Base-Catalysed Hydrogenation of Sulphur-Containing Aldehydes
FULL PAPERS
Table 1. Comparison of the results[a] of hydrogen transfer hydrogenations using 2-butanol at its boiling point and
hydrogenation on Rh and Pd catalysts from ref.[2]
Substrate
Catalyst
Alcohol Yield [%]
Reaction Time [h]
1a Thiophenecarboxaldehyde
Rh(DiPFc)/Al2O3
10% Pd/C
HDT
Rh(DiPFc)/Al2O3
10% Pd/C
HDT
Rh(DiPFc)/Al2O3
Pd/C
HDT
100
8
100
37.2
0
100
100
5
18
18
3
18
18
3
18
18
3
1b 5-Bromothiophene-2-carboxaldehyde
2 4-Methylthiobenzaldehyde
100
[a]
Reaction conditions: for the immobilised complex: 293 K, 100 psi H2; molar ratio substrate/Rh 300 350, concentration:
0.1 M, solvent 10 mL of 2-PrOH/H2O (1/1 v/v); for Pd/C: 293 K, 14 psi H2, concentration 0.1 M, 10 mg of 10% Pd w/w for
reaction 1 (X H) and 2.5 mg for reaction 1 (X Br), solvent 10 mL of 2-PrOH/H2O (1/1 v/v); for hydrotalcites standard
conditions: 371 K, atmospheric pressure, concentration 0.1 M, 0.1 g of catalyst corresponding to 1.2 wt % of the reaction
mixture, solvent 10 mL of 2-PrOH.
necked glass reactor using 2-propanol or 2-butanol as hydro-
gen donors at their boiling points. The pre-treated catalyst
(0.15 g of original solid, reduced to about 0.1 g after activation)
was carefullytransferred to the reactor swept bya small flow of
nitrogen and containing the alcohol (10 mL, Aldrich) at the
reaction temperature without exposure to air. Afterwards the
substrate (1 mmol, purity99% from Aldrich) was introduced.
The amount of catalyst in the reaction mixture is therefore
about 1.2 wt %. Periodicallywithdrawn samples were analysed
on a Shimadzu GC 14B gas chromatograph using a Super-
cowax capillarycolumn (30 m, 0.5 mm ID and 0.3 mm film
thickness). The products were identified byGC-MS analysis
and calibrations were done using authentic samples of the
corresponding alcohols.
alcohol) leads to the maximum in the rate observed here.
A lower but still acceptable rate is, however, observed in
more concentrated solutions. Since MgAl mixed oxides
contain about 0.4 meq/g of active sites,[16] at the optimal
concentration, the catalyst converts 25 mol per active
site in 2 h, therefore the reaction is trulycatalytic. On the
same reaction, the supported Rh catalyst converts 320
mol per mol Rh in 16 h, therefore the activities per active
site are of the same magnitude.
Conclusion
We have shown for the first time that the mixed oxides
obtained bycalcination of MgAl hydrotalcites can be
successfullyused to selectivelyreduce carbonyl com-
pounds to alcohols in the presence of sulphur moieties.
The selectivityis also excellent, with retention of Br,
with 5-bromothiophenecarboxaldehyde. The process
uses an inexpensive catalyst, operates at atmospheric
pressure and low temperature. The hydrotalcite can be
regenerated several times bycalcination provided it
contains no chlorine at the surface.[17] These catalysts are
suitable for aldolisation[18] and hydrogen transfer hydro-
genation[4] and can be reused several times after
calcination at 723 K in air.
Acknowledgements
TTHV acknowledges a grant from the French government.
References and Notes
[1] a) G. Ertl, H. Knozinger, J. Weitkamp, (Eds.), Handbook
of Heterogeneous Catalysis, Wiley-VCH, Weinheim,
1997; b) H.-U. Blaser, H. Steiner, M. Studer, in Tran-
sition Metals for Organic Synthesis, (Eds.: M. Beller, C
Bolm), Wiley-VCH, Weinheim, 1998, Vol. 2, Chapter 1.2.
[2] M. J. Burk, A. Gerlach, D. Semmeril, J. Org. Chem. 2000,
65, 8933 8939.
[3] M. J. Burk, A. Gerlach (Chirotech Technology), WO 01/
26807, 2001.
Experimental Section
[4] P. S. Kumbhar, J. Sanchez-Valente, J. Lopez, F. Figueras,
Chem. Commum. 1998, 535 536.
[5] J. Lopez, J. Sanchez Valente, J.-M. Clacens, F. Figueras, J.
General Remarks
An MgAl hydrotalcite [HDT][19] with a ratio Mg/Al 2 was
prepared according to experimental procedures described in
detail earlier, and activated bycalcination in air at 723 K. The
physico-chemical characterisation of this solid has been also
reported previously.[16] The catalytic test was carried out in a 3-
Catal. 2002,208,30 37.
[6] M. A. Aramendia, V. Borau, C. Jimenez, J. M. Marinas,
¬
J. R. Ruiz, F. J. Urbano, J. Colloid Int. Sci. 2001, 238,
385 389.
Adv. Synth. Catal. 2003, 345, 493 496
495