A study of hydrogenation of benzhydrols in the presence of catalytic amount of triflic acid

Article abstract of DOI:10.1139/v00-082

The ionic hydrogenation of diarylcarbinols by triethylsilane, in the presence of catalytic amount of triflic acid proceeds via dibenzhydryl ethers. These ethers do not disproportionate to benzophenones and diphenylmethanes but are reduced by triethylsilane to diarylmethanes.

Full text of DOI:10.1139/v00-082

1242
A study of hydrogenation of benzhydrols in the
presence of catalytic amount of triflic acid
Christophe Waterlot, Daniel Couturier, Marc De Backer, and Benoît Rigo
Abstract: The ionic hydrogenation of diarylcarbinols by triethylsilane, in the presence of catalytic amount of triflic
acid proceeds via dibenzhydryl ethers. These ethers do not disproportionate to benzophenones and diphenylmethanes
but are reduced by triethylsilane to diarylmethanes.
Key words: catalytic reduction, benzhydrols, dibenzhydryl ethers, triethylsilane.
Résumé : L’hydrogénation ionique des benzhydrols par le triéthylsilane en présence d’une quantité catalytique d’acide
triflique, passe par la formation d’éthers de dibenzhydryles. Ces éthers ne se dismutent pas en benzophénones et
diphénylméthanes correspondants mais sont directement réduit en diarylméthanes par le triéthylsilane.
Mots clés : réduction catalytique, benzhydrols, éthers de dibenzhydryles, triéthylsilane. Waterlot et al. 1246
1. Introduction
Scheme 1.
During the course of a program requiring the use of
diarylcarbinols (1), we were puzzled by some differences
between the reported chemistry (2–6) of these compounds
and some of our results (7).
Ionic hydrogenation of ketones (2) and alcohols (3) by
triethylsilane can be realized by using four molar equiv. of
triflic acid, and is assumed to proceed through carbocations
(3, 4) (Scheme 1).
In similar reactions, no symmetrical ethers were observed
during the reductions of diaryl ketones by triethylsilane –
trifluoroacetic acid (5). In the case of aliphatic or aryl aliphatic
ketones only, the reduction performed with triethylsilane –
trimethylsilyl triflate yielded symmetrical ethers (6).
It is also reported that in acidic media diarylcarbinols 1
easily yielded dibenzhydryl ethers 2 (8) which rapidly dis-
proportionated into diphenylmethanes 3 and benzophenones
4 (9). These triflic acid-catalyzed disproportionations pro-
ceeded via hydride transfer (10). In a previous work (7), it
was found that a quantity as low as 0.06 molar equiv. of
triflic acid in a chloroform solution of dibenzhydryl ethers 2
promoted this disproportionation. (Scheme 2).
tal conditions were selected to answer the following
questions
(1) Are the benzhydrols 1, after protonation, directly re-
duced to diphenylmethanes 3 (Scheme 3, path A)?
(2) Or do dibenzhydryl ethers 2, formed in a first step,
disproportionate into diphenylmethanes 3 and benzophen-
ones 4, whose further reduction recycles the starting benz-
hydrols 1 (Scheme 3, path B)?
The work reported here is an attempt to elucidate which
reaction route is the most probable with the very small amount
of triflic acid utilized in the previous works (7). Experimen-
(3) Alternatively, are the dibenzhydryl ethers 2 reduced to
diphenylmethanes 3 and triethylsilyl benzhydryl ethers
which again disproportionate to dibenzhydryl ether 2 (7)
(Scheme 3, path C)?
Received June 19, 1999. Published on the NRC Research
Press website on September 1, 2000.
B. Rigo.1 Laboratoire d’Ingéniérie Moléculaire, Ecole des
Hautes Etudes Industrielles, 13 rue de Toul, 59046 Lille,
France.
2. Results and discussions
M. De Backer. LASIR-HEI, UMR 8516, 13 rue de Toul,
59046 Lille, France.
C. Waterlot and D. Couturier. Laboratoire d’Ingéniérie
Moléculaire, Université des Sciences et Technologies de Lille,
59655 Villeneuve d’Ascq, France.
2.1 Reduction of substituted benzhydrols to
diarylmethane
To lower the reaction rates and to observe the intermediate
products, the reduction of substituted benzhydrols 1 to
diarylmethanes 3 was realized with a small amount of triflic
acid (benzhydrol – triethylsilane – triflic acid, 1:3:0.3) that
is thirteen times lower than that used by others (3) for the
1Author to whom correspondence may be addressed.
Telephone: 03 28 38 48 58. e-mail: rigo@hei.fr
Can. J. Chem. 78: 1242–1246 (2000)
© 2000 NRC Canada

Waterlot et al.
1243
Scheme 2.
Table 1. Reduction of benzhydrols 1 by Et3SiH and triflic acid.a
Temp
(°C)
Time
(min)
Benzhydrol 1
Ether 2
ArCH2Ar 3
Entry
R1
R2
(%)b
(%)b
(%)b
1
2
3
4
5
6
7
8
9e
10
11
12
H
H
H
H
4′-Cl
4′-Br
20
20
20
20
20
60
60
60
5
240
240
240
90
360
15
30
45
15
45
0
67
18
10
0
0
0
0
0f
0
30
4
70
29
71
17
78
11
73
22
22
5
0
29
0
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
2,5-(OMe)2
2,5-(OMe)2
2,5-(OMe)2
2,5-(OMe)2
4′-OMe
4′-OMe
4′-OMe
4′-OMe
4′-OMe
4′-Clc
4′-Clc
4′-Brc
4′-Ic
78
95
100/(96)d
0
e,f
f
f
f
20
20
20
(84)d
(90)d
(92)d
45
45
0
0
0
0
aReaction conditions: benzhydrol 1 10 mmol, triethylsilane 30 mmol, triflic acid 3 mmol, CHCl3 40 mL.
1
bYields were determined by H NMR, 13C NMR, or HPLC.
cTriflic acid 3 mmol, triethylsilane 25 mmol.
dIsolated yield.
eThe corresponding benzophenone 4 was used; 71% of the starting ketone was also observed in the reaction mixture.
fSynthesis of the starting ketones will be described in a future publication.
reduction of benzophenones (1:3:4) as it was described in
Scheme 1. It is shown in Table 1 that dibenzhydryl ethers 2
are important intermediates in the reaction. This is well evi-
denced by comparing the results in rows 4 and 5; 6–8 where
the same reaction has been performed at two different tem-
peratures and for various lengths of time. The same conclu-
sion may be drawn from rows 9 and 10. Thus the already
known path A is not the only route for this reaction.
2.2 Reduction of benzhydryl ethers
The reduction of dibenzhydryl ethers2 2 to diarylmethanes
3 with 0.01 molar equiv. of triflic acid, in the absence or
2 The dibenzhydryl ethers 2 were obtained by adding a catalytic amount of triflic acid to a solution of benzhydrol in chloroform (1): R1, R2 =
H, 86% (20°C, 2 h); R1 = H, R2 = Cl, 93% (60°C, 2 h); R1 = H, R2 = Br, 95% (60°C, 2 h); R1, R2 = OMe, 85% (20°C, 1.25 h).
© 2000 NRC Canada

1244
Can. J. Chem. Vol. 78, 2000
Scheme 3.
Table 2. Reaction of dibenzhydryl ethers 2 with triflic acid in the presence or absence of triethylsilane.a
Temp
(°C)
Time
(h)
Ether 2
Ketone 4
ArCH2Ar 3
(%)b
(%)b
(%)b
Et3SiH
Entry
R1
R2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
H
H
H
H
H
H
H
H
H
H
H
H
no
60
60
20
20
60
20
20
60
20
20
60
20
60
20
0.25
0.25
4
4
5
48
4
5
48
4
0.25
4
0
0
100
23
0
100
71
0
100
28
0
50
0
0
0
50
100
0
77
yesc
no
yesc
no
no
yesc
no
no
yesc
no
4′-Cl
4′-Cl
4′-Cl
4′-Br
4′-Br
4′-Br
4′-OMe
4′-OMe
4′-OMe
4′-OMe
50/(38)d
50/(34)d
0
0
0
29
50/(39)d
0
50/(40)d
H
H
0
0
72
4-OMe
4-OMe
4-OMe
4-OMe
50/(42)d
50/(44)d
26
no
48
0
45
26
0
0
yesc
0.5
4
100/(96)d
55
yesc
aReaction conditions: dibenzhydryl ether 2 10 mmol, triflic acid 0.1 mmol.
1
bYields were determined by H NMR, 13C NMR, or HPLC.
cTriethylsilane 22 mmol.
dIsolated yield.
presence of triethylsilane, was studied. Four dibenzhydryl
ethers bearing various groups were chosen in such a way
that it was possible to identify the reaction intermediates.
The reaction was studied at two different temperatures and
for various lengths of time. In the absence of triethylsilane,
the disproportionation reaction leading to 3 and 4 is the most
important while reduction to 3 is preponderant in its pres-
ence.
Path B implies that substituted 4,4′-dimethoxybenzophen-
one 4 should be an important intermediate in this reaction.
© 2000 NRC Canada

Waterlot et al.
1245
However under experimental conditions similar to ours, it
has been shown that the reduction of 4 (R = OMe) did not
take place3 due to the extreme stability of the intermediate
carbocation formed by the protonation of the carbonyl group
(3). If path B was important, there should be in this case an
accumulation of 4 (R = OMe) in the course of the reaction.
As it can be seen in Table 2 during the reduction of
tetramethoxy-dibenzhydryl ether 2 (R1 = R2 = OMe) (rows
13 and 14) benzophenone 4 was never observed when
triethylsilane was present; this result allows us to exclude
path B.
This point can be further strengthened by observing that
in the presence of triethylsilane the production of other sub-
stituted benzophenone 4 was never observed. Also, the rate
of reduction of dibenzhydryl ethers 2 was greater than the
rate of the disproportionation reaction (compare entries 4, 7,
10 with entries 3, 6, 9).
60 min at 5°C and brought to room temperature. After re-
moving the methanol, the crude product was quenched with
a solution of 1N HCl. The mixture was extracted with di-
ethyl ether (3 × 20 mL), dried over CaCl2, and solvents were
evaporated in vacuo; mp 65°C (petroleum ether – diethyl
ether), yield 93%. IR (KBr) ν (cm–1): 3419, 3003, 2836,
1
1591, 1500, 1447, 1276, 1217, 1046, 711. H NMR (CDCl3)
δ (ppm): 3.24 (s, 1H), 3.73 (s, 6H), 5.95 (s, 1H), 6.80 (m,
3H), 7,27 (m, 4H). 13C NMR (CDCl3) δ (ppm): 55.7, 55.9,
71.6, 111.9, 112.9, 114.0, 127.9, 128.3, 132.6, 132.9, 141.7,
150.7, 153.8 Anal. calcd. for C15H15ClO3: C 64.64, H 5.42,
O 17.22; found: C 64.60, H 5.40, O 17.23.
4′-Iodo-2,5-dimethoxybenzhydrol (1:R1 = H, R2 = I)
This compound was obtained following the same proce-
dure as 4′-chloro-2,5-dimethoxy-benzhydrol; mp 86°C (pe-
troleum ether – diethyl ether), yield 88%. IR (KBr) ν (cm–1):
3345, 3000, 2831, 1590, 1502, 1451, 1278, 1223, 1048, 505.
1H NMR (CDCl3) δ (ppm): 3.35 (s, 1H), 3.64 (s, 6H), 5.83
(s, 1H), 6.68 (m, 2H), 6.77 (d, J = 2.7 Hz, 1H), 7.02 (d, J =
8.3 Hz, 2H), 7.53 (d, J = 8,3 Hz, 2H). 13C NMR (CDCl3) δ
(ppm): 55.7, 55.9, 71.6, 92.7, 111.8, 112.9, 113.9, 128.6,
132.5, 137.2, 143.0, 150.7, 153.7.; Anal. calcd. for
C15H15IO3: C 48.67, H 4.08, O 12.97; found: C 48.69, H
4.10, O 12.95.
3. Conclusion
Although no formal kinetic studies were reported here, the
results presented in Tables 1 and 2 indicate that path C is a
favored process when only a small amount of triflic acid is
used. In a first step of the ionic reduction of benzhydrols by
Et3SiH–CF3SO3H, dibenzhydryl ethers are formed and re-
duction of these ethers is faster than their disproportionation
reaction (path C). However, the formation of dibenzhydryl
ether is not always considerably faster than the reduction
(Table 1, entries 2 and 3). It is then possible that path A con-
tributes in part, to the reduction process. It also must be
stressed that the experimental conditions used here are dif-
ferent from previous reported works (3, 11) where super
acidic conditions promoted other reaction paths.
4′-Bromo-2,5-dimethoxybenzhydrol (1:R1 = H, R2 = Br)
Sodium borohydride (0.85 g, 0.225 mol) was added at
25°C, over a period of 15 min, to a solution of 2,5-
dimethoxy-1-(4′-bromobenzoyl)benzene (6 g, 0.187 mol) in
methanol (80 mL). The reaction mixture was then stirred for
another 90 min at room temperature. After removing the
methanol, the crude product was washed with a solution of
1N HCl and the product was extracted with diethyl ether
(3 × 10 mL). After drying the organic phase over CaCl2 and
evaporating the solvents, the benzhydrol was obtained as an
oil which crystallized in a petroleum ether and diethyl ether
mixture; mp 70°C (petroleum ether – diethyl ether), yield
92%. IR (KBr) ν (cm–1): 3300, 3000, 2831, 1583, 1499,
4. Experimental
Materials
All benzhydrols were commercially available, except for
compounds 1 (R1 = H, R2 = Cl; R1 = H, R2 = I; R1 = H,
R2 = Br). Melting points were determined with a Mettler
1
1456, 1271, 1223, 1042, 701. H NMR (CDCl3) δ (ppm):
1
FP1 and are uncorrected. H NMR and 13C NMR spectra
3.12 (s, 1H), 3.71 (s, 3H), 3.72 (s, 3H), 5.93 (s, 1H), 6.78
(m, 2H), 6.85 (d, J = 2.4 Hz, 1H), 7.24 (d, J = 8.4 Hz, 2H),
7.41 (d, J = 8.4 Hz, 2H). 13C NMR (CDCl3) δ (ppm): 55.7,
55.9, 71.6, 111.9, 112.9, 113.9, 121.0, 128.3, 131.2, 132.6,
142.3, 150.7, 153.7. Anal. calcd. for C15H15BrO3: C 55.75,
H 4.68, O 14.85; found: C 55.72, H 4.69, O 14.87.
were recorded on a Brücker AC300 spectrometer operating
at, respectively, 300.133 and 75.47 MHz using trimethyl-
silane as an internal reference. Coupling constants are given
in Hz. IR spectra were recorded on a Brücker IFS 48 spec-
1
trometer. The reaction mixtures were analyzed by H NMR
and 13C NMR and by liquid chromatography (C column and
a 60:40 acetonitrile–water as mobile phase). E1le8mental anal-
yses were performed by the “Service Central de Microanalyses”
of CNRS, in Vernaison, France.
Disproportionation of bis-4,4′-dimethoxybenzhydryl
ether (2:R1 = R2 = OMe)
A solution of bis-4,4′-dimethoxybenzhydryl ether (4.7 g,
10 mmol) and triflic acid (0.09 mL, 0.1 mmol) in chloro-
form (5 mL) was heated at 60°C for 15 min. Methylene
dichloride (40 mL) and water (40 mL) were added. After
washing, the organic phase was dried then evaporated. The
solid obtained was filtered, then washed with heptane giving
a 42% yield in 4,4′-dimethoxybenzophenone identical to the
commercial compound. The heptane solution was evaporated,
Procedure
4′-Chloro-2,5-dimethoxybenzhydrol (1:R1 = H, R2 = Cl)
To a stirred solution of 2,5-dimethoxy-1-(4′-chloroben-
zoyl)benzene (17 g, 0.615 mol) in methanol (160 mL), so-
dium borohydride (2.8 g, 0.738 mol) was added at 5°C over
a period of 15 min. The reaction mixture was then stirred for
3 We have repeated the reduction of 4,4′-dimethoxybenzophenone in the described conditions (3). After 30 h of reflux, the yield of 4,4′-
dimethoxydiphenylmethane was less than 10%.
© 2000 NRC Canada

1246
Can. J. Chem. Vol. 78, 2000
giving a 44% yield in dianisylmethane, identical to the
known compound (9b).
ganic layer was dried with magnesium sulfate, and evapo-
rated; mp 42°C (petroleum ether – diethyl ether), yield 84%.
IR (KBr) ν (cm–1): 2998, 2947, 2831, 1590, 1495, 1459,
1
1437, 1219, 1045, 711. H NMR (CDCl3) δ (ppm): 3.74 (s,
Typical procedure: reduction of bis-4,4′-
3H), 3.76 (s, 3H), 3.91 (s, 1H),), 6.66 (d, J = 3.0 Hz, 1H),
6.73 (dd, J = 8.8 Hz, J = 3.0 Hz, 1H), 6.80 (d, J = 8.8 Hz,
1H), 7.14 (d, J = 8.5 Hz, 2H), 7.20 (d, J = 8.5 Hz, 2H). 13C
NMR (CDCl3) δ (ppm): 35.5, 55.6, 56.0, 111.4, 116.8,
128.3, 130.2, 130.3, 131.6, 139.1, 151.6, 153.5. Anal. calcd.
for C15H15ClO2: C 68.57, H 5.75, O 12.18; found: C 68.60,
H 5.71, O 12.20.
dimethoxybenzhydryl ether (2:R1 = R2 = OMe)
Triflic acid (0.09 mL, 0.1 mmol) was slowly added to a
solution of bis-4,4′-dimethoxybenzhydril ether (4.7 g,
10 mmol) and triethylsilane (3.5 mL, 22 mmol) in chloro-
form (5 mL). The mixture was stirred at room temperature
for 45 min. The organic phase was washed with a sodium
carbonate solution (30 mL) and dried. After evaporation, the
solid was filtered to give a 96% yield in 4,4′-dianisyl-
methane, identical to the known compound (9b).
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Typical procedure: reduction of 4,4′-
dimethoxybenzhydrol (1:R1 = R2 = OMe)
Triflic acid (0.25 mL, 0.3 mmol) was slowly added
(2 min) via syringe, to a stirred solution of 4,4′-dimethoxy-
benzhydrol (2.5 g, 1 mmol) and triethylsilane (4.8 mL,
3 mmol) in chloroform (40 mL), and the mixture was stirred
at 20°C for 90 min. The composition of the solution was ob-
tained by HPLC analysis (C18 column, 60:40 acetonitrile–wa-
ter as the mobile phase, flow 1.3 mLmin–1): (i) 4.2 min (4,4′-
dimethoxybenzhydrol, 10%); (ii) 10.8 min (4,4′-
dimethoxydiphenylmethane, 73%); and (iii) 38.3 min (4,4′-
dimethoxydibenzhydryl ether, 17%).
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2,5-Dimethoxy-1-(4′-chlorobenzyl)benzene (3:R1 = H, R2 =
Cl)
Triflic acid (0.26 mL, 3 mmol) was added (syringe) at
room temperature, to a stirred solution of 2,5-dimethoxy-4′-
chlorobenzhydrol (2.8 g, 10 mmol) in dichloromethane
(30 mL). The color of the solution turned yellow-red. Subse-
quently, a solution of triethylsilane (4.02 mL, 25 mmol) in
dry dichloromethane (10 mL) was added dropwise. An exo-
thermic reaction took place while the temperature was main-
tained at 20°C. After 45 min, the mixture was poured into
cold saturated sodium hydrogen carbonate solution (20 mL)
and extracted with dichloromethane (3 × 10 mL). The or-
10. P.D. Bartlett and J.D. McCollum. J. Am. Chem. Soc. 78, 1441
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© 2000 NRC Canada