8092 J . Org. Chem., Vol. 61, No. 23, 1996
Saito et al.
stirred vigorously for 5 h under Ar. The solution was added
slowly to 200 mL of ice-water and extracted with 200 mL of
CH2Cl2. The organic layer was separated, washed with brine,
dried over Na2SO4, and evaporated to give a brown oily solid.
The residue was purified by repeated column chromatography
(hexane and/or hexane:AcOEt ) 10:1) to give 6 mg (0.02 mmol,
2%) of 2 as a colorless solid, 13 mg (0.08 mmol, 8%) of 3 as a
colorless oil, and 26 mg (0.11 mmol, 11%) of 4 as a colorless
solid. The fluorobenzaldehyde fraction (69 mg) was also
obtained as a pale brown oil which contained only 5 (0.55
mmol, 55%), but no benzaldehyde (examined by NMR).
4-(trifluoromethyl)triphenylmethane may be slower
(Scheme 7).15 The facile formation of these triphenyl-
methanes from nitro- and (trifluoromethyl)benzaldehyde
can be reasonably interpreted in terms of the participa-
tion of protonated benzaldehyde, which is highly reactive.
Though we could not directly study the mechanism of
the formation of anthracene (Scheme 1) at high concen-
tration of benzaldehyde in benzene,8 our results provide
some basis for speculation. The formation of diphenyl-
methane (3) and biphenyl (7) in the reaction of 4-ben-
zylbiphenyl (9) could be reasonably interpreted in terms
of the formation of benzyl cation and its reaction with
benzene. When the concentration of substrate is rela-
tively high, the benzyl cation may react with the diphe-
nylmethane formed to give 1,2-dibenzylbenzene, which
again eliminates benzene and undergoes intramolecular
Friedel-Crafts reaction to give dihydroanthracene. Hy-
dride transfer would afford the final product, anthracene
(Scheme 8).
Rea ction of 1,1′-Bip h en yl-4-ca r boxya ld eh yd e (6) w ith
Ben zen e in th e P r esen ce of TF SA. TFSA (8.8 mL, 100
mmol) was added slowly to a solution of 6 (182 mg, 1.0 mmol)
in dry benzene (39 g, 500 mmol) at room temperature. The
heterogeneous yellow solution was heated to 70 °C and stirred
vigorously for 16 h under Ar. The solution was added slowly
to 200 mL of ice-water and extracted with 200 mL of CH2Cl2.
The organic layer was separated, washed with brine, dried over
Na2SO4, and evaporated to give a brown semisolid. The
residue was purified by repeated column chromatography
(hexane and/or hexane:AcOEt ) 10:1) to give 14 mg (0.06
mmol, 6%) of 2 as a colorless solid, 45 mg (0.27 mmol, 27%) of
3 as a colorless oil, 65 mg (0.25 mmol, 25%) of 4 as a pale brown
powder, 27 mg (0.15 mmol, 15%) of 6 as a brown solid, and
107 mg (0.70 mmol, 70%) of 7 as a colorless solid. The NMR
spectra and TLC behavior of 2-4, 6, and 7 were identical with
those of authentic samples.
Con clu sion
We propose a mechanism for the reaction of substituted
benzaldehyde with benzene, which can explain the puz-
zling results in previously reported reactions. Trans-
formylation or formation of carbon monoxide is not a
likely process. The key step is transalkylation after the
formation of a substituted triphenylmethane as the
intermediate.
Syn t h esis of r,r-Dip h en yl-1,1′-b ip h en yl-4-m et h a n ol
(10). To a suspension of Mg (0.75 g, 31 mmol) in dry ether
(10 mL) was added a solution of 4-bromo-1,1′-biphenyl (6.20
g, 27 mmol) in dry ether (40 mL) under Ar, followed by the
addition of a small amount of I2. The solution was heated to
reflux for 3 h (red-brown suspension). To the suspension was
added a solution of benzophenone (3.64 g, 20 mmol) in dry
ether (40 mL) over 5 min at room temperature. After 15 min,
the solution was added slowly to ice-water (100 mL), followed
by the addition of 100 mL of 2 N aqueous HCl. The solution
was extracted with 500 mL of ethyl acetate. The organic layer
was separated, washed with brine, dried over Na2SO4, and
evaporated to give a yellow oil. The oil was purified by column
chromatography (hexane:AcOEt ) 10:1) to give 4.71 g (14
mmol, 52%) of 10 as a pale yellow amorphous solid. The
product was recrystallized from MeOH.
Exp er im en ta l Section
Gen er a l Meth od s. The instrumentation used for this
study was described previously.8
Ma t er ia ls. Trifluoromethanesulfonic acid (TFSA) was
purchased from Central Glass Co. (J apan) and was purified
as reported.16 Trifluoroacetic acid (TFA) was also purified as
reported.16 4-Fluorobenzaldehyde (5) (bp 75 °C/18 mmHg) and
4-(trifluoromethyl)benzaldehyde (13) (bp 75-76 °C/16 mmHg)
were purified by distillation and stored in glass ampules under
Ar. 4-(Phenylmethyl)-1,1′-biphenyl (9) (mp 84-85 °C) was
purified by recrystallization from ethanol. 4-Nitrobenzalde-
hyde (11) (mp 105-106 °C) was purified by recrystallization
from water.
Rea ction of 4-F lu or oben za ld eh yd e (5) w ith Ben zen e
in th e P r esen ce of TF SA. (a ) Lon ger Rea ction Tim e (50
°C, 19 h ). TFSA (8.8 mL, 100 mmol) was added slowly to a
solution of 5 (124 mg, 1.0 mmol) in dry benzene (39 g, 500
mmol) at room temperature. The heterogeneous yellow solu-
tion was heated to 50 °C and stirred vigorously for 19 h under
Ar. The solution was added slowly to 200 mL of ice-water
and extracted with 200 mL of CH2Cl2. The organic layer was
separated, washed with brine, dried over Na2SO4, and evapo-
rated to give a red oily solid. The residue was purified by
column chromatography (hexane f hexane:AcOEt ) 10:1) to
give 64 mg (0.36 mmol, 36%) of 3 as a colorless oil and 75 mg
(0.29 mmol, 29%) of 4 as a pale yellow powder.
Da ta for 10: mp 134-135 °C, pale yellow cubes (recrystal-
lized from MeOH) (lit.17 136-137 °C); 1H NMR (CDCl3) δ 7.59
(d, 2 H, 7.3 Hz), 7.54 (d, 2 H, 8.4 Hz), 7.43 (t, 2 H, 7.7 Hz),
7.3-7.4 (m, 13 H), 2.83 (s, 1 H). Anal. Calcd for C25H20O: C,
89.25; H, 5.99. Found: C, 89.50; H, 6.04.
Syn th esis of 4-(Dip h en ylm eth yl)-1,1′-bip h en yl (8).18
A
mixture of 10 (1.34 g, 4 mmol) and NaBH4 (powder, 1.52 g, 40
mmol) was added in portions over 5 min to TFA (50 mL) at 0
°C under a stream of Ar (red solution). The solution was
stirred vigorously. An additional 6 mL of TFA was added to
complete the addition, and 40 min later, the solution was
evaporated. To the residue was added 100 mL of water, and
the solution was extracted with 300 mL of hexane. The
organic layer was separated, washed with saturated aqueous
NaHCO3, dried over Na2SO4, and evaporated to give 8 (1.25
g, 98%) as a pale yellow solid.
In another run, fluorobenzene was detected in the reaction
mixture by gas chromatography (column, Shimadzu PEG 20
M 25%, 4 m; column temp 60 °C).
(b) Sh or ter Rea ction Tim e (50 °C, 5 h ). TFSA (8.8 mL,
100 mmol) was added slowly to a solution of 5 (124 mg, 1.0
mmol) in dry benzene (39 g, 500 mmol) at room temperature.
The heterogeneous yellow solution was heated to 50 °C and
Da ta for 8: mp 112-113 °C (“melted” once at 102-103 °C),
pale yellow needles (recrystallized from hexane) (lit.19 112-
113 °C); 1H NMR (CDCl3) δ 7.58 (d, 2 H, 8.4 Hz), 7.51 (d, 2 H,
8.4 Hz), 7.43 (t, 2 H, 7.7 Hz), 7.3-7.4 (m, 5 H), 7.1-7.3 (m, 7
H), 5.59 (s, 1 H). Anal. Calcd for C25H20: C, 93.71; H, 6.29.
Found: C, 93.76; H, 6.21.
(15) The formation of a small amount of 15 in the reaction of
4-(trifluoromethyl)benzaldehyde (13) with benzene could be explained
by the solvolytic transformation of 14 to 4-(diphenylmethyl)benzen-
ecarboxylic acid or its equivalent, followed by reaction with benzene.
See also, Saito, S.; Sato, Y.; Ohwada, T.; Shudo, K. J . Am. Chem. Soc.
1994, 116, 2312-2317.
(17) Lichtin, N. N.; Glazer, H. J . Am. Chem. Soc. 1951, 73, 5537-
5541.
(18) Gribble, G. W.; Leese, R. M.; Evans, B. E. Synthesis 1977, 172-
176.
(19) Schlenk, W.; Weickel, T.; Herzenstein, A. J ustus Liebig Ann.
Chem. 1910, 372, 1-20.
(16) Saito, S.; Saito, S.; Ohwada, T.; Shudo, K. Chem. Pharm. Bull.
1991, 39(10), 2718-2720.