pH-dependent catalytic activity and chemoselectivity in transfer hydrogenation catalyzed by iridium complex with 4,4′-dihydroxy-2,2′- bipyridine

Article abstract of DOI:10.1002/chem.200801568

Transfer hydrogenation catalyzed by an iridium catalyst with 4,4′dihydroxy-2,2′-bipyridine (DHBP) in an aqueous formate solution exhibits highly pH-dependent catalytic activity and chemoselectivity. The substantial change in the activity is due to the electronic effect based on the acid-base equilibrium of the phenolic hydroxyl group of DHBP. Under basic conditions, high turnover frequency values of the DHBP complex, which can be more than 1000 times the value of the unsubstituted analogue, are obtained (up to 81000 h^-1 at 80°C). In addition, the DHBP catalyst exhibits pH-dependent chemoselectivity for α,β-unsaturated carbonyl compounds. Selective reduction of the C=C bond of enone with high activity are observed under basic conditions. The ketone moieties can be reduced with satisfactory activity under acidic conditions. In particular, pH-selective chemoselectivity of the C=O versus C=C bond reduction was observed in the transfer hydrogenation of cinnamaldehyde.

Full text of DOI:10.1002/chem.200801568

DOI: 10.1002/chem.200801568
pH-Dependent Catalytic Activity and Chemoselectivity in
Transfer Hydrogenation Catalyzed by Iridium Complex with
4,4’-Dihydroxy-2,2’-bipyridine
Yuichiro Himeda,*[a] Nobuko Onozawa-Komatsuzaki,[a] Satoru Miyazawa,[b]
Hideki Sugihara,[a] Takuji Hirose,[b] and Kazuyuki Kasuga[a]
Abstract: Transfer hydrogenation cata-
lyzed by an iridium catalyst with 4,4’-
dihydroxy-2,2’-bipyridine (DHBP) in
an aqueous formate solution exhibits
highly pH-dependent catalytic activity
and chemoselectivity. The substantial
change in the activity is due to the
electronic effect based on the acid–
base equilibrium of the phenolic hy-
droxyl group of DHBP. Under basic
conditions, high turnover frequency
values of the DHBP complex, which
can be more than 1000 times the value
of the unsubstituted analogue, are ob-
tained (up to 81000 hꢀ1 at 808C). In
addition, the DHBP catalyst exhibits
pH-dependent chemoselectivity for
a,b-unsaturated carbonyl compounds.
Selective reduction of the C=C bond of
enone with high activity are observed
under basic conditions. The ketone
moieties can be reduced with satisfac-
tory activity under acidic conditions. In
particular, pH-selective chemoselectivi-
ty of the C=O versus C=C bond reduc-
tion was observed in the transfer hy-
drogenation of cinnamaldehyde.
Keywords: ligand design
·
pH-
dependent catalyst substituent
·
effects · transfer hydrogenation ·
water chemistry
Introduction
cause it avoids the use of explosive molecular hydrogen and
harmful organic solvents.[6–8,13–20] Although highly efficient
TH catalysts in iPrOH have been reported recently,[21–25]
aqueous TH reactions, which usually require high reaction
temperatures and low substrate/catalyst (S/C) ratios, seem
to be sluggish except for an iridium complex with monotosy-
lated ethylenediamine (turnover frequency (TOF) up to
132000 hꢀ1 at 808C).[26] On the other hand, attempts have
been made at the chemoselective TH of a,b-unsaturated car-
bonyl compounds in iPrOH.[27–31] The pH-dependent chemo-
selective TH of a,b-unsaturated carbonyl compounds in an
aqueous formate solution was reported by Frost for the first
time in 2007.[32] However, further improvements in catalyst
efficiency, selectivity, and versatility are required.
The use of water as a reaction medium is of great interest
for organometallic catalysis.[1] Among various solvents,
water is the most preferred solvent, because it is inexpen-
sive, safe, clean, and abundant.[2] Moreover, owing to the
amphoteric behavior in the Brønsted sense, the reaction
rates and the selectivities can be varied by changing the pH
of the solution.[3–8] Extensive studies by Joꢀ on pH-switcha-
ble chemoselective hydrogenation of cinnamaldehyde are
ACHTUNGTRENNUNG
a hydrogen donor in water has received much attention, be-
Recently, we have reported two important developments
in aqueous catalysis. First, the half-sandwich bipyridine com-
[a] Dr. Y. Himeda, Dr. N. Onozawa-Komatsuzaki, Dr. H. Sugihara,
Dr. K. Kasuga
plexes [MCp*
2,2’-bipyridine) serve as efficient catalysts for the TH of
AHCTUNGTRENNUNG
ketones in an acidic formate solution.[6,33] Although it is
ACHTUNGRTENN(NUG bpy)Cl]Cl (Cp*=C5Me5; M=Rh, Ir; bpy=
Energy Technology Research Institute
National Institute of Advanced Industrial Science and Technology
Tsukuba Central 5-2, 1-1-1 Higashi
Tsukuba, Ibaraki 305-8565 (Japan)
Fax : (+81)29-861-4687
E-mail: himeda.y@aist.go.jp
known that the rhodium complex shows poor activity in TH
(TOF=0.2 hꢀ1) under neutral formate conditions,[34] we
have revealed that the reactivity toward ketones is strongly
pH-dependent (TOF=66 hꢀ1 at pH 2.5). Second, remark-
able catalytic activation has been achieved by the introduc-
tion of a hydroxyl group into the pyridine ligand (i.e., 4,4’-
[b] S. Miyazawa, Prof. Dr. T. Hirose
Graduate School of Science & Engineering
Saitama University, Shimo-ohkubo 255, Sakura-ku
Saitama, Saitama 338–8570 (Japan)
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FULL PAPER
dihydroxy-2,2’-bipyridine (DHBP) and 4,7-dihydroxy-1,10-
phenanthroline (DHPT)) for the hydrogenation of bicarbon-
ates.[35] This is attributed to the strong electron-donating
ability of the oxyanion generated by deprotonation of the
phenolic hydroxy group under basic conditions.[36,37] In par-
ticular, the iridium series is most strongly affected by the
electronic substituent effect among ruthenium, rhodium,
and iridium complexes. In fact, in comparison with the ini-
tial TOF of the unsubstituted bipyridine complex, the TOF
of the iridium–DHBP complex is increased about 1300
times.
These two findings have prompted us to develop efficient
and chemoselective TH by using DHBP complexes. Herein,
we report that the catalytic activity and chemoselectivity of
the TH catalyzed by an iridium–DHBP complex in an aque-
ous formate solution is strongly pH-dependent. The catalytic
activity is discussed on the basis of the acid–base equilibri-
um of the hydroxyl group of the ligand and the Hammett
rule under acidic and basic conditions. The pH-dependent
chemoselective TH of a,b-unsaturated carbonyl compounds
is also investigated.
Figure 1. pH-Dependent profile of initial TOFs for the TH of a) 2-cyclo-
hexen-1-one (2.0 mmol) with 1-SO4 (0.5 mmol), b) cyclohexanone
(2.0 mmol) with 1-SO4 (1.0 mmol), and c) 2-cyclohexen-1-one (2.0 mmol)
with 4-SO4 (2.0 mmol) in an aqueous 1m formate solution (20 mL) at
408C.
The pH of the reaction solution changed during the
course of the reaction, due to the consumption of HCO2H
as a hydrogen donor and/or the generation of CO2, which
produces bicarbonates and carbonates under basic condi-
tions.[4,38] For example, the initial pH values changed from
7.3 and 2.6 to 8.5 and 2.8, respectively, at the end of the re-
action. The TH in air without inert gas protection proceeded
with high initial TOF at the beginning of the reaction. How-
ever, the catalyst degraded gradually and lower yields were
obtained.[26] The use of an excess of formate was required to
obtain a high yield of the desired reduction product.
Results and Discussion
pH-Dependent TH of 2-cyclohexen-1-one: To probe the
effect of pH, the TH of a water-soluble cyclic enone,
2-cyclohexen-1-one, was select-
ed as the model reaction. The
TH
of
2-cyclohexen-1-one
(2 mmol) was carried out with
the water-soluble iridium aqua
catalyst 1-SO4 (0.5 mmol) at
408C in 1m formate solution
(20 mL) at various pH values
by adjusting the HCO2H/
HCO2Na/NaOH [formate/sub-
strate/catalyst (F/S/C) mole
ratio=40000:4000:1]. The ini-
Correlation with the Hammett substituent constant (sp+) at
pH 2.6 and 7.3: Next, we systematically examined the elec-
tronic substituent effect on the TH of 2-cyclohexen-1-one by
using a series of iridium complexes 1–5 at pH 2.6 and 7.3.
As expected, the catalytic activities were strongly affected
by the substituents in the bipyridine ligand. At pH 2.6, the
TOF of 1 was 19 and 1.9 times higher than the TOFs of 2
and 4, respectively. Figure 2a shows a good correlation be-
tween log TOF and the sþp value[39] of the respective sub-
tial TOFs of TH were measured after 10 min (Figure 1a).
From pH 6.7 to 9.0, cyclohexanone was exclusively formed
with high TOFs (>4000 hꢀ1) through 1,4-reduction, but the
substrate was not consumed completely under these condi-
tions (turnover number (TON) >ca.3500). An almost quan-
titative yield was obtained under an S/C ratio of 2000.[38]
However, the activity of the chemoselective TH of 2-cyclo-
hexen-1-one is far superior to earlier studies.[28,29]
At pH 4 and below, the TOFs decreased below 1400 hꢀ1.
After 8 h, a mixture of cyclohexanone, cyclohexanol, and a
small amount of 2-cyclohexen-1-ol as a 1,2-reduction prod-
uct was obtained. An analysis of the progress of the reaction
by GC showed that cyclohexanone was generated at first
and then the amount of cyclohexanol increased gradually
with a TOF of up to 700 hꢀ1. After 24 h, 2-cyclohexen-1-one
was consumed completely at pH 2.6, and a mixture of cyclo-
hexanol (94%) and 2-cyclohexen-1-ol (6%) was obtained.
Figure 2. Plot for the correlation between sþp and the initial TOF for the
TH of 2-cyclohexen-1-one at pH 2.6 and 7.3 with catalyst 1–5 (20–
0.5 mmol) at 408C.
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11077

Y. Himeda et al.
stituents (R). A similar result was observed for the TH of
cyclohexanone at pH 3.0.
hexen-3-one (entry 3 at pH 2.6) yielded a mixture of 1,4-re-
duction products (58%) and 1,2-reduction product (41%;
Scheme 1). To avoid the formation of the mixed product,
the reaction was carried out in 1m HCO2Na solution at first,
followed by the addition of formic acid to give 3-hexanol
At pH 7.3, although the TOF of 1 was approximately
three times higher than that at pH 2.6, the TOFs of 2–5
were decreased significantly. Although the correlation
among the TOFs of 2–4 was unchanged, 1 and 5 showed a
different correlation due to the deprotonation of the acidic
substituents. Namely, the sþp values for 1 and 5 are regarded
AHCTUNGTREGUNeNN xclusively.
ꢀ
as ꢀ2.30 (-Oꢀ) and ꢀ0.02 (-CO2 ), respectively.[37] The Ham-
mett plot according to the interpretation shows a good cor-
relation (Figure 2b). In addition, high reaction constants
(1=ꢀ1.3 and ꢀ1.2 at pH 2.6 and 7.3, respectively) of the
Hammett plots were observed, which were similar to those
observed for the hydrogenation of bicarbonates (1=
ꢀ1.3).[37] The strong electronic substituent effect has never
been observed in other catalyst ligands (e.g., phosphorus li-
gands and 1,2-dephenylethylendiamine).[30,40–42] It is apparent
that the remarkable activation of 1 can be attributed to the
strong electron-donating ability of the oxyanion (i.e., low spþ
value) and the strong substituent effect (i.e., high 1 value).
Scheme 1. TH of 4-hexen-3-one catalyzed by 1.
The TH of cinnamaldehyde at pH 2.6 afforded cinnamyl
alcohol as the major 1,2-reduction product in 85% yield
(entry 4). The minor 1,4-reduction product was further re-
duced to give 3-phenyl-1-propanol in 15% yield. On the
other hand, the TH at pH 7.3 showed highly selective 1,4-re-
duction with high TOF, and the product distributions and
the yields were temperature-dependent (Table 2). The gen-
erated hydrocinnamaldehyde was tolerated at 408C
(entry 1), while the aldehyde moiety was smoothly reduced
at 808C (entry 3). It is interesting that the pH-dependent
product distribution is the inverse of that of biphasic hydro-
genation using ruthenium complexes with sulfonated phos-
phine ligands.[9] Unfortunately, the C=C bond in ethyl croto-
nate was not reduced.
Next, other substrates were examined. The TH of a-keto
acid, pyruvic acid, gave lactic acid under both acidic and
basic conditions (entry 5 in Table 1).[43,44] The reduction of
benzaldehyde proceeded with a high TOF at pH 7.3 (entry 6
in Table 1). The highest TOF of 81000 hꢀ1 and almost quan-
titative yield were obtained at 808C. However, high sub-
TH with various substrates: The scope of TH using 1 was
examined with various substrates at pH 2.6 and 7.3, and the
results are summarized in Table 1. In the TH of all the
enones at pH 7.3, the 1,4-reduction proceeded exclusively
with high TOFs to give ketones (entries 1–3 at pH 7.3). The
generated ketones were tolerated under basic conditions.
Equilibrium, which was dependent on reaction conditions,
seems to exist.[38] Lowered S/C ratio led to increased yield.
On the other hand, although at pH 2.6 the substrates were
consumed completely, the distributions of 1,4- and 1,2-re-
duction products were influenced by a steric hindrance at
the b-position of the enones. While 2-cyclohexen-1-ol was
generated in 6% yield from 2-cyclohexen-1-one (entry 1 at
pH 2.6), no 1,2-reduction product was observed in the case
of methyl vinyl ketone (entry 2 at pH 2.6). The TH of 4-
Table 1. TH catalyzed by 1[a]
pH 2.6
Yield [%]
pH 7.3
Yield [%]
Substrate
t [h]
TOF [hꢀ1
]
Product
t [h]
TOF [hꢀ1
4300
]
Product
1
2
24
1100
94[b]
2[c]
>99
24[c]
1500
72[d]
0.5[c]
5300
>99
3
4
24[c]
8[c]
700
29[e]
85[f]
3
2700
4100
>99
1200
0.2[c]
17[g]
5
6
MeC(=O)CO2H
PhCHO
8
6
620
1400
>99
>99
MeCH(OH)CO2H
PhCH2OH
8
1100
81000
>99
>99
MeCH(OH)CO2H
PhCH2OH
0.2[h]
7
8
8[c]
8[c]
820
480
>99
>99
8[c]
6[c]
15
80
6
7
PhCOMe
PhCH(OH)Me
PhCH(OH)Me
[a] The reaction was carried out at 408C in an aqueous 1m formate solution (20 mL) with a substrate (2 mmol) and 1-SO4 (0.5 mmol); F/S/C=
40000:4000:1; yields were determined by a GC or HPLC analysis. The initial TOF was measured after 10 min. [b] 2-Cyclohexenol (6%) was formed.
[c] The reaction was carried out by using 1-SO4 (1 mmol); F/S/C=20000:2000:1. [d] 2-Butanone (28%) was formed. [e] 4-Hexen-3-ol (41%) and 3-hexa-
none (29%) were formed. [f] 3-Phenyl-1-propanol (15%) was formed. [g] The reaction reached equilibrium. [h] The reaction was carried out at 808C in
an aqueous 2m formate solution (20 mL) with a substrate (4 mmol) and 1-SO4 (0.5 mmol); F/S/C=80000:8000:1] The initial TOF was measured after
5 min.
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Chem. Eur. J. 2008, 14, 11076 – 11081

Water Chemistry
FULL PAPER
Table 2. TH of cinnamaldehyde[a]
Table 3. TH of cyclic enones into ketones at pH 7.3[a]
T
[8C]
TOF
Entry
Substrate
t [h]
TOF [hꢀ1
3600
]
Yield [%]
A
]
1
2
>99
1
2
40
5000 79
19
48
27
0
1
16
72
15
<1
1
1
60 17000 35
3[b] 80 28000
0
0
2
3
4
6
8
8
2200
130
>99
14
4[c] 40
1200
85
[a] The reaction was carried out in an aqueous 2m HCO2Na solution
(20 mL) with a substrate (2 mmol) and 1-SO4 (0.5 mmol) for 3 h; F/S/C=
80000:4000:1; yields were determined by a GC and 1H NMR analyses.
The initial TOF was measured after 5 min. [b] The reaction was carried
out for 8 h. [c] The reaction was carried out in an aqueous 1m formate so-
lution (pH 2.6) with a substrate (2 mmol) and 1-SO4 (1 mmol) for 8 h;
F/S/C=20000:2000:1.
560
31
5
1.5
2000
>99[b]
strate concentration led to decreased TOF and yield due to
phase separation.[38] In the TH of the water-soluble cyclo-
hexanone and insoluble acetophenone at pH 2.6, the TOFs
improved significantly compared with that of unsubstituted
analog 2 (entries 7, 8 in Table 1). The high TOF of 4000 hꢀ1
at pH 2.6 was obtained in the TH of acetophenone at 808C.
On the other hand, the TH at pH 7.3 showed poor activities
toward ketones. The pH-dependent profiles for the TH of
cyclohexanone with 1, in which the TOFs improved signifi-
cantly compared with the TOFs of unsubstituted analog 2
under acidic conditions, are consistent with a previous obser-
vation (Figure 1b).[6] The pH dependence of the reduction
of ketones can be explained by the activation of the carbon-
yl moiety by protons.[44] Consequently, the order of the reac-
tivity at pH 2.6 is aldehyde >C=C bond of a,b-unsaturated
carbonyl compounds>ketone, while that at pH 7.3 is C=C
bond of a,b-unsaturated carbonyl compounds>aromatic
[a] The reaction was carried out at 408C in an aqueous 1m formate solu-
tion (20 mL) with a substrate (2 mmol) and 1-SO4 (0.5 mmol); F/S/C=
40000:4000:1; yields were determined by a GC analysis. [b] The reaction
was carried out using a substrate (1 mmol) and 1-SO4 (1 mmol); F/S/C=
20000:1000:1; dihydrocarvone was isolated as
a mixture (trans/cis=
1.7:1).[45]
ACHTUNGTRENNUNG
pH 7.3 were investigated (Table 3). The TH of 2-cyclopent-
en-1-one showed high activity similar to that of 2-cyclohex-
en-1-one (entry 1). While the activity for 2-metlyl-2-cyclo-
penten-1-one somewhat decreased (entry 2), the THs of 3-
methyl-substituted analogues were sluggish (entries 3 and
4). The decrease in activity due to steric hindrance in TH is
known.[28,29] Since the influence of a-substituted enone was
relatively small, (R)-(ꢀ)-carvone could be efficiently con-
verted to dihydrocarvone as a trans/cis mixture (entry 5).
Figure 3. Absorption spectra of 1 (0.1 mm) in an aqueous solution as a
function of pH. Curves 1, 2, 3, 4, 5, 6, 7, 8, and 9 were obtained at
pH 3.0, 4.0, 4.5, 5.0, 5.5, 6.0, 8.0, 9.0, and 10.0, respectively.
The degree of pH dependence of the catalytic activation
of 1 was examined by comparing it with the methoxy ana-
logue 4, in which the electronic effect of the methoxy group
was independent of pH. The pH-dependent profile for the
TH of 2-cyclohexen-1-one with 4, the trend for which is op-
posite compared to 1, is shown in Figure 1c. The TOF ratio
of 1/4 on a logarithmic scale as a function of pH is shown in
Figure 4b. A sharp increase of the TOF ratio from 2 to 85
times was observed from pH 3 to 7. Interestingly, the curve
overlapped appreciably with the above-mentioned curve of
the absorbance change. This result strongly suggests that the
increase in activity can be attributed to the increase in the
electron-donating effect due to the deprotonation of the
Effect of acid-base equilibrium of DHBP: The acid–base
equilibrium of 1, which has two types of proton-donating
groups (i.e., two phenolic hydroxyl groups and an aqua
ligand), was examined using absorption spectra as a function
of pH adjusted by H2SO4/KOH (Figure 3). Figure 4a shows
the percentage change in the optical density of 1 at 265 nm
against the pH. From our previous studies, the change from
pH 3 to 6 can be attributed to the acid–base equilibrium of
the hydroxy groups in 1.[36] On the other hand, the change
from pH 8 to 10 may be attributed to that of the aqua
ligand.[46] Thus, the complex seems to exist as a protonated
form 1 at pH 2.6, a deprotonated form 6 at pH 7.3, and a
hydoxo complex 7 at pH above 10 (Scheme 2).
AHCTUNGTRENNUNG
plained by the reaction path depicted in Scheme 2. In the
pH range from 2 to 10, the iridium complex reacted easily
with formate to generate the corresponding hydride com-
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11079

Y. Himeda et al.
complex 1 were controlled by pH. We demonstrated that
catalyst activation by the electronic effect of the oxyanion
was applicable to TH in an aqueous formate solution. It was
confirmed that the substantial change in the catalytic activi-
ty of 1 was due to the electronic effect based on the acid–
base equilibrium of the hydroxyl group by means of the
comparison with that of methoxy analogue 4. The TOF
values of 1 at pH 7.3 can be more than 1000 times the value
of the unsubstituted analogue 2. The highest TOF of
81000 hꢀ1 was obtained in the TH of benzaldehyde at 808C.
In addition, the DHBP catalyst exhibited pH-dependent
chemoselectivity for a,b-unsaturated carbonyl compounds.
Selective reductions of the C=C bond of enone with high
TOF were observed under basic conditions. The ketone moi-
eties can be reduced with satisfactory activity under acidic
conditions. In particular, pH-selective chemoselectivity of
the C=O versus C=C bond reduction was observed in the
TH of cinnamaldehyde. From the viewpoint of green
chemistry, we believe that TH, through improvement in re-
action efficiency, has the potential to be a safe and innocu-
ous alternative to the hydrogenation reaction.
Figure 4. a) Absorbance change of 1 at 265 nm as a function of pH. b)
TOF ratio of 1 vs. 4 as a function of pH on a logarithmic scale.
Experimental Section
General considerations: All manipulations were carried out under an
argon atmosphere. All aqueous solutions were degassed prior to use. 1H
and 13C NMR spectra were recorded on a Varian INOVA 400 spectrome-
ter using sodium 3-(trimethylsilyl)-1-propanesulfonate (DSS) as an inter-
nal standard. Mass spectra were recorded on Waters ZQ2000 (ESI). Ele-
mental analyses were carried out on an Eager 200 instrument. FT-IR
spectra were recorded on a Perkin–Elmer Spectrum One spectrometer.
The pH values were measured on an Orion Model 290 A pH meter with
a glass electrode after calibration to standard buffer solutions. [IrCp*-
[SO4] was prepared according to the literature procedure.[48] The
ACHUTNGREN(NUG H2O)3]ACHTUNGTRENNGUN
[50]
4,4’-dihydroxy-2,2’-bipyridine[49] and 4-SO4 were prepared according to
the literature procedures.
DHBP complex (1-SO4): A aqueous solution (30 mL) of [IrCp*
ACHTUNGTRENNUNG(H2O)3]-
AHCTUNGTRENNUNG
0.84 mmol) was stirred at 408C for 12 h. The solution was filtered. The
volume of the filtrate was reduced to ꢁ5 mL in vacuo. The solution was
placed in a refrigerator to give a yellow solid 1-SO4 (370 mg, 70%).
1H NMR (400 MHz, D2O): d=8.77 (d, J=6.4 Hz, 2H), 7.77 (brs, 2H),
7.24 (dd, J=6.4, 2.6 Hz, 2H), 1.66 ppm (s, 15H); 13C NMR (100 MHz,
D2O): d=170.37, 160.38, 154.94, 119.13, 114.41, 90.94, 10.48 ppm;
1H NMR (400 MHz, D2O/KOD): d=8.25 (d, J=6.6 Hz, 2H), 7.13 (d, J=
2.7 Hz, 2H), 6.63 (dd, J=6.6, 2.7 Hz, 2H), 1.56 ppm (s, 15H); 13C NMR
(100 MHz, D2O/KOD): d=178.61, 159.76, 152.91, 121.50, 115.95, 87.47,
10.23 ppm; IR (KBr): n˜ =1622, 1498, 1457, 1027 cmꢀ1; ESIMS: m/z: 515
[MꢀSO4ꢀH2OꢀH]+; elemental analysis calcd (%) for C20H25IrN2O7S: C
38.15, H 4.00, N 4.45, S 5.09; found: C 38.16, H 4.09, N 4.42, S 5.03.
Scheme 2. Behavior of 1 at various pH values.
plexes 8 or 9 as active species. It is worth noting that the for-
mation of hydride complex 9 may be unaffected by the hy-
droxo complex 7.[44] The generated hydride complexes
showed catalytic activities that were dependent on the elec-
tronic effect of the substituent in the bipyridine ligand. Al-
though the catalytic activities decreased significantly under
basic conditions due to the increase in the protonic charac-
ter of the hydride,[4447] the activation of 9 by the strong elec-
tronic effect of the oxyanion surpassed the inherent
4,4’-Dimethyl-2,2’-bipyridine complex (3-SO4): This compound was pre-
pared from [IrCp*ACHTNUTRGENN(UG H2O)3]ACHTUTGNREN[NUNG SO4] (192 mg, 0.4 mmol) and 4,4’-dimethyl-
2,2’-bipyridine (74 mg, 0.4 mmol) according to the procedure described
for the preparation of 1-SO4. Recrystallization from water gave yellow
powder; yield 200 mg (80%). 1H NMR (400 MHz, D2O): d=8.92 (d, J=
5.9 Hz, 2H), 8.35 (brs, 2H), 7.71 (dd, J=5.9, 1.0 Hz, 2H), 2.64 (s, 3H),
1.66 ppm (s, 15H); 13C NMR (100 MHz, D2O): d=158.31, 157.56, 153.38,
132.56, 127.53, 91.60, 23.44, 10.43 ppm; IR (KBr): n˜ =1621, 1484, 1452,
1031 cmꢀ1; ESIMS: m/z: 511 [MꢀSO4ꢀH2OꢀH]+; elemental analysis
calcd (%) for C22H29IrN2O5S: C 42.23, H 4.67, N 4.48, S 5.12; found: C
42.14, H 4.72, N 4.50, S 5.29.
ACHTUNGTRENNUNGdeactivation.
Conclusion
The results clearly showed that the reaction activity and
chemo
ACHTUNGTRENNUNGselectivity in the TH catalyzed by the iridium–DHBP
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Water Chemistry
FULL PAPER
4,4’-Dicarboxy-2,2’-bipyridine complex (5-SO4): This compound was pre-
pared from [IrCp*ACHTUNGTRENNUNG(H2O)3]ACHTUTGNREN[NUNG SO4] (192 mg, 0.4 mmol) and 4,4’-dicarboxy-
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2,2’-bipyridine (98 mg, 0.4 mmol) according to the procedure described
for the preparation of 1. Purification by chromatography over Sephadex
LH-20 with water gave yellow powder; yield 145 mg (53%). 1H NMR
(400 MHz, D2O): d=9.38 (d, J=5.7 Hz, 2H), 9.18 (d, J=1.6 Hz, 2H),
8.44 (dd, J=5.7, 1.7 Hz, 2H), 3.7–3.4 (brs, 2H), 1.78 ppm (s, 15H);
13C NMR (100 MHz, D2O): d=169.32, 159.41, 155.38, 146.59, 131.54,
126.93, 92.83, 10.48 ppm; 1H NMR (400 MHz, D2O/KOD): d=9.05 (d,
J=5.7 Hz, 2H), 8.80 (d, J=1.6 Hz, 2H), 8.10 (dd, J=5.7, 1.6 Hz, 2H),
1.65 ppm (s, 15H); 13C NMR (100 MHz, D2O/KOD): d=173.14, 158.72,
154.12, 150.62, 130.23, 125.51, 90.24, 10.14 ppm; IR (KBr): n˜ =1725, 1560,
1454, 1408 cmꢀ1; ESIMS: m/z: 571 [MꢀSO4ꢀH2OꢀH]+; elemental analy-
sis calcd (%) for C22H25IrN2O9S·H2SO4·H2O: C 32.95, H 3.65, N 3.49, S
8.00; found: C 32.71, H 3.46, N 3.46, S 8.20.
General procedure for transfer hydrogenation: A solution of complex
(0.5 mmol) in water (0.1 ml) was added to a degassed aqueous 1m formate
solution (20 mL) of a substrate (2 mmol). The reaction was carried out at
408C in an argon atmosphere with vigorous stirring. The reaction solu-
tion was analyzed by GC (PEG-HT 5%, Uniport HT 60/80, 2 m packed
column, 1008C) or HPLC (Tosoh TSKgel SCX(H+)).
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UV studies of acid–base equilibrium: Stock solutions of 1-SO4 (0.1 mm)
in 0.05m H2SO4 and 0.1m KOH were prepared. The pH of the solutions
was adjusted by mixing. The UV/Vis spectra were measured after reading
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Received: August 1, 2008
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Published online: November 6, 2008
Chem. Eur. J. 2008, 14, 11076 – 11081
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11081