Synthesis of the [(η6-p-cymene)Ru(dppb)Cl]PF6 complex and catalytic activity in the transfer hydrogenation of ketones

Article abstract of DOI:10.1080/00958972.2017.1390226

Catalysis under mild conditions is of great importance to various chemistry areas, particularly for the development of novel active compounds and for natural products modifications, among others. In this study, the synthesis, characterization, and evaluation of the catalytic activity of a new ruthenium(II) compound, [(η⁶-p-cymene)Ru(dppb)Cl]PF₆ (A) where dppb=1,4-bis(diphenylphosphine)butane, is presented. Catalytic activity of the new Ru(II) compound was tested on hydrogen transfer reaction in various substrates, acetophenone, benzophenone, cyclohexanone, and methyl-ethyl-ketone. Potassium hydroxide was used as base, whereas isopropanol served as both solvent and hydrogen source. Samples comprising substrate: base: catalyst at a 200 : 20 : 1 ratio were poured into 5?mm tubes and monitored in situ at 40, 50, and 60?°C in a 600?MHz NMR spectrometer. The complex was active in the transfer hydrogenation of ketones, achieving conversions superior to 90% within 4?h at 60?°C, which suggests under mild conditions. Therefore, in situ monitoring the reactions through ¹H NMR was a valuable technique to establish the possible catalytic mechanism of Ru(II) precatalyst.

Full text of DOI:10.1080/00958972.2017.1390226

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
6
Synthesis of the [(η -p-cymene)Ru(dppb)Cl]PF6
complex and catalytic activity in the transfer
hydrogenation of ketones
Angel Ruben Higuera-Padilla, Alzir Azevedo Batista, Legna Colina-Vegas,
Wilmer Villarreal & Luiz Alberto Colnago
To cite this article: Angel Ruben Higuera-Padilla, Alzir Azevedo Batista, Legna Colina-Vegas,
6
Wilmer Villarreal & Luiz Alberto Colnago (2017): Synthesis of the [(η -p-cymene)Ru(dppb)Cl]PF6
complex and catalytic activity in the transfer hydrogenation of ketones, Journal of Coordination
Chemistry, DOI: 10.1080/00958972.2017.1390226
To link to this article: http://dx.doi.org/10.1080/00958972.2017.1390226
Accepted author version posted online: 09
Oct 2017.
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Date: 10 October 2017, At: 14:23

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://doi.org/10.1080/00958972.2017.1390226
6
Synthesis of the [(η -p-cymene)Ru(dppb)Cl]PF complex and catalytic activity
6
in the transfer hydrogenation of ketones
ANGEL RUBEN HIGUERA-PADILLA*†, ALZIR AZEVEDO BATISTA‡, LEGNA COLINA-VEGAS‡,
WILMER VILLARREAL‡ and LUIZ ALBERTO COLNAGO*†
†
‡
Laboratório de Ressonância Magnética Nuclear, Embrapa Instrumentação, São Carlos, Brazil
Laboratório de Estrutura e Reatividade de Compostos Inorgânicos, UFSCar, São Carlos, Brazil
Catalysis under mild conditions is of great importance to various chemistry areas, particularly for
the development of novel active compounds and for natural products modifications, among
others. In this study, the synthesis, characterization, and evaluation of the catalytic activity of a
6
new ruthenium(II) compound, [(η -p-cymene)Ru(dppb)Cl]PF (A)where dppb =
6
1
,4-bis(diphenylphosphine)butane, is presented. Catalytic activity of the new Ru(II) compound
was tested on hydrogen transfer reaction in various substrates, acetophenone, benzophenone,
cyclohexanone, and methyl-ethyl-ketone. Potassium hydroxide was used as base, whereas
isopropanol served as both solvent and hydrogen source. Samples comprising substrate : base :
catalyst at a 200 : 20 : 1 ratio were poured into 5 mm tubes and monitored in situ at 40, 50 and
6
0 °C in a 600 MHz NMR spectrometer. The complex was active in the transfer hydrogenation
of ketones, achieving conversions superior to 90% within 4 h at 60 °C, which suggests under
1
mild conditions. Therefore, in situ monitoring the reactions through H NMR was a valuable
technique to establish the possible catalytic mechanism of Ru(II) precatalyst.
Keywords: Ruthenium complexes;Homogeneous catalysis;Transfer hydrogenation; NMR
Reaction monitoring
1
. Introduction
*
Corresponding authors. Email: angelruben@hotmail.com (A.R. Higuera-Padilla); luiz.colnago@embrapa.br
(L.A. Colnago)
1

There is a significant need for new catalysts with improved performance that are
environmentally friendly. In other words, catalysts with higher activity, superior selectivity, and
enhanced stability are highly pursued [1]. Ruthenium complexes have been reported to be
excellent precursor catalysts for hydrogenation of ketones and alkenes [2,3], olefin metathesis
[4,5], and hydrogen transfer reaction [6-8], among many other reactions [9-12]. Ru(II) catalysts
are considerably cheaper than other metals that are employed in such reactions (e.g.iridium,
rhodium, rhenium, and osmium)[13,14]. Moreover, ruthenium compounds have also been
studied as potential anticancer [15-18], antibacterial [19], and antiviral drugs [20]. In this study,
high-field NMR spectroscopy has been used to monitor the mechanism in situ of the hydrogen
transfer reaction catalyzed by Ru complexes [21-26]. Understanding the chemical reaction
mechanism may lead to remarkable practical advancements, such as process cost reduction,
selective enhancement, and superior environmental consideration [27].
The hydrogen-transfer reaction or transfer hydrogenation is the process of reduction of
multiple bonds through abstraction of hydrogen from a reagent (hydrogen-donor) by means of a
catalyst, followed by hydrogen addition to the unsaturated functional group of the hydrogen-
acceptor (substrate) [28]. There are several substrates that have been successfully reduced
employing the hydrogen transfer reaction in the presence of both heterogeneous and
homogeneous catalysts. The list includes unsaturated hydrocarbons (alkenes, alkynes), carbonyl
(ketones, aldehydes, acids, esters), and nitro compounds (imines, imides, oxides). As catalyst
and/or precatalyst, the list includes many complexes of various metals, among them Rh, Ir, Ru,
etc. [28]. The hydrogen transfer reaction has numerous advantages over conventional
hydrogenation processes. The former does not require pressurized hydrogen gas, neither produce
hazardous waste, uses an inexpensive hydrogen-donor (2-propanol), and forms a single side-
product (i.e. acetone), which is easily removed by distillation. Therefore, the hydrogen transfer
reaction denotes an environmentally-friendly process [29]. Despite recent progress, there is a
demand for the development of more sustainable, economic catalytic systems to the hydrogen
transfer reaction [30].
In this study, we present an approach using high-field NMR spectroscopy to in situ
monitor the hydrogen transfer reaction in a real-time fashion using integration of static with rapid
injection NMR spectroscopy in conventional 5-mm tubes. NMR spectroscopy is a very useful
tool for the structural analysis and quantitative monitoring of complex reaction processes and
2

mixtures. There are several approaches to conducting NMR spectroscopy reaction monitoring
31]. In the method proposed in this paper, the NMR tubes containing substrate and base are
[
placed in the equipment probe for locking, shimming, and temperature settings. The catalyst in
the isopropanol solution is injected in the NMR reaction tube via a long cannula connected to a
plastic syringe. Measurements began once catalysts had been injected and the system had been
mechanically stirred. This procedure avoids data loss in the initial portion of reaction. Therefore,
1
in situ H NMR spectroscopy was demonstrated to be a valuable technique for monitoring the
hydrogen transfer reactions as well as for establishing the possible catalytic mechanism of Ru(II)
precatalyst.
2
2
. Experimental
.1. Materials and methods
All reactions were carried out under argon using standard Schlenk techniques. Solvents were
purified by standardized methods and degassed prior to use. RuCl ·3H O was obtained from
3
2
MetalPrecious, whereas PPh , dppb, α-phellandrene and NH PF were obtained from Sigma-
3
4
6
Aldrich and used as received (i.e. without further treatment).
Elemental analyses were performed in a Perkin Elmer Series II CHNS/O Analyzer,
model 2400. The FT-IR spectra of the powder complexes were recorded from KBr pellets at
-1
wavenumbers ranging from 4000 to 200 cm in a Bomen-Michelson FT MB-102 instrument.
1
13
1
31
1
H, C{ H} and P{ H} NMR spectra were recorded in the following spectrometers: BRUKER
DRX 400 MHz, using a BBO 5 mm probe, and BRUKER Ascend 600 MHz, using a BBO 5 mm
1
13
1
31
1
probe at 298 K. (CD ) CO, CDCl ( H, C{ H}) and, in some cases, CH Cl ( P{ H}) were
3
2
3
2
2
3
1
1
1
used as solvents, as well as PF for internal reference related to P{ H} and TMS for H and
6
1
3
1
C{ H}.
6
2
.2. Synthesis of [(η -p-cymene)Ru(dppb)Cl]PF (A)
6
A solution of 1.00 g of RuCl ·3H O dissolved in methanol with 5 mL of α-phellandrene was
3
2
maintained under reflux during 5 h (see figure 1). The solution was concentrated and the
6
obtained dark red precipitate [(η -p-cymene) Ru Cl ] (a) was filtered off and vacuum dried [32].
2
2
4
6
The red solid [(η -p-cymene)Ru(PPh )Cl ] (b) was obtained by placing 850 mg of a in methanol
3
2
with 804 mg of PPh under reduced pressure during 2 h. To 200 mg of b in 10 mL of toluene
3
3

were added 121 mg of NH PF and 3 mL of ethanol. The solution was stirred, slightly warmed
4
6
for 10 min, and added by 132 mg of dppb. This solution was placed under reflux for 3 h.
Subsequently, the solution was cooled and the obtained yellow solid was filtered off, washed
with diethylether, and vacuum dried. Yield: 76%. Anal. Calcd for C H ClF P Ru·H O; C,
3
8
42
6
3
2
-
1
5
8
3.06; H, 5.16. Found: C, 53.49; H, 4.74. IR (KBr, cm ): ν(C-H), 3069; ν(C=C), 1479; ν(P-F),
1
41; ν(Ru-P), 512; ν(Ru-Cl), 295. HNMR [(CD ) CO], δ(ppm) (m, I): 7.73-7.53 (m, 20H,
3
2
Phdppb), 6.06 (d, 2H,JHH = 5.18 Hz,3-CHp-cymene), 5.31 (d, 2H,J = 5.35 Hz,4-CHp-cymene), 3.56
HH
(
m, 2H, a-CH2dppb), 2.62 (sep, 1H,JHH = 6.91 Hz,6-CHp-cymene). 2.27 (m, 2H, d-CH2dppb), 1.94 (m,
H, b-CH2dppb), 1.59 (m, 2H, c-CH2dppb), 1.19 (s, 3H, 1-CH3p-cymene), 1.04 (d, 6H,JHH = 6.95 Hz,7-
2
CH3p-cymene). P{ H} NMR [(CD ) CO] δ (ppm) (m, I): 31.19 (s, dppb), -144.10 (sep, PF ).
31
1
3 2
6
1
3
C{ H} NMR [(CD ) CO] δ(ppm) (m, I): 134.12 (t, CH-Phdppb), 132.81 (t, CH-Phdppb), 132.46
1
3
2
(
s, C-Phdppb), 131.31 (s, C-Phdppb), 129.94 (t, CH-Phdppb), 129.32 (t, CH-Phdppb), 99.23 (s,
C-2p-cymene), 97.05 (s, CH-3p-cymene), 91.76 (s, CH-4p-cymene), 31,27 (s, CH-6p-cymene), 30.25 (s,
a-CH2dppb), 28.31 (s,d-CH2dppb), 28.21 (s,b-CH2dppb), 28.12 (s,c-CH2dppb), 23.28 (s, CH -7p-cymene),
3
p-cymene
), 15,29 (s, CH -1p-cymene).
2
1.35 (s, CH -7
3
3
2
.3. Catalytic studies by NMR
NMR analyses of the hydrogen transfer reaction from 2-propanol to selected ketones were
1
carried out in 5-mm NMR tubes in a Bruker Ascend 600 MHz equipment. H NMR spectra were
acquired with eight scans, a single 30° pulse angle, an acquisition time of 1.36 s, and a relaxation
delay equal to 1 s. Spectra were recorded at every 30 s (around 500 spectra). The chemicals were
added to NMR tubes, mixed by inversion, and inserted into the spectrometer. When the tube was
placed in the probe, it is mandatory to allow a suitable time (7-10 minutes) to achieve thermal
equilibrium. Sample temperature was controlled by the variable temperature heater of the probe.
After locking and shimming operations, the catalyst was rapidly injected through a cannula
coupled to a syringe and mechanically mixed by rapid pushing and pulling. This simpler
procedure incorporates modifications to the standard previously reported method [33,34].
Measurements began upon catalyst injection. The yield of the process was calculated taking into
account the relative integrals of ketone and alcohol.
3
. Results and discussion
4

3
.1. Synthesis and characterization
Complex A was stable in light and in air. Elemental analysis was consistent with the proposed
formula. The IR spectrum of the complex displayed characteristic bands that were assigned to
the main functional groups of ligands, and to the metal-ligand interaction: aromatic C-H at
-1
-1
-1
-1
-1
3
069 cm , C=C at 1479 cm , P-F at 841 cm , Ru-P at 512 cm , and Ru-Cl at 295 cm . This
clearly evidences the complex formation [31]. NMR signals were assigned on the basis of 1-D
(1
H, C and P NMR) and 2-D NMR experiments, including Correlation Spectroscopy ( H- H,
13
31
1
1
1
13
COSY) and Heteronuclear Single Quantum Coherence ( H- C, HSQC). Complex A showed a
1H NMR spectrum in deuterated acetone comprising ten signals. A multiplet that integrates for
2
0 protons was observed at 7.6 ppm and assigned to dppb phenyl groups. A doublet that
integrates for two protons was noticed close to 6.06 ppm and attributed to CH of p-cymene ortho
to CH - group. Another doublet that integrates for two protons was observed at 5.31 ppm and
3
attributed to CH of p-cymene meta to CH - group. A multiplet for two protons was observed at
3
3
.56 ppm and assigned to dppb -CH - adjacent to a P atom. The signal septet at 2.62 ppm that
2
integrates for one proton was attributed to CH adjacent to two p-cymene CH - groups. The
3
multiplet signals at 2.27, 1.94 and 1.59 ppm, which integrate for two protons each, are attributed
to dppb -CH - groups. The singlet at 1.19 ppm that integrates for three protons corresponds to the
2
alone p-cymene CH group, whereas the doublet at 1.04 ppm for six protons belongs to the two
3
1
1
1
13
p-cymene CH - groups. All signals were confirmed by the H- H COSY and H- C HSQC
3
experiments. The presence of dppb and PF in ruthenium(II) complex A was confirmed by
6
31P NMR spectroscopy, in which one singlet and one septet were observed at 31.19 and
-144.10 ppm, respectively. Based on these data, we proposed a piano-stool structure with a
6
ruthenium atom that binds η to a p-cymene ligand, a dppb bidentate ligand, a chloridoligand,
and a PF anion as counter ion (figure 2).
6
3
.2. Catalytic studies by NMR
Complex A was designed as catalyst precursor to study the activity for the hydrogen transfer
reaction from isopropanol (2-propanol) to ketones (figure 3). NMR techniques provide valuable
mechanistic information to comprehend this reaction. Complex A was evaluated within an air
atmosphere employing isopropanol as hydrogen source and different ketone substrates
(acetophenone, benzophenone, cyclohexanone, and methyl-ethyl-ketone), 0.5, and 1 mol% of
5

precatalyst concentration. A KOH content of 10 mol% and temperatures of 40, 50 and 60 °C
were investigated. Although reaction duration may vary from 3 to 6 h depending on temperature
and precatalyst combination, the measuring time was fixed in 4 h.
Initially, a mixture consisting of isopropanol, substrate and base, and then deuterated
solvent was added into the 5-mm NMR tube. This tube was then placed in the equipment (probe)
and heated to the desirable reaction temperature (40, 50 or 60 °C). When thermal equilibrium
was reached (7-10 min), locking and shimming processes were performed and the spectrum for
t < 0 was acquired. After these adjustments, the catalyst solution was rapidly injected through a
cannula and mechanically mixed by pushing and pulling the syringe plumber for approximately
3
0 seconds. This procedure was intended to avoid the loss of important data at the beginning of
1
the reaction. Once mixed, the system was analyzed through H NMR by means of acquiring
spectra at every 30 s until achieved 4 h duration.
Figure4 presents NMR spectrum of the solution containing isopropanol, base and the
precatalyst A. The inset shows the triplet signal at approximately -11 ppm. This weak triplet was
attributed to the coupling between hydrogen and phosphorous atoms of the hydride intermediate,
1
which occurs in a low concentration (0.1%). Therefore, the H NMR spectrum demonstrates the
formation of ruthenium-hydride specie as the initial stage of hydrogen transfer reaction [35]. The
hydride may be observed until the end of the reaction [36].
NMR techniques are widely known to enable direct observation of multiple reactive
intermediates, providing important information concerning the involved reaction mechanism. As
excess of isopropanol was used, it was uncomplicated to evaluate the reaction rate by the
1
formation of acetone and the reduced substrate. Figure5 shows the expansion of H NMR spectra
of hydrogen transfer reaction using precatalyst A, andusing cyclohexanone as substrate. The
NMR spectra from 1.3 to 2.2 ppm show an acetone singlet at 1.9 ppm, which increases
throughout the reaction. The spectra also denoted three cyclohexanone signals at 2.25, 1.64 and
1
1
.52 ppm that decayed throughout the reaction. Three cyclohexanol signals also appeared at
.74, 1.58 and 1.38 ppm, which consequently increase with reaction time. The spectra
demonstrate the continuous formation of these products during 4 h. This figure clearly shows
1
that H NMR spectroscopy can be easily used to in situ monitor the hydrogen transfer reaction.
The NMR data may also be used to extract quantitative information about relative
concentration and reaction rates. Figure 6 displays the increase in the relative methyl
6

concentration in phenylethanol at 40, 50 and 60C. In accordance to expectations, this figure
corroborates the elevated reaction rates at higher temperatures. The data were fitted using an
-1
exponential curve to obtain reaction half-life time (t , min) and rate constants (k, s ) (table
1
/2
†
†
1
).These information were used to calculate energy of activation, ΔH , and ΔS (table 1). The
energy of activation (Ea) was calculated by Arrhenius plots (lnk = -Ea/RT + lnA), wherein k is
the equation rate constant, R is the universal gas constant, and T is the temperature (K). A ln(k)
-1
versus T plot results in a straight line, whose gradient was used to determine Ea. The
thermodynamic parameters of such equation (ΔH and ΔS) were determined by the Eyring-
†
†
Polanyi equation (ln(k/T) = (-ΔH /RT) + [ln(k /h)+(ΔS /R)]), wherein k and h are the
B
B
-1
Boltzmann’s and Planck’s constant, respectively. The ln(k/T) versus T plot gives a straight line
†
†
with a slope equal to -ΔH /RT and an intercept equal to ln(k /h)+(ΔS /R), from which the
B
enthalpy and entropy of activation, respectively, may be derived (table 1). Belkova etal.
previously suggested that these values indicate an ordered (associative) transition state that is
typical of proton transfer reactions [37]. Although significant conversions (90%) were obtained
in a relatively short period of time (4 h) without continuous mixing, it is always possible to
enhance the yield by stirring the reaction mixture.
Table 2 presents the efficiency of the Ru(II) precatalyst on the hydrogen transfer reaction.
The conversion to product was observed to vary remarkably (i.e. from 24 to 99%) depending on
the applied conditions. Most of the reported works use nitrogen atmosphere, temperature of
8
2 C, and catalyst load between 0.1 to 1 mol % [29,38,39].A recent work employed
temperatures of 40 and 50 C, a catalyst load of 5%, and reaction time between 24 to 30 h [40].
These results indicate that the conditions carried out here are very mild, because we do not use
inert atmosphere, the temperature is lower, the reaction time is acceptable, the catalyst load is
similar, and the in situ conditions were completely done in the NMR equipment, approaching a
combined methodology between rapid injection and static tube. The reaction itself occurs only in
the presence of both precatalyst and base. Table 2 also illustrates that the conversion to product
is clearly dependent on the catalyst concentration: the higher the catalyst content, the higher the
conversion to ketone and vice versa.
A broadly accepted mechanism for catalytic activity of ruthenium complexes involves the
dissociation of the chloride ligand as well as the coordination of the hydride species [41]. In the
case of base-assisted transfer hydrogenation from isopropanol to ketones, a widely accepted
7

mechanism follows a monohydride route, in which the base transports isopropanol protic
hydrogens and the catalyst delivers the hydrides from the carbinol to the carbonyl carbons of the
studied ketones. A plausible mechanism for the catalytic activity of the Ru(II) complex is
presented in figure 7. It was previously proposed by Aydemir et al. that the first step comprised
the removal of isopropanol protic hydrogens by the base [7], as well as the formation of ions that
react with precatalyst A, forming the Ru-iso-propoxyde species B. The B intermediate undergoes

-hydride elimination and forms the ruthenium metal species C and acetone. Then, the hydride
intermediate C reacts with the carbonyl carbons of ketones substrate and leads to the alkoxyde
substrate D. Finally, the alkoxide substrate reacts with another isopropanol molecule (in excess),
restoring the intermediate B by releasing the respective alcohol derived from the substrate.
3
1
In order to corroborate the proposed catalytic mechanism, P NMR spectroscopy
3
1
1
experiments were performed. Figures 8(a) and 8(b) display the P{ H} NMR spectra of the
precatalyst Aas well as of isopropanol in CDCl , and after addition of the KOH solution in
3
isopropanol, respectively. The difference between the spectra before and after KOH addition
becomes evident. The spectra acquired after KOH incorporation denote new peaks with different
1
chemical shifts, suggesting the formation of the intermediate reactive specieB. Therefore, the H
3
1
and P NMR spectra played an important role in the understanding of base-assisted transfer
hydrogenation of ketones catalyzed by Ru(II) complexes as well as the catalytic mechanism
proposed in figure 7.
4
. Conclusion
A new organometallic Ru(II) complex was synthesized, characterized, and tested regarding
hydrogen transfer reaction, showing notable catalytic activity in the transfer hydrogenation of
ketones, achieving conversions superior to 90% within 4 h at 60 °C, which denote mild
1
conditions. The reaction in situ monitored by rapid injection and in real time through H NMR
spectroscopy was demonstrated to be a valuable technique for the establishment of the complex
catalytic mechanism. This involves the formation of a hydride specie and acetone followed by
the reduction of ketone substrates to obtain the desirable compounds. This in situ NMR
spectroscopy method comprised by probe tuning, locking, magnetic field shimming, and reaction
temperature setting may also be easily applied to investigate several other chemical reactions.
8

Acknowledgement
The authors are grateful to the personnel of laboratories of EMBRAPA Instrumentation. This
work was supported by CNPq grant numbers 163394/2015-6, 303837/2013-6 and 2017/02197-7,
Sao Paulo Research Foundation (FAPESP).
Disclosure statement
No potential conflict of interest was reported by the authors.
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1
1

Figure captions
Figure 1. Synthetic pathway of complex A.
Figure 2. Optimized molecular structure of the Ru(II) complex (hydrogens omitted for clarity).
Figure 3. Schematic Ru(II) base-assisted transfer hydrogenation reaction of ketones from
isopropanol.
1
Figure 4. H NMR spectra showing the triplet signal for complex A.
Figure 5. Monitoring of the hydrogen transfer reaction on cyclohexanone, during 4 h, by
1
H NMR spectroscopy.
Figure 6. Variation of phenylethanol relative concentration (%) throughout time, measured for
1
4
h by H NMR spectroscopy. Acetophenone, isopropanol and compound A were used as
substrate, hydrogen donor, and catalyst, respectively.
Figure 7. Proposed mechanism for the transfer hydrogenation of ketones catalyzed by the Ru(II)
complex A.
3
1
1
Figure 8. P{ H} NMR spectrum of precatalyst A in CDCl with a) isopropanol and b) KOH
3
(0.9 M).
1
2

1
3

1
4

1
5

1
6

1
7

1
8

Table 1. Activation parameters for the catalytic activity of the catalyst A.
5
-1
-1
-1
§
S§
T (K)
t (min)
k (×10 s )
Ea (J mol K ) H
1
/2
3
3
3
13
23
33
248
182
82
4.65
6.34
14.08
4.62×104
43.50
-2.74
Table 2. Performance of the Ru(II) complex in the transfer hydrogenation of ketonesa.
b
TONc
40 °C
Conversion (%)
Entry
Substrate
4
0 °C
50 °C
60 °C
50 °C
60 °C
O
1
24
32
55
91
48
64
110
182
O
2
3
88d
90
88
180
O
40
29
58
99e
87
80
58
116
69
198
174
O
4
69f
a
Reactions were carried out at 60 °C during 4 h using isopropanol (520 L), ketone (0.5 mmol),
base (10 mol%), and catalyst (0.5 mol%).
b
c
d
e
f
1
Conversion was monitored by H NMR spectroscopy using C D as internal standard.
6
6
TON: Turnover number: ratio of formed product moles to consumed catalyst moles.
Using 1 mol% of catalyst.
After 6 h.
After 3 h with 1 mol% of catalyst.
1
9