Enthalpy and entropy of conjugative interaction in a nearly coplanar styrene and cinnamyl radical

Article abstract of DOI:10.1021/ja983088d

Conjugative interaction in styrenes designed to constrain the dihedral angle between the planes of the phenyl and olefinic groups near to coplanarity (21° by X-ray) is greater by 0.7 kcal mol^-1 than that in a styrene in which the phenyl group is unconstrained (ad libitum) (-35° by MM2 molecular mechanical calculation). Conjugative interaction in a cinnamyl radical similarly constrained to maintain the phenyl and allyl moieties nearly coplanar (15° by MM2) is greater by 1.1 kcal mol^-1 than in a cinnamyl radical in which the phenyl group is ad libitum. Efforts to separate the contribution of π-electron delocalization from the nonbonded steric factor by employing MM2 calculations proved quantitatively unsatisfactory.

Full text of DOI:10.1021/ja983088d

962
J. Am. Chem. Soc. 1999, 121, 962-968
Enthalpy and Entropy of Conjugative Interaction in a Nearly Coplanar
Styrene and Cinnamyl Radical
W. v. E. Doering,* Johannes Benkhoff, and Liming Shao
Contribution from the Department of Chemistry and Chemical Biology, HarVard UniVersity,
Cambridge, Massachusetts 02138-2902
ReceiVed August 27, 1998
Abstract: Conjugative interaction in styrenes designed to constrain the dihedral angle between the planes of
the phenyl and olefinic groups near to coplanarity (21° by X-ray) is greater by 0.7 kcal mol-1 than that in a
styrene in which the phenyl group is unconstrained (ad libitum) (-35° by MM2 molecular mechanical
calculation). Conjugative interaction in a cinnamyl radical similarly constrained to maintain the phenyl and
allyl moieties nearly coplanar (15° by MM2) is greater by 1.1 kcal mol-1 than in a cinnamyl radical in which
the phenyl group is ad libitum. Efforts to separate the contribution of π-electron delocalization from the
nonbonded steric factor by employing MM2 calculations proved quantitatively unsatisfactory.
Conjugative interaction between phenyl group and olefin in
Chart 1
styrenes has been found to weaken as steric factors cause an
increase in dihedral angle.1 A phenyl group free to assume
whatever dihedral (torsional) angle optimizes interaction (ad
libitum) varies in conjugative interaction vis-a`-vis alkyl from
2.5 kcal mol-1 in unhindered trans-â-substituted styrenes,
through 1.1 kcal mol-1 in cis-â-substituted styrenes to 0.0 kcal
mol-1 in an R-substituted styrenes. In this paper, an effort is
made to enforce near coplanarity by incorporating phenyl group
and double bond into a bicyclic system.
Conjugative interaction between a phenyl group and the
π-system of an allyl radical is closely related. How such
interaction in the cinnamyl radical compares with that of the
olefinic group in pentadienyl is of fundamental theoretical
interest. Here, too, elucidation of the change in the magnitude
of the interaction as the dihedral angle approaches zero or
coplanarity is the focus.
Results and Discussion
Coplanarity in Conjugative Interaction in Styrenes. To
explore the possibility of enforcing coplanarity in a styrene,
compounds of the type 6 and 8 shown in Chart 1 have been
selected. The three-atom system classically used for the estima-
tion of free energy of conjugation is represented in these
compounds by carbon atoms 4a, 4, and 3. Synthesis, although
uncomplicated, contains a delightful interplay of kinetic and
thermodynamic control. Following literature procedures,2,3
preparation begins with R-tetralone and proceeds through
compounds 1 and 2 to racemic 3 (Scheme 1). Reduction over
palladium on charcoal affords a mixture of four stereoisomers
(4), one of which, isolated in pure form, mp 112 °C, can be
oxidized with chromic acid to ketone 5 of mp 72 °C. Treatment
with phenylmagnesium bromide affords a pair of phenyl-
carbinols (not separated), which, on being heated with p-
toluenesulfonic acid, are dehydrated to a pair of olefins
consisting for the most part of a crystalline isomer, mp 116 °C.
Its structure has been determined by X-ray crystallographic
analysis to be trans-7 (Scheme 2).4 Acid-catalyzed equilibration
confirms an equilibrium consisting of 96% of trans-7 and 4%
of trans-6. That the integrity of the trans configuration of the
hexalin ring has not been compromised in this equilibration (not
to be expected on mechanistic grounds) is confirmed by
repeating the sequence starting from the mixture of carbinols
(1) Doering, W. v. E.; Benkhoff, J.; Carleton, P. S.; Pagnotta, M. J. Am.
Chem. Soc. 1997, 119, 10947-10955.
(2) Cainelli, G.; Panunzio, M.; Umani-Ronchi, A. Tetrahedron Lett. 1973,
2491-2494.
(4) Staples, R. J.; Doering, W. v. E.; Benkhoff, J. Z. Krystallogr. 1997,
212, 435-436.
(3) DeBoer, C. D. J. Org. Chem. 1974, 39, 2426-2327.
10.1021/ja983088d CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/22/1999

Enthalpy and Entropy of ConjugatiVe Interaction
J. Am. Chem. Soc., Vol. 121, No. 5, 1999 963
Table 1. Ratios among cis-8, cis-6, trans-8, and trans-6 at
Equilibrium at Various Temperatures Relative to cis-8
Scheme 1
T, °C
Kcis-8/trans-8
Kcis-8/cis-6
Kcis-8/trans-6
61.0
97.3
2.14
2.04
2.00
1.97
1.93
1.93
1.90
1.86
1.87
2.05
1.79
1.77
1.75
1.65
1.66
1.59
1.55
1.54
102.0
86.3
70.2
71.2
68.4
57.5
51.7
46.7
46.4
111.0
121.1
136.3
137.2
152.8
165.0
177.5
(∆SE) calculated by MM2 (see below) of -4.07 and -0.07 kcal
mol-1, respectively.
Entry into conjugated compounds of type 8 is made by
treating the readily accessible trans-7 with a strongly basic
system consisting of potassium tert-butoxide in hexameth-
ylphosphorictriamide (HMPT) scrupulously free of water and
tert-butyl alcohol. Quenching the deep purple solution with
water produces in high yield a crystalline material, mp 113 °C,
the structure of which is determined by X-ray crystallographic
analysis to be that of cis-8.5
That this catalytic system is too strongly basic to be suitable
for the determination of the temperature-dependence of the
equilibria in Chart 1 is demonstrated by quenching the purple
solution with deuterium oxide. The resulting samples of cis-8
and cis-6 contain 76 and 71% of deuterium, respectively, while
recoverd trans-7 contains less than 1% deuterium. These
observations invite the conclusions that 70-75% of cis-8 and
cis-6 is present as the stable, common anion, 6/8 (Scheme 2),
and that a more weakly basic catalytic system will not only
suffice to establish equilibria among the neutral isomers but
will not lead to kinetic contamination from neutralization of a
common anion. In practice, replacement of HMPT as solvent
by anhydrous tert-butyl alcohol meets the requirements. In other
respects, the experimental procedure is the same as that
previously described in detail.1
Equilibrium among the four isomers is established cleanly
without the formation of disturbing byproducts. Good fortune
has made quantitative discrimination among these isomers easy,
thanks to the sharp singlets of their bridgehead methyl groups,
the chemical shifts (δ) of which are noted in Scheme 1 and
Chart 1. Ratios of the most stable isomer, cis-8, to the other
three at various temperatures are given in Table 1. Note that
the analytical error associated with trans-6 is inevitably larger
but still of a magnitude compatible with useful conclusions. A
plot of the experimental data, given in Figure S1 of Supporting
Information, as the natural logarithm of the ratios of the
concentration of an individual isomer to the sum of all four
Scheme 2
isomers (in %) against the reciprocal of temperature (K-1
)
provides slopes and intercepts, “∆H” and “∆S”, relatable to
∆∆H and ∆∆S, each with its own standard deviation. These
values are recorded in Chart 1, relative to cis-8 as zero.
Among the four isomers, the styrene systems in cis-8 and
trans-8 are more nearly coplanar than the similarly substituted,
ad libitum styrenes, cis- and trans-6. From the X-ray crystal-
lographic analysis of its structure, the styrene system in cis-8
has a dihedral angle of 21°, further from coplanarity than hoped
for at the inception of this work. By comparison, the dihedral
angle in trans-7, alone among the four ad libitum styrenes to
have its structure determined by crystallographic analysis, is
-36°. For the rest, we find molecular mechanics, as developed
4. Along with trans-6 and trans-7, two additional compounds,
reasonably presumed both to have the cis configuration, can be
identified in the NMR spectrum. Parenthetically, the values of
∆∆G at 136 °C for the pairs, trans-6 to trans-7, and cis-6 to
cis-7, are -2.6 and +0.7 kcal mol-1, respectively. These values
compare with the corresponding differences in “steric energy”
(5) Staples, R. J.; Doering, W. v. E.; Benkhoff, J. Z. Krystallogr. 1997,
212, 433-434.

964 J. Am. Chem. Soc., Vol. 121, No. 5, 1999
Doering et al.
by Allinger6 and made available by Cambridge Software,7 a
satisfying way of resolving conformational questions. In its
application, painstaking care involving systematic variation of
initial conformations has been taken in an effort to avoid
entrapment in local minima.8 By MM2, not only is the styrene
dihedral angle in cis-8 reproduced (21°) but also is the entire
conformation. Similarly well reproduced is the dihedral angle
in trans-7 (-36°) and its conformation (global minimum). The
remaining MM2-calculated, styrene dihedral angles in the global
minima follow: trans-8, 21°; cis-7, 36°; cis-6, -35°; trans-6,
+39°.
Scheme 3
Empirically, the ∆1 isomers, cis-6 and trans-6, have a higher
enthalpy of formation than their corresponding ∆8a isomers, cis-8
and trans-8, by 0.74 and 1.82 kcal mol-1, respectively. Were it
not for steric factors, these values might be expected to be very
nearly equal. To estimate any differences in the contribution of
the π-electron component, correction for the steric component
is essential.
Attempts to make this correction by application of molecular
mechanics have not succeeded at the required level of accuracy.
The compounds in this work seem as favorable as can be
imagined. The pairs, cis- and trans-6 and cis- and trans-8, are
each in fact “isoBensonian”; that is, they are constituted of
identical sets of group equivalent values and therefore have
identical “Bensonian” heats of formation. The pairs, cis-6 and
cis-8 and trans-6 and trans-8, almost qualify! Their difference
is likely small but cannot be estimated by the Benson method
because of the lack of one (empirically based) group equivalent
value: [C-(H)2(C)2 ) -4.93] and [C-(Cd)(C)3 ) ?] for
compounds of type 8 and [C-(Cd)(C)(H)2 ) -4.76] and
[C-(C)4 ) +0.50] for compounds of type 6.
The steric energy of the global minimum calculated by MM2
for trans-6 (+8.98 kcal mol-1) is so much larger than that
calculated for trans-8 (+5.72 kcal mol-1) that any effort to
extract a quantitative estimate of π-electron delocalization
energy is problematic, particularly in the face of the large
experimental uncertainty in the relative enthalpy of formation
of trans-6. An uncritical comparing of the calculated difference
in steric energy of 3.26 kcal mol-1 to the experimental difference
of 1.82 kcal mol-1 leads to the conclusion that the difference in
π-electron delocalization amounts to -1.44 kcal mol-1 (1.82 -
3.26) in favor of the ad libitum styrene, trans-6. A similar
procedure applied to cis-6 and cis-8 leads to the conclusion that
the difference in π-electron delocalization between ad libitum
cis-6 and the more nearly coplanar cis-8 had disfavored cis-6
by +0.46 kcal mol-1! These conclusions can scarcely both be
true. As a cautionary note, had relative steric energies from
MM2 calculations been used as the basis for assignment of
structure, cis-6 and trans-8 would have been mistakenly
interchanged.
delocalization energy as dihedral angle is reduced from ∼35°
to 21°.9 This work, however it may reinforce the view expressed
earlier1 that conjugative interaction be a combination of at least
two factors, also illustrates once again the hazards inherent in
the time-honored strategy of introducing a structural perturbation
for the purpose of exploring a single mechanistic hypothesis
only to find that the strategem has led to additional perturbations
by inadequately anticipated factors as well!
Coplanarity in the Cinnamyl Radical. Interaction of phenyl
with the allyl radical (the cinnamyl radical) has been elucidated
through studies of thermal cis-trans rearrangements about a
carbon-carbon double bond in syn- and anti-bis-(3-phenyl-
cyclohex-2-enylidene) (10) by Teles10 (Scheme 3), and the
thermal rearrangements of 1-, 2-, and 3-phenyl- and 1,3- and
3,4-diphenylhexa-1,3-trans-5-trienes to the corresponding cy-
clohexadienes by Wiktor.11,12 In studies of this type, the rate-
determining transition state is assumed to be a 90°-twisted,
diradical-like double bond that is stabilized as completely as
possible by the energy of π-electron delocalization of a phenyl
group free to adopt, ad libitum, its energetically most favorable
dihedral angle. It is further assumed that any concomitant
reduction in enthalpy of strain in the educt will also contribute
to lowering the energy in the transition region. An example is
the comparison of syn- and anti-10 with the allyl system 11
(Scheme 3).13 The difference in their enthalpies of activation
of 5 kcal mol-1 is an indication that an ad libitum phenyl group
in the 1-position of a cyclohexene ring is 2.5 kcal mol-1 more
effective than an alkyl substituent in stabilizing an allyl radical.
Until either of two things come to pass, molecular mechanical
methods reach a level of thermochemical accuracy com-
mensurate with the needs of defining positions of equiilibrium
or quantum chemical mechanical methods attain the ability to
calculate enthalpies of formation of larger molecules with an
accuracy, for example, of (e0.3 kcal mol-1, the empirical
difference of 0.74 kcal mol-1 in favor of the more nearly planar
cis-8 may be a qualitative indication of a small increase in
(9) For the most part, the dihedral angles calculated by MM2 for the
global minima are quite soft: (12-14° corresponding to +0.2 kcal mol-1
in steric energy (SE).
(6) Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 8127-8134.
(7) Cambridge Scientific Computing, Inc., 875 Massachusetts Avenue,
Cambridge, MA 02139, as “MM2” modified in TINKER by Ponder, J. W.
in CSC Chem 3D Plus 3.5.1.
(8) For example, 16 local minima were located for cis-6, of which that
shown in Chart 1 having the lowest steric energy (5.0 kcal mol-1) is taken
to be the global minimum.
(10) 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.
(11) R. Wiktor in footnote 129, Scheme 17 and Table 39 of ref 12.
(12) Roth, W. R.; Staemmler, V.; Neumann, M.; Schmuck, C. Liebigs
Ann. 1995, 1061-1118.
(13) Doering, W. v. E.; Shi, Y.-q.; Zhao, D.-c. J. Am. Chem. Soc. 1992,
114, 10763-10766.

Enthalpy and Entropy of ConjugatiVe Interaction
J. Am. Chem. Soc., Vol. 121, No. 5, 1999 965
Table 2. Thermal Interconversion of anti- and syn-9 in
Toluene-d8: Equilibrium and Specific Rate Constants and Derived
Activation Parameters
atoms of the cyclohexenylidene ring and the o-hydrogens of
the phenyl group resists attainment of coplanarity and leads to
a dihedral angle between benzene ring and double bond
calculated by MM2 to be -38°. In 9, the hexahydronaphthalene
system opposes attainment of full coplanarity but nonetheless
leads to a smaller dihedral angle of 15°.
The fused benzene ring in 9 is quite similar to the comparable
olefinic group in 12 (dihedral angle of 2.5°) in ability to augment
enthalpy of stabilization. Both are identically positioned in the
same conformationally rigid bicyclic system and both contribute
∼3.5 kcal mol-1 to enthalpy of stabilization (cinnamyl and
(E,E)-pentadienyl radical, respectively). Similarly, phenyl (ben-
zyl ∼12 kcal mol-1) and vinyl (allyl ∼13 kcal mol-1) contribute
almost identically to the stabilization of the ethyl radical.16
Unfortunately, the trend in stabilization energy in passing from
benzyl to R-methylbenzyl to R,R-dimethylbenzyl has not been
elucidated in the literature.
T, °C
106 k1, s-1 a
K
144.5 ( 0.5
144.5 ( 0.5
144.5 ( 0.5
152.3 ( 0.3
152.3 ( 0.3
152.3 ( 0.3
164.7 ( 0.2
164.7 ( 0.2
164.7 ( 0.2
176.9 ( 0.2
176.9 ( 0.2
176.9 ( 0.2
184.3 ( 0.1
184.3 ( 0.1
184.3 ( 0.1
6.37 ( 0.07b
6.83 ( 0.06b
6.62 ( 0.07c
14.5 ( 0.2b
14.8 ( 0.2b
15.4 ( 0.2c
42.5 ( 0.9b
43.7 ( 0.7b
45.6 ( 1.5c
118 ( 2b
0.56
0.56
0.56
0.56
0.56
0.60
0.59
0.59
0.59
0.60
0.59
0.60
0.61
0.60
0.61
125 ( 1b
127 ( 1c
229 ( 3b
237 ( 1b
224 ( 1c
Cinnamyl stabilization has also been evaluated by Wiktor11
and Roth12 in a study of the thermal rearrangement of 1-phe-
nylhexa-1-trans-3,5-trienes to an equilibrium mixture of phe-
nylcyclohexadienes. Activation parameters determined between
245 and 309 °C are ∆Hq ) 36.9 ( 0.8 kcal mol-1 and ∆Sq )
-8.75 ( 1.5 e.u. Comparison with the enthalpy of activation
of hexatriene (∆Hq ) 43.2 ( 1.2 kcal mol-1, ∆Sq ) -2.7 (
2.2 e.u.,17 and ∆Hq ) 42.1 ( 0.5 kcal mol-1, ∆Sq ) -2.0 (
0.4 e.u.)18 leads to an extra stabilization energy of 6.3 ( 2.0
kcal mol-1, or 5.3 ( 1.3 kcal mol-1, respectively, values which
are greater than that of 3.6 ( 0.2 kcal mol-1 found in this work.
If an explanation for the lack of congruence does not lie in the
experimental uncertainties, we can offer no other.19
Arrhenius Parameters
Ea ) 33.64 ( 0.21 kcal mol-1
log A ) 12.43 ( 0.10
Eyring Parameters
∆Hq ) 32.77 ( 0.21 kcal mol-1
∆Sq ) -4.40 ( 0.48 cal mol-1 K-1 d
a Calculated by linear regression of the standard expression for
reversible first-order reactions. b Starting with anti-9. c Starting with
syn-9. d Calculated at 164.4 °C.
The styrene system selected to constrain the interacting phenyl
group to near coplanarity consists of anti- and syn-9 (Schemes
1 and 3). Close analogy to anti- and syn-12 permits a
straightforward comparison of phenyl and ethenyl. Preparation
involves reductive coupling of two molecules of 3 by the method
of McMurry, Mukaiyama, and Tyrlik. Applied to racemic 3,
four stereoisomers can be expected by analogy with prior
work,14 whereas applied to a single enantiomer no more than
two products can be formed. Accordingly, racemic 3 has been
resolved by chromatography on a chiral column. Coupling of
the (S) isomer produces a mixture of anti- and syn-(S,S)-9, which
has been separated by crystallization.
Rates of thermal equilibration of anti- and syn-9 in hexa-
deuteriobenzene are followed directly by NMR spectroscopy
thanks to a satisfactory separation of the resonances of the two
pairs of vinyl and ortho protons (noted in Scheme 3). The data
acquired at various times and temperatures are collected in the
Supporting Information. Specific rate constants are calculated
in the usual manner on the basis of the equation governing
reversible reactions of the first-order. In that calculation,
equilibrium constants are taken from experiment and are thereby
fixed rather than treated as a variable, even though statistically
a better fit would inevitably have resulted by allowing the
equilibrium constant to float. Specific rate and equilibrium
constants and derived activation parameters are collected in
Table 2.
Conclusions
The results of an attempt to evaluate the dependence of
conjugative interaction in styrenes on dihedral angle in the
direction of coplanarity1 is suggestive of a small increase in
stabilization energy (∼-0.4 kcal mol-1) in response to a
decrease in angle from ∼35° (cis-6) to ∼21° (cis-8) (Chart 1).
But reservations about the quantitative reliability of the conclu-
sion remain owing to a lack of confidence in the ability of
molecular mechanical estimations to correct for the steric
component at an apposite level of thermochemical accuracy.
The effect of dihedral angle on stabilization energy in a
cinnamyl radical is more secure. A similar decrease in dihedral
angle, from 38° to 15°, between a benzene ring and an allyl
radical leads to an increase in stabilization energy from -2.5
kcal mol-1 (10) to -3.6 kcal mol-1 (9), respectively, vis-a`-vis
allyl (11) (Scheme 3).
Experimental Section
1
General Procedures. H and 13C NMR (125.8 MHz) spectra are
measured in the noted solvents by a Bruker AM-500 instrument (500
MHz). Spin-lattice relaxation times (T1) are determined by the
inversion-recovery method with use of vacuum-sealed solutions in
toluene-d8. Chemical shifts are reported in ppm (δ) with respect to TMS;
coupling constants, J, are reported in hertz. Infrared spectra are
measured on a Nicolet Impact 400D FT-IR instrument and reported in
cm-1. UV-vis spectra are measured on a Varian Cary 1 E UV-visible
spectrophotometer and reported as λmax in nm (extinction coefficients
The more nearly coplanar benzene ring in 9 is more effective
(∼1.1 kcal mol-1 per phenyl group) at lowering the empirical
enthalpy of conjugative interaction than is the ad libitum phenyl
group in 10.15 In 10, opposition between the proximate hydrogen
(16) Ellison, G. B.; Davico, G. E.; Bierbaum, V. M.; DePuy, C. H. Int.
J. Mass Spectrosc. Ion Proc. 1996, 156, 109-131.
(17) Doering, W. von E.; Beasley, G. H. Tetrahedron 1973, 29, 2231-
2243.
(18) Orchard, S. W.; Thrush, B. A. J. Chem. Soc. Chem. Commun. 1973,
14.
(14) Doering, W. v. E.; Kitagawa, T. J. Am. Chem. Soc. 1991, 113,
4288-4297.
(15) The activation parameters reported in ref 10 for the equilbration of
anti- and syn-10 have been confirmed by repetition at 164.5 °C (k1 ) 7.78
( 0.5 s-6) and 184.3 °C (k1 ) 45.8 ( 0.9 s-6): Ea ) 35.6 kcal mol-1 and
log A ) 12.7; ∆Hq ) 34.7 kcal mol-1 and ∆Sq ) -3.3 e.u.
(19) Persistent concern about a possible difference between gas-phase
and liquid-phase remains unresolved.

966 J. Am. Chem. Soc., Vol. 121, No. 5, 1999
Doering et al.
as log ꢀ). Optical rotations are measured on a Perkin-Elmer Polarimeter
241. Enantiomeric excess (ee) is determined on a Hewlett-Packard 5890
Series 11 gas chromatograph with a Chiraldex GTA column: 20 m ×
0.25 mm i.d. × 0.125 mm film, Advanced Separation Technology, Inc.
High-resolution mass spectra (HR-MS) are measured on a JEOL AX
505 spectrometer equipped with a data-recovery system.
of (10aS)-(+)-3 (0.51 g, 2.4 mmol) in THF (10 mL) was added
dropwise, the reaction being continued at 0 °C for 1 h. The mixture
was diluted with 30 mL of pentane and 30 mL of ethyl ether and then
filtered through Celite. The organic layer was washed with aqueous
NH4Cl (2 × 20 mL), brine (2 × 20 mL), and dried over Na2SO4.
Removal of the solvent afforded, after flash chromatography on silica
gel (hexane), 0.49 g of a yellow oil, recrystallization of which from 1
mL of pentane gave 250 mg of (S,S)-(+)-anti-9. A second recrystal-
2-Methyl-1,2,3,4-tetrahydro-(2H)-naphthalen-1-one (1). This com-
pound was prepared by a procedure modified from that of Cainelli et
al.2 by Rae and Umbrasas.20 A mixture of iron pentacarbonyl (52.0
mL, 0.396 mol), potassium hydroxide (66.6 g, 1.19 mol), and ethanol
(500 mL) was stirred and heated at reflux for 2 h. A solution of 1,2,3,4-
dihydro-(2H)-naphthalen-1-one (R-tetralone, 38.4 g, 0.263 mol) and
37.2% aqueous formaldehyde (22.0 mL) was then added. The resulting
mixture was boiled under reflux for 20 h, poured into water, and
extracted with hexane. The hexane extract was washed with water and
dried over anhydrous Na2SO4. Two distillations of the extract gave
24.4 g (58%) of colorless liquid: bp 122-125 °C (10 Torr) [lit.3 134-
lization from ethyl acetate afforded 98 mg (42%) of yellow needles:
1
mp 209-210 °C; [R]20 +86° (c 1.12, CHCl3); H NMR (CDCl3)
589
7.80 (d, 2H, J ) 7.3), 7.30 (s, 2H), 7.14-6.97 (m, 6H), 2.90 (ddd, 2H,
J ) 6.1, 12.3, 17.1), 2.83 (dt, 2H, J ) 3.3, 14.7), 2.62 (ddd, 2H, J )
2.2, 5.0, 17.0), 2.48-2.40 (m, 2H), 1.65-1.48 (m, 8H), 1.00 (s, 6H);
13C NMR (CDCl3) 141.31, 136.24, 135.15, 130.95, 129.75, 127.21,
126.40, 124.70, 119.54, 38.62, 37.89, 33.52, 26.60, 22.49, 21.88; IR
2958, 2916, 2846, 1482, 1468, 1453, 1380, 1268, 1186, 1106, 885,
769, 758, 724; UV-vis 383 (4.780), 199 (4.719); HR-MS calcd for
C30H32 392.2504, found 392.2494.
1
138 °C (16 Torr)]; H NMR (CDCl3) 8.04 (d, 1H, J ) 7.6), 7.45 (t,
1H, J ) 7.5), 7.30 (t, 1H, J ) 7.5), 7.23 (d, 1H, J ) 7.7), 3.05 (ddd,
1 H, J ) 4.4, 11.1, 16.8), 2.97 (dt, 1H, J ) 4.4, 16.7), 2.59 (m, 1H),
2.20 (dq, 1H, J ) 4.4, 13.2), 1.89 (ddd, 1H, J ) 4.7, 12.1, 24.3), 1.27
(d, 3H, J ) 6.7); IR 2963, 2931, 2860, 1685, 1602, 1455, 1433, 1374,
1358, 1323, 1267, 1228, 968, 907, 739.
Concentration of the mother liquor to ca. 0.5 mL and storing at 10
°C overnight yielded 110 mg of a more soluble, crystalline product.
Recrystallization from pentane afforded 81 mg of (S,S)-(-)-syn-9 (17%)
as yellow crystals: mp 162-164 °C; [R]20 -4.4° (c 1.08, CHCl3);
589
1H NMR (CDCl3) 7.70 (d, 2H, J ) 7.6), 7.66 (s, 2H), 7.03-6.95 (m,
6H), 2.88 (ddd, 2H, J ) 6.0, 12.7, 17.0), 2.62 (ddd, 2H, J ) 2.0, 5.3,
17.1), 2.54 (ddd, 2H, J ) 2.9, 4.4, 15.8), 2.44 (dt, 2H, J ) 4.7, 14.9),
1.63-1.50 (m, 8H), 1.00 (s, 6H); 13C NMR (CDCl3) 140.66, 136.12,
135.12, 131.36, 129.59, 127.16, 126.58, 124.73, 117.97, 38.69, 38.07,
33.55, 26.63, 23.92, 21.83; IR 2958, 2916, 2846, 1482, 1468, 1453,
1380, 1268, 1186, 1106, 885, 769, 758, 724; UV-vis: 379 (4.732),
199 (4.712); HR-MS calcd for C30H32 392.2504, found 392.2513.
Distinction between anti-9 and syn-9 relied on nuclear Overhauser
enhancement: irradiation at 7.30 ppm (H-4 of anti-9) showed an
enhancement at 2.83 ppm (H-2 of anti-9) of 12%, while irradiation at
7.66 ppm (H-4 of syn-9) showed no enhancement at 2.62 ppm (H-2 of
syn-9).
Kinetics of the Anti-Syn lsomerization of (S,S)-10a,10a′-Di-
methyl-1,1′,9,9′,10,10′,10a,10a′-octahydro-3,3′-bis-(2H)-phenanthryli-
dene (9). A solution of 5.0 mg of crystalline (S,S)-(+)-anti-9 and ca.
5% mol 18-crown-6 ether as internal standard in 0.5 mL of toluene-d8
in an NMR tube (treated with 10% aqueous KOH for 20 h and washed
with distilled water, and analytical grade acetone) was degassed by
three freeze-pump-thaw cycles and sealed under vacuum (10-5 Torr).
An NMR reading at zero time was taken, and the sealed NMR tube
was heated in a constant temperature vapor bath for the designated
periods of time.
The temperature of the heating bath was measured with a J-type
thermocouple connected to a Leeds and Northrup Model 8686 millivolt
potentiometer using ice-water as a reference junction. Temperature
was recorded immediately before and immediately after each heating
period. However, during longer runs, temperature was recorded every
12-18 h without disturbing the kinetic experiment. The temperature
of each heating period was taken as the mean of the temperature
recorded before and after the kinetic run. The mean of the temperatures
of the individual intervals was considered the average temperature of
the kinetic experiment.
The boiling solvents used as the constant temperature vapor bath (a
convenient method which suffers from variations in atmospheric
pressure) in the various kinetic experiments were as follows: o-xylene
(143.9-145.5 °C), cumene (151.7-152.7 °C), mesitylene (164.4-164.9
°C), p-cymene (176.5-177.2 °C), and diethyl oxalate (184.2-184.5
°C).
(()-10a-Methyl-1,9,10,10a-tetrahydro-(2H)-phenanthren-3-one
(3). This compound was prepared by the method reported by DeBoer.3
A stream of nitrogen saturated with methyl vinyl ketone was bubbled
through a solution of 2-methyl-1,2,3,4-tetrahydro-(2H)-naphthalen-1-
one (1) (5.0 g, 31.2 mmol) and 1 mL of 1,5-diazabicyclo[4.3.0]non-
5-ene (DBN) in 15 mL of benzene. The reaction was monitored by
gas chromatography until >96% of 1 had reacted (ca. 2 h). The mixture
was diluted with 1% HCl and extracted with CH2Cl2. The organic layer
was washed with NaHCO3, dried over anhydrous Na2SO4, and
concentrated. Purification by flash chromatography (silica gel, elution
with hexane/ethyl acetate 6:1) afforded 5.5 g of Michael adduct 2: 1H
NMR (CDCl3) 8.02 (d, 1H, J ) 7.7), 7.46 (t, 1H, J ) 7.5), 7.30 (t, 1H,
J ) 7.5), 7.22 (d, 1H, J ) 7.7), 3.01 (m, 1H), 2.51 (ddd, 1H, J ) 5.7,
10.4, 17.4), 2.41 (ddd, 1H, J ) 5.7, 10.2, 17.4), 2.13 (s, 3H), 2.08
(ddd, 1H, J ) 5.2, 7.2, 13.6), 1.91 (m, 1H), 1.19 (s, 3H); IR 2962,
2929, 2857, 1715, 1679, 1600, 1454, 1355, 1223, 743.
Compound 2 was cyclized by boiling under reflux with 100 mL of
2% NaOH in 100 mL of 50:50 water/methanol for 2 h. The cooled
mixture (0 °C) was neutralized with NaHCO3. An extract with CH2Cl2
(3 × 60 mL) was washed with saturated NaCl (30 mL), dried (Na2-
SO4), and passed through a silica gel column (elution with hexane/
ethyl acetate, 4:1). Removal of solvent in vacuo gave 4.3 g of (()-3 as
a slightly yellow oil (65% overall): 1H NMR (CDCl3) 7.70 (d, 1H, J
) 8.0), 7.33 (t, 1H, J ) 7.4), 7.25 (t, 1H, J ) 7.8), 7.20 (d, 1H, J )
7.6), 6.51 (s, 1H), 3.09 (m, 1H), 2.89 (dt, 1H, J ) 3.7, 17.5), 2.67
(ddd, 1H, J ) 5.7, 14.6, 17.4), 2.47 (ddd, 1H, J ) 2.3, 4.6, 17.4), 1.98
(ddd, 1H, J ) 4.6, 13.4, 14.5), 1.91 (ddd, 1H, J ) 2.3, 4.6, 13.4), 1.80
(m, 2H), 1.21 (s, 3H); IR 2964, 2923, 2853, 1662, 1588, 1449, 1455,
1345, 1325, 1277, 1249, 1234, 1209, 782, 768.
Resolution of (()-3 was effected by HPLC on a Chiralcel OJ
semipreparative column (hexane/isopropyl alcohol, 95:5). The first
compound to separate was (10aR)-(-)-10a-methyl-1,9,10,10a-tetrahy-
dro-(2H)-phenanthren-3-one, (10aR)-(-)-3: 100% ee, [R]20589 -406°;
[R]20578 -432° (c 3.09, ethanol) (lit.21 [R]589 -436°, CHCl3). The second
was (10aS)-(+)-3: 100% ee, [R]20 +408°; [R]20 +433° (c 3.12,
589
578
ethanol) (lit.21 [R]589 +440°, CHCl3; (lit.22 [R]25 +428°, CHCl3).
578
10a,10a′-Dimethyl-1,1′,9,9′,10,10′,10a,10a′-octahydro-3,3′-bi-(2H)-
phenanthrylidene (9). This compound was prepared by the reaction
of McMurry-Mukaiyama-Tyrlik closely following a published pro-
cedure,13 in which TiCl4 (3.0 mL, 5.2 g, 27 mmol) was added with a
syringe to cooled (0 °C, ice water), freshly distilled THF (30 mL) under
nitrogen followed by activated Zn dust (3.5 g, 54 mmol) in portions.
After 10 min of stirring, 1 mL of dry pyridine was added by syringe.
The resulting mixture was stirred for an additional 10 min. A solution
Quantitative 1H NMR spectra were measured allowing an interpulse
time-delay (relaxation delay and saturation period) of at least five times
the longest spin lattice relaxation time of the concerned protons. NMR
analysis (500 MHz) was effected by using the H-4 signals at 7.30 ppm
of anti-9 and 7.66 ppm of syn-9. Recovery was determined from the
ratio of the sums of the areas of the H-4 peaks [7.30 ppm (anti-9) and
7.66 ppm (syn-9)] and the H-5 peaks [7.30 ppm (anti-9) and 7.66 ppm
(syn-9)] to the area of the internal standard, 18-crown-6 (3.50 ppm).
Results are given in Tables 1 and 2. Reaction rates and equilibrium
constants at each of the individual temperatures were optimized
(20) Rae, I. D.; Umbrasas, B. N. Aust. J. Chem. 1975, 28, 2669-2680.
(21) Meyer, A.; Hofer, O. J. Am. Chem. Soc. 1980, 102, 4410-4414.
(22) Touboul, E.; Brienne, M.-J.; Jacques, J. J. Chem. Res. 1977, (S)
106; (M) 1182-1190.

Enthalpy and Entropy of ConjugatiVe Interaction
J. Am. Chem. Soc., Vol. 121, No. 5, 1999 967
simultaneously by the nonlinear least-squares method to fit the reversible
first-order kinetic equation to the observed data. The activation
parameters were calculated from the observed rate and equilibrium
constants at the various temperatures.
7.03 (m, 2H), 6.94 (d, 1H), 6.15 (s, 1H), 4.20 (m, 1H), 2.76 (dq, 1H),
2.53 (dd, 1H), 1.86-1.81 (m, 2H), 1.59-1.51 (m, 1H), 1.43-1.32 (m,
3H), 1.23 (dt, 1H), 0.90 (s, 3H); 13C NMR (C6D6) 142.48, 136.26,
135.05, 129.84, 127.97, 126.82, 126.01, 125.64, 68.93, 38.63, 37.52,
33.70, 29.48, 26.72, 22.93; IR(KBr) 3410, 2964, 2935, 1455, 1052;
UV (hexanes) 294 (3.089), 286 (3.217), 256 (4.129), 216 (4.239); HR-
MS calcd for C15H18O 214.1358, found 214.1359; MS (70 eV) 214
(100), 196 (33), 181 (70), 157 (35), 144 (50). Minor isomer: 1H NMR
(C6D6) 7.49 (dd, 1H), 7.08 (m, 2H), 6.96 (d, 1H), 6.06 (d, 1H, J )
4.9), 4.12 (s, 1H), 2.77 (dq, 1H), 2.57 (dd, 1H), 1.74-1.70 (m, 2H),
1.70-1.61 (m, 1H), 1.52-1.41 (m, 2H), 1.28 (dt, 1H), 1.09 (s, 1H),
0.83 (s, 3H); 13C NMR (C6D6) 136.77, 128.78, 128.60, 128.40, 128.26,
126.76, 125.90, 122.60, 65.48, 38.09, 34.06, 33.80, 28.29, 26.55, 21.74.
1,2,3,4,4ar,9,10,10a-Octahydro-10aâ-methylphenanthrene-3-ol (4).
Three grams of Pd/C (1%) was added under argon to a stirred solution
of 10 g (47.1 mmol) of 3 in 30 mL of ethanol in a hydrogenation bomb.
After being sealed, the bomb was freed of remaining air/argon by
charging and releasing five times with hydrogen (20 bar). Finally, the
mixture was stirred for 18 h at room temperature under 20 bar of
hydrogen. The black suspension was filtered and washed thoroughly
with 250 mL of ethanol. Solvent was removed; 100 mL of hexanes
was added to the light yellowish oil, and the cloudy liquid was filtered
and concentrated in vacuo. Dissolving in 60 mL of hexanes and
crystallization at 0 °C afforded 9.0 g (41.6 mmol, 88%) of a mixture
of diastereomers of 4. Recrystallization of the colorless crystals at room
temperature from hexanes/ethyl acetate (slow evaporation of solvent)
yielded 6.6 g (30.6 mmol, 65%) of colorless needles of the major
isomer: mp 112 °C; Rf (hexane/ethyl acetate 5:1) 0.15; 1H NMR (C6D6)
7.20-6.95 (m, 4H), 3.44 (m, 1H), 2.75 (m, 1H), 2.62 (m, 1H), 2.34
(dm, 1H), 2.20 (dd, 1H), 1.71 (dm, 1H), 1.46 (m, 1H), 1.35-1.15 (m,
5H), 1.00 (td, 1H), 0.61 (s, 3H); 13C NMR (C6D6) 139.77, 136.69,
129.48, 129.02, 126.50, 126.40, 125.53, 72.16, 44.52, 39.65, 38.78,
34.83, 32.76; IR (KBr) 3381, 2934, 2908, 1492, 1050; UV (hexanes)
268 (2.662), 214 (3.948), 202 (4.237); HR-MS calcd for C15H20O
216.1514, found 216.1507; MS (70 eV) 216 (100), 198 (21), 157 (40),
129 (36), 115 (33).
1,9,10,10a-Tetrahydro-10a-methylphenanthrene. Commercial grade
SOCl2 (0.7 mL, 9.5 mmol) was added under argon within 5 min to a
stirred solution of anhydrous pyridine (0.8 g, 10.1 mmol) in 1 mL of
CHCl3 cooled in an ice/water bath. The pyridine/SOCl2 complex was
added under argon slowly with a syringe to a stirred solution of 1.7 g
(7.9 mmol) of the mixture of carbinols above at -10 to -5 °C in an
ice/NaCl/water bath. The yellow solution was stirred 30 min, diluted
with 50 mL of cold CHCl3, washed with cold saturated brine (3 × 50
mL), filtered, and concentrated in vacuo (bath 20 °C). The residual
yellow oil was dissolved in 2 mL of cyclohexane and purified by
chromatography on 20 g of silica gel (cyclohexane as eluent): first
fraction, 0.5 g (2.6 mmol, 33%) of diene as a slightly yellow oil; second
fraction (cyclohexane/ethyl acetate 10:1 as eluent), 0.7 g of an intensely
yellow, unidentified solid; third fraction (cyclohexane/ethyl acetate 5:1
as eluent), 0.18 g (0.8 mmol, 11%) of the reverse isomer of the starting
carbinol. The diene was further purified by vacuum distillation: Rf
1,4,4ar,9,10,10a-Hexahydro-10aâ-methylphenanthren-3-one (trans-
5). A solution of 6.9 g of CrO3 in 13 mL of H2O was mixed with 6 mL
of concentrated sulfuric acid.23 Addition of 1.5 mL of water provided
a bright orange solution, of which 7.5 mL was added slowly to a stirred
solution of 4.5 g (20.8 mmol) of trans-4, mp 112 °C, in 60 mL of
acetone cooled to 0 °C. After completion of the addition, stirring was
continued 1 h at 0 °C and 2 h at room temperature. The strongly acidic,
blue suspension was poured into 300 mL of saturated aqueous NaHCO3.
Extraction of the resulting mixture with ethyl ether (3 × 100 mL)
afforded a yellow organic layer, which was washed with brine (2 ×
100 mL) and dried over anhydrous sodium sulfate. Filtration and
concentration in vacuo afforded a yellow oil, which was crystallized
by the addition of hexanes to provide 3.8 g (17.6 mmol, 85%) of
1
(cyclohexane/ethyl acetate 5:1) 0.75; H NMR (C6D6) 7.65 (dd, 1H),
7.03 (m, 2H), 6.95 (d, 1H), 6.46 (d, 1H, J ) 5.6), 6.01 (m, 1H, J )
9.4), 5.68 (dq, 1H), 2.82 (dq, 1H), 2.48 (dq, 1H), 2.23 (dt, 1H), 1.88
(dd, 1H), 1.53 (dt, 1H), 1.42 (dq, 1H), 1.00 (s, 3H); 13C NMR (C6D6)
140.03, 137.03, 133.71, 130.24, 127.67, 126.91, 125.88, 125.68, 124.47,
116.25, 40.32, 37.67, 33.92, 27.39, 21.13; IR(film) 3036, 2919, 2851,
1560, 1485; UV (hexanes) 326 (4.068), 312 (4.035), 248 (3.773), 242
(3.849), 204 (4.209); GC-MS 196 (70), 181 (100), 165 (60).
1,4,4ar,9,10,10a-Hexahydro-10aâ-methyl-3-phenylphenan-
threne (trans-7). A 3 M solution of phenylmagnesium bromide (8 mL,
24 mmol) in ether was added over a 30-min period to a solution of
3.29 g (15.4 mmol) of ketone, trans-5, mp 72 °C, in 65 mL of anhydrous
THF at 0 °C. The mixture was stirred for 2 h at 0 °C and 3 h at room
temperature, quenched with 100 mL of aqueous NH4Cl, and extracted
with ether (3 × 100 mL). The combined organic layers were washed
with 100 mL of brine, filtered, and concentrated in vacuo to provide a
slightly yellow oil, which was mixed with 200 mL of toluene and 0.8
g of p-toluenesulfonic acid. After the solution was boiled under reflux
for 2 h and the evolved water was separated, the cooled, reddish brown
solution was poured into 100 mL of NaHCO3 and extracted with
hexanes (2 × 100 mL). The combined organic layers were washed
with 150 mL of brine, dried over potassium carbonate, filtered, and
concentrated in vacuo. The yellow oil was diluted with 1 mL of hexanes
and purified by chromatography on 50 g of silica gel with hexanes as
the eluent. Removal of the solvent left 3.7 g (13.5 mmol, 86%) of a
96:4 colorless mixture of phenylphenanthrenes trans-7 and trans-6,
crystallization of which from hexanes yielded 3.3 g (12.0 mmol, 78%)
1
colorless trans-5: mp 72 °C; Rf (hexane/ethyl acetate 5:1) 0.45; H
NMR (C6D6) 7.10-6.70 (m, 4H), 2.91 (m, 1H), 2.67 (m, 1H), 2.51
(m, 1H), 2.40 (dd, 1H), 2.20 (m, 1H), 2.10 (dd, 1H), 1.98 (t, 1H), 1.35-
1.00 (m, 4H), 0.51 (s, 3H); 13C NMR (C6D6) 208.63, 138.09, 135.58,
129.07, 126.27, 126.23, 125.13, 44.68, 41.10, 39.11, 37.66, 37.16, 31.86,
26.34, 14.66; IR(KBr) 2918, 1709, 1490; UV(hexanes) 266 (2.571),
256 (2.528), 214 (3.931), 202 (4.106); HR-MS calcd for C15H18O
214.1358, found 214.1359; MS (70 ev) 214 (100), 181 (35), 144 (50),
129 (40), 115 (38).
Starting with the mixture of both isomers of 4 led to an 88:12 mixture
of trans-5 and cis-5. Recrystallization (hexanes/ethyl acetate 5:1 at 0
°C) enriched the trans isomer in the mother liquor and concentrated
the cis isomer in the crystalline material. Therefore, pure trans-5 can
only be obtained from the mother liquors after complete removal of
cis-5.
1,2,3r,9,10,10a-Hexahydro-10aâ-methylphenanthren-3-ol and
1,2,3â,9,10,10a-Hexahydro-10aâ-methylphenanthren-3-ol. A solution
of 0.4 g (10.6 mmol) of NaBH4 in 2 mL of methanol was added to a
stirred solution of 3 (2.0 g, 9.4 mmol) in 15 mL of methanol. The
yellow solution warmed to 40-45 °C, became colorless within a few
minutes, and was stirred for an additional 30 min at room temperature.
After addition of 50 mL of water and extraction with ethyl ether (3 ×
50 mL), the combined organic solutions were washed with 50 mL of
saturated brine, filtered, and concentrated in vacuo to a slightly yellow
oil. Chromatography on 20 g of silica (cyclohexane/ethyl acetate 5:1)
afforded 1.7 g (7.9 mmol, 84%) of an 88:12 mixture of carbinols as a
colorless oil. Recrystallization from hexane/ethyl acetate provided 1.1
g (5.1 mmol, 54%) of the major isomer as colorless crystals: mp 127
1
of pure trans-7: mp 116 °C; H NMR (C6D6) 7.40 (d, 2H), 7.23 (m,
3H), 7.13 (m, 4H), 7.03 (m, 1H), 6.00 (m, 1H), 2.99 (dd, 1H, J )
16.8, 4.8), 2.82 (m, 1H, J ) 17.0, 12.1, 6.5), 2.66 (m, 1H, J ) 11.6,
5.3), 2.58 (m, 1H, J ) 17.1, 5.3), 2.31 (m, 1H, J ) 14.6, 13.9, 2.0),
1.92 (dm, 1H), 1.90 (dm, 1H, J ) 17.6, 5.3, 2.0), 1.41 (dm, 1H, J )
13.8, 6.5, 1.8), 1.41 (td, 1H, J ) 12.7, 12.7, 6.0), 0.76 (s, 3H); 13C
NMR (C6D6) 142.71, 139.28, 136.35, 136.22, 129.24, 128.53, 128.30,
127.92, 126.99, 126.46, 126.25, 125.95, 125.57, 123.36, 42.10, 41.86,
37.42, 30.79, 29.96, 26.52, 16.35; IR(KBr) 3081-2820, 1650, 1599,
1492, 749, 692; UV (hexanes) 248 (4.072), 214 (4.219); HR-MS calcd
for C21H22 274.1722, found 274.1721; GC-MS 274 (100), 259 (15),
157 (60), 144 (37), 129 (45).
1
°C; Rf (hexane/ethyl acetate, 5:1) 0.2; H NMR (C6D6) 7.59 (dd, 1H),
For X-ray crystallographic analysis, a single crystal was grown from
(23) Schlubach, H. H.; Repenning, K. Liebigs Ann. 1958, 614, 37-46.
hexanes at 0 °C.

968 J. Am. Chem. Soc., Vol. 121, No. 5, 1999
Doering et al.
1,2,3a,9,10,10a-Hexahydro-10aâ-methyl-3-phenylphenanthrene
(cis-8). A solution of 0.9 g (3.28 mmol) of trans-7 in 2 mL of
cyclohexane was added to 80 mL (8 mmol) of a 0.1 M solution of
potassium tert-butoxide in HMPT. The solution became dark violet
instantly. After being stirred under argon for 2 days at 55-60 °C, the
solution was quenched with 50 mL of water, treated with 50 mL of
brine, and extracted with hexanes (3 × 100 mL). The combined organic
layers were washed with 100 mL of brine, dried over sodium sulfate,
filtered, and concentrated in vacuo. The remaining yellow oil was
purified on 40 g of silica gel, hexanes as eluent, to provide 0.85 mg
(3.1 mmol, 95%) of a colorless oil consisting mainly of cis-8, which
could be crystallized from pentane at 0 °C to yield 0.71 g (2.59 mmol,
Note that any mixture of the isomers of the phenylphenanthrenes
can be converted to cis-8 by this procedure followed by crystallization
as described above.
Acknowledgment. We express our thanks to Professor E.
J. Corey for the use of his Chiralcel OJ semipreparative column
and to Professor Martin Saunders for having checked our MM2
calculations for local minima. This work has been supported
by the National Science Foundation, grant CHE 94-11248.
Support to the Department of Chemistry and Chemical Biology
of Harvard University for its Mass Spectrometry Facility comes
from the NSF (CHE 90-20043) and the NIH (S10-RR06716),
and for its NMR Facility from the NIH (S10-RR01748 and S10-
RR04870) and from the NSF (CHE 88 14019).
1
79%) of a single isomer: mp 113 °C; H NMR (C6D6) 7.44 (d, 1H),
7.21 (m, 4H), 7.14 (m, 1H), 7.06 (td, 1H), 7.00 (t, 2H), 6.14 (d, 1H, J
) 2.0), 3.39 (m, 1H, J ) 6.0, 11.0), 2.85 (m, 1H, J ) 16.6, 12.8, 6.0),
2.61 (dd, 1H, J ) 5.0), 1.85 (m, 1H), 1.71 (m, 1H), 1.49 (m, 4H), 1.01
(s, 3H); 13C NMR (C6D6) 147.11, 141.27, 135.42, 135.04, 129.36,
128.88, 128.29, 127.93, 127.44, 127.13, 126.50, 126.30, 125.36, 124.72,
44.76, 39.19, 38.22, 32.95, 29.34, 26.28, 23.05; IR (KBr) 3072-2866,
1634, 1600, 1488, 755, 703; UV (hexanes) 256 (4.273), 214 (4.387);
HR-MS calcd for C21H22 275.1800, found 275.1799; GC-MS 275 (100),
259 (2), 183 (1), 157 (4).
Supporting Information Available: Plot of the equilibrium
data among cis-8, cis-6, trans-8, and trans-6 over the temper-
ature range 61-178 °C, kinetic data of the thermal equilibration
of syn- and anti-10 at 164.5 and 184.3 °C in benzene-d6, and
kinetic data of the equilibration of syn- and anti-9. This material
is available free of charge via the Internet at http://pubs.acs.org.
For X-ray crystallographic analysis, a single crystal was grown from
a dilute solution in ethanol by slow evaporation at room temperature.
JA983088D