Synthesis of α-hydroxy ketones from vicinal diols by selective dehydrogenation over Ir-ReOx/SiO2 catalyst

Article abstract of DOI:10.1246/cl.130983

Rhenium oxide-modified Ir/SiO₂ (Ir-ReO_x/SiO ₂) catalyst shows high activity and selectivity for the dehydrogenation of trans-1,2-cyclohexanediol to 2-hydroxycyclohexanone in water solvent under Ar. Linear vicinal diols bearing two secondary hydroxy groups or both a primary hydroxy group and a secondary hydroxy group can also be transformed to the corresponding α-hydroxy ketones in high selectivities. Ir-ReO_x/SiO₂ can be reused at least four times without loss of activity and selectivity.

Full text of DOI:10.1246/cl.130983

CL-130983
Received: October 22, 2013 | Accepted: November 9, 2013 | Web Released: November 15, 2013
Synthesis of α-Hydroxy Ketones from Vicinal Diols
by Selective Dehydrogenation over Ir-ReOx/SiO2 Catalyst
Hiraku Sato, Masazumi Tamura, Yoshinao Nakagawa, and Keiichi Tomishige*
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University,
6-6-07 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579
(E-mail: tomi@erec.che.tohoku.ac.jp)
In our previous studies15 on hydrogenolysis of polyols or
hydrogenation of aldehydes, Ir-ReOx/SiO2 catalyst showed high
activity for these reactions. The high activity can arise from the
high ability for heterolytic H2 activation. Considering the same
dissociation mechanism of C-H bonds as that of H-H bonds in
coordination chemistry, Ir-ReOx/SiO2 catalyst will be active for
heterolytic dissociation of C-H. Therefore, Ir-ReOx/SiO2
catalyst may be a new catalyst for dehydrogenation of alcohols
because dissociation of C-H is a key step in dehydrogenation of
alcohols. In this communication, we demonstrated that Ir-ReOx/
SiO2 catalyst is an effective heterogeneous catalyst for the
selective dehydrogenation of diols in water under Ar.
Rhenium oxide-modified Ir/SiO2 (Ir-ReOx/SiO2) catalyst
shows high activity and selectivity for the dehydrogenation of
trans-1,2-cyclohexanediol to 2-hydroxycyclohexanone in water
solvent under Ar. Linear vicinal diols bearing two secondary
hydroxy groups or both a primary hydroxy group and a secondary
hydroxy group can also be transformed to the corresponding α-
hydroxy ketones in high selectivities. Ir-ReOx/SiO2 can be reused
at least four times without loss of activity and selectivity.
α-Hydroxy ketones are valuable in organic chemistry
because these compounds are important intermediates for
syntheses of natural products and fine chemicals such as
fragrances, agrochemicals, and medicines.1 As conventional
methods, acyloin condensation of diesters or benzoin condensa-
tion of aldehydes are very well known for the synthesis of α-
hydroxy ketones. However, these reaction systems have serious
drawbacks such as use of hazardous reagents like sodium metal
or cyanide compounds and production of metal salts by
neutralization after the reaction. To overcome these problems,
various methods for preparation of α-hydroxy ketones such as
dihydroxylation of enol ethers,2 oxidation of silylenol ethers3 or
epoxides,4 α-oxidation of ketones,5 oxidation of olefins,6 and
selective transformation of vicinal diols7-13 have been devel-
oped. Among these methods, selective transformation of diols is
promising from the viewpoints of availability of substrates
because diols can be easily produced from biomass and biomass-
derived compounds.14 However, selective transformation of
diols to α-hydroxy ketones is a quite challenging reaction
because of poor chemoselectivity caused by competitive C-C
cleavage, over-oxidation, and dehydration. Among various
diols, 1,2-cyclohexanediol (1,2-CHD) is frequently used as a
motif of sugars.9a Various transformation methods of 1,2-
CHD to 2-hydroxycyclohexanone (1) with stoichiometric
dioxiranes,7 organotin,8 NaBrO3/NaHSO3,9 Pd complex,10 Cu
catalyst,11 and electrocatalyst12 have been reported. From the
viewpoints of green chemistry and energy consumption, direct
and selective dehydrogenation of diols by a catalyst will be
the most preferable because only H2 is produced in the
reaction.
Ir-ReOx/SiO2 catalyst was prepared by
a sequential
impregnation method using SiO2, H2[IrCl6](aq), and
NH4ReO4(aq), followed by drying at 383 K and calcination at
773 K. The loading amount of Ir was 4 wt %, and the molar ratio
of Re to Ir was 2. Carbon-supported noble metal (5 wt %)
catalysts (Pd/C, Ru/C, Rh/C, and Pt/C) are commercially
available. For the pretreatment of these catalysts, a catalyst in
water was heated at 473 K with 8 MPa H2 for 1 h in an autoclave.
After the pretreatment, the autoclave was cooled down, and H2
was removed from the autoclave. trans-1,2-CHD was put into
the autoclave, and the autoclave was heated to 493 K under Ar.
Products were analyzed by GC and GC-MS. The isomerization
product, cis-1,2-CHD, was excluded in the calculation of
conversion and selectivity because cis-1,2-CHD can be con-
verted to 1 at the same reactivity (Table S1).16 Conversion and
selectivity were defined as follows: conversion (%) = 100 ©
[1 ¹ {produced cis-1,2-CHD (mol) + unreacted trans-1,2-CHD
(mol)}/trans-1,2-CHD before the reaction (mol)] and selectivity
(%) = 100 © product (mol)/{trans-1,2-CHD before the reaction
(mol) ¹ produced cis-1,2-CHD (mol) ¹ unreacted trans-1,2-
CHD (mol)}. The details of the experimental method are
described in the Supporting Information.16
The dehydrogenation of trans-1,2-CHD to 1 was examined
over various catalysts (Table 1). Compared with the conven-
tional carbon-supported hydrogenation catalysts such as Pd/C,
Ru/C, Rh/C and Pt/C, Ir-ReOx/SiO2 catalyst showed higher
selectivity to 1 and higher or similar conversion (Entries 2 and
5-8), resulting in providing higher yield of 1. In the case of
carbon-supported catalysts, the selectivities to phenol and linear
alkanes were relatively high, indicating that dehydrogenation of
cyclohexane and C-C bond cleavage easily take place. On the
other hand, the selectivity of copper oxide was as high as that of
Ir-ReOx/SiO2, however, the activity was much lower (Entry 9).
It should be noted that the activity of Ir-ReOx/SiO2 catalyst
was much higher than that of Ir/SiO2 (Entry 3) and ReOx/
SiO2 (Entry 4). As above, Ir-ReOx/SiO2 catalyst is the most
effective for dehydrogenation of trans-1,2-CHD to 1 in water
under Ar.
OH
O
+ H2
ð1Þ
OH
OH
1
There is only one report for dehydrogenation of 1,2-CHD by a
heterogeneous catalyst, the system of which consists of a Cu
catalyst.11 However, this catalyst system suffers from high
reaction temperature (²533 K). Therefore, development of a
new heterogeneous catalyst for selective dehydrogenation of
diols to α-hydroxy ketones is desirable.
334 | Chem. Lett. 2014, 43, 334–336 | doi:10.1246/cl.130983
© 2014 The Chemical Society of Japan

Table 1. Production of 2-hydroxycyclohexanone (1) by dehydrogenation of trans-1,2-CHD over various catalystsa
OH
O
Catalyst
Water 10 g
493 K, 1 h, Ar (1 MPa)
OH
OH
0.1 g
1
Selectivity/%
cis/trans
ratio of
1,2-CHD
t
/h
Conv. Yield of 1
Entry Catalyst
2-Hydroxy-
cyclohexanone (1)
Linear
alkanes
Cyclohexanone Cyclohexanol Phenol
Othersb
/%
/%
1
2
3
4
5
6
7
8
9
Ir-ReOx/SiO2 0c
25.8
35.8
19.3
1.1
12.7
38.1
36.9
40.7
5.0
19.8
24.7
17.0
0.8
78.4
68.9
85.8
71.5
63.8
42.5
42.1
31.5
72.4
12.3
16.9
7.2
6.2
13.5
8.6
22.2
24.6
10.4
7.6
8.8
4.5
6.6
6.1
10.2
17.0
24.8
10.0
0.0
1.6
0.6
0.0
2.6
18.4
15.2
16.0
2.5
1.3
1.1
1.3
13.9
2.2
10.2
0.0
0.5
2.8
0.5
0.2
0.4
0.1
0.0
0.1
0.6
0.5
0.6
0.0
1
Ir/SiO2
ReOx/SiO2
Pd/C
Ru/C
Rh/Cd
Pt/Ce
1
1
1
1
1
1
1
1.8
8.2
11.8
10.0
3.5
3.1
0.2
16.0
15.8
13.0
3.7
0.1
4.5
CuO
aReaction conditions: Ir-ReOx/SiO2 0.1 g, trans-1,2-CHD 0.5 g, water 10 g, T = 493K, PAr = 1 MPa. bOthers: cyclohexane + benzene + cyclopentane-
c
d
e
carbaldehyde + 1,2-cyclohexanedione. The reaction was stopped when the temperature was raised to 493 K. Rh/C 50 mg. Pt/C 20 mg.
cis/trans ratio
OH
OH
O
0.20.4 0.5 0.5
0.4
0.4
O
OH
H2
-H2
H2
of 1,2-CHD
100
80
60
40
20
0
-H2O
OH
2-Hydroxy
cyclohexanone
trans-1,2-CHD
Cyclohexanone
Cyclohexanol
(1)
Consecutive reaction
Main reaction
Scheme 1. Reaction pathway for dehydrogenation of trans-1,2-
CHD over Ir-ReOx/SiO2 catalyst.
as the reaction proceeds. To clarify the cause of deactivation of
Ir-ReOx/SiO2, the catalyst after the reaction was characterized
by TG-DTA (Figure S1).16 The TG-DTA profile showed two
signals at 330 and 560 K, which are assigned to water and
products, respectively. The sample for TG-DTA was obtained by
flashing the recovered catalyst with ethanol. Therefore, the
products detected at 560 K will be strongly adsorbed on the
catalyst, resulting in a poison for the catalyst.
0
5
10
Time /h
15
20
Figure 1. Time-course of dehydrogenation of trans-1,2-CHD over
Next, effects of concentration of trans-1,2-CHD and the
reaction temperature were investigated (Figures S2 and S3).16
Effect of concentration of trans-1,2-CHD was examined within
the range from 3.2 to 9.0 wt % of trans-1,2-CHD. The concen-
tration of trans-1,2-CHD was adjusted by addition of water
(5-15 g) to the fixed amount of trans-1,2-CHD (0.5 g). The
conversion was almost constant in the above range. However,
the selectivity to 1 was decreased with increasing the concen-
tration of trans-1,2-CHD, and those to cyclohexanone and linear
alkanes were increased. The hydrogenolysis of 1 must readily
take place at the high concentration of the substrate because
concentration dependence of the reaction rate in hydrogenation
is positive (reaction order: +0.7).15g Therefore, lower concen-
tration of trans-1,2-CHD is more preferable. Effect of the
reaction temperature was investigated within the range from 433
to 533 K. The conversion was increased with increasing the
reaction temperature. The selectivity to 1 was almost the same in
the range from 433 to 493 K, but the selectivity was drastically
decreased at 533 K. Taking into consideration that the yield of
cyclohexanone was increased, the consecutive hydrogenolysis
reaction of 1 will proceed at the temperature.
Ir-ReOx/SiO2 catalyst
selectivity to cyclohexanone; , selectivity to cyclohexanol;
selectivity to others). Reaction conditions: Ir-ReOx/SiO2 0.1 g, trans-
1,2-CHD 0.5 g, water 10 g, T = 493 K, PAr = 1 MPa. Others: phe-
nol + linear alkanes + cyclohexane + benzene + cyclopentanecarbal-
dehyde + 1,2-cyclohexanedione.
(
: conversion; : selectivity to 1;
,
,
The time-course of the trans-1,2-CHD dehydrogenation
over Ir-ReOx/SiO2 was examined as shown in Figure 1. At the
initial stage, the selectivity to 1 was very high (78% at 0 h). The
selectivity to 1 gradually decreased with increasing the reaction
time, and those to cyclohexanone and cyclohexanol increased
simultaneously. These results indicate that cyclohexanone and
cyclohexanol are formed by the consecutive reactions of 1
(Scheme 1): cyclohexanone was produced by hydrogenolysis of
the hydroxy group of 1, and cyclohexanol was produced by
hydrogenation of cyclohexanone.15g On the other hand, after 5 h
reaction time, dehydrogenation of trans-1,2-CHD hardly pro-
ceeded and the conversion was saturated at about 50%, however,
the consecutive hydrogenolysis reaction proceeded. These
results suggest that the active site for dehydrogenation is
different from that for hydrogenolysis, and the active site for
dehydrogenation will be decomposed or covered by the products
The scope of diols in dehydrogenation at 453 K over Ir-
ReOx/SiO2 catalyst was examined (Table 2). 2,3-Butanediol,
Chem. Lett. 2014, 43, 334–336 | doi:10.1246/cl.130983
© 2014 The Chemical Society of Japan | 335

a
Table 2. Dehydrogenation of various diols over Ir-ReOx/SiO2
least four times without loss of its high catalytic performance,
and the selectivity was rather improved with increasing the reuse
time. From XRD and XAFS analyses of the catalysts before and
after the reaction, the structure of Ir-ReOx/SiO2 was hardly
changed during the course of the reaction (Figures S4-S7 and
Tables S2 and S3).16 This phenomenon is of great interest,
however, the details remain unclear. Further investigation is
necessary to clarify this behavior of Ir-ReOx/SiO2 catalyst.
In summary, Ir-ReOx/SiO2 catalyst acted as an efficient
catalyst for the dehydrogenation of diols to the corresponding
α-hydroxy ketones in water under Ar. Ir-ReOx/SiO2 catalyst
can be reused at least four times without loss of activity or
selectivity.
Entry Substrate Conv./%
Selectivity/%
O
OH Othersb
O
OH
1
2
3
17.1
19.1
12.6
OH
56.2
OH
OH
OH
16.5
20.9
6.3
O
OH
0
O
OH
OH
84.9
13.9
1.2
1.5
HO
O
OH
O
OH
83.0
8.2
7.2
aReaction conditions: Ir-ReOx/SiO2 0.1 g, substrate 4.3 mmol, water
10 g, T = 453 K, t = 1 h,
Ar = 1 MPa. bOthers: phenol + linear
alkanes + cyclohexane + benzene + cyclopentanecarbaldehyde +
1,2-cyclohexanedione.
This work was supported by the Cabinet Office, Government
of Japan through its “Funding Program for Next Generation
World-Leading Researchers.” The XAFS spectra were obtained
at the BL01B1 station at SPring-8 (JASRI; Proposal No.
2013A1048).
P
cis/trans ratio
0.4 0.3 0.4 0.4 0.5
of 1,2-CHD
100
References and Notes
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1
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which has two secondary hydroxy groups, reacted to afford 3-
hydroxy-2-butanone in high selectivity (Entry 2). 1,2-Butane-
diol, which has one primary hydroxy group and one secondary
hydroxy group, was also converted to 1-hydroxy-2-butanone in
high selectivity (Entry 3). These results indicate that dehydro-
genation of the secondary hydroxy group is more preferable. The
reactivities were almost the same as that of trans-1,2-CHD.
However, the selectivities of linear vicinal diols to the
corresponding α-hydroxy ketones were higher than that of
trans-1,2-CHD to 2-hydroxycyclohexanone. This result is
probably because the linear α-hydroxy ketones have no ring
strain and the structures are more stable than that of 2-
hydroxycyclohexanone.
Reusability of the catalyst is very important from an
industrial viewpoint. We evaluated the reusability of Ir-ReOx/
SiO2 catalyst for the selective dehydrogenation of trans-1,2-
CHD (Figure 2). The catalyst can be easily retrieved from the
reaction mixture by filtration. For each successive use, the
catalyst was washed with 10 mL of ethanol and dried at 383 K
for 12 h. After the treatment, the recovered catalyst was reused at
336 | Chem. Lett. 2014, 43, 334–336 | doi:10.1246/cl.130983
© 2014 The Chemical Society of Japan