Wilkinson-Type Catalyst for Transfer Hydrogenation
mobilization made it possible to recycle the expensive noble FTIR Spectroscopy: FTIR spectra of the support and the catalyst
were measured on a Varian 670-IR spectrometer, equipped with an
metal catalyst without significant loss of activity.
attenuated total reflection (ATR) device (Goldengate by Specac
with a single reflection diamond ATR element and KRS-5 lenses)
Experimental Section
and a room temperature detector of deuterated triglycine sulfate
(
DTGS). The spectra were recorded in the frequency range 390–
1
General: H NMR spectra of isolated products in CDCl
3
were re-
–1
–1
4000 cm at a spectral resolution of 4 cm .
corded with a Varian spectrometer. The product yields of the trans-
fer hydrogenation reaction were determined by GC with a CP-Chir-
asil-Dex CB capillary column (30 mϫ0.25 mmϫ0.25 μm) by using
a Varian 3800 gas chromatography equipped with a flame ioniza-
tion detector. Lithium, chlorodiphenylphosphane, and (3-chlo-
ropropyl)triethoxysilane were supplied by Sigma Aldrich.
31P Solid-State NMR Spectroscopy: The 31P NMR spectrum was
recorded by using a Chemagnetics Infinity 400 spectrometer with
a 9.4 T wide-bore magnet attached, operating at 161.97 MHz. A
double-resonance 6 mm probe head acquiring was used with a
magic angle spinning (MAS) at 8.00 kHz. The P NMR chemical
shift scale was externally calibrated to the signal of a concentrated
3
1
3 3
RhCl(PPh ) was purchased from Strem. Chromatographic silica
3 4
solution of aqueous orthophosphoric acid (H PO , 85%, δ =
gel was obtained from commercial sources. All the substrate
ketones are commercially available and were used without purifica-
tion. All the solvents used in the present study were distilled prior
to use. 2-Propanol was HPLC grade and purchased from Sigma
Aldrich. Reactions involving air-sensitive compounds were per-
formed by using Schlenk-line techniques under a positive pressure
of argon.
0
.0 ppm). Direct-polarization experiments were conducted to re-
3
1
cord a P NMR NMR spectrum. A recycling delay of 100 s was
used in summing up the detected free induction decays to avoid
spectral distortion from uneven saturation patterns; 512 transients
were added. An excitation pulse of 4 μs was used. An exponential
apodization of 500 Hz was applied on the FID as a trade-off in
between high resolution and high signal-to-noise ratio in the spec-
trum.
Synthesis of the Silica-Bound RhCl(PPh
a) Synthesis of the Phosphane Linker: The phosphane linker (di-
phenylphosphanyl)triethoxysilane was synthesized according to a
3 3
) Catalyst
(
ICP-AAS Analysis: The rhodium content in the catalyst was deter-
mined by ICP-AAS analysis.[
20]
[
8]
literature procedure. Granulated lithium (0.5 g, 72 mmol) was
added to a solution of chlorodiphenylphosphane (4.9 g, 22 mmol)
in dry THF (20 mL) in a 50 mL flame-dried round-bottomed
Schlenk-flask at room temperature under argon. The mixture was
stirred vigorously for 3 h, meanwhile the light yellow color of the
mixture turned into deep reddish orange. The mixture was filtered,
and the filtrate was added dropwise to a solution of (3-chloro-
propyl)triethoxysilane (7.2 g, 30 mL) in dry THF (25 mL) in a
Transfer Hydrogenation of Cyclohexanone: In a 5 mL Schlenk-tube
the silica-bound Wilkinson-type catalyst (200 mg), cyclohexanone
(49 mg, 0.5 mmol) and Na CO (53 mg, 0.5 mmol) were mixed in
2 3
2-propanol (0.5 mL). The Schlenk-tube was closed with a stopper,
the mixture was evacuated and backfilled with argon three times,
then heated to 82 °C in an oil bath. After 2 h at 82 °C, the mixture
was cooled to room temperature, and an aliquot (50 μL) was ana-
lyzed by GC.
100 mL flame-dried round-bottomed Schlenk flask at 0 °C under
argon over a period of 30 min. The mixture was stirred for 16 h
and warmed up to ambient temperature. Then it was filtered care-
fully under argon, and the filtrate was used directly for silica func-
tionalization.
Recycling Studies on the Transfer Hydrogenation Reaction of Cyclo-
hexanone with the Silica-Bound Wilkinson-Type Catalyst: When the
above described reaction became complete, the dark mixture was
poured onto a sintered glass filter under argon. The filter cake was
washed with a copious amount of 2-propanol containing 1 vol-%
of water and dried under reduced pressure at ambient temperature.
The obtained fine light yellow powder was recycled in the consecu-
tive catalytic runs under argon. Once this recycled catalyst became
wet by 2-propanol containing 1 vol-% of water during the reaction
setup it turned dark again.
(b) Phosphane Functionalization of the Silica Gel: Commercially
available chromatographic silica gel (5 g, 35–70 μm) was activated
at 250 °C in vacuo for 2 h, then suspended in dry degassed THF
(
40 mL) and P-functionalized by adding 3-(diphenylphosphanyl)-
propyltriethoxysilane (6.24 g, 22 mmol), dissolved in dry THF
150 mL), and the mixture was stirred at room temperature under
(
argon overnight. Then the solid was filtered off, washed with dry
THF (100 mL) under argon and dried under reduced pressure to
give a white powder.
Acknowledgments
(
c) Anchoring of RhCl(PPh
change: P-Functionalized silica gel (3 g) was suspended in dry de-
gassed toluene (60 mL), and RhCl(PPh (350 mg, 0.378 mmol)
3 3
) on the Silica Surface by Ligand Ex-
The Berzelius Center EXSELENT, AstraZeneca R&D Södertälje,
and the K & A Wallenberg foundation are gratefully acknowledged
for financial support.
3 3
)
was added and the mixture was stirred at room temperature under
argon overnight. During stirring the color slowly changed from
brown to orange. The solid was filtered off, washed with toluene
until the filtrate became colorless, and dried under reduced pressure
to give a fine yellow powder.
[1] a) R. L. Augustine, Heterogeneous Catalysis for the Synthetic
Chemist, Marcel Dekker, Inc., New York, 1996; b) E. Klabu-
novskii; G. V. Smith; Á. Zsigmond, Heterogeneous enantiose-
lective hydrogenation, Springer, Dordrecht, 2006.
[2] B. Cornils; W Herrmann, Applied Homogeneous Catalysis with
Organometallic Compounds, Wiley-VCH, Weinheim, 1996.
Characterization of the Catalyst
Brunauer–Emmett–Teller (BET) Surface Area Analysis: The BET
surface areas of the support and the catalyst were determined from
nitrogen adsorption/desorption isotherms at 77 K by using a
Micromeritics ASAP2020 volumetric adsorption analyzer. The
samples were heated in vacuo at a temperature of 383 K for 6 h.
The BET specific surface areas were calculated from the recorded
[
3] a) D. E. DeVos, I. F. J. Vankelecom, P. A. Jacobs (Eds.), Chiral
Catalyst Immobilization and Recycling, Wiley-VCH, Weinheim,
2000; b) K. Ding, Y. Uozumi (Eds.), Handbook of Asymmetric
Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2008; c) M.
Bartók, Chem. Rev. 2010, 110, 1663.
[4] B. M. Trost, I. Fleming, Comprehensive Organic Synthesis, El-
sevier, Oxford, 1991, vol. 8.
0
data between p/p = 0.05 and 0.15.
Eur. J. Org. Chem. 2011, 4409–4414
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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