The surprising formation of structurally distorted ap-9-(o-tert-butylphenyl)-9-methylthiofluorene and its facile homolysis into sp-9-(o-tert-butylphenyl)-3-methylthiofluorene and sp-9-(o-tert-butylphenyl)fluorene

Article abstract of DOI:10.1139/v99-088

The continued study of rotationally restricted 9-(o-tert-butylphenyl)fluorenes has provided surprising results. Treatment of sp-9-(o-tert-butylphenyl)-9-fluorenol (1) with ethanol or methanol under acidic conditions affords sp-9-(o-tert-butylphenyl)-9-eth

Full text of DOI:10.1139/v99-088

960
The surprising formation of structurally
distorted ap-9-(o-tert-butylphenyl)-9-
methylthiofluorene and its facile homolysis into
sp-9-(o-tert-butylphenyl)-3-methylthiofluorene
and sp-9-(o-tert-butylphenyl)fluorene
Yuqing Hou and Cal Y. Meyers
Abstract: The continued study of rotationally restricted 9-(o-tert-butylphenyl)fluorenes has provided surprising results.
Treatment of sp-9-(o-tert-butylphenyl)-9-fluorenol (1) with ethanol or methanol under acidic conditions affords
sp-9-(o-tert-butylphenyl)-9-ethoxyfluorene (2b) and sp-9-(o-tert-butylphenyl)-9-methoxyfluorene (3b), respectively, but
similar treatment with methanethiol converts 1 into the rotamerically opposite ap-9-(o-tert-butylphenyl)-9-
methylthiofluorene (4a). While all three products reflect reaction with inversion at C-9, 2b and 3b reflect subsequent
rotation, which is not the case with 4a. X-ray diffraction shows 4a to be highly distorted and strained, but apparently
favored thermodynamically over its sp rotamer. Homolysis of 4a is observed at room temperature, and at elevated
temperatures accounts for the formation of sp-9-(o-tert-butylphenyl)fluorene (6b) and sp-9-(o-tert-butylphenyl)-3-
methylthiofluorene (7c) as major products. X-ray diffraction shows 6b and 7c to be virtually devoid of distortion.
Methylation of 6b via its anion also proceeds with inversion without rotation to form ap-9-(o-tert-butylphenyl)-9-
methylfluorene (4c). Dynamic NMR unexpectedly showed that in these ap configurations 9-CH3 (of 4c) has a greater
bulk effect than 9-CH3S (of 4a) in forcing the o-tert-butyl group into the fluorene plane.
Key words: fluorene, rotamer, distortion, homolysis, carbanion, carbocation, radical.
Résumé : La suite de nos études sur les 9-(o-tert-butylphényl)fluorènes à rotation empêchée a fourni des résultats
surprenants. Les réactions du sp-9-(o-tert-butylphényl)-9-fluorénol (1) avec l’éthanol et le méthanol dans des conditions
acides conduisent respectivement au sp-9-(o-tert-butylphényl)-9-éthoxyfluorène (2b) et sp-9-(o-tert-butylphényl)-9-
méthoxyfluorène (3b); toutefois, une réaction avec le méthanethiol transforme le composé 1 au rotamère opposé, le ap-
9-(o-tert-butylphényl)-9-méthylthiofluorène (4a). Même si les trois produits sont le reflet d’une inversion en C-9, les
produits 2b et 3b reflètent une rotation subséquente, ce qui n’est pas le cas avec 4a. La diffraction des rayons X
montre que le composé 4a est tendu et très déformé, mais qu’il est apparemment thermodynamiquement favorisé par
rapport à son rotamère sp. On observe de l’homolyse du produit 4a à la température ambiante et, à des températures
plus élevées, elle peut expliquer la formation des sp-9-(o-tert-butylphényl)fluorène (6b) et sp-9-(o-tert-butylphényl)-3-
méthylthiofluorène (7c) comme produits majeurs. La diffraction des rayons X montre que les composés 6b et 7c sont
pratiquement sans distorsion. La méthylation du produit 6b par le biais de son anion se produit avec inversion, sans
rotation, pour former le ap-9-(o-tert-butylphényl)-9-méthylfluorène (4c). La RMN dynamique a montré d’une façon
inattendue que, dans des configurations ap, le 9-CH3 (du composé 4c) présente un effet stérique plus important que
celui du 9-CH3S (du composé 4a) pour forcer le groupe o-tert-butyle dans le plan du fluorène.
Mots clés : fluorène, rotamère, distorsion, homolyse, carbanion, carbocation, radical.
[Traduit par la Rédaction] Hou and Meyers 966
Introduction
ture in our series, 9-(o-tert-butylphenyl)fluorene (6b), as
well as 9-(o-tert-butylphenyl)-9-fluorenol (1), exists only in
their sp form in solution at ambient temperatures and crys-
talline state (2).2 While treatment of 1 with CH3CH2OH or
As part of our recent studies of rotationally restricted 9-
substituted fluorenes (1), we reported that the parent struc-
Received November 18, 1998.
This paper is dedicated to Jerry Kresge in recognition of his many achievements in chemistry.
Y. Hou and C.Y. Meyers.1 Department of Chemistry and Biochemistry, Southern Illinois University-4409, Carbondale, IL 62901,
U.S.A.
1Author to whom correspondence may be addressed. Telephone: (618) 453-6474. Fax: (618) 453-6408. e-mail: cal@chem.siu.edu
2The designations sp (syn periplanar) and ap (anti periplanar) for these conformations are in accord with Rule E-6.6 (see ref. 14).
Can. J. Chem. 77: 960–966 (1999)
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Hou and Meyers
961
Scheme 1.
CH3OH in the presence of strong acid also provided only the
corresponding sp-9-alkoxy rotamers, sp-9-(o-tert-butylphe-
nyl)-9-ethoxyfluorene (2b), and sp-9-(o-tert-butylphenyl)-9-
methoxyfluorene (3b), respectively, similar treatment of 1
with CH3SH unexpectedly provided only the rotamerically
opposite ap-9-methylthio derivative, ap-9-(o-tert-butyl-
phenyl)-9-methylthiofluorene (4a), in solution and crystalline
form (3, 4) (Scheme 1).
In contrast, in the sp conformation these methyls are main-
tained in the deshielding zone of the fluorene ring, giving
rise to a much lower field resonance, near 1.8 ppm. This dis-
tinction has provided a ready means to differentiate between
the two rotameric configurations of these fluorene deriva-
tives in solution (3, 5–7). For the rotameric structure charac-
terization of their solid states and the corresponding bonding
parameters, we have relied on X-ray crystallography (2, 4,
8–10).
The ap conformation in the series of 9-(o-tert-butyl-
phenyl)fluorenes provokes a strong steric repulsion between
the tert-butyl group and the fluorene π cloud, which raises
the ground-state energy making it, in most cases, highly un-
stable relative to the sp conformation (3, 5–7). This strong
shielding of the tert-butyl methyls by the fluorene-ring p
electrons in the ap conformation accounts for their corre-
Results and discussion
Highly distorted/strained structure of ap-9-(o-tert-
butylphenyl)-9-methylthiofluorene (4a)
The X-ray structure of 4a (Fig. 1), whose geometric pa-
rameters have been reported in detail (4), clearly displays
1
spondingly high upfield H NMR resonance, near 0.6 ppm.
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Can. J. Chem. Vol. 77, 1999
Fig. 1. X-ray structure of 4a.
In this regard, the very close contact of S1···H6′, 2.42 Å,
and S1···C6′, 2.96 Å (4), could give rise to a “hydrogen
bond” reflected in the unusually low field signal of H6′, near
1
δ 9.0, in its H NMR spectrum (1, 3). Similar aromatic-H
downfield shifts have been previously reported but were not
associated with hydrogen bonding (11; cf. 12). While hydro-
gen bonding involving an aromatic H is unusual, it has been
suggested from time to time (for related studies see ref. 13)
and is supported in 4a by comparison to a related structure,
9-(o-tert-butylphenyl)-9-methylfluorene (4c), in which such
a possibility is absent. Compound 4c is the only other
9-substituted 9-(o-tert-butylphenyl)fluorene we have so far
prepared that exists solely in its ap conformation in solution,
eq. [1]. While the tert-butyl group of 4c, like that of 4a,
impinges onto the face of the fluorene ring (viz. the upfield
1H NMR resonance of these methyls, δ 0.63 and 0.65, re-
spectively) and thereby forces the 9-methyl and H6′ into
close proximity, the 1H NMR resonance of H6′, δ 8.0, is nor-
mal for an aromatic H held in the deshielding zone of the
fluorene moiety. In comparing the thermodynamic
favorability of the ap rotamer over the sp rotomer of both 4a
and 4c, it is important to note that the 9-S atom and 9-CH3
group exert a similar steric influence, which is not exerted
by the 9-O atom in related structures whose sp rotamers are
thermodynamically preferred (vide infra).
extensive distortions. Our finding that 4a exists exclusively
in its ap conformation tells us that the 9-methylthio group
enforces this conformation, which, although highly energetic
per se, is thermodynamically favored over the sp conforma-
tion in this case.3 Nonetheless, ap 4a is highly distorted,
provoking the strain that is clearly responsible for its un-
usual instability, which promotes its facile homolytic de-
composition at room temperature, in contrast to the thermal
stability of the sp compounds, e.g., 1, 2b, 3b, and 6b (2, 3).
The major source of the strain emanates from the enforced
impinging of a methyl of the tert-butyl group on the face of
the fluorene ring. As a result, the bond between the tert-
butyl group and phenyl, C2′—C7′, is considerably bent, an-
gle C1′-C2′-C7′ being almost 18° larger than the correspond-
ing angle C3′-C2′-C7′. In turn, angle C2′-C7′-C10′ is greatly
distorted relative to the corresponding angles C2′-C7′-C9′
and C2′-C7′-C8′. For the same reason, the bond between the
phenyl and fluorene rings, C1′—C9, is bent, making the an-
gle C2′-C1′-C9 some 13° larger than the corresponding an-
gle C6′-C1′-C9 and forcing the S1—C9 bond more towards
perpendicularity with the fluorene plane, the angle between
the S1-C9 vector and the fluorene ring plane being 99.09 (7)°.
In turn, torsion angle C4b-C8a-C9-C1′ is almost 53° larger
than its counterpart S1-C9-C9a-C4a. These enforced strains
provoke an unusual distortion from planarity of the phenyl
ring, the average deviation of the atoms of the C1′-to-C6′
ring from the least-squares plane being 0.028 (2) Å, while
the corresponding values for the fluorene C1-to-C9a and
C4b-to-C8a phenyl rings are 0.010 (2) and 0.005 (2) Å, re-
spectively. The molecular distortion is manifested in unusu-
ally intimate intramolecular nonbonding contacts, e.g.,
C9a···C10′, C1···C10, C9a···H10′c, C8′···H3′, C3′····H8′b,
and C4a···H10′c, all of whose distances are smaller than the
sum of their respective van der Waals radii.
Facile homolysis of 4a into sp-9-(o-tert-
butylphenyl)fluorene (6b) and sp-9-(o-tert-butylphenyl)-
3-methylthiofluorene (7c)
It was not unexpected that such a highly strained molecule
like 4a would undergo facile decomposition. Such a conse-
quence indeed was initially exhibited by the appearance of a
blue cast taken on by the originally colorless crystals on
standing in a sealed vial at room temperature over the period
1
of a day or two. In attempting a dynamic H NMR study of
4a in DMSO-d6 to examine the effect of temperature on the
rotation of the sterically encumbered tert-butyl group (vide
infra), little change was observed between 20 and 100°C.
Slightly above this temperature, however, the resonances of
4a began to deteriorate as other resonances appeared. Subse-
quently, a sample from the NMR tube was examined by
TLC, which indicated the absence of 4a but the presence of
two new compounds. In a larger scale study, a DMSO solu-
tion of crystalline 4a, heated for 30 min in a 155°C oil bath,
afforded the same TLC results. The two products were iden-
tified as sp-9-(o-tert-butylphenyl)fluorene (6b, 24%) and sp-
9-(o-tert-butylphenyl)-3-methylthiofluorene (7c, 48%), sug-
gesting that the decomposition of 4a was homolytic
(Scheme 2). In contrast to 4a, both products have the sp con-
formation, and their X-ray diffraction parameters (6b (2); 7c
(8)) show them to be nearly free of distortion and strain, as
illustrated in Figs. 2 and 3.
Mechanisms
Our recent studies with the enantiomers of 9-(o-tert-
butylphenyl)-2-methylfluorenes show that the 9-anion, -cat-
ion, and -radical of 9-(o-tert-butylphenyl)fluorene are
3 That the ap conformation is thermodynamically preferred is supported by energy minimization calculations of both the ap (4a) and sp (4b)
conformations using the AM1 method with a closed-shell (restricted) wave function and then comparing the energies of the heats of forma-
tion. The results showed that the ap conformation is favored over the sp conformation by 21.3 kJ/mol. We are indebted to a referee of our
X-ray paper (4) for performing these calculations and transmitting them to us.
© 1999 NRC Canada

Hou and Meyers
963
Scheme 2.
Fig. 3. X-ray structure of 7c.
Fig. 2. X-ray structure of 6b.
vored sp rotamers. In rare instances, the initially formed ap
rotamer is thermodynamically preferred and is the one iso-
lated.
Shown in Scheme 1 is the conversion of sp-1 into cation
1a and its subsequent reaction with ethanol, methanol, or
methanethiol leading to the respective ap products, 2a, 3a,
and 4a. The former two quickly undergo rotation to their
more stable sp isomers, 2b and 3b, while ap-4a, being ther-
modynamically preferred over its sp rotamer, 4b, remains
largely sp2 hybridized and are generally attacked from the
less hindered face of the fluorene plane (1, 3). It follows,
and we indeed found, that sp-9-(o-tert-butylphenyl)fluorenes
that undergo 9-substitution reactions involving the respec-
tive 9-anion, -cation, or -radical will produce the products of
inversion and ap conformation. In most cases, however,
these products undergo rapid rotation to the energetically fa-
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964
Can. J. Chem. Vol. 77, 1999
Scheme 3.
the exclusive product. In a similar manner, but via anion 1b,
sp-6b is methylated with inversion into ap-4c, which, like
4a, does not undergo rotation and consequently is the exclu-
sive product, eq. [1].
thereby providing 9-(o-tert-butylphenyl)-3H-fluorenone (8).
Both products were formed in 50% yield, which supports the
suggested disproportionation mechanism. Moreover, the fact
that 6b was formed exclusively bearing 9-H and not 9-D
proves that the H transfer emanated solely from the fluorene
3-H even when the solvent was CHCl3–H2O.
As illustrated in Scheme 2, the concurrent formation of 6b
and 7c is homolytically initiated. This is the consequence of
the highly distorted and strained structure of 4a, the low
C—S bond strength, and the formation of the relatively sta-
ble methylthio free radical and free radical 5, whose forma-
tion greatly relieves the distortion and strain characterizing
4a. Hydrogen-atom transfer from CH3S· to 5 produces ap-
6a, which undergoes rapid rotation to sp-6b (24% yield).
Concurrently, CH3S· also adds to 5 at its unhindered 3-
position forming 7a; 1,5-H migration provides ap-7b, which
undergoes rotation to the more stable sp-7c (48% yield).
Via a similar but heterolytic pathway shown in Scheme 3,
1 is converted to cation 1a by treatment with triflic anhy-
dride. We utilized CDCl3 containing some D2O as the sol-
vent to determine the mechanism more precisely. Addition
of D2O to 1a provides 1b reversibly and permits a hydride
transfer from 1b to 1a, converting the latter into 6a, which
undergoes rapid rotation to 6b. The transfer of hydride from
1b is followed by loss of D+ from the residual cation,
Dynamic NMR study
As already noted, in the ap conformation of these struc-
tures, the o-tert-butyl is forced into the fluorene plane, an ef-
fect which is amplified with 9-methylthio and 9-methyl
substituents. Surprisingly, a dynamic NMR study showed
1
that 9-methyl exerts a larger effect than 9-methylthio. H
NMR spectra of 4a in CD2Cl2 taken between 20 and –85°C
and in DMSO-d6 between 20 and 100°C consistently exhib-
ited one sharp singlet for the tert-butyl methyls, showing
that rotation of the tert-butyl group was faster than NMR ac-
quisition time, even at –85°C, and therefore was not
sterically impeded. The 6′-H remained a doublet within this
temperature range. However, a similar study with 4c in
CDCl3 showed at 20°C, after a broad singlet of nine protons
for the tert-butyl methyls, but whose signal height was simi-
lar to that of the three protons for the 9-methyl. The 6′-H ap-
[1]
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Hou and Meyers
965
peared to be a broad doublet. At 50°C, the tert-butyl signal
had narrowed, and its height was commensurate with nine
protons relative to that for three protons of the 9-methyl, and
the 6′-H became a much sharper doublet. These results indi-
cate that, at 20°C, rotation of the tert-butyl group of 4c is
noticeably impeded by steric encumbrance, becoming close
to or slower than NMR acquisition time, so that the methyls
are detected as being “almost” nonequivalent.
87–89°C. Recrystallization from methanol provided crystals
mp 89.5–90.5°C. H NMR, δ: 1.76 (s, 9 H); 2.76 (s, 3 H),
1
6.43 (dd, 1 H, J = 8.1, 1.5 Hz), 6.72 (ddd, 1 H, J = 1.5,
8.4 Hz), 7.02 (ddd, 1 H, J = 1.5, 7.2, 8.1 Hz), 7.18–7.27 (m,
4 H), 7.31–7.37 (m, 2 H), 7.52 (dd, 1 H, J = 8.1, 1.5 Hz),
7.67 (dd, 2 H, J = 7.5, 0.9 Hz). 13C NMR, δ: 33.93, 37.66,
49.62, 93.05, 119.59, 124.88, 125.56, 125.99, 127.72,
128.39, 128.54, 131.93, 140.36, 140.53, 141.16, 150.26.
Anal. calcd. for C24H24O: C 87.76, H 7.36, found: C 87.86,
H 7.32.
Experimental
Melting points are corrected. NMR spectra were taken on
a Varian VXR-300 spectrometer, 300 MHz for 1H and
75 MHz for 13C, in CDCl3 with TMS as internal standard,
except for the dynamic NMR studies, which were carried
out in CD2Cl2, CDCl3, or DMSO-d6, as described in the dis-
cussion.
ap-9-(o-tert-Butylphenyl)-9-methylthiofluorene (4a)
A stoppered 10 mL round-bottomed flask containing 1
(33.6 mg, 0.107 mmol) was kept in the freezer (–15°C) for
2 h. Methanethiol (–15°C, 3 mL) was then added with a
precooled pipet; the solution remained colorless. Precooled
48% HBr solution (ca. 1 mL, 8.8 mmol) was added, and the
reaction mixture immediately turned light yellow. This mix-
ture was maintained at –15°C overnight, the residual
methanethiol was then evaporated in the hood and the resi-
due extracted with hexanes (3 × 20 mL). The combined ex-
tracts were washed with water, dilute aq NaHCO3, and water
again. Rotary evaporation left a light yellow solid (36 mg,
ca.100% yield); mp 110–113.5°C (dec., bubbling, probably
MeSSMe or CH2෇S; color deepens at 90°C). 1H NMR
showed the solid was essentially pure 4a, δ: 0.65 (s, 9 H),
1.70 (s, 3 H), 7.14 (d, 2 H, J = 7.5 Hz), 7.27 (ddd, 2 H, J =
7.5, 1.2 Hz), 7.30 (ddd, 1 H, J = 1.8, 8.1 Hz), 7.37 (ddd, 1
H, J = 1.8, 7.5 Hz), 7.39 (ddd, 2 H, J = 7.5, 1.2 Hz), 7.59
(dd, 1 H, J = 7.8, 1.8 Hz), 7.80 (dd, 2 H, J = 7.5, 0.6 Hz),
8.92 (d, 1 H, J = 8.4 Hz). 13C NMR, δ: 16.69, 32.81, 37.42,
67.60, 120.64, 123.79, 125.36, 127.08, 127.30, 127.49,
131.43, 134.78, 133.47, 139.27, 150.71, 150.79.
Recrystallization from hexanes provided colorless single
crystals for X-ray analysis (4). During X-ray diffraction, the
crystal became yellow. Within a few days standing on the
desktop under normal fluorescent lighting, crystalline 4a de-
veloped a deep blue coloration suggestive of homolytic de-
composition.
sp-9-(o-tert-Butylphenyl)-9-fluorenol (1)
Following the method recently reported for the X-ray
study (2), 1 was prepared from the reaction of o-tert-
butylphenylmagnesium bromide with fluorenone in 76%
yield, 97% conversion based on unrecovered fluorenone;
colorless crystals (from isooctane), mp 158–159°C (lit.
(5) mp 156.5–158°C).
sp-9-(o-tert-Butylphenyl)-9-ethoxyfluorene (2b)
To a colorless solution of 9-(o-tert-butylphenyl)-9-fluorenol
(1) (50.2 mg, 0.16 mmol) and 5 mL of ethanol in a 10 mL
round-bottomed flask, 2 g of 48% HBr (9.9 mmol) was
added. The reaction mixture, which became light yellow,
was stirred overnight at room temperature and then extracted
with hexanes (20 mL × 3). The combined extracts were
washed with water, aq NaHCO3, and again with water, dried
(anhyd MgSO4), and rotary evaporated to give a sticky oil
that slowly crystallized (53.6 mg, 98.5%); colorless crystals,
mp 100.5–101.5°C. 1H NMR showed them to be almost pure
(2b), δ: 1.08 (t, 3 H, J = 6.9 Hz), 1.81 (s, 9 H), 2.92 (q, 2 H,
J = 6.9 Hz), 6.41 (dd, 1 H, J = 8.1, 1.5 Hz), 6.71 (ddd, 1 H,
J = 1.5, 8.1, 7.2 Hz), 7.01 (ddd, 1 H, J = 1.5, 7.2, 8.1 Hz),
7.20 (ddd, 2 H, J = 1.2, 7.5, 7.2 Hz), 7.27 (dd, 2 H, J = 1.2,
7.5 Hz), 7.33 (ddd, 2 H, J = 1.5, 7.2, 7.5 Hz), 7.55 (dd, 1 H,
J = 8.1, 1.5 Hz), 7.67 (dd, 2 H, J = 7.5, 1.2 Hz). 13C NMR,
δ: 15.06, 34.16, 37.61, 58.48, 92.74, 119.62, 124.95, 125.48,
126.01, 127.72, 128.46, 132.09, 140.37, 140.61, 149.31,
151.15. Anal. calcd. for C25H26O: C 87.68, H 7.65; found: C
87.37, H 7.62.
ap-9-(o-tert-Butylphenyl)-9-methylfluorene (4c)
A
10 mL round-bottomed flask containing 6b
(2) (140 mg, 0.47 mmol) was flushed with argon and sealed
with a rubber septum. Freshly distilled THF (3 mL) was sy-
ringed into the flask and to this colorless solution an n-BuLi
solution (1.0 mL, 1.6 M, 1.6 mmol) was added via syringe.
The mixture, which turned red immediately, was stirred at
room temp for 6 h after which time iodomethane (0.10 mL,
1.6 mmol) was syringed dropwise into the flask. The red so-
lution became colorless within 5 min. TLC produced two
spots, the minor one matching that of the starting material.
The solvent was removed in vacuo leaving a pale yellow
solid. Water (5 mL) was added, the resulting slurry was ex-
tracted with hexanes, and the combined extracts were con-
centrated in vacuo to give a white solid, 135 mg, shown by
1H NMR to be essentially 4c and a small amount of residual
6b. Purification via column chromatography (silica gel, hex-
anes) yielded colorless crystalline ap-4c, 120 mg (82%
sp-9-(o-tert-Butylphenyl)-9-methoxyfluorene (3b)
To an Erlenmeyer flask containing a colorless solution of
1 (500 mg, 1.6 mmol) in methanol (10 mL), 1 mL of 9 M
sulfuric acid (9 mmol) was added. After the flask was
swirled at room temperature for 10 min, TLC showed that
all the starting material was consumed; only one spot, re-
flecting a less polar compound, was observed. The reaction
mixture was transferred to a separatory funnel with the aid
of 40 mL of hexanes to rinse the flask. The mixture was
washed with water, aq NaHCO3, and water again. The or-
ganic layer was separated, dried over anhyd MgSO4, and
evaporated in vacuo, leaving a colorless oil (3b) (510 mg,
97.6%) that slowly crystallized at room temperature; mp:
1
yield), mp 89.5–90.5°C. H NMR, δ: 0.63 (bs, 9 H), 1.72
(s, 3 H), 7.10 (d, 2 H, J = 7.5 Hz), 7.12 (ddd, 2 H, J = 7.5,
1.5 Hz), 7.30 (m, 2 H), 7.33 (ddd, 2 H, J = 7.5, 1.2 Hz), 7.59
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966
Can. J. Chem. Vol. 77, 1999
(m, 1 H), 7.77 (d, 2 H, J = 7.8 Hz), 8.05 (br d, 1 H, J =
6.3 Hz). 13C NMR, δ: 32.92, 37.46, 38.62, 56.89, 120.61,
123.12, 125.23, 126.56, 126.74, 127.33, 130.85, 131.45,
138.73, 139.23, 150.74, 156.42. 1H NMR at 50°C: the chem-
ical shifts of the peaks were essentially the same, but the
peaks at δ 0.63, 1.72, and 8.05 were much sharper. Anal.
calcd. for C24H24: C 92.26, H 7.74; found: C 92.21, H 7.46.
(s, 9 H), 6.03 (dd, 1 H, J = 1.8, 9.6 Hz), 6.61 (d, 1 H, J =
1.8 Hz), 6.77 (dd, 1 H, J = 1.5, 6.9 Hz), 6.84 (d, 1 H, J =
9.9 Hz), 7.00 (dd, 1 H, J = 1.8, 7.5 Hz), 7.12 (ddd, 1 H, J =
1.2, 7.5 Hz), 7.17 (ddd, 1 H, J = 1.5, 7.8 Hz), 7.26 (ddd, 1
H, J = 1.2, 7.5 Hz), 7.39 (dd, 1 H, J = 1.8, 8.1 Hz), 7.42
(ddd, 1 H, J = 1.2, 7.5 Hz), 7.62 (dd, 1 H, J = 1.2, 8.1 Hz).
13C NMR, δ: 32.30, 36.58, 121.53, 121.94, 124.36, 125.63,
127.27, 127.64, 128.79, 129.09, 129.71, 130.03, 130.51,
131.99, 134.10, 136.40, 146.67, 149.25, 150.92, 157.05,
189.03. Anal. calcd for C23H20O: C 88.43, H 6.45; found: C
88.59, H 6.47.
Homolysis of 4a. Formation of sp-9-(o-tert-
butylphenyl)fluorene (6b) and sp-9-(o-tert-butylphenyl)-
3-methylthiofluorene (7c)
A solution of 4a (115 mg, 0.334 mmol) in 5 mL of DMSO
in a 25 mL round-bottomed flask was magnetically stirred
for 30 min while heated in an oil bath maintained at 155°C.
The reaction mixture was cooled to room temperature and
transferred to a separatory funnel, the residue was trans-
ferred by several washings with ether, and the combined so-
lution was washed with water to remove DMSO. The
ethereal solution was dried (anhyd MgSO4) and evaporated
Acknowledgments
Partial support of this research from Southern Illinois
University through doctoral fellowship (Y.H.) and Distin-
guished Professorship (C.Y.M.) funding and from the Uni-
versity Research Foundation – La Jolla is graciously
acknowledged.
1
in vacuo, leaving a black sticky oil. H NMR showed the
References
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4a. The two products were separated and isolated by “dry-
column chromatography” (silica gel; hexane). From the first
fraction 6b (24 mg, 24.1% yield) was isolated and identified
by mp (179.5–180.5°C), NMR, and X-ray crystallography
(2). Evaporation of the second fraction produced 7c (55 mg,
47.8% yield), which had almost the same Rf value as 4a in
TLC (10:1 hexanes:ether). Recrystallization from hexanes
gave almost colorless crystals of 7c, mp 156.5–157.5°C (no
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1
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To a stirred colorless solution of 1 (63.2 mg, 0.20 mmol)
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1
(m), 1562 (m), 755 (m), 740 (s) cm–1. H NMR of 8, δ: 1.32
© 1999 NRC Canada