Disproportionation of diarylmethanol derivatives by using supercritical water

Article abstract of DOI:10.1016/S0040-4039(02)01163-2

Non-catalytic disproportionation of diarylmethanol derivatives was found to proceed efficiently in supercritical water. This method was also applied to various diarylmethylamine derivatives to give the disproportionation products in good yields.

Full text of DOI:10.1016/S0040-4039(02)01163-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 5859–5861
Disproportionation of diarylmethanol derivatives by using
supercritical water
Bunpei Hatano,* Jun-ichi Kadokawa and Hideyuki Tagaya
Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University,
Yonezawa, Yamagata 992-8510, Japan
Received 11 March 2002; revised 12 June 2002; accepted 14 June 2002
Abstract—Non-catalytic disproportionation of diarylmethanol derivatives was found to proceed efficiently in supercritical water.
This method was also applied to various diarylmethylamine derivatives to give the disproportionation products in good yields.
© 2002 Elsevier Science Ltd. All rights reserved.
Water has been attracting increased attention as a
medium for chemical reactions in recent years.1 The
physicochemical properties of supercritical and near-
critical water are quite different from those of ordinary
water,2 and the reactivity of organic compounds is
often enhanced greatly. Early interest in supercritical
water utilization was focused on waste treatment of
such as polymer wastes and toxic compounds, and the
complete air combustion of these wastes with little or
no toxic emission is possible in supercritical water.3 On
the other hand, only a few reports have described the
utility of supercritical or near-critical water from the
viewpoint of organic synthesis.4,5 We now report the
first example of disproportionation6 of diarylmethanol
derivatives (1) in supercritical water. This transforma-
tion is applicable to various diarylmethanol and diaryl-
methylamine derivatives (1) to give the corresponding
diarylmethanes (2) and diaryl ketones (3) in excellent
yields, respectively.
During the course of our study to investigate the reac-
tivity of alcohols in supercritical water, we found that
(1)
Table 1. The reaction of 1a in high-temperature watera
Entry
Temp.
(°C)
Water
(g)
Density
Ion product
(−log Kw)
Reaction period
(min)
Conv.
(%)
Isolated yield (%)
(g/cm3)b
2a
3a
4a
1
2
3
4
5
6c
7
435
435
435
435
435
300
200
0.5
3.0
5.4
0
3.0
3.0
3.0
0.05
0.30
0.54
–
0.30
–
22.0
13.8
11.1
–
13.8
–
90
90
90
90
15
90
90
100
100
100
47
69
44
42
46
43
11
27
15
–
40
48
46
12
28
16
–
–
–
–
–
–
Trace
27
–
–
30
a Reaction conditions: 2.0 mmol of 1a, H2O, under Ar in a tubular steel bomb reactor (10 mL), unless otherwise stated.
b The value of supercritical water density: water density=water (g)/the volume of reactor (10 mL).
c A trace amount of 4a was detected by 1H NMR.
* Corresponding author.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01163-2

5860
B. Hatano et al. / Tetrahedron Letters 43 (2002) 5859–5861
(2)
Table 2. Disproportionation of 1 in supercritical watera
1
Ar1
Ar2
X
Isolated yield (%)
Alkane 2
Ketone 3
1b
1c
1d
1e
1f
1g
1h
1i
C6H5
C6H5
4-MeC6H4
C6H5
C6H5
C6H5
C6H5
4-MeC6H4
C6H5
3-MeC6H4
4-MeC6H4
4-MeC6H4
2-Naphtyl
4-Pyridyl
C6H5
4-MeC6H4
4-MeC6H4
Cyclohexyl
OH
OH
OH
OH
OH
NH2
NH2
NH2
OH
46
47
34
39
49b
49
41
37
43
42
34
38
49b
49
42
38
1j
Complex mixture
a Reaction conditions: 2.0 mmol of 1, 3.0 mL of H2O, at 435°C, under Ar, unless otherwise stated.
b The yield was determined by 1H NMR.
(3)
diphenylmethanol (1a) was transformed into diphenyl-
methane (2a) and benzophenone (3a). In order to reveal
the scope of the present unique disproportionation, 1a
was treated under several conditions summarized in Eq.
(1) and Table 1.7 In supercritical water, the present
disproportionation proceeded smoothly over the exam-
ined water density range (0.05, 0.30, and 0.54 g/cm3)
(entries 1, 2, and 3). Although change in the density of
supercritical water is reported to influence ion product
at the same temperature,8,9 it did not affect the reac-
tion. When 1a was heated at 435°C without water, the
yields of the desirable products greatly decreased (entry
4). This fact indicates that supercritical water acceler-
ates the disproportionation. The shorter reaction time
gave the corresponding disproportionation products in
low yields along with unchanged 1a (entry 5). On the
other hand, the reaction temperature affected the selec-
tivity of products. At 300°C, we detected a trace
amount of bis(diphenylmethyl) ether (4a), along with 2a
and 3a (entry 6). Ether 4a was obtained in 27% yield
instead of the disproportionation products at 200°C
(entry 7).
trace amount of 1g and 1a (17%) was observed along
with 2a (35%) and 3a (37%). This fact suggests the
amine derivatives are first transformed to the corre-
sponding alcohol derivatives (1). Cyclohexylphenyl-
methanol (1j), bearing
a hydrogen atom at the
a-position of the hydroxyl group, gave a complex mix-
ture containing benzylidenecyclohexane and 1-benzyl-
cyclohexene along with a trace amount of the dispro-
portionation products. The unexpected products were
yielded through intramolecular dehydration of 1j. The
alkenes formation from alcohols was reported to be
accelerated in supercritical water.11
When ether 4a was treated under the supercritical con-
dition, the corresponding products were obtained in
excellent yields (Eq. (3)).
Based on these results, the disproportionation is consid-
ered to proceed through the pathway shown in Scheme
1. It was already reported that diarylmethanols were
transformed into the corresponding diarylmethanes and
diarylketones in the presence of p-toluenesulfonic acid
This disproportionation was effected with other diaryl-
methanols (Eq. (2), Table 2). Similar treatment of
diarylmethanols (1b–d) with supercritical water gave the
corresponding disproportionation products in good
yields. The substrates bearing a naphthyl (1e) or 4-
pyridyl group (1f)10 also gave the disproportionation
products. In addition to the diarylmethanols, primary
amines such as diphenylmethylamine derivatives (1g–i)
were found to give 2 and 3 in good yields. When 1g was
treated under supercritical condition for 15 minutes, a
OH
Ar2
Ar1
Ar2
Ar1
Ar2
O
Ar1
1
4
O
Ar2
Ar1
Ar2
Ar1
2
3
Scheme 1.

B. Hatano et al. / Tetrahedron Letters 43 (2002) 5859–5861
5861
or trifluoromethanesulfonic acid, and the reaction is
considered to proceed via a hydride transfer mechanism
from a bis(diarylmethyl) ether 4 intermediate.12 Conse-
quently, we consider the present disproportionation to
proceed through the intramolecular hydrogen transfer
of 4 to give alkane 2 and ketone 3. During the pro-
cesses, supercritical water not only works as a reaction
medium but also accelerates the reaction.
5. For examples of organic synthesis using high-temperature
water: (a) Ikushima, Y.; Hatakeda, K.; Sato, O.;
Yokoyama, T.; Arai, M. Angew. Chem., Int. Ed. 2001, 40,
210; (b) Bro¨ll, D.; Kaul, C.; Kra¨mer, A.; Krammer, P.;
Richter, T.; Jung, M.; Vogel, H.; Zehner, P. Angew.
Chem., Int. Ed. 1999, 38, 2998; (c) Ikushima, Y.;
Hatakeda, K.; Sato, O.; Yokoyama, T.; Arai, M. Angew.
Chem., Int. Ed. 1999, 38, 2910; (d) Sato, O.; Ikushima,
Y.; Yokoyama, T. J. Org. Chem. 1998, 63, 9100; (e) Junk,
T.; Catallo, W. J. Tetrahedron Lett. 1996, 37, 3445; (f)
Yao, J.; Evilia, R. F. J. Am. Chem. Soc. 1994, 116, 11229.
6. Another type of disproportionation in supercritical water,
i.e. Cannizzaro-type disproportionation, has been
reported in the previous study.5a,b
7. A typical procedure is as follows: in a tubular steel bomb
reactor (10 mL) were placed diphenylmethanol 1a (368
mg, 2.0 mmol) and water (3.0 mL) under argon atmo-
sphere and kept at 435°C for 90 min in a sand bath. After
the reactor was cooled to room temperature in a water
bath, water (20 mL) and ethyl acetate (30 mL) were
added to the resulting mixture, and the two liquid layers
were separated. The organic layer was washed with brine
(20 mL), dried over Na2SO4, and concentrated. The
residue was purified by column chromatography on silica
gel (30 g; eluent, petroleum ether:chloroform=50:0, 48:2,
46:4, 44:6, 42:8, 40:10, 35:15, 30:20, 25:25, and 0:50, each
×50 mL), giving 2a (162 mg, 0.96 mmol) and 3a (168 mg,
0.92 mmol) in the yields as shown in Table 1, entry 1.
8. The ion product value (−log Kw) was calculated by water
density and reaction temperature. For reference see: (a)
Wagner, W.; Kruse, A. Properties of Water and Steam;
Springer-Verlag: Berlin–New York–Tokyo, 1998; (b)
Marshall, W. L.; Franck, E. U. J. Phys. Chem. Ref. Date
1981, 10, 295.
In conclusion, we have developed a novel dispropor-
tionation of diarylmethanol derivatives and diaryl-
methylamine derivatives. In supercritical water, this
disproportionation proceeds under neutral conditions
efficiently. Although the present method can be applied
only to limited substrates, this report reveals the syn-
thetic utility of supercritical water as a reaction
medium. More detailed study on the reaction mecha-
nism as well as other synthetic applications of supercrit-
ical water are now in progress.
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