Synthesis and conformation of 2-aminophenyldiarylperfluoroalkylmethanes (molecular propellers)

Full text of DOI:10.1021/jo001001c

J . Org. Chem. 2000, 65, 7703-7706
7703
Sch em e 1
Syn th esis a n d Con for m a tion of
2-Am in op h en yld ia r ylp er flu or oa lk ylm eth a n es
(Molecu la r P r op eller s)
Lucjan Strekowski,*,† Hyeran Lee,† Shou-Yuan Lin,†
Agnieszka Czarny,† and Donald Van Derveer‡
Department of Chemistry, Georgia State University, Atlanta,
Georgia 30303, and School of Chemistry and Biochemistry,
Georgia Institute of Technology, Atlanta, Georgia 30332
Lucjan@gsu.edu
Received J uly 3, 2000
In tr od u ction
Molecular propellers, such as triarylmethanes, exist in
chiral helical conformations in which the helicity and the
correlated rotation of the planar substituents (blades) are
imposed by severe steric hindrance in the molecule (see
6-8 in Scheme 1). Depending on the nature of the aryl
groups, a single molecule may exist in a large number of
conformational isomers.1,2 Studies on molecular propel-
lers were pioneered by Mislow in the early 1970s, and
his fundamental work resulted in an understanding of
conformational transitions of this complex class of
molecules.1a Here, we report a simple synthetic route to
perfluoroalkyl-substituted triarylmethanes such as 6-8
(Scheme 1) and conformational studies of this previously
unknown class of molecular propellers. The easy avail-
ability of a series of compounds substituted with a
2-aminophenyl moiety permitted a critical examination
of two controversial proposals. Specifically, a stabilizing
intramolecular interaction between the amino group and
the perfluoroalkyl substituent3 and the intramolecular
hydrogen bonding of the amino group to the π-electron
system of the phenyl group have been suggested for
related molecules.1e
aniline with a perfluoroalkyl iodide in the presence of
Zn and SO2.5 The former method was used in this work.
The treatment of 1-3 with phenylmagnesium bromide
furnished the respective compounds 6-8 in yields of 51-
83%. These are isolated yields, and the lowest efficiency
of 51% for the relatively volatile CF3 derivative 6 is due
to its partial loss during workup and purification.
The mechanistic pathway to 6-8 is a particular case
of the chemistry of the anionically activated perfluoro-
alkyl group.6,7 Briefly, experimental evidence has been
accumulating that the anion derived from 1-3 undergoes
elimination of fluoride to generate an intermediate
product 4 which is then aromatized in the addition
reaction with a nucleophile. In the pathway leading to
6-8, the anionic adduct 5 undergoes a similar sequence
of the elimination/addition reactions with the intermedi-
ary of 4′ to give, after quenching the mixture with water,
the final product 6-8.
An attempt was made to deaminate 8 to a triphenyl-
methyl derivative by treatment with EtONO in DMF.8
Instead of the expected formal replacement of the amino
group by hydrogen, this reaction resulted in deaminative
cyclization with the formation of a fluorene 9. A driving
force for this cyclization is a close proximity of the
intermediate phenyl cation or radical8 to the phenyl
group in the sterically congested molecular propeller.
A reaction leading to a heteroaryl analogue of 8 is given
in Scheme 2. Thus, the treatment of 3 with 2-thienyl-
Syn th esis
The starting materials 1-3 for the synthesis of 6-8
are readily available by a coupling reaction of 2-iodo-
aniline with a perfluoroalkyl iodide in the presence of
copper bronze4 or by reductive perfluoroalkylation of
* To whom correspondence should be addressed. Tel: 404-651-0999.
Fax: 404-651-1416. E-mail: Lucjan@gsu.edu.
† Georgia State University.
‡ Georgia Institute of Technology.
(1) For reviews, see: (a) Mislow, K. Acc. Chem. Res. 1976, 9, 26. (b)
Eliel, E. L.; Wilen, S. H.; Mander, L. N.; Stereochemistry of Organic
Compounds; Wiley: New York, 1994. (c) Oki, M. In Topics in Stereo-
chemistry; Allinger, N. L., Eliel, E. L., Wilen, S. H., Eds.; Wiley: New
York, 1983. (d) Oki, M. In Applications of Dynamic NMR Spectroscopy
to Organic Chemistry; Marchand, A. P., Ed.; VCH: Deerfield Beach,
1985. (e) Oki, M. In The Chemistry of Rotational Isomers; Hafner, K.,
Rees, C. W., Trost, B. M., Lehn, J .-M., Schleyer, P. v. R., Zahradnik,
R., Eds.; Springler-Verlag: Berlin, 1993.
(2) When the aryl rings are on a double bond, a “vinyl propeller”
results: Gur, E.; Kaida, Y.; Okamoto, Y.; Biali, S. E.; Rappoport, Z. J .
Org. Chem. 1992, 57, 3689.
(3) (a) Howard, J . A. K.; Hoy, V. J .; O’Hagen, D. H.; Smith, G. T.
Tetrahedron 1996, 52, 12613. (b) Kovacs, A.; Hargittai, I. Int. J .
Quantum Chem. 1997, 62, 646.
(5) (a) Tordeux, M.; Langlois, B.; Wakselman, C. J . Chem. Soc.,
Perkin Trans. 1 1990, 2293. (b) Strekowski, L.; Hojjat, M.; Patterson,
S. E.; Kiselyov, A. S. J . Heterocycl. Chem. 1994, 31, 1413.
(6) For reviews, see: Kobayashi, Y.; Kumadaki, I. Acc. Chem. Res.
1978, 11, 197. (b) Strekowski, L.; Kiselyov, A. S. Trends Heterocycl.
Chem. 1993, 73, 3. (c) Kiselyov, A. S.; Strekowski, L. Org. Prep. Proc.
Int. 1996, 28, 289.
(7) (a) Strekowski, L.; Lin, S.-Y.; Lee, H.; Mason, J . C. Tetrahedron
Lett. 1996, 27, 4655. (b) Strekowski, L.; Lin, S.-Y.; Lee, H.; Zhang,
Z.-Q.; Mason, J . C. Tetrahedron 1998, 54, 7942.
(4) Fuchikami, T.; Ojima, I. J . Fluorine Chem. 1983, 22, 541. (b)
Yoshino, N.; Kitamura, M.; Seto, T.; Shibata, Y.; Abe, M.; Ogino, K.
Bull. Chem. Soc. J pn. 1992, 65, 2141.
(8) Doyle, M. P.; Dellaria, J . F.; Siegfried, B.; Bishop, S. W. J . Org.
Chem. 1977, 42, 3494.
10.1021/jo001001c CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/07/2000

7704 J . Org. Chem., Vol. 65, No. 22, 2000
Notes
Sch em e 2
g 280 Hz), the relatively weak 1-2 and 1-3 interactions
(J ∼ 10 Hz) cause only a slight broadening of the
corresponding signals in the chemical shift scale of 19F
NMR. The only exception is a dithienyl derivative 13, for
which even at -60 °C the individual signals are much
broader than those for its diphenyl analogue 8. This
result is consistent with an increased conformational
complexity of 13, which is due to two possible, syn and
anti, relative orientations of the 2-thienyl groups in the
molecular propeller. Indeed, the AB-type signals for the
diastereotopic difluoromethylene moieties in the cyclized
product 15 are narrow in the tested range of tempera-
tures from -60 to +80 °C and comparable in shape to
those for 7 and 8. The geminal fluorine atoms of the
achiral phenyl analogue 9 are isochronous at -60 °C.
This important result implies that the temperature-
dependent anisochronism of the perfluoroalkyl substit-
uents in 6-8 and 13 is mainly due to location of these
groups in a chiral environment of the molecular propeller,
rather than their restricted rotation. Consistent with this
suggestion are small rotation barriers, typically 5-7 kcal/
mol reported previously for perfluoroalkyl groups in
highly congested molecules.9 By comparison, the barriers
for conformational changes based on the disappearance
of diastereotopicity for the perfluoroalkyl moieties were
calculated in this work as follows: 6, 12 kcal/mol; 7, 14
kcal/mol; 8, 15 kcal/mol; and 13, 13 kcal/mol. These
values were obtained by using the coalescence data
discussed above.10 The estimated error is (0.5 kcal/mol.10c
The observed single AA′X pattern for CF3 of 6, one AB
system for geminal fluorines in 7, and two AB absorptions
for 8 are consistent with the presence of a single racemic
pair in each case. This is an unusual finding because two
racemic pairs, due to two different orientations of the
aminophenyl moiety relative to the perfluoroalkyl group
in the molecular propeller (syn and anti), can be pre-
dicted.1
To obtain a better insight into the preferred conforma-
tion of these molecular propellers, additional studies with
the selected compound 7 were conducted. The UV spec-
trum taken in hexane shows absorptions at λmax 244 nm
(ꢀ 10 000) and λmax 298 nm (ꢀ 3900) that can safely be
assigned to the respective E2- and B-band. By compari-
son, the respective B-band in the UV spectra of aniline
and o-toluidine, taken in hexane, is at λmax 291 nm (ꢀ
2500) and λmax 289 nm (ꢀ 3200). The bathochromically
shifted B-band of 7 is indicative of strong conjugation
between the nonbonding electrons on the nitrogen atom
and the aromatic π-electron system in the aniline portion
of the molecule.11 It can be suggested that the nitrogen
atom acquires sp2-hybridization that flattens the aniline
moiety and which, in turn, minimizes steric hindrance
within the molecular propeller.
lithium gave a mixture of the desired compound 13 and
a thieno[3,2-b]quinoline 14 in a ratio of 3:2 and a total
yield of 54%. Products 13 and 14 were separated by
chromatography. In the unified mechanism for 13 and
14 it is suggested that the initial adduct 10 can undergo
a dual elimination of fluoride ion. The elimination
through the benzene moiety, as discussed above, gener-
ates 11, which is a precursor to 13. This reaction is
accompanied by an SN2′ substitution of fluoride ion with
the involvement of the thiophene ring in 10, which results
in cyclization to 12. Alternatively, 12 may be generated
by electrocyclization of 11. The intermediate product 12
is apparently oxidized (aromatized) to 14 by molecular
oxygen during workup.
The yield of 13 was improved to 73% by conducting
the reaction of 3 with 2-thienylmagnesium bromide
instead of 2-thienyllithium. The formation of 14 was
completely suppressed in the Grignard reaction. The
treatment of 13 with EtONO in DMF resulted in deami-
native cyclization to give 15.
Con for m a tion a l Stu d ies
The IR spectra of 7 taken in the solid state (KBr) and
in 0.7 and 0.07 M solutions in CCl4 all show virtually
identical absorptions for NH2 at 3402 ( 1 cm-1 and at
3498 ( 1 cm-1. The absorption bands are relatively
strong, symmetric, and narrow with a half-width of
approximately 50 cm-1. These results clearly demonstrate
The three fluorine atoms of the CF3 group of 6 are
anisochronous at -20 °C, and they give rise to a clearly
defined AA′X absorption pattern. As the temperature is
raised, the spectrum gradually broadens and then coa-
lesces at 21 °C. In a similar way, AB-type signals
observed at -15 °C for the difluoromethylene moiety of
7 coalesce at 43 °C, analogous resonances at 25 °C for
each of two sets of geminal fluorines of 8 coalesce to two
independent broad signals at 81 °C, and the two AB-type
resonances for 13 at -5 °C become broad singlets as the
temperature is increased to 35 °C. Due to the large values
of coupling constants for geminal fluorines in 7 and 8 (J
(9) Weigert, F. J .; Mahler, W. J . Am. Chem. Soc. 1972, 94, 5314.
(10) (a) Kurland, R. J .; Rubin, M. B.; Wise, W. B. J . Chem. Phys.
1964, 40, 2426. (b) Oki, M.; Iwamura, H.; Hayakawa, N. Bull. Chem.
Soc. J pn. 1964, 37, 1865. (c) Sandstro¨m, J . Dynamic NMR Spectros-
copy; Academic Press: London, 1982.
(11) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric
Identification of Organic Compounds, 4th ed.; Wiley: New York, 1981.

Notes
J . Org. Chem., Vol. 65, No. 22, 2000 7705
were analyzed and mass spectra of pure components were
obtained on a GC-MS instrument equipped with an on-column
injector, a poly(dimethylsiloxane)-coated capillary column, and
a mass selective detector operating at 70 eV. Melting points
(Pyrex capillary) are uncorrected. The standard 1H NMR (400
MHz) and 19F NMR (282 MHz) spectra were taken at 25 °C in
CDCl3 solutions with TMS and C6F6 as the respective internal
references. H-H coupling constants smaller than 2 Hz are not
reported. In the chemical shift scale of 19F NMR the small F-F
vicinal coupling constants for a perfluoroalkyl group (J ∼ 10 Hz)
result in only a slight broadening of signals. As such, these
coupling constants are not reported as well.
the lack of any intermolecular hydrogen bonding inter-
action of the amino group. They also strongly argue
against the intramolecular NH2‚‚‚Ph hydrogen bonding
that has been suggested previously for sterically crowded
molecules.1e
Since the IR spectra of 7 taken in solution and the solid
state are identical, the solid-state structure appears to
be an excellent approximation of the conformation in
solution. Accordingly, the X-ray diffraction analysis for
7 was conducted (Figure 1, Supporting Information). To
the best of our knowledge, this is the first X-ray structure
of a triarylmethane that contains a 2-aminophenyl
moiety.
In the solid state, the molecule is chiral and adopts a
single conformation with the amino group and the
perfluoroalkyl substituent anti to each other. The short
distance of 1.38 Å for the C-N bond indicates sp2
hybridization of the nitrogen atom, as obtained from UV
studies in solution. The three aryl groups are essentially
planar, and distances between the adjacent carbon atoms
in the two unsubstituted phenyl rings (1.375 ( 0.019 Å)
are in the normal range. By contrast, the C(1)-C(2) bond
of the aniline moiety is elongated to 1.42 Å, which
minimizes steric interactions of the amino group. The
C-CF2-CF3 subunit is in an almost ideal all-staggered
conformation. On the other hand, the molecular propeller
is highly distorted as quantified by the following torsion
angles with the C(F2)-C(central) bond taken as a refer-
ence: 171.8° for C(F2)-C-C(1-PhNH2)-C(2-PhNH2) and
108.0° and 116.8° for the analogous chains C(F2)-C-C(1-
Ph)-C(2-Ph). Again, the unusually large twist of the
aminophenyl moiety minimizes steric interactions of the
amino group with the adjacent phenyl rings. A similar
conformation in solution should result in a relatively low
energy barrier for racemization, as observed. Specifically,
the high torsion angle C(F2)-C-C(1-PhNH2)-C(2-Ph-
NH2) approximates that of 180 ° in the transition state
for racemization of 7 by a one-ring flip mechanism.1a
2-P er flu or oa lk yla n ilin es 1-3. 2-Iodoaniline was allowed
to react with a perfluoroalkyl iodide in the presence of copper
bronze in DMF by using a general procedure.4a Products 1-3
were purified by distillation on a Kugelrohr (80-110 °C/1.5
mmHg) and found to be at least 96% pure by GC.
1
2-P en ta flu or oeth yla n ilin e (1): yield 54%; an oil; H NMR
δ 4.28 (bs, exchangeable with D2O, 2H), 6.78 (d, J ) 8 Hz, 1H),
6.88 (t, J ) 8 Hz, 1H), 7.36 (d, J ) 8 Hz, 1H), 7.42 (t, J ) 8 Hz,
1H); 19F NMR δ 48.5 (2F), 76.8 (3F); HRMS exact mass calcd
for C8H6F5N 211.0420, found 211.0418.
2-Hep ta flu or op r op yla n ilin e (2): yield 58%; an oil; 1H NMR
δ 4.24 (bs, exchangeable with D2O, 2H), 6.70 (d, J ) 8 Hz, 1H),
6.81 (t, J ) 8 Hz, 1H), 7.28 (d, J ) 8 Hz, 1H), 7.32 (t, J ) 8 Hz,
1H); 19F NMR δ 35.2 (2F), 52.2 (2F), 81.7 (3F); HRMS exact mass
calcd for C9H6F7N 261.0388, found 261.0380.
2-Non a flu or obu tyla n ilin e (3): yield 88%; an oil; the NMR
data were virtually identical with those reported.4b
Rea ction s of 1-3 w ith Or ga n om eta llic Rea gen ts. Gen -
er a l P r oced u r e. A solution of a Grignard or lithium reagent
(10 mmol) in THF (25 mL) was stirred at -70 °C and treated
dropwise with a solution of a 2-perfluoroalkylaniline 1-3 (3
mmol) in THF (10 mL). Then the mixture was allowed to reach
23 °C within 1 h and stirred at 23 °C for an additional 1 h before
being quenched with water. Standard workup was followed by
chromatography on silica gel eluting with hexanes/ether (9:1).
Products 6-9 and 13-15 were crystallized from pentanes and
ether, respectively.
2-(2,2,2-Tr iflu or o-1,1-d ip h en yleth yl)a n ilin e (6, from 1 and
PhMgBr): yield 51%; mp 92-93 °C; 1H NMR δ 3.17 (bs,
exchangeable with D2O, 2H), 6.55 (d, J ) 8 Hz, 1H), 6.75 (t, J )
8 Hz, 1H), 7.15 (t, J ) 8 Hz, 1H), 7.25 (d, J ) 8 Hz, 1H), 7.33
(m, 6H), 7.43 (m, 4H); 19F NMR (CDCl3, -60 °C, AA′X) δ 99.4
(dd, J ) 125 and 115 Hz, 1F), 103.3 (d, J ) 115 Hz, 1F), 103.7
(d, J ) 125 Hz, 1F); 19F NMR (CDCl3, 21 °C, coalescence) δ
102.5 (bs); MS m/z 211 (100), 231 (40, M+). Anal. Calcd for
C20H16F3N: C, 73.38; H, 4.93; N, 4.28. Found: C, 73.44; H, 4.82;
N, 4.18.
2-(2,2,3,3,3-P en t a flu or o-1,1-d ip h en ylp r op yl)a n ilin e (7,
from 2 and PhMgBr): yield 72%; mp 98-99 °C; 1H NMR δ 3.20
(bs, exchangeable with D2O, 2H), 6.51 (d, J ) 8 Hz, 1H), 6.78 (t,
J ) 8 Hz, 1H), 7.15 (t, J ) 8 Hz, 1H), 7.32 (m, 7H), 7.58 (m,
4H); 19F NMR (CDCl3, -15 °C, AB for CF2) δ 60.5 (d, J ) 280
Hz, 1F), 60.9 (d, J ) 280 Hz, 1F), 87.8 (s, 3F); 19F NMR (CDCl3,
43 °C, coalescence) δ 61.7 (bs, 2F), 88.0 (s, 3F); MS m/z 180 (100),
258 (40), 377 (70, M+). Anal. Calcd for C21H16F5N: C, 66.84; H,
4.27; N, 3.71. Found: C, 66.61; H, 4.07; N, 3.62.
2-(2,2,3,3,4,4,4-Hep ta flu or o-1,1-d ip h en ylbu tyl)a n ilin e (8,
from 3 and PhMgBr): yield 83%; mp 108-110 °C; 1H NMR δ
3.19 (bs, exchangeable with D2O, 2H), 6.54 (d, J ) 8 Hz, 1H),
6.82 (t, J ) 8 Hz, 1H), 7.18 (t, J ) 8 Hz, 1H), 7.34 (m, 7H), 7.54
(m, 2H), 7.64 (m, 2H); 19F NMR (DMSO-d6, 25 °C, 2AB for CF2-
CF2) δ 43.9 (d, J ) 285 Hz, 1F), 49.1 (d, J ) 285 Hz, 1F), 63.2
(d, J ) 290 Hz, 1F), 66.8 (d, J ) 290 Hz, 1F), 82.2 (s, 3F); 19F
NMR (DMSO-d6, 81 °C, coalescence) δ 46.3 (bs, 2F), 65.8 (bs,
2F), 82.5 (s, 3F); MS m/z 180 (100), 258 (50), 427 (40, M+). Anal.
Calcd for C22H16F7N: C, 61.83; H, 3.75; N, 3.28. Found: C, 61.69;
H, 3.60; N, 3.22.
Con clu sion s
The results of the solution and X-ray crystallographic
studies of 7 complement each other and demonstrate that
the molecule exists in essentially identical conformations
in solution and in the solid state. No unusually short
distances between nonbonded atoms in the molecule were
found by the X-ray crystallographic analysis. The mol-
ecule adopts a distorted helical conformation in which
repulsive steric interactions including the NH2‚‚‚Ph
interactions are minimized. This finding positively rules
out the controversial NH2‚‚‚Ph hydrogen bonding that
has been suggested previously for closely related mol-
ecules. An additional argument strongly supporting this
conclusion was obtained by molecular modeling. Thus,
lowering the distortion of the molecular propeller by
rotating the aminophenyl substituent while maintaining
the remaining conformational parameters intact greatly
increases distances of the aminophenyl moiety to C2F5
and one of the phenyl rings and, at the same time, brings
the amino group to a closer proximity to the second
phenyl ring. This conformation is not observed because
the interaction NH2‚‚‚Ph is repulsive, not stabilizing.
2-[2,2,3,3,4,4,4-H e p t a flu o r o -1,1-(2-d it h ie n y l)b u t y l]-
a n ilin e (13): yield 33% from 3 and 2-thienyllithium; yield 73%
from 3 and 2-thienylmagnesium bromide; mp 83-85 °C; 1H NMR
δ 2.80 (bs, exchangeable with D2O, 2H), 6.61 (d, J ) 8 Hz, 1H),
6.82 (t, J ) 8 Hz, 1H), 7.02 (dd, J ) 5.2, 3.6 Hz, 2H), 7.20 (t, J
) 8 Hz, 1H), 7.35 (m, 4H), 7.46 (bd, J ) 8 Hz, 1H); 19F NMR
(CDCl3, -5 °C, 2AB for CF2CF2) δ 42.2 (bd, J ) 280 Hz, 1F),
45.4 (bd, J ) 280 Hz, 1F), 57.5 (bd, J ) 280 Hz, 1F), 62.0 (bd, J
Exp er im en ta l Section
Gen er a l Meth od s. THF was distilled from sodium ben-
zophenone ketyl immediately before use. All reactions were
conducted under an atmosphere of nitrogen. Crude mixtures

7706 J . Org. Chem., Vol. 65, No. 22, 2000
Notes
) 280 Hz, 1F), 80.2 (s, 3F); 19F NMR (CDCl3, 35 °C, coalescence)
δ 43.8 (bs, 2F), 59.5 (bs, 2F), 80.3 (s, 3F); MS m/z 270 (100), 439
(40, M+). Anal. Calcd for C18H12F7NS2: C, 49.20; H, 2.75; N, 3.19.
Found: C, 49.26; H, 2.72; N, 3.04.
9-Hep ta flu or op r op ylth ien o[3,2-b]qu in olin e (14, from 3
and 2-thienyllithium): yield 21%; mp 72-73 °C; 1H NMR δ 7.67
(t, J ) 8 Hz, 1H), 7.69 (d, J ) 8 Hz, 1H), 7.83 (t, J ) 8 Hz, 1H),
8.00 (d, J ) 6 Hz, 1H), 8.29 (m, 2H); 19F NMR δ 37.4 (s, 2F),
57.0 (s, 2F), 81.8 (s, 3F); MS m/z 234 (100), 353 (50, M+). Anal.
Calcd for C14H6F7NS: C, 47.60; H, 1.71; N, 3.96. Found: C,
47.66; H, 1.76; N, 3.92.
Dea m in a tive Cycliza tion of 8 a n d 13. A solution of 8 or
13 (1 mmol) and EtONO (1.5 mmol) in DMF (30 mL) was heated
to 80 °C for 2 h and then cooled to 23 °C and quenched with
aqueous HCl (20%, 10 mL). The mixture was extracted with
ether (3 × 25 mL), and the extract was dried (MgSO4) and
concentrated. Silica gel chromatography (pentanes/ether, 3:1)
followed by crystallization from ether gave 9 or 15.
treatise by Sandstro¨m.10c Errors in ∆Gq for 6-8 and 13 were
evaluated to be (0.5 kcal/mol.
X-r a y Cr ysta l Str u ctu r e for 7. A chiral crystal of dimen-
sions 0.476 × 0.408 × 0.204 mm was mounted on thin glass fiber
with epoxy cement. Data were collected on a Bruker 1K CCD
diffractometer using Mo KR radiation (0.710 73 A) at 293 K. The
material crystallizes in the orthorhombic space group P212121
with a ) 9.4598(3) Å, b ) 12.8487(2) Å, c ) 14.4550(5) Å, and Z
) 4. Data collection, reduction, solution, and refinement were
all carried out using Bruker AXS software.12 All non-H atoms
were refined anisotropically; 2522 observations, 260 variables;
R1 ) 0.032 for F2 > 4σ(F2) and wR2 ) 0.066 for all F2.
Ch ir a lity of 7. Racemic compound 7 crystallizes from pen-
tane in two enantiomorphic crystals and, as shown by X-ray
crystallographic analysis, the crystals are composed of single
enantiomers of 7. By mimicking the early work by Pasteur on
tartaric acid, it was possible to separate manually the two
enantiomorphic sets. Due to the low racemization barrier of 14
kcal/mol, no CD signal for 7 in solution was observed (pentanes,
23 °C, total time of 5 min for the preparation of the solution
and analysis).
9-Hep ta flu or op r op yl-9-p h en ylflu or en e (9, from 8): yield
1
37%; mp 64-66 °C; H NMR δ 7.22 (m, 3H), 7.32 (t, J ) 8 Hz,
2H), 7.39 (m, 2H), 7.45 (t, J ) 8 Hz, 2H), 7.59 (d, J ) 8 Hz, 2H),
7.77 (d, J ) 8 Hz, 2H); 19F NMR δ 40.6 (s, 2F), 56.8 (s, 2F), 80.9
(s, 3F); MS m/z 241 (100), 412 (10, M+). Anal. Calcd for
Ack n ow led gm en t. We thank the donors of the
Petroleum Research Fund, administered by the Ameri-
can Chemical Society, for partial support of this re-
search.
C
22H13F7: C, 64.40; H, 3.17. Found: C, 64.43; H, 3.07.
8 -H e p t a fl u o r o p r o p y l -8 -(2 -t h i e n y l )i n d e n o [2 ,3 -b ]-
th iop h en e (15, from 13): yield 56%; mp 23-25 °C; 1H NMR δ
6.95 (dd, J ) 5.2, 4.0 Hz, 1H), 7.23 (m, 2H), 7.29 (m, 2H), 7.42
(t, J ) 8 Hz, 1H), 7.52 (m, 2H), 7.79 (bd, J ) 8 Hz, 1H); 19F
NMR δ 38.3 (d, J ) 285 Hz, 1F), 40.9 (d, J ) 285 Hz, 1F), 53.0
(d, J ) 282 Hz, 1F), 55.5 (d, J ) 282 Hz, 1F), 80.7 (s, 3F); MS
m/z 253 (100), 422 (40, M+). Anal. Calcd for C18H9F7S2: C, 51.18;
H, 2.13. Found: C, 51.49; H, 2.46.
Su p p or tin g In for m a tion Ava ila ble: Temperature-de-
pendent 19F NMR spectra of 6-8 and 13, full crystallographic
data for 7, and different ORTEP perspectives of 7. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Deter m in a tion of th e Rota tion Ba r r ier s. The rotation
barriers (the free energies of activation, ∆Gq) for 6-8 and 13
were derived from temperature-dependent 19F NMR spectra (282
MHz) at the coalescence temperatures. The equations10a,b used
for the determination of the barriers and a detailed analysis of
the accuracy of the determination are given in the excellent
J O001001C
(12) Smart 5.054, SAINT 6.01, SHELXTL 5.1, 1998-1999, Bruker
AXS, Madison, WI.