soc. 1999, 121, 10112-10118
Perturbation of Cope’s Rearrangement: 1,3,5-Triphenylhexa-l,5-diene.
Chameleonic or Centauric Transition Region?
W. v. E. Doering* and Yonghui Wang
Contribution from the Department of Chemistry and Chemical Biology, HarVard UniVersity,
Cambridge, Massachusetts 02138-2902
ReceiVed March 17, 1999
Abstract: Two types of perturbations of Cope’s rearrangement are distinguished by their occupancy of sets of
four “active” and two “nodal” positions. A “chameleonic” model of a continuum of chair-like transition regions
is defined as extending from two noninteracting allyl radicals at one extreme to cyclohexa-1,4-diyl diradical
at the other. Perturbations are analyzed quantitatively in terms of obligatory corrections for conjugative interaction
in the educt, and a model of the transition region that specifies transference of stabilization energies of the
perturbing substituents on allyl radicals if occupying active positions, and on secondary radicals if occupying
nodal positions. When this model is applied to the chameleonic 2,5- (nodal) and 1,4- (active) diphenylhexa-
l,5-dienes, good agreement with empirical lowering of enthalpies of activation per phenyl group of -8.7 and
-4.4 kcal mol-1, respectively, is obtained. In a perturbation of mixed type, 1,3,5-triphenylhexa-1,5-diene (1,3-
diphenyl-active; 5-phenyl-nodal), a novel question is addressed: Will the stronger of the two types alone
prevail (transition region remaining chameleonic), or will the stabilizing capacity of both be realized (centauric
domain)? The result is close to, but perhaps somewhat shy of, the full additivity expected of the centauric
model.
Introduction
Cope’s rearrangement of hexa-1,5-dienes1 is a major, quint-
essentially degenerate reaction, which leaves the seeker after
mechanistic insight little scope beyond the fundament of its
thermochemistry and response to perturbations. If chemistry is
the science of the transformation of one structure of matter into
another, its ultimate goal is the quantitative prediction of
positions of equilibrium and rates of reaction. The perturbational
approach seeks conceptual schemes that permit substitutions on
an archetype to be translated into changes in enthalpies and
entropies of formation of educt and product, and of the transition
region leading to rate-determining intermediates if multistep,
or directly to products if single step.2-4 The goal of the quantum
chemical, theoretical approach is the same, but its realization
involves de novo independent calculation of heats of atomization
of educt and transition structure, their difference then being the
enthalpy of activation predicted directly without reference to
the archetype, which plays no essential role. These consider-
ations are pictured in Figure 1.
Figure 1. Enthalpies of formation of the educts and transition regions
for archetypal hexa-1,5-diene and a perturbed derivative illustrate the
relations of conjugative interaction (∆Hci), differences in enthalpy of
activation (∆∆fHq), and their sum, stabilization energy in the transition
region (SEtr). A comparable depiction of the theoretically calculated
perturbed system is included at an arbitrary energy level.
(1) See reviews by (a) Rhoads, S. J. In Molecular Rearrangements; de
Mayo, P., Ed.; Wiley: New York, 1963; Vol. 1, pp 684-696; (b) Gajewski,
J. J. Hydrocarbon Thermal Rearrangements; Academic: New York, 1981;
pp 166-176.
Discussion
Mechanistic Hypotheses. Three models shown in Scheme
1 have been the historic focus of mechanistic attention and
continue to be “the usual suspects”. At one extreme, as an
alternative to the single-step, concerted mechanism, Cope
posited dissociation into a pair of noninteracting allyl radicals
followed by their random recombination, the homolytic-colli-
gative mechanism, which turned out to be inconsistent with a
cross-over experiment.5 At no subsequent time, during which
the empirical enthalpy of formation of the allyl radical fluctuated
widely, did this mechanism acquire thermochemical credibility.
(2) “Whereas it has been adequate to consider the mechanism, or the
transition state, for a reaction, as molecules become more complicated, the
bond-breaking process may need to be viewed as a family of individual
processes, the number of which increases with the complexity of the
empirical formula...”3 If “transition state” is reserved for individual “state-
to-state” transitions and “transition structure” for the “point” resulting from
theoretical computations, the entire family of closely related paths encoun-
tered in reactions of molecules of many internal degrees of freedom is
perhaps better differentiated by “transition region”, a term recently
introduced by Professor John E. Baldwin4 (see also Houk et al.13a).
(3) Doering, W. v. E.; Sachdev, K. J. Am. Chem. Soc. 1974, 96, 1168-
1187.
(4) Baldwin, J. E. J. Comput. Chem. 1998, 19, 222-231; Baldwin, J.
E.; Bonacorsi, S. J., Jr.; Burrell, R. C. J. Org. Chem. 1998, 63, 4721-
4725.
(5) Cope, A. C.; Hofmann, C. M.; Hardy, E. M. J. Am. Chem. Soc. 1941,
63, 1852-1857.
10.1021/ja9908568 CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/14/1999

Perturbation of Cope’s Rearrangement
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10113
Scheme 1
structure) to 2.19 Å, the spread in enthalpy is no more than ∼3
kcal mol-1 (at 0° K).14 The continuum, as originally formulated
in detail by Dewar and Wade,10 elaborated by Wehrli, Schmid,
Bellusˆ, and Hansen,15 and Gajewski and Conrad,16 is here
designated “chameleonic”. The core six-membered ring of the
transition structure in its chairlike conformation is characterized
by C2h symmetry and equality of the critical bond distances,
C-3-C-4 and C-1-C-6. Owing to its calculated flatness, this
depiction of the transition region offers little prospect of serving
usefully as a model for the perturbational approach. There are
no rules either for deciding where to hang a particular perturba-
tion along the continuum from 1.64 to 2.19 Å or beyond, or for
estimating the magnitude of a hypothetical optimum thermo-
chemical response. These remarks do not detract from the
notable successes achieved in calculating de novo the enthalpies
of activation of Cope rearrangements perturbed by nitrile and
vinyl groups.17,18
A conceptual scheme compatible with the calculated flatness
can be envisioned and evaluated quantitatively on the basis of
a division of radical-stabilizing perturbations according to their
location in hexa-1,5-diene. By analogy with their expected effect
on an allyl radical, an “active” category located at C-1, C-3,
C-4, or C-6 and an inactive, “nodal” category at C-2 or C-5
can be defined. Radical-stabilizing perturbations in active
positions may be expected to shift the transition region toward
the bis-allyl radical extreme, whereas in nodal positions they
become active by shifting the transition region toward the
cyclohexadiyl extreme. Testing the validity of this approach
begins with an examination in the chameleonic domain of
perturbations by the phenyl group in active and nodal locations.
Only three patterns of substitution satisfy the specification: 2,5-
diphenyl- (nodal), 1,4-diphenyl- (active),19 and 1,3,4,6-tetraphen-
ylhexa-1,5-diene (active) (see Figure 2).20
Dewar and Wade21 selected 2,5-diphenyl-3,3,4,4-d4-hexa-1,5-
diene as a candidate to bring about a change in mechanism
toward the cyclohexa-l,4-diyl biradicaloid. The rearrangement
occurred so much faster than the archetype (∆Hq ) 21.3 ( 0.2
kcal mol-1; ∆Sq ) -20.8 ( 0.5 cal mol-1 K-l; reproduced using
the 1,6-13C-labeled isotopomer)22 that this supposition was
considered verified. It remained to ask whether the full
stabilizing capacity of phenyl had been realized.
The currently best value for the enthalpy of formation of the
allyl radical, +39.9 ( 0.7 kcal mol-1,6 places two allyl radicals
26.1 kcal mol-1 above the experimental enthalpy of activation
of the archetype, hexa-1,5-diene: ∆fH0 ) +20.1 ( 0.2 kcal
mol-1;7 ∆Hq ) 33.5 ( 0.5 kcal mol-1; ∆Sq -13.8 ( 1.0 cal
mol-1 K-1 8
.
At the other extreme, another two-step model is imagined to
generate by covalent bond formation between C-1 and C-6 an
intermediate “cyclohexa-1,4-diyl” diradical, which by cleavage
either reverts to the original hexa-1,5-diene or proceeds to the
product of rearrangement (associative-dissociative mechanism).
This hypothetical “diradical” is strictly defined as cyclohexane
from which hydrons have been removed from the 1 and 4
positions at the cost of twice the difference in heats of formation
of propane (-25.0 kcal mol-1 7
) and isopropyl radical. A
coetaneous value, 16.7 kcal mol-1,9 for the latter pointed to an
estimated enthalpy of formation for cyclohexa-1,4-diyl (54.0
kcal mol-1) seductively close to the experimental enthalpy of
formation of the transition region (53.6 kcal mol-1) of the
archetypal Cope rearrangement. This model attracted extensive
attention, particularly in its theoretically more proper reformula-
tion by Dewar et al. as a biradicaloid.10 As the enthalpy of
formation of isopropyl radical gradually rose with time, finally
(12) Hrovat, D. A.; Morokuma, K.; Borden, W. T. J. Am. Chem. Soc.
1994, 116, 1072-1076.
(13) (a) Houk, K. N.; Li, Y.; Evanseck, J. D. Angew. Chem., Int. Ed.
Engl. 1992, 31, 682-708. (b) Wiest, O.; Black, K. A.; Houk, K. N. J. Am.
Chem. Soc. 1994, 116, 10336-10337. (c) Wiest, O.; Montiel, D. C.; Houk,
K. N. J. Phys. Chem. A 1997, 101, 8378-8388. (d) Black, K. A.; Wilsey,
S.; Houk, K. N. J. Am. Chem. Soc. 1998, 120, 5622-5627.
(14) Kozlowski, P. M.; Dupuis, M.; Davidson, E. R. J. Am. Chem. Soc.
1995, 117, 774-778; Davidson, E. R. J. Phys. Chem. 1996, 100, 6161-
6166.
to reach the value 21.3 kcal mol-1 11
, the estimated enthalpy of
formation of cyclohexa-1,4-diyl rose to 63.5 kcal mol-1, some
10 kcal mol-1 above the experimental transition region.
Phenyl in the “Chameleonic” Domain. “State-of-the-art”
theoretical calculations of the “aromatic” (concerted) transition
structure of the archetype define its geometry and enthalpy.12-14
All agree that the “potential energy surface ... is flat”. Over the
range from 1.64 Å (“diradicaloid” but far from “diradical”)
through a minimum at 1.85 Å (the “aromatic” transition
(15) Wehrli, R.; Schmid, H.; Bellusˆ, D.; Hansen, H.-J. HelV. Chim. Acta
1977, 60, 1325-1356.
(16) Gajewski, J. J.; Conrad, N. D. J. Am. Chem. Soc. 1979, 101, 6693-
6704.
(17) Hrovat, D. A.; Borden, W. T.; Vance, R. L.; Rondan, N. G.; Houk,
K. N.; Morokuma, K. J. Am. Chem. Soc. 1990, 112, 2018-2019.
(18) Beno, B. R.; Hrovat, D. A.; Lange, H.; Yoo, H.-y.; Houk, K. N.;
Borden, W. T. J. Am. Chem. Soc., in press.
(6) Roth, W. R.; Bauer, F.; Beitat, A.; Ebbrecht, T.; Wu¨stefeld, M. Chem.
Ber. 1991, 124, 1453-1460.
(7) Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of
Organic Compounds, 2nd ed.; Chapman and Hall: London, U.K., 1986.
(8) Doering, W. v. E.; Toscano, V. G.; Beasley, G. H. Tetrahedron 1971,
27, 5299-5306.
(19) Doering, W. v. E.; Birladeanu, L.; Sarma, K.; Teles, J. H.; Kla¨rner,
F.-G.; Gehrke, J.-S. J. Am. Chem. Soc. 1994, 116, 4289-4297.
(20) Doering, W. v. E.; Birladeanu, L.; Sarma, K.; Blaschke, G.;
Scheidemantel, U.; Boese, R.; Benet-Buchholz, J.; Kla¨rner, F.-G.; Gehrke,
J.-S.; Zimny, B. U.; Sustmann, R.; Korth, H.-G. J. Am. Chem. Soc. 1998,
submitted Aug. 10, 1998; rejected, Oct. 5, 1998.
(21) Dewar, M. J. S.; Wade, L. E., Jr. J. Am. Chem. Soc. 1977, 99, 4417-
4424.
(22) Roth, W.-R., Lennartz, H.-W.; Doering, W. v. E.; Birladeanu, L.;
Guyton, C. A.; Kitagawa, T. J. Am. Chem. Soc. 1990, 112, 1722-1732.
(9) Tsang, W. Int. J. Chem. Kinet. 1969, 1, 245-278.
(10) Dewar, M. J. S.; Ford, G. P.; McKee, M. L.; Rzepa, H. S.; Wade,
L. E., Jr. J. Am. Chem. Soc. 1977, 99, 5069-5073.
(11) (a) Seetula, J. A.; Russell, J. J.; Gutman, D. J. Am. Chem. Soc.
1990, 112, 1347-1353. (b) Seakins, P. W.; Pilling, M. J.; Niiranen, J. T.;
Gutman, D.; Krasnoperov, L. N. J. Phys. Chem. 1992, 96, 9847-9855.

10114 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
Doering et al.
89.8 ( 0.6 kcal mol-l, most recently favored by Ellison inter
alios,28 Denisov’s value is corrected to 85.4 kcal mol-1
.
Subtraction of the standard enthalpy of formation of hydron,
52.1 kcal mol-1, leads to ∆∆fHo ) +33.3 kcal mol-1 as the
change in enthalpy of formation in the transformation of
phenylcyclohexane to phenylcyclohexyl radical (and one half
dihydrogen). The difference between this value and +44.3 kcal
mol-1, ∆∆fHo for the transformation of isobutane into the tert-
butyl radical,7 is -11.0 kcal mol-1. Greater than the value of
-8.7 kcal mol-1 per phenyl group deduced above, it points to
substantial, but not full, realization of the stabilizing potential.
The validity of having selected the “aromatic” archetype as
model (33.5 kcal mol-1) is arguable. If the two phenyl groups
have indeed shifted the transition region close to the diradical
extreme of the chameleonic continuum, a more apppropriate
model is unstabilized 1,4-diphenylcyclohexa-1,4-diyl.29 From
an enthalpy of formation of 2,5-diphenylhexa-1,5-diene,30
addition of the experimental enthalpy of activation (21.3 kcal
mol-1) leads to an “experimental” enthalpy of formation of the
transition region of +87.9 kcal mol-1. This value is lower by
-22.2 kcal mol-1 or -11.1 kcal mol-l per phenyl group than
the enthalpy of formation of 110.1 kcal mol-1 estimated for
the unstabilized diradical.29 The phenyl groups have stabilized
the transition region quantitatively as if it were simulating
cyclohexa-1,4-diyl (overview in Supporting Information, Figure
S5).
Figure 2. 1,4-Diphenyl- and 2,5-diphenyl-hexa-1,5-diene are perturbed
symmetrically in active (“a”) and nodal (“n”) positions, respectively.
Activation parameters are reported: ∆Hq in kcal mol-1 and ∆Sq in cal
mol-1 K-1. Discrepancies from the archetype (33.5 kcal mol-1) are
given uncorrected (uncorr. ∆∆Hq), and corrected (corr. ∆∆Hq) for the
lowering in enthalpy of formation of the educt by conjugative
interaction.
To the extent that a chair model of 1,4-diphenylcyclohexa-
1,4-diyl is consistent with experiment, conformations in the
bicyclo[2.2.0]hexane domain of the Cope potential energy
surface are also viable (see Scheme 1).31 Although Goldstein
and Benzon in their elegant investigation of the archetype
rejected the intervention of twist-boat-cyclohexa-l,4-diyl32 in
stereochemical inversion and cleavage to hexa-1,5-diene on the
basis of the erroneously low enthalpy of formation of the
isopropyl radical, it is now a serious contender, the more so in
light of recent theoretical results of Baumann and Voellniger-
Borel.33 The proposition is developed at greater length in
Supporting Information, Appendix S1, Figures S3, S4, and S5.
Although the phenyl group may still be too large for
successful calculations at the level required to handle hexa-1,5-
diene, Beno et al. have made theoretical calculations on the
The answer requires quantitatiVe examination of the enthalpy
of activation. Analysis begins obligatorily with correction for
conjugation in the educt. Reliable values for conjugative
interactions of phenyl (and alkyl) Vis-a`-Vis hydron, inter alia
in four distinct types of environment, are reproduced in Figure
S1 of Supporting Information.23,24 With phenyl, values range
from -5.1 kcal mol-1 for â-alkyl substituted to -2.6 kcal mol-1
for R-alkyl substituted styrenes, the type involved in this
example.25,26 Were conjugation the only effect of perturbations
in the 2 and 5 positions, the predicted enthalpy of activation
would be raised to 38.7 (33.5 + 2 × 2.6) kcal mol-1 (∆∆Hq
o
+ ∆Hci in Figure 1). The experimental value, 21.3 kcal mol-1
,
is lower by -17.4 kcal mol-1 or -8.7 kcal mol-1 per phenyl
group. As large as this stabilization may seem, does it reflect
the full stabilization that a phenyl group is able to provide a
simple radical?
(28) Ellison, G. B.; Davico, G. E.; Bierbaum, V. M.; DePuy, C. H. Int.
J. Mass Spectrosc. 1996, 156, 109-131.
Although there is an extensive literature on the stabilization
energy in a benzyl radical, there is no direct evaluation of the
bond dissociation enthalpy (BDE) of the tertiary benzylic CH
bond. An indirect estimate is available from Denisov’s collection
and analysis of the rates of all the radical abstractions from
hydrocarbons in the literature.27 In particular, he has deduced a
BDE for phenylcyclohexane of 83.6 kcal mol-1 relative to that
(29) An estimated ∆fHo ) +21.5 kcal mol-1 for 1,4-diphenylcyclohexane
is derived from cyclohexane (-29.49 kcal mol-1) and phenylcyclohexane
(-3.99 kcal mol-1): ∆∆fHo ) +25.50 kcal mol-1.7 Removal of two hydrons
using the currently “best” value for the tertiary CH bond in isobutane (∆∆fHo
) +44.3 kcal mol-1) leads to an enthalpy of formation of +110.1 kcal
mol-1 (Supporting Information, Figure S5).
(30) Although the heat of formation of 2,5-diphenylhexa-1,5-diene is
reported to be +68.0 kcal mol-1 on the basis of its heat of hydrogenation
(∆HH2 56.65 kcal mol-1), reservations stem from finding ∆HH2 ) 55.65
kcal mol-1 for its homolog, 2,6diphenylhepta-1,6-diene.22 An altenative
value is derived from 2,5-diphenylhexane: calculated ∆fHo ) +11.88 kcal
mol-1 [n-hexane, -39.94 kcal mol-1]; ∆∆fHo ) +25.91 kcal mol-1
(isopropylbenzene, +0.96 kcal mol-1/propane, -25.02 kcal mol-1; 2-phe-
nylbutane, -4.16 kcal mol-1/n-butane; -30.00 kcal mol-1).7 Addition of
twice the appropriate heat of dehydrogenation, 27.34 kcal mol-1, estimated
from van’t Hoff studies, generates ∆fHo for 2,5-diphenylhexa-1,5-diene of
+66.6 kcal mol-1. By the same method, ∆fHo ) +7.0 kcal mol-1 is
calculated for 2,6-diphenylheptane and +61.7 kcal mol-1 for 2,6-diphen-
ylhepta-1,6-diene.
(31) Roth, W. R.; Kla¨rner, F.-G.; Lennartz, H.-W. Chem. Ber. 1980, 113,
1818-1829.
(32) (a) Goldstein, M. J.; Benzon, M. S. J. Am. Chem. Soc. 1972, 94,
5119-5121; (b) Goldstein, M. J.; Benzon, M. S. J. Am. Chem. Soc. 1972,
94, 7147-7149; (c) Goldstein, M. J.; Benzon, M. S. J. Am. Chem. Soc.
1972, 94, 7149-7151; (d) Benzon, M. S., Ph.D. Dissertation, Diss. Abstr.
Intern. B 1972, Vol. 45, p 1468B (Order-No. 72-18547).
of toluene as standard, taken to be ∆H298 ) 88.0 kcal mol-1
.
This value being 1.8 kcal mol-1 lower than the value for toluene,
(23) In the publication,24 heats of formation of a few of the relevant
phenyl hydrocarbons were based redundantly on a mean of values that
included those few experimental values noted and those calculated by the
method of group equivalents, which, of course, had been based on the same
data but gave somewhat different results. Recalculations that omit the latter
values have led to a few quite minor modifications.
(24) Doering, W. v. E.; Benkhoff, J.; Carleton, P. S.; Pagnotta, M. J.
Am. Chem. Soc. 1997, 119, 10947-10955.
(25) Note the discrepancy, for which we offer no explanation, of 0.6
kcal mol-1 between the heats of hydrogenation of styrene predicted from
the equilibrium data (-27.52) (Figure S1 of Supporting Information) and
determined by combustion (-28.20)7 and hydrogenation (-28.01).26
(26) Abboud, J.-L.; Jime´nez, P.; Roux, M. V.; Turrio´n, C.; Lopez-
Mardomingo, C.; Podosenin, A.; Rogers, D. W.; Liebman, J. F. J. Phys.
Org. Chem. 1995, 8, 15-25.
(27) Denisov, E. T. Russ. J. Phys. Chem. (Engl. Transl.) 1993, 67, 2416-
2422.
(33) Baumann, H.; Voellinger-Borel, A. HelV. Chim. Acta 1997, 80,
2112-2122.

Perturbation of Cope’s Rearrangement
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10115
Scheme 2
rearrangement of apposite vinylhexa-1,5-dienes. Conventionally
estimated thermochemical properties are outlined in Figure S6
of Supporting Information. On the entirely reasonable assump-
tion that the vinyl group is a good simulacrum of the phenyl
group,18 the theoretically predicted enthalpy of activation of
-20.9 kcal mol-1 agrees well with the experimental value for
2,5-diphenylhexa-1,5-diene, 21.3 kcal mol-1. A value of 24.4
kcal mol-1, reported by Roth et al.34 for the rearrangement of
3-ethylidene-5-methyl-6-methylene-octa-1,7-diene, after cor-
rection for the single element of methyl conjugative interaction
in the educt (2.7 kcal mol-1), is also in excellent agreement
with that predicted by Beno et al.18
Substitution in active positions in the chameleonic domain
is exemplified by l,4-diphenylhexa-1,5-diene (∆Hq ) 29.9 +
0.2 kcal mol-1; ∆Sq ) -15.0 ( 0.8 cal mol-1 K-1).19 After
the single correction for conjugative interaction of 5.1 kcal
mol-1, the enthalpy of activation is lower than that of the
archetype (29.9 - 33.5 - 5.1 ) -8.7 kcal mol-1) by -4.4
kcal mol-1 per phenyl group (Figure 2). This stabilization
corresponds satisfactorily to three values reported for cinnamyl
radical relative to allyl radical: -3.6,35 -5.3,36,37 and -5.4 kcal
Figure 3. Three examples in the centauric domain involve the pairs
in rows l and 2, 3, and 4, and 5 and 6. The lowering in enthalpy of
activation on replacing an allyl moiety by a 2-phenylallyl moiety is
mol-1 38
Perturbation in active positions like those in nodal
.
positions appear to have behaved “as if” full stabilization had
been realized.
interpolated as ∆∆Hq. Deviations from the archetype (-33.5 kcal mol-1
)
are given uncorrected (uncorr. ∆∆Hq) and corrected (corr. ∆∆Hq) for
conjugative interaction in the educt. In the lower part, the results of
theoretical calculations on vinyl analogs by Beno et al. are recorded.
Another, perhaps flawed, example is that of rac-3,4-diphe-
nylhexa-1,5-diene, which is afflicted with a large thermochemi-
cal bias of -10.2 kcal mol-1 in favor of the product. Studied
by Koch,39 Lutz and Berg,40 and Kla¨rner et al.,41 the lowering
in enthalpy of activation corresponds to -4.7 per phenyl group
(∆Hq ) 24.0 ( 0.2 kcal mol-1; ∆Sq ) -12.4 ( 0.6 cal mol-1
K-1) in excellent agreement with the lowering seen in 1,4-
diphenylhexa-1,5-diene.
Phenyl in the Centauric Domain. In the centauric domain,
two sets of substituents are brought into conflict: one set in
“active” positions draws toward the allylic transition region,
while a second in the “nodal” position draws toward the
cyclohexadiyl region (Scheme 2).42 Is the transition region
obliged to retain chameleonic symmetry with the possible
∆Hq values are in kcal mol-1; ∆Sq values are in kcal mol-1 K-1
.
consequence that the stronger of the perturbations alone will
prevail? Or will the symmetry change to “centauric” so that
one half functions as if it were an allylic radical, while the other
functions as if it were a tertiary aliphatic radical? Perhaps a
compromise is reached that denies each competing perturbation
from realizing its full stabilizing potential. Resolution comes
in the form of quantitatiVe distinctions.
In the literature, the example of 2,4-diphenylhexa-1,5-diene
was designed by Dewar and Wade to test the hypothesis of
addivity. Based on full use of the stabilization energies of -4.4
kcal mol-1 per “a” phenyl and -8.7 kcal mol-1 per “n” phenyl
(Figure 2), restriction to the chameleonic domain leads to a
predicted enthalpy of activation of 27.4 kcal mol-1 (33.5 + 2.6
- 8.7 kcal mol-1). In the centauric domain, a minimum enthalpy
of activation of 23.0 kcal mo1-1 (33.5 + 2.6 - 4.4 - 8.7 kcal
mol-1) is predicted. Based on the values from 2- and 3-phe-
nylhexa-1,5-dienes in Figure 3, another predicted value is 23.9
kcal mol-1 (33.5 + 2.6 - 5.4 - 6.8 kcal mol-1). Both are in
satisfactory agreement with experiment (∆Hq) 24.6 + 0.8 kcal
mol-1) and support the centauric model, but not the chameleonic
(34) Roth, W. R.; Wollweber, D.; Offerhaus, R.; Rekowski, V.; Lennartz,
H.-W.; Sustmann, R.; Mu¨ller, W. Chem. Ber. 1993, 126, 2701-2715.
(35) Doering, W. v. E.; Benkhoff, J.; Shao, L.-m. J. Am. Chem. Soc.
1999, 121, 962-968.
(36) R. Wiktor in footnote 129, Scheme 17 and Table 39 of ref 37. See
also ref 34.
(37) Roth, W. R.; Staemmler, V.; Neumann, M.; Schmuck, C. Liebigs
Ann. 1995, 1061-1118. See especially pp 1095, 1100, 1101; Table 48, p
1104.
(38) Saltiel, J.; Crowder, J. M.; Wang, S. J. Am. Chem. Soc. 1999, 121,
895-902.
(39) Koch, H. P. J. Chem. Soc. 1948, 1111-1117.
(40) Lutz, R. P.; Berg, H. A. J. Org. Chem. 1980, 45, 3915-3916.
(41) Diedrich, M. K.; Hochstrate, D.; Kla¨rner, F.-G.; Zimny, B. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1079-1081.
(42) Centauric, half man, half horse, is intended as a mnemonically keen
sobriquet. A referee suggests a happy alternative: minotauric.

10116 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
Doering et al.
Table 2. Specific Rate Constants and Derived Activation
Table 1. Specific Rate Constants and Derived Activation
Parameters for the Reversible Rearrangement of
1,3-Diphenylhexa-1,5-diene-6-13C in Benzene-d6
Parameters for the Reversible Rearrangement of
1,3,5-Triphenylhexa-1,5-diene-6-13C in Benzene-d6
T, °C
k1 × 106 s-1 a
T, °C
k1 × 106 s-1 a
154.0 ( 0.3
165.0 ( 0.5
176.8 ( 0.2
185.2 ( 0.1
196.0 ( 0.1
1.51 ( 0.01
3.72 ( 0.04
9.60 ( 0.09
19.07 ( 0.11
40.37 ( 0.34
121.2 ( 0.1
132.0 ( 0.1
144.6 ( 0.1
152.1 ( 0.1
164.8 ( 0.1
1.73 ( 0.01
4.65 ( 0.06
13.33 ( 0.10
24.61 ( 0.11
64.81 ( 0.31
Arrhenius parameters
Eyring parametersb
Ea ) 31.36 ( 0.28 kcal mol-1
log A ) 10.22 ( 0.14
Arrhenius parameters
Eyring parametersb
Ea ) 28.49 ( 0.09 kcal mol-1
log A ) 10.03 ( 0.09
∆Hq ) 30.5 ( 0.3 kcal mol-1
∆Sq ) -14.6 ( 0.6 kcal mol-1 K-1
∆Hq ) 27.7 ( 0.09 kcal mol-1
∆Sq ) -15.4 ( 0.3 kcal mol-1 K-1
a Calculated by linear regression of the standard expression for
a Calculated by linear regression of the standard expression for
reversible first-order reactions. b Calculated at 175 °C.
reversible first-order reactions. b Calculated at 143 °C.
model. Although the superiority of the centauric model seems
clear, there is a small shortfall from full centauric expectation
that may be real or little more than a reflection of experimental
uncertainties. If real, it is important for theory and for the use
of the model for predictive purposes.
27.7 ( 0.1 kcal mol-1 (∆Sq ) -15.4 ( 0.3 cal mol-1 K-l), is
lower by -13.5 kcal mol-1. This stabilization is substantially
greater than that expected of either the 1,3-diphenyl moiety
(-8.1 kcal mol-1) or the 5-phenyl moiety (-6.8 kcal mol-1
)
working alone in a C2h chameleonic domain (Figure 2). Here,
too, there is a possibly significant shortfall from the full centauric
expectation of their sum, -14.9 kcal mol-1, based on the two
stabilizing groups working optimally and independently.44
In a new test of the centauric model presented here,
1,3-diphenylhexa-1,5-diene and 1,3,5- triphenylhexa-1,5-diene
are compared. Both are degenerate and therefore uncompro-
mised by thermochemical bias. For kinetic examination, 1,3-
diphenylhexa-1,5-diene is labeled at C-6 with 13C. The resulting
specific rate constants and derived activation parameters are
given in Table 1 (175.0 + 21.0 °C). The enthalpy of activation,
∆Hq ) 30.5 ( 0.3 kcal mol-1 (∆Sq ) -14.6 ( 0.9 cal mol-1
K-1),43 after correction by -5.1 kcal mol-1 for conjugative
interaction in the educt, reveals a lowering of -8.1 kcal mol-1
by the 1,3-diphenylallyl group (rows 1 and 5 of Figure 3). This
lowering compares with that of -5.4 kcal mol-1 by the
3-phenylallyl group (row 3).
Because 1,3-diphenylhexa-1,5-diene is centauric and not a
model chameleon, 1,3,4,6-tetraphenylhexa-1,5-diene might be
considered a better model, but appears to present special
problems as a consequence of its closeness to the homolytic-
colligative regime. Racemization of the optically active (E,E)
isomer has activation parameters, ∆Hq ) 21.3 ( 0.1 kcal mol-1
and ∆Sq ) -13.2 ( 0.3 cal mol-1 K-l.20 After correction by
-10.2 kcal mol-1 for conjugative interaction in the educt, an
empirical lowering of -22.4 kcal mol-1 by the two 1,3-
diphenylallyl groups emerges. That the additional lowering by
the second 1,3 diphenyl group should have been so large as
-14.3 kcal mol-1 finds no conventional explanation in our
hands. Beno et al. have, however, uncovered a significant
cooperative augmentation in their comparison of 1,3-dicyano-
and l,3,4,6-tetracyano-hexa-1,5-diene. In their credible ration-
alization, the first set of perturbations moves the transition region
in the allylic direction and thereby allows the second concordant
set to be more effective than the first alone.
Recent theoretical calculations by Beno et al. on vinyl-, 1,3-
divinyl-, and 1,3,5-trivinyl-hexa-1,5-diene as reasonable sur-
rogates for the trio of phenylhexadienes predict enthalpies of
activation of 30.4, 29.9, and 30.4 kcal mol-1, respectively. Direct
comparison with the archetype (33.5 kcal mol-1) (uncorr. ∆∆Hq)
reveals that 1,3,5-trivinylhexa-1,5-diene is slightly destabilized
relative to the chameleonic model, -3.1 kcal mol-1 versus either
-3.1 kcal mol-1 (2-vinyl-) or -3.6 kcal mol-1 (1,3-divinyl),
and strongly destabilized vis-a`-vis the value predicted on the
basis of the centauric model (-6.7 kcal mol-1). A quite different
picture in gratifying agreement with the thesis of this work
emerges if credit is given for the enthalpy consumed in
overcoming the estimated conjugative interaction in the educts.45
The requisite enthalpies of conjugation of vinyl vis-a`-vis hydron
are estimated to be -6.5 kcal mol-1 for trans-piperylene and
-5.5 kcal mol-1 for isoprene, models for 1,3-divinyl- and
2-vinyl-hexa-1,5-diene, respectively (for details, see Figure S2
of Supporting Information). The corrected value of ∆∆Hq given
in the lower part of Figure 3 (-15.1 kcal mol-1 predicted) is
now closer to the centauric model (-18.7 kcal mol-1; chame-
leonic, -10.1 kcal mol-1). Comparison of experimental corr.
∆∆Hq for phenyl perturbations and theoretical for corresponding
vinyl compounds in Figure 3 is remarkably favorable, with the
exception of the experimentally questionable 2-phenylhexadiene.
Conclusions
A conceptual scheme for handling perturbations on Cope’s
rearrangement of hexa-1,5-diene recognizes two types: one
located at positions 1, 3, 4, or 6, the “active” positions, and a
second at 2 or 5, the “nodal” positions. Radical-stabilizing
substituents in the first locus seem to behave as if they draw
1,3,5-Triphenylhexa-l,5-diene incorporates the competing
effects of the doubly “active” 1,3-diphenyl group and the
“nodal” 5-phenyl group. Specific rate constants are determined
in benzene solution on a sample labeled with 13C at C-6 over
the temperature range, 143.0 ( 21.8 °C (Table 2). The system,
which is perturbed by two elements of conjugative interaction
(33.5 + 5.1 + 2.6 kcal mol-1), is predicted to have an enthalpy
of activation of 41.2 kcal mol-1, absent stabilization in the
transition region. The experimental enthalpy of activation, ∆Hq)
(44) Note that the values from the purely chameleonic models could be
used, whereupon the expected lowering from the 1,3-diphenyl portion, -11.2
kcal mol-1, and the 5-phenyl portion, -8.7 kcal mol-1, together would
have amounted to -19.9 kcal mol-1. Not only have we stated reservations
about the appropriateness of 1,3,4,6-tetraphenylhexa-1,5-diene as model,
but the cooperational augmentation indicated by theoretical calculations also
argues against its use.
(45) Note that replacement of allyl by 2-vinylallyl in 1,3-divinylhexa-
1,5-diene is destabilizing by +0.5 kcal mol-1 if uncorrected enthalpies of
activation are considered.
(43) Emrani, J., Ph.D. Dissertation, Indiana University, 1985 (Gajewski,
J. J., Research Sponsor); Diss. Abstr. Intern. 1985, Vol. 46, p 1922B (Order
No.: 85 16636) (∆Hq ) 29.0 ( 1.4 kcal mol-1; ∆Sq ) -16.3 ( 3.1 cal
mol-1 K-1, our calculation from his data).

Perturbation of Cope’s Rearrangement
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10117
the procedure of Wibaut et al.:47 1H NMR (300 MHz) 3.12 (d, J ) 7.0,
2H), 4.38 (m, 1H), 6.36 (m, 2H), 7.09 (m, 13 H), 7.76 (d, J )7.0, 2H).
A sample (0.53 g, 1.70 mmol) in 5 mL of THF was added dropwise
with stirring to a solution of methyltriphenylphosphonium ylid, prepared
from 0.88 g (2.48 mmol) of methyltriphenylphosphonium bromide and
0.99 mL of BuLi (2.5 M in hexane) in 20 mL of THF at 0 ˚C. After
having been stirred at 0 °C for 1.5 h, the reaction mixture was diluted
with 15 mL of THF and subjected to the standard work-up: 0.3 g (57%)
the transition region toward the extreme of a pair of allyl
radicals, even though that end of the continuum is the more
difficultly achievable for two reasons: first, because a total
stabilization of 26 kcal mol-1 is needed (quite apart from
enthalpy to counter whatever deceleration might have been
occasioned by conjugative interaction in the educt); and, second,
because perturbation in the active locus of an allyl radical is
considerably less effective than is perturbation of a tertiary
radical. That a perturbation of the first type should be
comparable in magnitude to its effect on a simple allyl radical
seems all the more remarkable. By contrast, perturbations in
the second locus have to overcome only 10 kcal mol-1 in order
to approach the other, cyclohexa-1,5-diyl diradical end of the
continuum. In this type also, the major part of the potential
stabilization of a tertiary alkyl radical seems to be realized.
Multiple perturbations are divided into two classes. One is
defined as the “chameleonic”, in which every perturbation
occupies the same type of locus, and the core of the chair-like
six-membered ring of the transition region retains C2h-like
symmetry. The other is defined as the “centauric” of C1h
symmetry, in which one half of the transition region is perturbed
at “active” positions, the other at a “nodal” position. In the
chameleonic domain, the two identical halves are pulled toward
either the bis-allylic or the cyclohexa-1,5-diyl extreme; in the
centauric domain, the two halves are free to be drawn in
opposing directions (Figure 3). The question, “How are the
conflicting demands resolved?”, is answered in favor of the
centauric. Each perturbation seems to contribute its full stabiliz-
ing potential independently, or close to it.
1
as a colorless viscous oil; H NMR 2.90 (d, J ) 7.6, 2H), 3.55 (dt, J
) 7.2, J ) 7.6, 1H), 5.04 (d, J ) 13.6, 2H), 6.21 (d; J ) 15.9, 1H),
6.27 (dd, J ) 7.2, J ) 15.9, 1H), 7.01-7.16 (m, 13H), 7.26 (d, J )
7.1, 2H); MS m/e 311 (M + 1, 5.6); 310 (M, 15.9), 251 (M - 59,
17.6), 219 (M - 91, 6.8), 193 (M - 117, 100), 178 (13.5), 115 (30);
HRMS calcd for (M+) 310.1722, found 310.1724.
(E)-1,3,5-Triphenyl-1,5-hexadiene-6-13C was prepared similarly using
methyl-13C-triphenylphosphonium iodide: 1H NMR 2.90 (d, J ) 7.6,
J ) 5.8, 2H), 3.55 (dt, J ) 7.2, J ) 7.6, 1H), 5.04 (d, J ) 133.7, J )
155.9, 2H), 6.21 (d, J ) 15.9, 1H), 6.27 (dd, J ) 7.2, J ) 15.9, 1H),
7.01-7.16 (m, 13H), 7.26 (d, J ) 7.1, 2H); MS m/e 311 (M, 9.8), 220
(M - 91, 6.4), 193 (M - 117, 100), 178 (18), 115 (36), 91 (7.4);
HRMS calcd for (M+) 311.1756, found 311.1742.
(E)-3,5-Diphenyl-4-pentenal. To a stirred solution of LDA, prepared
from 7 mL (50 mmol) of diisopropylamine and 20 mL (50 mmol) of
BuLi (2.5 M in hexane) in 40 mL of THF at -78 °C, was added a
solution of 9.7 g (50 mmol) of (E)-l,3-diphenylpropene48 in 50 mL of
THF, dropwise. The resulting red solution was added dropwise with
stirring at 78 °C for 2 h to a solution of 5.2 mL (50 mmol) of
2-bromomethyl-1,3-dioxotane at room temperature. The rate of addition
was controlled by observing the disappearance of the red color. After
completion of the addition, the reaction mixture was stirred for 1 h.
Standard work-up without flash chromatography afforded an oil, which
had an NMR spectrum consistent with the expected intermediate acetal.
Without further purification, this material was treated with 5% H2SO4
in refluxing dioxane for 1 h. Standard work-up (10:1 hexane/EtOAc
as the eluting solvent) afforded 9.44 g (80 %) of (E)-3,5-diphenyl-4-
pentenal: 1H NMR 2.36 (d, J ) 7.4, 2H), 3.81 (dt, J ) 7.4, J ) 7.1,
1H), 6.13 (dd, J ) 15.9, J ) 7.1, 1H), 6.26 (d, J ) 15.9, 1H), 7.03-
7.18 (m, 10H), 9.31 (s, 1H); MS 237 (M + 1, 12), 236 (M, 59), 208
(M - 28, 38), 193 (M - 43, 78), 178 (M - 58, 34), 115 (100), 91
(34); HRMS calcd for (M+) 236.1201, found 236.1195.
(E)-1,3-Diphenyl-1,5-hexadiene. A solution of the aldehyde above
(0.54 g, 2.3 mmol) in 5 mL of THF was added dropwise with stirring
to a solution of methyl triphenylphosphonium ylid, prepared from 0.89
g (2.5 mmol) of methyltriphenylphosphonium bromide and 10 mL of
BuLi (2.5 M in hexane) in 20 mL of THF at 0 °C. After 1 h at 0 °C,
standard work-up gave 0.3 g (57%) of (E)-1,3-diphenyl-1,5-hexadiene
as a colorless oil: 1H NMR 2.48 (dd, J ) 6.9, J ) 7.3, 2H), 3.39 (dt,
J ) 15.9, J ) 7.3, 1H), 5.00 (dd, J ) 21.1, J ) 17.2, 2H), 5.74 (m,
1H), 6.29 (dd, J ) 15.9, J ) 6.9, 1H), 6.35 (d, J ) 15.9, 1H), 7.04-
7.23 (m, 10H); MS 234 (M, 1.7), 193 (M - 41, 100), 178 (M - 56,
27), 165 (M - 69, 15), 115 (M - 119, 70), 91 (29), 78 (59); HRMS
calcd for (M+) 234.1409, found, 234.1416.
In similar fashion, (E)-1,3-diphenylhexa-1,5-diene-6-13C was pre-
pared using methyl-13C-triphenylphosphonium iodide: 1H NMR 2.48
(ddd, J ) 6.9, J ) 7.3, J ) 5.8, 2H), 3.39 (dt, J ) 15.9, J ) 7.3, 1H),
5.00 (ddd, J ) 155.1, J ) 22.7, J ) 17.1, 2H-), 5.74 (m, 1H), 6.29
(dd, J ) 15.9, J ) 6.9, 1H), 6.35 (d, J ) 15.9, 1H), 7.04-7.23 (m,
10H); MS 235 (M, 3.8), 194 (M - 41, 58), 193 (M - 42, 100), 178
(M - 57, 37), 165 (14), 115 (86), 91 (20); HRMS calcd for (M+)
235.1443, found 235.1441.
Kinetic Measurements. Kinetic studies were effected in degassed
C6D6 (0.6 mL) in vacuum-sealed NMR tubes with CHDCl2 as internal
standard. The concentration of the diene used in the thermal rearrange-
ments ranged from 0.034 M to 0.055 M. Heating was effected in the
vapors of appropriate liquids boiling under reflux. The chosen liquids
were distilled before use: undecane (196 °C), diethyl oxalate (185 °C),
4-methylanisole (176 °C), mesitylene (164 °C), anisole (154 °C),
cumene (152 °C), o-xylene (144 °C), chlorobenzene (132 °C), and
Reviewed oversimplistically, replacement of an allyl moiety
in hexa-1,5-diene, 3-phenyl-, and 3,5-diphenyl-hexa-l,5-diene
by competing moiety 2-phenylallyl, lowers the enthalpy of
activation vis-a`-vis the archetype by ∆∆Hq ) -4.2, -3.5, and
-2.8 kcal mol-1, respectively. Were this sequence a trend
toward less effective competition by the nodal half as the active
half becomes strongersone should caution that (1 kcal mol-1
may already be a generous assessment of accuraciess
substantiation by further experiment and theory would be
required prior to its incorporation into the conceptual scheme
for handling perturbations.
Experimental Section
General Procedures. 1H NMR and 13C NMR (125.8 MHz) spectra
were measured in benzene-d6 on a Bruker AM-500 instrument (500
MHz unless otherwise noted). Spin-lattice relaxation times (T1) were
determined by the inversion-recovery method with use of vacuum-sealed
solutions in benzene-d6. Chemical shifts are reported in parts per million
(δ); coupling constants, J, are reported in hertz. High-resolution mass
spectra (HRMS) were measured on a JEOL AX 505 spectrometer
equipped with a data-recovery system and reported as m/z (density as
percent of major peak). Melting points are uncorrected. Solvents were
redistilled before use: THF from sodium/benzophenone, benzene from
P2O5. A standard work-up consisted of quenching with saturated
aqueous NH4Cl, extraction first with diethyl ether, then with CH2Cl2,
drying the combined organic layers over anhydrous MgSO4, filtration,
concentration to an oil, and flash column chromatography (hexane as
eluting solvent).
(E)-1,3,5-Triphenyl-1,5-hexadiene. The starting material, (E,E)-
cinnamylideneacetophenone, was prepared from trans-cinnamaldehyde
and acetophenone following Scholtz:46 1H NMR (300 MHz) 6.43 (d, J
) 15.6, 1H), 6.63 (dd, J ) 15.7, J ) 11.0, 1H), 6.78 (d, J ) 14.9,
1H), 7.05 (m, 8H), 7.64 (dd, J ) 14.9, J ) 11.0, 1H), 7.92 (d, J )
10.3, 2H). It was converted to (E)-1,3,5-triphenyl-4-pentenone following
(47) Wibaut, J.; Overhoff, J.; Jonker, E. Recl. TraV. Chim. Pays-Bas 1943,
62, 31-45.
(48) Raunio, E.; Bonner, W. J. Org. Chem. 1966, 31, 396-399.
(46) Scholtz, M. Chem. Ber. 1895, 28, 1726-1733.

10118 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
Doering et al.
tetrachloroethane (121 °C). The tubes were periodically withdrawn from
the bath for analysis by 1H-NMR and returned for further heating.
Temperature was monitored by a thermocouple and read periodically
during heating. An average value was taken for each analysis of data.
For the kinetic measurements at higher temperatures, correction was
made for the time taken for the reintroduced tube to reach the reaction
temperature. This lag time (∼40 s) was determined beforehand by
introducing similar tubes containing a thermocouple.
Acknowledgment. This paper is dedicated with profound
gratitude to the memory of Michael James Steuart Dewar.
W.v.E.D. expresses his warm thanks to Professor John E.
Baldwin for calling attention to the work of E. T. Denisov and
to Professors Weston Thatcher Borden and Kenneth R. Houk
for their liberal and constructive help and sharing of information.
This work has been supported generously by the Norman Fund
in Organic Chemistry in memory of Ruth Alice Norman Weil
Halsband. Support to the Departments of Chemistry and
Chemical Biology of Harvard University for its NMR Facility
comes from the NIH (S10-RR01748 and S10-RR04870) and
from the NSF (CHE 88-14019).
1
Quantitative analysis by H NMR took advantage of 13C coupling
with vinyl hydrons at C-6 and methylene hydrons at C-4 in both starting
material and product. Recovery was monitored by comparing the
benzylic hydron at C-3 (3.4 ppm) with the singlet at 4.2 ppm in the
standard (CH2Cl2). Each datum was generated from at least three
integrations while the final percentage of starting material and product
was the average of the two sets of signals. Relaxation times were
determined for each group of hydrons. The protocol involved an
acquisition time of 2.77 s, relaxation delay times of 12 s for 1,3,5-
triphenylhexa-1,5-diene-6-13C and 19 s for 1,3-diphenylhexa-1,5-diene-
6-13C, and 32 scans for each measurement. Specific rate constants and
activation parameters are given in Tables 1 and 2, based on data in
Tables S1 and S2, respectively. Products of thermal rearrangement were
Supporting Information Available: Six figures, an ap-
pendix, and two tables give a collection of heats of hydrogena-
tion of styrenes and alkenes, a derivation of the thermochemistry
of 1- and 2-vinylhexa-l,5-dienes, a consideration of the possible
role of bicyclo[2.2.0]hexane in the rearrangement of 2,5-
diphenylhexa-l,5-diene, and kinetic data for Tables 1 and 2. This
material is available free of charge via the Internet at
http://www.pubs.acs.org.
1
identified by H NMR: 1,3,5-triphenylhexa-1,5-diene-4-13C 2.90 (dd,
J ) 7.6, J ) 128.4, 2H), 3.55 (m, 1H), 5.04 (dd, J ) 133.7, J ) 10.6,
2H), 6.21 (d, 15.9, 1H), 6.27 (dd, J ) 15.9, J ) 7.2, 1H), 7.01-7.16
(m, 13H), 7.26 (d, 2H). 1,3-Diphenylhexa-1,5-diene-4-13C: 2.48 (ddd,
J ) 127.7, J ) 6.9, J ) 7.3, 2H), 3.39 (m, 1H), 5.00 (m, 1H), 5.74 (m,
1H), 6.29 (dd, J ) 15.9, J ) 6.9, 1H), 6.35 (d, J ) 15.9, 1H), 7.04-
7.23 (m, 10H).
JA9908568