Studies of 9-Fluorenyl Carbocations
J . Org. Chem., Vol. 66, No. 4, 2001 1113
solvent gave 0.47 g (99%) of alkene 6; mp 228-229 °C (lit.26
mp 229.5 °C); 1H NMR δ 7.69 (d, J ) 7.50 Hz, 2H), 7.41-7.35
(m, 10H), 7.24 (d × d, 2H), 6.92 (t, J ) 7.73 Hz, 2H), 6.62 (d,
J ) 7.96 Hz, 2H); MS m/z: 330 (M, 100), 253 (30).
cation 8 in the LFP transient spectrum. Semiempirical
and high level calculations for cations 4 and 8, and the
transition state of the 1,2-hydride migration, indicated
that the periplanar geometry for this transformation is
not attained. The activation barrier is too large for the
hydride migration to occur at ambient conditions. The
hydrogen shift observed under the strong acid conditions
is most likely occurring via a deprotonation-protonation
equilibrium process. The enthalpy and free energy dif-
ference between cations 4 and 8 are surprisingly small
and suggests that 4 is not destabilized.
9-Meth oxy-9-ben zh yd r ylflu or en e (5). The alcohol 3 (0.1
g, 0.29 mmol) was dissolved in 25 mL of benzene. To this
solution were added 0.02 g of toluenesulfonic acid and 25 mL
methanol, and the solution was left to stir for 15 h. This
mixture was washed with 2 × 5 mL of saturated NaHCO3.
Separation of the organic layer and evaporation gave a residue
which crystallized in methanol, 0.06 g (60%); mp 188-190 °C;
1H NMR δ 7.58 (d, J ) 7.56 Hz, 2H), 7.36 (d, J ) 5.12 Hz,
4H), 7.27 (t, J ) 7.48 Hz, 2H), 7.18 (m, 6H), 6.99 (t, J ) 8.92
Hz, 2H), 6.58 (d, J ) 7.56 Hz, 2H), 4.17 (s, 1H), 2.79 (s, 3H);
13C NMR δ 144.7, 141.3, 140.9, 130.4, 128.7, 127.6, 126.7,
126.4, 126.1, 119.6, 90.7, 61.0, 51.4; MS m/z: 362 (M), 195
(100), 180, 167, 165, 152.
Exp er im en ta l Section
Continuous irradiations were performed using a Hanovia
450W medium-pressure mercury arc lamp in a water-cooled
quartz immersion well. Pyrex tubes containing samples dis-
solved in methanol were strapped around the immersion well
with the whole assembly immersed in an ice-water bath. The
samples were purged with nitrogen prior to irradiation. For
the radical-derived photoproduct studies, the solution was
purged with oxygen.
9-F lu or en yl-d ip en ylm eth oxym eth a n e (9). Alcohol 7 (0.1
g, 0.29 mmol) was dissolved in 25 mL of benzene and 25 mL
of methanol. To this solution was added 0.02 g of toluene-
sulfonic acid, and the solution was left to stir for 15 h. Workup
of the reaction mixture as for the preparation of 5 gave 0.08 g
1
(80%) of the title ether; mp 156-158 °C; H NMR δ 7.39 (d,
J ) 7.48 Hz, 4H), 7.26-7.09 (m, 14H), 5.12 (s, 1H), 3.03 (s,
3H); 13C NMR δ 143.6, 142.1, 138.0, 129.4, 127.2, 127.1, 127.08,
126.9, 126.1, 119.0, 87.9, 56.9, 52.0; MS m/z: 362 (M), 330,
252, 197 (100), 165, 105, 77.
Laser flash photolysis were carried out at room temperature
with 6-ns pulse, typically <40 mJ of 308 nm monochromatic
light from a Lumonics EX-510 excimer laser. The laser system
has been described in detail.19
F or m a tion a n d Meth a n ol Qu en ch in g of Ca tion 4. A
solution of alcohol 3 (5 mg) in CH2Cl2 (5 mL) was cooled in a
dry ice-acetone bath (-78 °C). This solution was added to a
cooled (-78 °C) solution of chlorosulfonic acid (1 mL) in 4 mL
of CH2Cl2 dropwise with stirring. A deep red color was seen
immediately after addition (λmax ) 494 nm). This cold solution
was immediately transferred to a flask containing 5 mL of
methanol whereupon the red color disappeared. Extraction
with chloroform (10 mL), washing with water and saturated
NaHCO3, separation of the organic layer, and evaporation gave
a residue. The 1H NMR spectrum of this residue was taken
and shown to consist only of alkene 6 and ether 5 in a 4:1
ratio, respectively, by comparison with the 1H NMR spectra
of authentic samples prepared above.
All chemicals used were purchased from Aldrich. The
solvents were dried and distilled freshly prior to use.
9-Hyd r oxy-9-ben zh yd r ylflu or en e (3). To a solution of
diphenylmethane (1.0 g, 6.0 mmol) in THF (50 mL) at 0 °C
under nitrogen was added n-butyllithium (2.4 mL of 2.5 M
solution in hexanes). The resulting orange-red solution was
left to stir for 30 min. To this solution was added 9-fluorenone
(1.08 g, 6.0 mmol) in THF (20 mL) and allowed to warm to
room temperature with stirring for 3 h. The reaction was
quenched by adding saturated aqueous NaCl, the organic layer
was separated, dried with anhydrous MgSO4, and filtered, and
the solvent was evaporated. The residue was purified by
column chromatography (silica gel; 8:1 hexane:ethyl acetate)
giving 1.5 g (74%) of the title alcohol; mp 185-186 °C (lit.18
mp 183 °C); 1H NMR δ 7.55 (d, J ) 7.6 Hz, 2H), 7.34-7.33
(m, 4H), 7.26 (d, J ) 7.39 Hz, 2H), 7.19-7.17 (m, 6H), 7.03 (t,
J ) 7.43 Hz, 2H), 6.75 (d, J ) 7.52 Hz, 2H), 4.39 (s, 1H), 2.32
(s, 1H, OH); 13C NMR δ 148.1, 140.7, 139.8, 130.2, 128.9, 127.9,
127.3, 126.7, 125.4, 119.8, 84.4, 60.3; MS m/z: 348 (M+) (25)
181 (100), 152, 105, 77.
9-F lu or en yl-d ip h en ylm eth a n ol (7). To a solution of fluo-
rene (1.0 g, 6.02 mmol) in THF (50 mL) at 0 °C under nitrogen
was added n-butyllithium (2.41 mL of 2.5 M solution in
hexanes). The resulting red-colored solution was left to stir
for 20 min. To this solution was added benzophenone (1.1 g,
6.0 mmol) dissolved in THF (10 mL), and the solution was
allowed to stir for 3 h at room temperature. The reaction was
quenched by the addition of an aqueous saturated NaCl
solution. The organic layer was separated, dried with anhy-
drous MgSO4, and filtered and the solvent evaporated to give
a residue which was purified by column chromatography (silica
gel; 8:1 hexane:ethyl acetate) to give 1.32 g (63%) of the title
alcohol; mp 230-232 °C;25 1H NMR δ 7.73-7.67 (m, 6H), 7.39-
7.28 (m, 8H), 6.98 (t, J ) 7.51 Hz, 2H), 6.52 (d, J ) 7.73 Hz,
2H), 5.29 (s, 1H), 1.95 (s, 1H, OH); 13C NMR δ 145.9, 142.8,
142.5, 128.2, 127.8, 127.1, 126.5, 126.4, 126.37, 119.6, 80.2,
56.8; MS m/z: 330 (M - H2O), 183 (100), 165, 105.
Qu en ch in g of Ca r boca tion 8 w ith Meth a n ol. Carbo-
cation 4 was prepared in the same way as above. However,
the carbocation solution was permitted to warm to room
temperature for 15 min, and a color change from red to yellow
was visible (λmax ) 410 nm). Quenching with methanol and
1
workup in identical fashion as above gave a residue whose H
NMR spectrum showed the presence of two main products
alkene 6 and ether 9 in a 1:4 ratio by comparison with
authentic samples prepared above.
P h otolysis of Alcoh ol 3. A solution containing alcohol 3
(0.05 g, 1.44 mmol) in 50 mL of methanol in a Pyrex tube was
irradiated for 1 h in a nitrogen atmosphere. After evaporation
of the solvent, the residue was chromatographed by thin-layer
chromatography (8:1 hexane/ethyl acetate).
The bands were separated and identified by comparison
with authentic samples: unreacted alcohol 3 (29%), fluorenone
(14%), ether 5 (5%), alkene 6, 9-diphenylmethylfluorene18 (3%),
alcohol 10 (12%), and hydrocarbon 11 (10%).
Alcoh ol 10. 1H NMR δ 7.70 (d, J ) 7.57 Hz, 2H), 7.37-
7.26 (m, 4H), 7.15 (t, J ) 7.70 Hz, 2H), 7.10-7.03 (m, 10H),
5.27 (s, 1H), 3.68 (s, 2H), 1.73, (s, 1H, OH), MS m/z 362 (M),
331, 167 (100).
Hyd r oca r bon 11. 1H NMR δ 7.52 (d, J ) 7.56 Hz, 2H),
7.28 (m, 10H), 7.18 (t, J ) 7.26 Hz, 2H), 7.01 (m, 18H), 6.96
(t, J ) 7.38 Hz, 2H), 6.87 (d, J ) 7.71 Hz, 2H), 4.91 (s, 2H);
MS m/z: 662 (M), 331 (95), 253, 167 (100).
9-Ben zh yd r ylid en eflu or en e (6). To a solution of alcohol
3 (0.5 g, 1.44 mmol) in C6H6 (25 mL) was added toluenesulfonic
acid (0.025 g, 0.14 mmol). The reaction was left to reflux
overnight. The mixture was washed using 2 × 10 mL of sat.
NaHCO3. Separation of the organic layer and evaporation of
Th eor etica l Ca lcu la tion s. All calculations were performed
using Gaussian 98.27,28 Hartree-Fock calculations in conjunc-
(25) Alcohol 7 has been previously reported cf Schlenk, W.; Bergman,
E. Ann. 1928, 463, 215, but no melting point was reported.
(26) J ennings, R. J . S.; Fowler-Williams, A. J . Appl. Chem. (London)
1953, 3, 426.