(Hive Bee)
05-31-04 12:31
No 510579
(Rated as: excellent)
Here is another great CTH method for reducing nitroarenes, benzaldehydes, and ketones if you want to avoid expensive palladium and get good results.
Catalytic Transfer Hydrogenation of Nitroarenes, Aldehydes, and Ketones with Propan-2-ol and KOH/NaOH over Mixed Metal Oxides
Sachin U. Sonavane, Radha V. Jayaram
Synthetic Communications 33(5), 843-849(2003)
Abstract
Mixed metal oxides such as ZrO2-NiO, ZrO2-CoO, and ZrO2-Fe2O3 effectively reduce nitroarenes, aldehydes and ketones with propan-2-ol and KOH/NaOH in liquid phase reaction. The catalysts also have considerable level of reusability.
Introduction
The selective and rapid reduction of nitro compounds is an interesting area of research particularly when other potentially reducible moieties are present in the molecule.1 Complete reduction of nitro compounds to amines is very important, as they are potential intermediates for a variety of fine chemicals. 2,3
A wide range of homogeneous and heterogeneous catalytic systems in combinations with different hydrogen donors have been employed for reduction of major functional groups attached to aliphatic and aromatic structures.4
Reduction of functional groups can be achieved by classical Meerwein Pondrof-Verley reduction. However the use of aluminium isopropoxide in stoichiometric amounts under severe reaction conditions often leads to undesired side reactions.5
Recently, transfer hydrogenation using homogeneous complexes of Cu, Fe, Rh, Pd, and Ni with propan-2-ol as hydrogen donor has gained much interest.6 But reduction rates are considerably low with these catalysts.7 On the other hand, use of heterogeneous catalysts has several advantages over these homogeneous systems, which includes ease of recovery and enhanced stability. In recent years different catalyst systems such as mixed metal oxides, hydrous oxides, metallic Sm, Indium metal and clay minerals have been exploited for reduction of nitro compounds.8-12
In this paper, we present the results of transfer hydrogenation of some aromatic nitro compounds, ketones and aldehydes with propan-2-ol over mixed metal oxides of the type ZrO2-NiO, ZrO2-CoO and ZrO2-Fe2O3
All chemicals and solvents used were of A.R. grade. The catalysts were characterized by XRD and FTIR spectroscopy. XRD patterns were recorded on a Siemens D-500 X-ray diffractometer using CuK radiation and a Ni filter.
FTIR spectra were recorded with a JASCO FTIR spectrophotometer by KBr pellet method. The progress of the reaction was monitored by TLC (SiO2-I2 vapor) for qualitative purpose. The products were further analyzed using a gas chromatograph (Model Esheeta, Eshika, India) fitted with an OV 101 column and FID detector.
Transfer Hydrogenation Reaction
Here we report catalytic transfer hydrogenation of nitro compounds (Scheme 1) (Table 1) and carbonyl functions (Scheme 2) in the liquid phase with propan-2-ol as hydrogen donor.
Scheme 1.
Scheme 2.
Table 1. Liquid phase catalytic transfer hydrogenation of nitrobenzene to aniline with propan-2-ol.
| Number | Catalyst (promotor KOH/NaOH) | Yield of aniline (%) |
| 1 | Catalyst without promotor | 0.0 |
| 2 | Promotor without catalyst | 0.0 |
| 3 | ZrO2 | 15.0 |
| 4 | ZrO2-NiO | 100.0 |
| 5 | ZrO2-CoO | 94.8 |
| 6 | ZrO2-Fe2O3 | 93.5 |
Since ZrO2-NiO gave maximum conversion to aniline this catalyst was chosen to study the reduction of other substrates (Table 2).
Table 2. Transfer hydrogenation of nitroarenes and carbonyl compounds with propan-2-ol on ZrO2-NiO.
| No. | Substrate | Product | Yield (%) |
| 1 | Nitrobenzene | Aniline | 100.0 |
| 2 | 4-Chloronitrobenzene | 4-Chloroaniline | 96.5 |
| 3 | 1,3-Dinitrobenzene | 3-Nitroaniline | 75.1 |
| 4 | 1-Chloro-2,4-dinitrobenzene | 4-Chloro-3-nitroaniline | 80.9 |
| 5 | 2-Nitrotoluene | 2-Aminotoluene | 81.0 |
| 6 | 4-Nitrotoluene | 4-Aminotoluene | 00.0 |
| 7 | 4-Nitrobenzoic acid | 4-Aminobenzoic acid | 00.0 |
| 8 | 4-Nitrophenol | 4-Aminophenol | 00.0 |
| 9 | 4-Nitroanisole | 4-Anisidine | 85.0 |
| 10 | 1,3-Dinitrobenzene | 3-Nitroaniline | 75.0 |
| 11 | Benzaldehyde | Benzyl alcohol | 97.0 |
| 12 | Acetophenone | 1-Phenylethanol | 99.2 |
| 13 | Cyclohexanone | Cyclohexanol | 95.1 |
| 14 | Benzophenone | Benzhydrol | 93.0 |
It is worth noting here that the reduction was not possible either in the absence of catalyst or promotor. Also equimolar quantities of either KOH or NaOH gave comparable yields. An increase in the promotor concentration increases the product selectivity/activity. The effect of promotor concentration on the reaction is shown in Table 3.
Table 3. Effect of promotor concentration on the product selectivity and nitrobenzene conversion.
| Amount of promotor (mmol) | Yield of aniline (%) |
| 28.0 | 100.0 |
| 25.0 | 100.0 |
| 20.0 | 100.0 |
| 15.0 | 94.8 |
| 10.0 | 69.0 |
The activity of the catalyst was maintained for at least 6 cycles without any regeneration treatment after which there was a gradual decrease. In a typical experiment the % yield to aniline decreased from 99% to 83% after six cycles and then to 54% after 8 cycles. Also the selectivity was 100%. There was no other side reaction as checked by material balance.
Results and Discussion
It is possible that propan-2-ol gets adsorbed on basic site of the catalyst and the substrates on an adjacent acidic site (Schemes 3 and 4).8-11 These basic sites could be active M0 sites (M = Ni, Fe, Co) promoted by KOH/NaOH. During adsorption of propan-2-ol the hydrogen from -OH gets adsorbed as proton and hydrogen of C-H migrates via hydride transfer to the substrate. Hence rate of reaction can show dependence on the strength of adsorption of both propan-2-ol and substrate.
Scheme 3.
Scheme 4.
It has been observed in the present case that the presence of electron releasing groups reduces the reduction of -NO2. If adsorbtion of the substrate is rate determining, presence of electron-releasing group at para positions should enhance the reduction. This is not found to be the case. However, an electron releasing substituent at para position will hinder the hydride transfer from the adsorbed propan-2-ol to the adsorbed species of nitro compound. Hence it is possible that the rate of reduction is more decided by the subsequent hydride transfer rather than the initial adsorbtion of the substrates. However more systems of nitro and carbonyl compounds have to be studied in detail to arrive at the complete mechanism of the reduction process.
Preparation of Catalysts
Appropriate amounts of respective soluble metals salts were dissolved in minimum amount of deionized water. To this, aqueous ammonia solution was added under vigorous stirring until pH 10.0 was achieved. Then the co-precipitated hydroxides were washed repeatedly to get neutral filtrate free of the anions of precursor materials. The precipitate was then dried in an air oven for 24 h at 110°C, ground below 100 mesh. The hydroxide was then calcined in a muffle furnace at 500°C for 8 h.
General Procedure for Liquid Phase Reactions
The liquid phase reaction was carried out in a 100 mL 2-necked round bottom flask equipped with a reflux condenser and a thermometer. The reaction mixture was stirred and heated to the desired temperature with a magnetic stirrer-heater and a Teflon coated magnetic bob. The products were analyzed by gas chromatographic method.
References
[1] Kabalka G.W., Varma R.S. Comprehensive Organic Synthesis, Trost B.M., Fleming I. Pergamon, Oxford, 1991, Vol. 8 p. p.363.
[2] Buist J.M. Developments in Polyurethanes, First Applied Science Publishers, London, 1978.
[3] Mortia E., Young E.I., A study of sulfenamide acceleration, Rubber Chem. and Technol., 36 (1963) 844.
[4] Johnstone R.A.W., Wilby A.H., Entwistle I.D., Heterogeneous catalytic transfer hydrogenation and its relation to other methods for reduction of organic compounds, Chem. Rev., 85 (1985) 129.
[5] Wilds A.L., Reduction with aluminium alkoxides, Org. React., 2 (1944) 178.
[6a] Choudhary R.L., Backvall J.E., Efficient ruthenium-catalysed transfer hydrogenation of ketones by propan-2-ol, J. Chem. Soc. Chem. Commun., (1991) 1063.
[6b] Wang G.Z., Backvall J.E., Ruthenium-catalysed transfer hydrogenation of imines by propan-2-ol, J. Chem. Soc. Chem. Commun., (1992) 980.
[6c] Iyer S., Varghese J.O., [NiCl2(PPh3)2] catalysed transfer hydrogenation of ketones and aldehydes by propan-2-ol, J. Chem. Soc. Chem. Commun., (1995) 465.
[6d] Nose A., Kudo T., Studies of reduction with the sodium borohydride-transition metal boride system, Chem. Pharm. Bull., 36 (1988) 1529.
[7] Pasto D.J., A theoretical study of the disproportionation reaction of N2H2 species, J. Am. Chem. Soc., 101 (1979) 6852.
[8] Upadhaya T.T., Katdare S.P., Sabded P., Ramaswamy V., Sudalai A., Chemoselective transfer hydrogenation of nitroarenes, aldehydes and ketones with propan-2-ol catalysed by Ni-stabilised zirconia, Chem. Commun., (1997) 1119.
[9a] Pitts M.R., Harrison J.R., Moody C.J., Indium metal as a reducing agent in organic synthesis, J. Chem. Soc., Perkin. Trans. 1, (2001) 955-977.
[9b] Banik B.K., Mukhopadhyay C., Becker F.F., A facile reduction of aromatic nitro compounds to aromatic amines by samarium and iodine, Tetrahedron Lett., 39 (1998) 7243.
[10] Kumbhar P., Svalente J.S., Figueras F., Reduction of aromatic nitro compound with hydrazine hydrate in the presence of iron(III) oxide-MgO catalyst prepared from a Mg-Fe hydrotalcite precursor, Tetrahedron Lett., 39 (1998) 2573.
[11] Jyothi T.M., Rao B.S., Reduction of nitroarenes with isopropanol and KOH over metal oxide catalysts, Ind. J. Chem., 39A (2000) 1041-1043.
[12] Waghoo G., Jayaram R.V., Joshi M.V., Heterogeneous catalytic conversions with hydrous SnO2, Synth. Commun., 29 (3), (1999) 513-520
(Heavyweight Chempion(eer))
05-31-04 17:00
No 510614
Very interesting indeed. Time to get to work.
Severe Aztecoholic and President of Sooty's fanclub - Sooty for President!!