Room-temperature Ru(II)-catalyzed transfer hydrogenation of ketones and aldehydes in air

Article abstract of DOI:10.1016/j.tetlet.2009.05.100

Transfer hydrogenation (TH) of ketones and aldehydes was efficiently carried out in 2-propanol at room temperature by means of a ruthenium(II) complex catalyst bearing a 2-(benzoimidazol-2-yl)-6-(pyrazol-1-yl)pyridine ligand. TH of the ketone substrates proceeded in air, reaching final TOFs of up to 59,400 h^-1, and the reduction of aldehydes proceeded under a nitrogen atmosphere to achieve final TOFs of up to 5940 h^-1.

Full text of DOI:10.1016/j.tetlet.2009.05.100

Tetrahedron Letters 50 (2009) 4624–4628
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Room-temperature Ru(II)-catalyzed transfer hydrogenation of ketones and
aldehydes in air
Miao Zhao a,b, Zhengkun Yu b, , Shenggang Yan , Yang Li
c
a
*
a
Department of Polymer Materials, College of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning 116012, China
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning 116023, China
Electromechanics and Materials Engineering College, Dalian Maritime University, Dalian, Liaoning 116026, China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Transfer hydrogenation (TH) of ketones and aldehydes was efficiently carried out in 2-propanol at room
Received 10 April 2009
Revised 22 May 2009
Accepted 27 May 2009
Available online 30 May 2009
temperature by means of a ruthenium(II) complex catalyst bearing a 2-(benzoimidazol-2-yl)-6-(pyrazol-
ꢀ1
1-yl)pyridine ligand. TH of the ketone substrates proceeded in air, reaching final TOFs of up to 59,400 h
,
and the reduction of aldehydes proceeded under a nitrogen atmosphere to achieve final TOFs of up to
ꢀ
1
5
940 h .
Ó 2009 Elsevier Ltd. All rights reserved.
Transfer hydrogenation (TH) is a potentially useful protocol for
the reduction of ketones and aldehydes to their corresponding alco-
hols and TH of ketones has been extensively studied. Ruthenium(II)
sition metal complex catalysts should be employed for the
catalytic reactions. Recently, we reported a new class of highly ac-
tive robust ruthenium(II) NNN complexes bearing a hemilabile
unsymmetrical pyridyl-based pyrazolyl–imidazolyl ligand which
can potentially provide a dynamic ‘on and off’ chelating effect for
1
complexes are usually applied as the most useful catalysts for trans-
fer hydrogenation of ketones. Ruthenium(II) complexes containing
monotosylated 1,2-diamines or aminoalcohols can offer high cata-
lytic activity and selectivity due to the presence of a N–H function-
1
0
the metal center during catalysis, and found that both complexes
1
0d
10f
1
and 2
can exhibit the same exceptionally high catalytic
2
ality (bifunctional catalysis). A great variety of related ligands and
activity in TH of ketones in refluxing 2-propanol under nitrogen
atmosphere, reaching 100% conversion for the ketone substrates
transition metal complex catalysts have been developed for this
3
5
ꢀ1
purpose. Although aldehydes seem to be more easily reduced to
and final TOFs of up to 7.2 ꢁ 10 h
with 0.05 mol % catalyst,
ꢀ1
their corresponding alcohols by transfer hydrogenation with 2-pro-
panol, reduction of aldehydes to primary alcohols with controlled
chemoselectivity of the products is considered rather difficult be-
cause catalytic TH of aldehydes usually leads to side reactions which
and 2 showed a final TOF value of 55,800 h in room-temperature
TH of ketones in nitrogen with 0.1 mol % loading. Herein, we report
the exploration of the catalytic behaviors of complex 1 in TH of ke-
tones and aldehydes in air at room temperature (Scheme 1).
4
occur under basic conditions. Thus, catalytic TH of aldehydes has
O
OH
OH
Me
O
been occasionally studied.5 Transition metal complex-catalyzed
TH of ketones is usually carried out in refluxing 2-propanol under
cat. 1
+
+
Ph
Me
Ph
ð1Þ
iPrOK/iPrOH
2
an inert atmosphere (N or Ar) in order to keep the catalysts active
in air
during the reaction and to reach a high conversion for the ketone
1
In our initial studies, TH of acetophenone was chosen as the mod-
el reaction to screen the reaction conditions (Eq. 1). With 0.05 mol %
1 as the catalyst in a 0.1 M solution of acetophenone in 2-propanol,
substrates. For asymmetric TH of ketones, the reactions may be car-
6
7
ried out in 2-propanol, HCO
2
H–Et
3
N, or water (usually an aqueous
8
solution of HCO
2
Na) at room temperature to obtain relatively sat-
isfactory enantioselectivity for the chiral alcohol products with very
low reaction rates. In the view of the practical application, room-
temperature catalytic TH of carbonyl compounds in air is strongly
desired for the production of alcohols in organic syntheses and
industries. Unfortunately, only a few examples of TH of ketones in
air at elevated temperatures have been documented.9
Me
Me
Me
Me
H
N
N
N
N
N
N
N
N
Cl
N
N
Ru
Cl
1
Ru
Cl
2
In order to realize TH of carbonyl compounds under mild condi-
tions such as at room temperature and/or in air, highly active tran-
Ph P
Ph P
3
3
*
Corresponding author. Tel./fax: +86 411 8437 9227.
E-mail address: zkyu@dicp.ac.cn (Z.K. Yu).
Scheme 1. Complexes 1 and 2.
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.05.100

M. Zhao et al. / Tetrahedron Letters 50 (2009) 4624–4628
4625
Table 1
Table 1 (continued)
Transfer hydrogenation of ketones catalyzed by complex 1 at room temperature in
air
ꢀ1
a,14
Entry
Ketone
Time (min)
0.5
Yieldb (%)
>99
Final TOF (h
)
0
.2 mol % 1
iPrOK
O
OH
OH
R2
O
16
O
59,400
58,800
+
+
(2)
R1
R2
R1
rt, in air
Entry
1
Ketone
Time (min)
Yieldb (%)
Final TOF (h )
ꢀ1
1
1
7
8
O
0.5
98
O
1 (0.5)c
95 (98)
28,500 (235,200)
39,200
Me
Et
O
O
O
O
1 (0.5)d
Me
73 (91)
21,900 (36,400)
0.5d
98
5
2
3
4
5
a
Reaction conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); 0.2 mol % 1,
Me
Cl
ketone/iPrOK/cat. 1 = 500:20:1; 28 °C, in air.
b
0.5 (0.5)c
>99 (97)
>99
59,400 (232,800)
59,400
By GC analysis.
Me
Me
Me
c
0
0
.05 mol % 1, 82 °C.
.3 mol % 1.
d
e
By 1H NMR determination.
0.5
1
OMe O
the TH reaction in air reached 35% and 49% conversions for the
ketone substrate within 10 min at room temperature (28 °C) and
93
27,900
4
0 °C, respectively, and then the reaction halted. On increasing the
catalyst loading to 0.1 mol %, the reaction proceeded faster to form
O
1
1
-phenylethanol in 73% and 91% yields at room temperature within
0 min and at 40 °C over a period of 2 min, respectively, and then
Me
1 (0.5)c
6
7
8
9
91 (97)
89e
27,300 (232,800)
26,700
Me
the reaction became very slow. However, in refluxing 2-propnaol
and with a lower catalyst loading (0.05 mol % 1) the same reaction
approached completion in air within 0.5 min, and under nitrogen
atmosphere acetophenone can be reduced to 1-phenylethanol
O
MeO
Br
1
Me
1
0d
within 10 s.
These results have demonstrated the possible deac-
O
O
1
1
tivation of complex 1 in air and the unpreferable generation of the
catalytically active species at room temperature under the TH con-
ditions, suggesting that higher catalyst loading should be used to
promote the TH of ketones at room temperature in air. When
0.2 mol % 1 was applied in the reaction at room temperature in
air, 95% conversion was obtained for acetophenone within 1 min,
revealing that considerable amount of the complex catalyst should
be present during the catalysis. It should be noted that 1-phenyl-
ethanol is the only product obtained from TH of acetophenone at
room temperature in air, and the most suitable molar ratio of iPrOK
base to Ru is 20:1.
0.5
98
58,800
Me
Cl
0.5
98
58,800
Me
O
10
11
12
13
14
15
Me
O
0.5 (0.5)c
93 (96)
55,800 (230,400)
Me
Thus, 0.2 mol % 1 was used as the catalyst for TH of a variety of ke-
tones at room temperature in air (Eq. 2 and Table 1). Acetophenone
was converted to 1-phenylethanol in 95% yield within 1 min (entry
Me
1
83
98
98
92
97e
24,900
58,800
58,800
55,200
29,100
MeO
Cl
O
O
1
). The TH of propiophenone was much slower under the same con-
ditions, affording the alcohol product in 80% yield within 1 min and
in 88% yield over a period of 10 min, while 98% conversion was
achieved for the ketone substrate by using 0.3 mol % catalyst within
Me
Me
0.5
0.5
0.5
1
3
0 s (entry 2). In most cases, TH of ketones reached 95–99% conver-
sion for the ketone substrates (Table 1) with final TOFs of up to
ꢀ1
5
9,400 h (entries 3, 4, and 16). To date, the known transition metal
Br
catalysts for TH and/or hydrogenation of ketones near room temper-
ature have only shown the highest TOFs in the range of 100–
O
ꢀ
1 12
4000 h
also efficiently reduced to the corresponding alcohols within 30 s
entries 16 and 17). However, aliphatic ketone 2-heptanone could
.
Cyclic ketones cyclohexanone and cyclopentanone were
Me
(
O
not be efficiently reduced under the same conditions or even with
a higher catalyst loading such as 0.3 mol % 1 (entry 17). It is notewor-
thy that the ketones could be more efficiently reduced to alcohols in
air with a lower catalyst loading such as 0.05 mol % 1 in refluxing 2-
ꢀ1
propanol, achieving final TOFs of up to 235,200 h (entries 1, 3, 6,
and 10).

4
626
M. Zhao et al. / Tetrahedron Letters 50 (2009) 4624–4628
Table 2
Table 3
Transfer hydrogenation of aldehydes catalyzed by complex 1 at room temperature
Transfer hydrogenation of aldehydes catalyzed by complex 1 at 82 °C under nitrogen
atmosphere
under nitrogen atmospherea
a
0
.05 mol % 1
PrOK
82 ºC, in N
Yieldb (%)
ꢀ1
O
Entry
1
Aldehyde
CHO
Time (min)
10
Final TOF (h
2940
)
OH
OH
O
i
+
+
(4)
R
H
R
H
2
98
ꢀ1
Entry
1
Aldehyde
Time (min)
Yieldb (%)
Final TOF (h )
CHO
0.5c
Me
98
>99
>99
235,200
594,000
712,800
d
0.5
CHO
CHO
2
3
4
5
6
7
8
9
5
5
>99
>99
97
39
45
98
96
49
32
98
25
98
5940
5940
2910
1170
1350
5880
5760
1470
960
1/6
Me
OMe
CHO
CHO
2
3
4
5
6
7
8
9
0.5
0.5
1
>99
>99
98
237,600
237,600
117,600
117,600
117,600
235,200
116,400
235,200
117,600
OMe
Me
CHO
10
10
10
5
Me
CHO
Cl
CHO
Cl
CHO
1
98
F C
3
CHO
CHO
F C
3
CHO
CHO
1
98
Me
MeO
Cl
CHO
CHO
0.5
1
98
5
Me
MeO
Cl
CHO
CHO
97
10
10
10
30
5c
CHO
0.5
1
98
10
11
12
13
Br
CHO
10
98
2940
250
Br
O
S
CHO
CHO
O
1
1
2
5
>99
(>99)
23,760
(118,800)
CHO
CHO
(
0.5)e
S
1
5f
95
5700
3920
CHO
a
Reaction conditions: aldehyde, 2.0 mmol (0.1 M in 20 mL iPrOH); 0.2 mol % 1,
aldehyde/iPrOK/cat. 1 = 500:20:1; 28 °C, in N
2
.
b
By GC analysis.
c
0
.3 mol % 1.

M. Zhao et al. / Tetrahedron Letters 50 (2009) 4624–4628
4627
Table 3 (continued)
Entry Aldehyde
2-furaldehyde, 2-thiophenecarboxaldehyde, and 2-methylbutyral-
dehyde were also efficiently reduced to their corresponding alco-
hols (entries 11–13).
Yieldb (%)
ꢀ1
Time (min)
2
Final TOF (h )
1
3
98
(>99)
58,800
(118,800)
Complexes 1 and 2 exhibited the same catalytic activity in TH of
ketones and aldehydes because 1 can be instantly transformed to 2
CHO
(
0.5)e
1
0d,f
under the reaction conditions.
The present transfer hydrogena-
a
Reaction conditions: aldehyde, 2.0 mmol (0.1 M in 20 mL iPrOH); 0.05 mol % 1,
13
tion may follow an inner-sphere mechanism as we proposed pre-
aldehyde/iPrOK/cat. 1 = 2000:20:1; 82 °C, in N
2
.
1
0d,f
b
viously.
Thus, TH of a ketone or aldehyde is presumably initiated
By GC analysis.
In air.
c
from 16-electron complex 2 in situ instantly generated by extrusion
of 1 equiv of hydrogen chloride from 1 with iPrOK base. Complex 2
interacts with the base to form Ru(II)-alkoxide which undergoes
b-H elimination to result in a Ru–H intermediate and the release of
acetone. Coordination of a ketone or aldehyde substrate to the
Ru–H species followed by insertion of the coordinated substrate
carbonyl into the Ru–H bond gives another Ru(II)-alkoxide which
is then reacted with 2-propanol to afford the alcohol product. The
Ru(II) hydride is presumably considered as the catalytically active
species although it is not successfully isolated by reacting 1 or 2 with
EtONa or iPrOK in refluxing ethanol (or 2-propanol).
d
0
0
0
.02 mol % 1.
.1 mol % 1.
.2 mol % 1.
e
f
0
.2 mol % 1
PrOK
rt, in N2
O
OH
OH
O
i
ð3Þ
+
+
Ph
H
Ph
H
Intrigued by the excellent catalytic activity of complex 1 in
room-temperature TH of ketones in air, we then tried the TH of
benzaldehyde under the same conditions as given in Table 1. The
reduction of benzaldehyde by 2-propanol at room temperature in
air only reached 50% conversion for the aldehyde under the stated
conditions, forming a mixture of benzyl alcohol and some un-
known side products within 5 min. However, with 0.2 mol % 1 as
the catalyst, TH of benzaldehyde at room temperature under nitro-
gen atmosphere afforded benzyl alcohol as the only product in 98%
yield within 10 min (Table 2, entry 1). If the reaction was carried
out with 0.05 mol % 1 as the catalyst in refluxing 2-propanol in
air, 98% conversion was obtained for the aldehyde to produce ben-
zyl alcohol over a period of 30 s, and the same reaction gave >99%
yield of the desired alcohol product within 10 s (final
In summary, we have developed an unusually active and effi-
cient Ru(II) complex catalyst for room-temperature TH of ketones
and aldehydes in air. The present TH methodology has demon-
strated potential application in reduction of ketones and aldehydes
to alcohols under mild conditions.
Acknowledgments
We are grateful to the National Natural Science Foundation of
China (20772124) and the National Basic Research Program of Chi-
na (2009CB825300) for the support of this research.
References and notes
ꢀ1
TOF = 712,800 h ) under nitrogen atmosphere. These results have
demonstrated that TH of aldehydes should be carried out under an
inert atmosphere or at elevated temperatures in air to reduce the
possible side reactions which usually take place under basic condi-
tions. Using 0.2 mol % 1 as the catalyst, unsubstituted and electron-
donating substituent-bearing aromatic aldehydes were efficiently
reduced to the corresponding alcohols by 2-propanol at room tem-
perature under nitrogen atmosphere, reaching up to >99% conver-
1. For selected recent reviews, see: (a) Ikariya, T.; Blacker, A. J. Acc. Chem. Res.
2007, 40, 1300; (b) Wu, X.; Xiao, J. L. Chem. Commun. 2007, 2449; (c) Gladiali, S.;
Alberico, E. Chem. Soc. Rev. 2006, 35, 226; (d) Samec, J. S. M.; B a¯ ckvall, J.-E.;
Andersson, P. G.; Brandt, P. Chem. Soc. Rev. 2006, 35, 237.
2. Ohkuma, T.; Utsumi, N.; Tsutsumi, K.; Murata, K.; Sandoval, C.; Noyori, R. J. Am.
Chem. Soc. 2006, 128, 8724.
3.
For selected recent reports, see: (a) Schlattera, A.; Woggon, W.-D. Adv. Synth.
Catal. 2008, 350, 995; (b) Baratta, W.; Ballico, M.; Esposito, G.; Rigo, P. Chem.
Eur. J. 2008, 14, 5588; (c) Liu, D. L.; Xie, F.; Zhao, X. H.; Zhang, W. B. Tetrahedron
2008, 64, 3561; (d) Lundgren, R. J.; Rankin, M. A.; McDonald, R.; Schatte, G.;
Stradiotto, M. Angew. Chem., Int. Ed. 2007, 46, 4732.
ꢀ1
sion for the aldehyde substrates and final TOFs of up to 5940 h
within 10 min (Table 2, entries 1–4, 7, 8, and 11). Aliphatic alde-
hyde 3-methylbutyraldehyde was also efficiently transformed to
the corresponding alcohol with 0.3 mol % 1 as catalyst under the
same conditions (entry 13). However, electron-withdrawing sub-
stituent-bearing aromatic aldehydes could not be efficiently re-
duced to the desired products due to their side reactions under
the TH reaction conditions (entries 5, 6, 9, and 10).
4
.
Baratta, W.; Siega, K.; Rigo, P. Adv. Synth. Catal. 2007, 349, 1633.
5. (a) Gnanamgari, D.; Moores, A.; Rajaseelan, E.; Crabtree, R. H. Organometallics 2007,
6, 1226.; (b) Maytum, H. C.; Tavassoli, B.; Williams, J. M. J. Org. Lett. 2007, 9, 4387;
c) Castarlenas, R.; Esteruelas, M. A.; Oñate, E. Organometallics 2008, 27, 3240.
2
(
6.
For selected recent reports, see: (a) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya,
T.; Noyori, R. . J. Am. Chem. Soc. 1995, 117, 7562; (b) Pastor, I. M.; Västilä, P.;
Adolfsson, H. Chem. Commun. 2002, 2046; (c) Hannedouche, J.; Clarkson, G. J.;
Wills, M. J. Am. Chem. Soc. 2004, 126, 986; (d) Schiffers, I.; Rantanen, T.;
Schmidt, F.; Bergamans, W.; Zani, L.; Bolm, C. J. Org. Chem. 2006, 71, 2320; (e)
Zaitsev, A. B.; Adolfsson, H. Org. Lett. 2006, 8, 5129; (f) Sui-Seng, C.; Freutel, F.;
Lough, A. J.; Morris, R. H. Angew. Chem., Int. Ed. 2008, 47, 940.
For selected recent reports, see: (a) Fujii, A.; Hashiguchi, S.; Uematsu, N.;
Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 2521; (b) Lennon, I. C.;
Ramsden, J. A. Org. Proc. Res. Dev. 2005, 9, 110; (c) Morris, D. J.; Hayes, A. M.;
Wills, M. J. Org. Chem. 2006, 71, 7035; (d) Cheung, F. K.; Graham, M. A.; Minissi,
F.; Wills, M. Organometallics 2007, 26, 5346.
(a) Wu, X. F.; Li, X. H.; Zanotti-Gerosa, A.; Pettman, A.; Liu, J. K.; Mills, A. J.; Xiao,
J. L. Chem. Eur. J. 2008, 14, 2209; (b) Ahlford, K.; Lind, J.; Mäler, L.; Adolsson, H.
Green Chem. 2008, 10, 832.
(a) Wu, X. F.; Vinci, D.; Ikariya, T.; Xiao, J. L. Chem. Commun. 2005, 4447; (b) Wu,
X. F.; Liu, J. K.; Li, X. H.; Zanotti-Gerosa, A.; Hancock, F.; Vinci, D.; Ruan, J. W.;
Xiao, J. L. Angew. Chem., Int. Ed. 2006, 45, 6718; (c) Xing, Y.; Chen, J.-S.; Dong,
Z.-R.; Li, Y.-Y.; Gao, J.-X. Tetrahedron Lett. 2006, 47, 4501; (d) Zhang, X.-Q.; Li,
Y.-Y.; Zhang, H.; Gao, J.-X. Tetrahedron: Asymmetry 2007, 18, 2049; (e) Chen, G.;
Xing, Y.; Zhang, H.; Gao, J.-X. J. Mol. Catal. A: Chem. 2007, 273, 284.
In order to get an efficient TH system for aldehydes, the applied
catalyst should be active enough to result in the fast reduction of
aldehydes at elevated temperatures to avoid base-promoted side
reactions of aldehydes.4 Surprisingly, 0.05 mol % 1 promoted
nearly complete reduction of benzaldehyde to benzyl alcohol in
refluxing 2-propanol in air over a period of 30 s, and the same reac-
tion could be finished within 10 s under nitrogen atmosphere,
7.
8
9
.
.
ꢀ1
reaching a final TOF value of 712,800 h (Table 3, entry 1). With
a lower catalyst loading such as 0.02 mol % 1, the reaction of benz-
aldehyde was also finished under nitrogen atmosphere within 30 s.
The TH reactions of other aldehydes in refluxing 2-propanol in air
usually gave the alcohol products as well as some unknown side
products, but they were efficiently reduced to the corresponding
alcohols under nitrogen atmosphere (Table 3). The reduction of
aromatic aldehydes was complete within 1 min (entries 1–10),
achieving 97–99% conversion for the aldehydes and exclusively
forming the alcohol products. By increasing the catalyst loading
to 0.1–0.2 mol % or by extending the reaction time to 2–5 min,
1
1
0. (a) Sun, X. J.; Yu, Z. K.; Wu, S. Z.; Xiao, W.-J. Organometallics 2005, 24, 2959; (b)
Deng, H. X.; Yu, Z. K.; Dong, J. H.; Wu, S. Z. Organometallics 2005, 24, 4110; (c)
Zeng, F. L.; Yu, Z. K. J. Org. Chem. 2006, 71, 5274; (d) Zeng, F. L.; Yu, Z. K.
Organometallics 2008, 27, 2898; (e) Zeng, F. L.; Yu, Z. K. Organometallics 2008,
2
7, 6025; (f) Zeng, F. L.; Yu, Z. K. Organometallics 2009, 28, 1855.
1. Sun, X. Y.; Manos, G.; Blacker, J.; Martin, J.; Gavriilidis, A. Org. Proc. Res. Dev.
2004, 8, 909.

4
1
628
M. Zhao et al. / Tetrahedron Letters 50 (2009) 4624–4628
2. (a) Sui-Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H. Angew. Chem., Int. Ed. 2008,
7, 940; (b) Nordin, S. J. M.; Roth, P.; Tarnai, T. T.; Alonso, D. A.; Brandt, P.;
of 0.1 M iPrOK (0.08 mmol) solution in 2-propanol was introduced to
initiate the transfer hydrogenation. At the stated time, 0.1 mL of the
reaction mixture was sampled and immediately diluted with 0.5 mL of
2-propanol precooled at 0 °C for immediate GC analysis. After the reaction
was complete, the reaction mixture was condensed under reduced pressure
and was subjected to purification by flash silica gel column chromatography
to afford the alcohol product. The alcohol products were identified by
comparison of their GC traces with the authentic samples and/or by proton
NMR measurements.
4
Andersson, P. G. Chem. Eur. J. 2001, 7, 1431.
1
1
3. Comas-Vives, A.; Ujaque, G.; Lledós, A. Organometallics 2007, 26, 4135.
4. A general procedure for the catalytic transfer hydrogenation of ketones and
aldehydes: The catalyst solution was prepared by dissolving complex
1
(
20.0 mol) in 2-propanol (40.0 mL). Under nitrogen atmosphere or in air,
l
the mixture of a ketone (or aldehyde) substrate (2.0 mmol), 8.0 mL of the
catalyst solution (0.2 mol % catalyst 1), and 2-propanol (11.2 mL) was
stirred at room temperature (ca. 28 °C) or 82 °C for 5 min. Then, 0.8 mL