Synthesis and rotational barriers of atropisomers of 1,2-bis[5-(11H- benzo[b]fluorenyl)]benzenes and related compounds

Article abstract of DOI:10.1016/j.tet.2005.10.062

Several 1,2-bis[5-(11H-benzo[b]fluorenyl)]benzenes and related compounds were synthesized via a cascade reaction sequence of the corresponding benzannulated enyne-allene precursors. The X-ray structures showed that the two benzo[b]fluorenyl moieties attac

Full text of DOI:10.1016/j.tet.2005.10.062

Tetrahedron 62 (2006) 1231–1238
Synthesis and rotational barriers of atropisomers
of 1,2-bis[5-(11H-benzo[b]fluorenyl)]benzenes
and related compounds
†
Hua Yang, Jeffrey L. Petersen and Kung K. Wang
*
C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, WV 26506-6045, USA
Received 23 August 2005; revised 22 October 2005; accepted 24 October 2005
Available online 2 December 2005
Abstract—Several 1,2-bis[5-(11H-benzo[b]fluorenyl)]benzenes and related compounds were synthesized via a cascade reaction sequence of
the corresponding benzannulated enyne-allene precursors. The X-ray structures showed that the two benzo[b]fluorenyl moieties attached via
the C5 carbons to the adjacent carbon atoms of the central benzene ring are oriented essentially perpendicular to the central benzene ring. The
rates of rotation around the carbon–carbon single bonds attaching the benzo[b]fluorenyl moieties to the central benzene ring are relatively
slow, allowing several anti and syn atropisomers to be separated at ambient temperature.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Scheme 1 could lead to compounds bearing two
benzo[b]fluorenyl moieties. Bridged bis(benzofluorenyl)
zirconium dichloride complexes and related compounds
Benzannulated enyne-allenes bearing an aryl substituent
at the alkynyl terminus have been shown to undergo a
cascade sequence of reactions, including a biradical-
forming Schmittel cyclization reaction,1–4 to produce
11H-benzo[b]fluorenes under mild thermal conditions.5–13
For instance, we reported that condensation between 1
and 2 followed by reduction of the resulting propargylic
alcohol
3 with triethylsilane in the presence of
trifluoroacetic acid provided the benzannulated enediyne
4, which on treatment with potassium t-butoxide
in refluxing toluene produced 10-(1,1-dimethylethyl)-5-
phenyl-11H-benzo[b]fluorene (8) in excellent yield
(Scheme 1).7 Presumably, the transformation involves
an initial prototropic rearrangement of 4 to form in situ
the benzannulated enyne-allene
5 followed by a
Schmittel cyclization reaction1–4 to generate biradical 6.
A subsequent intramolecular radical–radical coupling
then gave 7, which in turn underwent a prototropic
rearrangement to regain aromaticity leading to 8. It
occurred to us that by starting from compounds bearing
two enediynyl units for condensation with 2 equiv of 2
and related ketones, the synthetic sequence outlined in
Keywords: 1,2-Bis[5-(11H-benzo[b]fluorenyl)]benzenes; Atropisomers;
Benzannulated enyne-allenes.
*
Corresponding author. Tel.: C1 304 293 3068x6441; fax: C1 304 293
4904; e-mail: kung.wang@mail.wvu.edu
† To whom correspondence concerning the X-ray structures should be
addressed. Tel.: C1 304 293 3435x6423; fax: C1 304 293 4904.
Scheme 1.
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.10.062

1232
H. Yang et al. / Tetrahedron 62 (2006) 1231–1238
1
have been shown to exhibit high activities in catalytic
ethylene and propylene polymerization.14 The use of
tetraacetylene 9 bearing two benzannulated enediynyl
units (Scheme 2) to start the synthetic sequence could
provide easy access to a variety of bis(benzofluorenyl)ben-
zenes and related compounds as ligands with diverse
structural features and a new bridging connectivity for
metal complex formation.
The H NMR spectrum of the reaction mixture of 12a and
12b exhibits two sets of AB quartets at d 4.03/3.86
(JZ21 Hz) and d 3.92/3.80 (JZ21 Hz) in an 8:1 ratio
attributable to the diastereotopic hydrogens on the five-
membered rings of 12a and 12b, respectively. A single
crystal of 12a suitable for X-ray structure analysis was
obtained (Fig. 1). The X-ray structure showed that the two
benzo[b]fluorenyl moieties are oriented essentially perpen-
dicular to the central benzene ring, making the geminal
hydrogens on the five membered rings chemically and
magnetically nonequivalent. It is also worth noting that the
AB quartet signals of the anti atropisomer 12a appear to
shift downfield from those of the syn atropisomer 12b.
Figure 1. Perspective view of the molecular structure of 12a with the
thermal ellipsoids scaled to enclose 30% probability.
Apparently, at ambient temperature the rate of rotation
around the carbon–carbon single bonds attaching the
benzo[b]fluorenyl substituents to the central benzene ring
is relative slow, allowing 12a to be isolated. However,
heating 12a in refluxing toluene (110 8C) for 5 h resulted in
equilibration between 12a and 12b, again producing an 8:1
mixture. In refluxing benzene (80 8C), the rate constant of
this kinetic process leading to equilibrium (12a:12bZ9.1:1)
was determined to be 2.1!10K5 sK1, corresponding to a
free energy of activation of 28.3 kcal/mol to convert 12a to
12b and a half-life of 0.91 h to reach equilibrium.
Scheme 2.
2. Results and discussion
The preference for the anti atropisomer 12a is in contrast to
an earlier report showing essentially no preference for the
anti atropisomer 13a over the syn atropisomer 13b
(13a/13bZ1.1:1) after the equilibrium is reached at
150 8C.16 Unlike 13b, the syn atropisomer 12b appears to
suffer from unfavorable nonbonded steric interactions
between the two outer benzene rings on the same side of the
five-membered rings, making it the less stable rotational
isomer. Starting from 13b, the rate constant for the
transformation to 13a was determined to be 4.7!10K5 sK1
at 130 8C, corresponding to a free energy of activation of
31.2 kcal/mol and a half-life of 2.0 h to reach equilibrium.16
This rotational barrier is considerably higher than that of 12a.
Tetraacetylene 9 bearing two benzannulated enediynyl units
was prepared by the Sonogashira reactions between 1,2-
diethynylbenzene and 2 equiv of 1-iodo-2-[(trimethylsilyl)-
ethynyl]benzene followed by desilylation as reported
previously.15 Condensation between 9 and 2 equiv of 2
followed by the reduction of the resulting propargylic diol
10 then afforded 11. Treatment of 11 with potassium
t-butoxide in refluxing toluene for 5 h produced the anti
atropisomer (racemic) 12a and the syn atropisomer (meso)
12b bearing two benzo[b]fluorenyl substituents attached via
the C5 carbons to the adjacent carbon atoms of the central
benzene ring.

Table 1. Synthesis of 1,2-bis[5-(11H-benzo[b]fluorenyl)]benzenes and related compounds 26–29
Aryl ketone Propargylic alcohol and enediyne Product
Aryl ketone
Propargylic alcohol and enediyne
Product

1234
H. Yang et al. / Tetrahedron 62 (2006) 1231–1238
30 bearing two units of the chlorinated enyne-allene moiety
(Scheme 3). The subsequent cyclization reactions also
occurred at ambient or sub-ambient temperatures leading to
31, which was readily hydrolyzed to yield diol 32 as a
mixture of the rotational isomers and the configurational
isomers due to the presence of two chiral centers on the five-
membered rings. Because the relative reaction rates of the
steps of the SNi0 reactions and the subsequent cyclization
reactions have not been determined, it is also possible that
the first unit of the benzannulated enediynyl propargylic
alcohol moiety could undergo an SNi0 reaction followed by a
Schmittel cyclization reaction, a radical–radical coupling
reaction, and a prototropic rearrangement before the second
unit would begin its cyclization sequence. Oxidation of 32
with manganese dioxide at ambient temperature then
afforded the anti atropisomer 33a in 18% yield and the
syn atropisomer 33b in 53% yield. The structures of 33a and
33b were established by X-ray structure analyses. As
observed in the case of 28, the syn atropisomer 33b is the
kinetic product. On heating 33b in refluxing benzene for
186 h, the equilibrium between 33a and 33b was essentially
established, producing the anti atropisomer 33a predomi-
nantly (anti:synZ2.2:1). The rate constant for the transform-
Similarly, condensation between 9 and aryl ketones 14–17
produced the corresponding propargylic alcohols 18–21 for
subsequent reduction with triethylsilane in the presence of
trifluoroacetic acid to form the benzannulated enediynes
22–25 (Table 1). Treatment of 22 with potassium t-butoxide
in refluxing toluene for 4 h then furnished 26 bearing two
indeno[2,1-b]phenanthryl moieties on the adjacent carbon
atoms of the central benzene ring. Only the anti atropisomer
was detected (anti:synO98:2). The structure of the anti
atropisomer was established by X-ray structure analysis. It
showed that the two indeno[2,1-b]phenanthryl moieties are
nonplanar, bending away from each other in order to
minimize nonbonded steric interactions. Compared to 12a
and 12b, the presence of two more fused benzene rings on
the indeno[2,1-b]phenanthryl moieties appears to cause the
syn atropisomer of 26 to suffer from even more severe
nonbonded steric interactions.
ation of 33b to 33a was determined to be 2.3!10K6 sK1
,
corresponding to a free energy of activation of 29.9 kcal/mol
and a half-life of 58 h to reach equilibrium.
Treatment of 23 with potassium t-butoxide in refluxing
toluene for 6 h produced 27 as a mixture of the anti and syn
atropisomers (anti:synZ12:1) similar to what was observed
in 12a and 12b. The assignment of the anti atropisomer as
the major isomer is based on the observation that a major set
of the AB quartet signals occurred at d 4.01/3.75, downfield
from a minor set of the AB quartet signals at d 3.86/3.58, in
good correlation with those of 12a and 12b.
Interestingly, treatment of 24 with potassium t-butoxide in
refluxing benzene (80 8C) for 5 h furnished 28 in favor of
the syn atropisomer (anti:synZ1:1.7) with the isomeric
assignment also based on the chemical shift correlations of
the AB quartet signals at d 3.92/3.70 and 3.66/3.33.
However, after 20 h in refluxing benzene the equilibrium
was reached, and the anti atropisomer became the
predominant rotational isomer (anti:synZ5:1). Clearly,
the majority of the syn atropisomer was formed as the
kinetic product initially, which was then converted to the
thermodynamically more stable anti atropisomer after
prolonged heating. The anti:syn ratio of 28 became 4.8:1
after it had been heated in refluxing toluene for 24 h.
Similar results were obtained when 25 was treated with
potassium t-butoxide in refluxing toluene for 4 h to give 29
as a mixture of the anti and syn atropisomers (anti:synZ
14:1). The isomeric assignment is again based on the
chemical shift correlations of the AB quartet signals at d
3.58/3.30 and 3.44/3.04. Compared to 12a and 12b,
replacing the two t-butyl substituents with two phenyl
substituents in 29 showed no significant effect on the ratio of
the rotational isomers.
As reported earlier,6,7,9,12 the use of thionyl chloride to
induce two SNi0 reactions of propargylic diol 21 at ambient
or sub-ambient temperatures presumably furnished in situ
Scheme 3.

H. Yang et al. / Tetrahedron 62 (2006) 1231–1238
1235
gave 36 as a mixture of the anti and syn atropisomers
(anti:synZ2:1). The isomeric assignment is again based on
the chemical shift correlations of the AB quartet signals at d
4.25/4.12 and 4.00/3.85.
A synthetic sequence leading to 41 without additional
substituents on the two benzo[b]fluorenyl units was also
developed (Scheme 5). Coupling between 37 and 2 equiv of
1-bromo-2-iodobenzene produced 38, which was converted
to 39 by bromo to iodo exchange. Two Sonogashira
reactions between 39 and 3-phenylpropyne then produced
tetraacetylene 40. Treatment of 40 with potassium t-but-
oxide in refluxing toluene for 5 h furnished 41 as a mixture
of the anti and syn atropisomers (anti:synZ9:1) with the
isomeric assignment again based on the chemical shift
correlations of the AB quartet signals at d 3.90/3.71 and
3.65/3.46.
3. Conclusion
New synthetic pathways to 1,2-bis[5-(11H-benzo[b]-
fluorenyl)]benzenes and related compounds were
developed. These compounds could exist either as the anti
atropisomers or the syn atropisomers at ambient temperature
because of relatively high rotational barriers. The anti
atropisomers are the thermodynamically more stable
rotational isomers, whereas in a few cases the thermo-
dynamically less stable syn atropisomers could be produced
as the kinetic products predominantly. The X-ray structures
of several representative examples showed that the two
benzo[b]fluorenyl moieties are oriented essentially
perpendicular to the central benzene ring.
Scheme 4.
Similarly, the use of 9-fluorenone for condensation with 9
produced 34, which was then treated with thionyl chloride to
furnish in situ dichloride 35 (Scheme 4). Reduction of 35
with tributyltin hydride in refluxing benzene for 14 h then
4. Experimental
4.1. General
All reactions were conducted in oven-dried (120 8C)
glassware under a nitrogen atmosphere. Diethyl ether and
tetrahydrofuran (THF) were distilled from benzophenone
ketyl prior to use. n-Butyllithium (2.5 M) in hexanes,
t-butyllithium (1.7 M) in pentane, Pd(PPh3)2Cl2, copper(I)
iodide, CuBr$SMe2, pivalophenone (2), triethylsilane,
trifluoroacetic acid, potassium t-butoxide (1.0 M) in
2-methyl-2-propanol, 2-naphthoyl chloride, a-tetralone,
1-indanone, benzophenone, 9-fluorenone, and 3-phenyl-
propyne were purchased from chemical suppliers and were
used as received. 1,2-Bis[(2-ethynylphenyl)ethynyl]benzene
(9) was prepared as reported previously.15 t-Butyl
2-naphthyl ketone (14) was prepared by treatment of
2-naphthoyl chloride with t-butylcopper, prepared from
t-butyllithium and CuBr$SMe2, in quantitative yield as
described previously for a similar aryl t-butyl ketone.8
1-Oxo-2,2-dimethyl-1,2,3,4-tetrahydronaphthalene (15)7
and 2,2-dimethyl-1-indanone (16)7 were prepared as
reported previously. The kinetic studies of the rates of
rotations of 12a and 33b were conducted by following the
progress of the equilibration processes with the integrations
of pertinent 1H NMR signals. The rate constants
were obtained by regarding the equilibration processes
as reversible first-order reactions. The temperatures of
Scheme 5.

1236
H. Yang et al. / Tetrahedron 62 (2006) 1231–1238
the refluxing benzene and toluene solutions were deter-
mined by inserting a thermometer into the solutions and
were found to be 80G0.5 and 110G0.5 8C, respectively.
72%, 12a:12bZ8:1) as a pale yellow solid. 12a: 1H
(600 MHz) d 8.06 (2H, d, JZ9.0 Hz), 7.90–7.82 (4H, m),
7.43 (2H, d, JZ8.4 Hz), 7.30 (2H, d, JZ7.8 Hz), 7.20 (2H,
t, JZ7.2 Hz), 7.11 (2H, t, JZ7.2 Hz), 6.91 (2H, t,
JZ7.5 Hz), 6.82 (2H, d, JZ7.2 Hz), 6.20 (2H, t,
JZ7.5 Hz), 4.03 (2H, d, JZ21 Hz), 3.86 (2H, d,
JZ20.4 Hz), 1.57 (18H, s); 13C d 143.9, 140.8, 140.2,
139.5, 138.3, 136.5, 133.2, 132.7, 131.6, 130.0, 128.7,
128.1, 126.4, 126.3, 125.7, 124.3, 123.5, 122.4, 121.6, 39.3,
38.2, 34.0; MS m/z 618 (MC), 561, 547; HRMS calcd for
C48H42 618.3287, found 618.3299. Recrystallization from a
mixture of hexanes and methylene chloride produced a
crystal suitable for X-ray structure analysis.
1
Melting points were uncorrected. H (270 MHz) and 13C
(67.9 MHz) NMR spectra were recorded in CDCl3 using
CHCl3 (1H d 7.26) and CDCl3 (13C d 77.0) as internal
standards unless otherwise indicated for those recorded on
a 600-MHz NMR spectrometer.
4.1.1. Propargylic alcohol 10. To 0.471 g (1.44 mmol) of 9
in 40 mL of anhydrous diethyl ether under a nitrogen
atmosphere at 0 8C was added 1.15 mL of a 2.5 M solution
of n-butyllithium (2.88 mmol) in hexanes. After 30 min of
stirring, a solution of 0.470 g of pivalophenone (2,
2.90 mmol) in 20 mL of diethyl ether was introduced via
cannula, and the reaction mixture was allowed to warm to
room temperature. After an additional 2 h, 50 mL of water
was introduced, and the reaction mixture was extracted with
diethyl ether. The combined organic extracts were washed
with brine and water, dried over sodium sulfate, and
concentrated. The residue was purified by flash column
chromatography (silica gel/20% diethyl ether in hexanes) to
provide 0.854 g (1.31 mmol, 91%, 1:1 mixture of two
diastereomers) of 10 as a yellow liquid: IR 3436, 2085,
A minor set of 1H NMR signals attributable to 12b were also
observed at d (partial, 600 MHz) 7.96 (2H, dd, JZ5.4,
3.6 Hz), 7.40 (2H, d, JZ8.4 Hz), 7.15 (2H, d, JZ7.8 Hz),
7.08 (2H, d, JZ7.2 Hz), 7.02 (2H, t, JZ7.5 Hz), 6.86 (2H, t,
JZ7.2 Hz), 6.77 (2H, t, JZ7.5 Hz), 6.55 (2H, t, JZ7.8 Hz),
3.92 (2H, d, JZ21.0 Hz), 3.80 (2H, d, JZ21.0 Hz), and
1.58 (18H, s).
4.1.4. Rotational barrier of the transformation from 12a
to 12b. A solution of 0.016 g of 12a in 10 mL of benzene
was heated under reflux. At the intervals of 10, 20, 30, 80,
140, 200 min, 8, and 30 h, 1-mL aliquot of the reaction
mixture was withdrawn and cooled to room temperature
immediately. Benzene was removed under reduced pres-
sure, and the residue was dissolved in 0.75 mL of CDCl3.
The progress of the equilibration process was determined by
integrations of the well separated 1H NMR signals
(600 MHz) of 12a at d 6.20 and 12b at d 6.55. After 8 h,
the equilibrium was reached, and the equilibrium constant
([12b]/[12a]) was determined to be 0.11. A linear plot of
ln([12a]eqK[12a]0/[12a]eqK[12a]) versus time for the first
five data points of this reversible first-order reaction was
obtained, and the sum of the rate constants (k1Ck2) was
1637 cmK1 1H (1:1 mixture) d 7.78–7.72 (4H, m),
;
7.58–7.49 (4H, m), 7.42–7.38 (2H, m), 7.32–7.20 (12H,
m), 2.50 and 2.47 (2H, two singlets, 1:1 ratio), 1.09 (18H, s);
13C (1:1 mixture) d 142.0, 132.4, 132.1, 131.7, 128.1, 128.0,
127.8, 127.2, 127.0, 125.7, 125.0, 96.3, 92.5, 92.0, 84.4,
79.5, 39.7, 25.5.
4.1.2. Benzannulated enediyne 11. To a mixture of 10
(0.379 g, 0.583 mmol) and triethylsilane (0.203 g,
1.75 mmol) in 15 mL of methylene chloride was added
0.35 mL of trifluoroacetic acid (0.54 g, 4.7 mmol). After
5 min of stirring at room temperature, 0.48 g of sodium
carbonate (4.6 mmol) was added followed by 10 mL of
water and 40 mL of diethyl ether. The organic layer was
separated, dried over sodium sulfate, and concentrated.
Purification of the residue by flash column chromatography
(silica gel/10% CH2Cl2 in hexanes) provided 0.321 g
(0.519 mmol, 89%, 1:1 mixture of two diastereomers)
of 11 as a yellow solid: mp 56–58 8C; IR 2226, 1490,
determined from the slope of the plot to be 2.1!10K4 sK1
.
From the equilibrium constant K (k1/k2Z0.11) and the
sum of the rate constants, the rate constant k1 for
the transformation from 12a to 12b was calculated to be
2.1!10K5 sK1
.
1
756 cmK1; H d 7.56–7.52 (2H, m), 7.48–7.38 (6H, m),
From the rate constant k1, the free energy of activation
(DG‡) was calculated to be 28.3 kcal/mol using the Erying
equation of DG‡Z4.57 (T) (10.32Clog T/k). The half-life
of 0.91 h to reach equilibrium was calculated from the
equation t1/2Z0.693/(k1Ck2).
7.36–7.31 (2H, m), 7.25–7.14 (12H, m), 3.659 and 3.655
(2H, two singlets), 1.032 and 1.029 (18H, two singlets); 13C
d 139.2, 132.4, 132.0, 131.8, 129.8, 127.9, 127.8, 127.5,
127.3, 126.6, 126.2, 125.9, 125.7, 95.7, 92.8, 91.7, 82.4,
50.6, 35.5, 27.8.
4.1.5. anti Atropisomer 33a and syn atropisomer 33b.
To 21 (0.083 g, 0.12 mmol) in 5 mL of THF at 0 8C was
added via cannula a solution of thionyl chloride (0.06 mL,
0.8 mmol) and anhydrous pyridine (0.14 mL) in 3 mL of
THF. The reaction mixture then was allowed to warm to
room temperature. After an additional 12 h, 10 mL of water
was introduced, and the reaction mixture was extracted with
30 mL of diethyl ether. The combined organic extracts were
washed with water, dried over magnesium sulfate, and
concentrated to furnish the crude dichloride 31. The crude
dichloride was stirred with silica gel in methylene chloride
for 2 h at ambient temperature. Silica gel was filtered and
methylene chloride was evaporated to produce the crude
diol 32. To 32 dissolved in 4 mL of toluene was added 0.2 g
4.1.3. anti and syn Atropisomers of 1,2-bis[5-[10-(1,1-
dimethylethyl)-11H-benzo[b]fluorenyl]]benzene (12a
and 12b). To 0.389 g of 11 (0.629 mmol) in 15 mL of
anhydrous toluene under a nitrogen atmosphere was added
1.26 mL of a 1.0 M solution of potassium t-butoxide
(1.26 mmol) in 2-methyl-2-propanol. The reaction mixture
was then heated under reflux for 5 h. After the reaction
mixture was allowed to cool to room temperature, 10 mL of
water and 40 mL of methylene chloride were introduced,
and the organic layer was separated, dried over sodium
sulfate, and concentrated. The residue was purified by flash
column chromatography (silica gel/10% methylene chloride
in hexanes) to provide 0.279 g of 12a and 12b (0.451 mmol,

H. Yang et al. / Tetrahedron 62 (2006) 1231–1238
1237
of manganese dioxide. The reaction mixture was stirred for
48 h before manganese dioxide was filtered. Toluene was
evaporated, and the residue was purified by flash column
chromatography (silica gel/50% CH2Cl2 in hexanes) to
provide 0.015 g (0.022 mmol, 18%) of 33a and 0.043 g
(0.063 mmol, 53%) of 33b as yellow solids. Compound 33a.
The reaction mixture was heated under reflux for 14 h.
After the reaction mixture was allowed to cool to room
temperature, 5 mL of a 10% aqueous potassium fluoride
solution was introduced. The reaction mixture was stirred
for an additional 2 h and then filtered. The organic layer was
separated, washed with water, dried over sodium sulfate,
and concentrated to give a brown solid residue. The residue
was purified by flash column chromatography (silica gel/
10% methylene chloride in hexanes) to afford 36 (0.043 g,
0.066 mmol, 51%, anti:synZ2:1) as a yellow solid: IR
3056, 2923, 1444 cmK1; 1H d 8.00–7.83 (4H, m), 7.76–7.48
(8H, m), 7.46–7.02 (12H, m), 6.67 (syn, 0.6H, t, JZ7.5 Hz),
6.53 (anti, 1.4H, dd, JZ8.4, 6.9 Hz), 4.25 (anti, 1.4H, d,
JZ22.5 Hz), 4.12 (anti, 1.4H, d, JZ22.3 Hz), 4.00 (syn,
0.6H, d, JZ22.3 Hz), 3.85 (syn, 0.6H, d, JZ21.8 Hz); 13C d
143.9, 143.0, 141.7, 140.8, 139.9, 138.6, 138.5, 137.7,
137.5, 135.5, 133.4, 133.2, 133.0, 130.7, 130.5, 128.8,
128.6, 127.9, 127.1, 126.9, 126.7, 126.5, 125.8, 125.7,
125.20, 125.14, 124.5, 124.3, 122.7, 121.15, 121.07, 119.3,
119.1, 35.0, 34.7; MS m/z 654 (MC), 538; HRMS calcd for
C52H30 654.2348, found 654.2354.
1
IR 1739, 1366, 731 cmK1; H d 8.01–7.92 (4H, m), 7.54
(2H, d, JZ6.7 Hz), 7.48–7.37 (10H, m), 7.31–7.20 (6H, m),
6.97–6.91 (4H, m), 6.83–6.79 (2H, m), 6.39 (2H, ddd,
JZ8.2, 6.9, 1.2 Hz); 13C d 191.8, 144.8, 140.2, 138.3, 136.5,
136.3, 135.5, 135.4, 134.1, 132.7, 132.4, 129.3, 129.2,
129.0, 128.7, 128.05, 127.96, 127.92, 127.8, 126.8, 126.6,
124.1, 123.9. Recrystallization from a mixture of methylene
chloride and ethanol produced a crystal suitable for X-ray
structure analysis. Compound 33b. IR 1739, 1366,
1
1217 cmK1; H d 8.09–8.04 (2H, m), 7.97–7.93 (2H, m),
7.47–7.40 (6H, m), 7.36 (2H, d, JZ8.4 Hz), 7.28–7.17 (6H,
m), 7.08–7.02 (4H, m), 6.94–6.85 (4H, m), 6.81–6.76 (4H,
m); 13C d 191.5, 144.1, 140.1, 138.1, 136.1, 135.73, 135.69,
135.6, 133.4, 132.9, 132.7, 131.7, 129.2, 129.1, 128.6,
128.5, 128.2, 127.7, 127.6, 127.13, 127.06, 126.0, 124.8,
123.4. Recrystallization from a mixture of methylene
chloride and ethanol produced a crystal suitable for X-ray
structure analysis.
Acknowledgements
4.1.6. Rotational barrier of the transformation from 33b
to 33a. A solution of 0.014 g of 33b in 10 mL of benzene
was heated under reflux. At the intervals of 17, 40, 57, 74,
87, 128, 146, and 168 h, 1 mL aliquot of the reaction
mixture was withdrawn and cooled to room temperature
immediately. Benzene was removed under reduced press-
ure, and the residue was dissolved in 0.75 mL of CDCl3.
The progress of the equilibration process was determined
K.K.W. thanks the Petroleum Research Fund (38169-AC1),
administered by the American Chemical Society, and the
National Science Foundation (CHE-0414063) for financial
support. J.L.P. acknowledges the support (CHE-9120098)
provided by the National Science Foundation for the
acquisition of a Siemens P4 X-ray diffractometer. The
financial support of the NSF-EPSCoR (1002165R) for
the purchase of a 600 MHz NMR spectrometer is gratefully
acknowledged.
1
by integrations of the H NMR signals (270 MHz) of 33a
at d 6.39 and the overlapping signals of 33a and 33b at d
8.09–7.92. After 186 h, the equilibrium was essentially
reached, and the equilibrium constant ([33a]/[33b]) was
determined to be 2.2. A linear plot of ln([33b]eqK[33b]0/
[33b]eqK[33b]) versus time for the first four data points of
this reversible first-order reaction was obtained, and the sum
of the rate constants was determined from the slope of the
plot to be 3.3!10K6 sK1. From the equilibrium constant K
and the sum of the rate constants, the rate constant for
the transformation from 33b to 33a was calculated to be
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tet.2005.10.
062. Experimental procedures and spectroscopic data for
18–29, 34, 38, 39, 40, and 41; 1H and 13C NMR spectra of
compounds 10, 11, 12a, 18–29, 33a,b, 34, 36, and 38–41;
ORTEP drawings of the crystal structures of 12a, 26, 33a,
and 33b; Crystallographic data for the structures in this
paper have been deposited with the Cambridge Crystallo-
graphic Data Centre. The CCDC nos. 281747, 281748,
281749, and 281750 have been assigned for the compounds
12a, 26, 33a, and 33b, respectively. Copies of the data can
be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [fax: C44 1223
336033 or e-mail: deposit@ccdc.cam.ac.uk].
2.3!10K6 sK1
.
From the rate constant, the free energy of activation (DG‡)
of the rotational barrier to transform 33b to 33a was
calculated to be 29.9 kcal/mol, corresponding to a half-life
of 58 h to reach equilibrium.
4.1.7. anti and syn Atropisomers of 36. To 34 (0.088 g,
0.13 mmol) in 5 mL of THF at 0 8C was added via cannula a
solution of thionyl chloride (0.035 mL, 0.48 mmol) and
anhydrous pyridine (0.08 mL) in 4 mL of THF. The reaction
mixture then was allowed to warm to room temperature.
After an additional 12 h, 15 mL of water was introduced,
and the reaction mixture was extracted with 30 mL of
diethyl ether. The combined organic extracts were washed
with water, dried over magnesium sulfate, and concentrated
to furnish the crude dichloride 35. To a flask containing
AIBN (0.008 g) were added a solution of the crude 35 in
5 mL of benzene and tributyltin hydride (0.1 mL).
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