Thermoregulated phase-transfer ligands and catalysis(Part X):
CO selective reduction of aromatic nitro compounds catalyzed by Ru₃(CO)₉(PETPP)₃ in two-phase system

Mei Jianting, Jiang Jingyang, Li Yaming, Jin Zilin
(State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, 116012, China)

Received Oct. 30, 1999; Supported by the National Natural Science Foundation of China. (Grant No. 29876004) and the Doctorate Program Foundation of Higher Education .

Abstract CO selective reduction of aromatic nitro compounds catalyzed by Ru₃(CO)₉(PETPP)₃ complex ( PETPP= P[C₆H₄-p-(OC₂H₄)_nOH]₃ ) in aqueous/organic two-phase system was investigated. The catalyst displayed good catalytic activity for the reduction of o-ClC₆H₄NO₂ at 140ºC and Pco= 4 MPa : conversion of o-ClC₆H₄NO₂ was 99.2% and selectivity toward the desired amines was higher than 99%. After the reduction the catalyst in water phase could be easily separated from the organic phase containing product and could be employed in the successive reaction runs. The conversion of aromatic nitro compound could be kept higher than 88% after 4 recycles of the catalyst.
Keywords Water-soluble phosphine ruthenium complex, Biphasic catalysis, Aromatic nitro compounds, CO selective reduction, TRPTC

1. INTRODUCTION
Aromatic amines carrying carbonyl, chloro-, cyano and C=C groups are important intermediates for the dye, pigment, pharmaceutical and pesticide industries ^[1]. Generally, these amines have been produced from the corresponding nitro compounds by the reduction using iron powder, or alkali metal sulfides, or by catalytic hydrogenation. However, the problem of treatment of the by-products and loss of the selectivity toward the desired aromatic amines remains ^[2,3]. Recently it has been discovered that reduction of aromatic nitro compounds to their corresponding aromatic amines using CO and water with complex catalyst is an very attractive subject from the viewpoints of both high chemoselectivity and environmental friendliness ^[4,5]. Tafesh ^[6] studied CO selective reduction of aromatic nitro compounds in aqueous/organic biphasic system catalyzed by water-soluble complex PdCl₂/TPPTS (TPPTS = P(C₆H₄-3-SO₃Na)₃ ). Under the conditions of 15MPa and 100ºC, the conversion of the aromatic nitro compounds reached 70% with 15% of the by-products.
    Jin et al proposed a conception of thermoregulated phase-transfer catalysis (TRPTC) based on the nonionic water-soluble phosphine ligands, which possess an inverse temperature-dependent solubility in water and thus a distinct cloud point. Promising results have been achieved in the aqueous/organic two-phase hydroformylation of higher olefins catalyzed by Rh/PETPP ^[7-9] . In this work Ru₃(CO)₉(PETPP)₃ (PETPP = P[C₆H₄-p-(OC₂H₄)_nOH]₃ n = 6 ) was synthesized and applied in the CO selective reduction of aromatic nitro compounds in the aqueous/organic two-phase system. Under the reaction conditions of 140ºC, Pco=4MPa, and t=10h, the conversion of o-ClC₆H₄NO₂ was 99.2% and the selectivity of o-ClC₆H₄NH₂ was higher than 99%. The catalyst retained in water layer could be easily recovered and employed successively. Furthermore, when the system was applied in the reaction of aromatic nitro compounds containing C=O or C=N groups, the reduction proceeded with good activity and high selectivity too.
2. EXPERIMENTAL
2.1 General
All solvents and liquid aromatic nitro compounds were distilled prior to use. Distilled deionized water was used. Gas chromatographic analysis was performed on a SP-09 instrument (OV-101, 50m capillary column, carrier gas: 0.2MPa N₂, FID detector) equipped with a Shimadzu C-R3A integrator. n-Decane was used as an internal standard. IR and ³¹P-NMR were recorded on a Nicolet 200X-BFT-IR spectrometer and on a JEOLFX-90Q-NMR spectrometer separately. Mass spectra were measured on a Finnagan 312/ss 200 GC-mass spectrometer.
2.2 Catalysts preparation
PETPP, Ru₃(CO)₁₂ were prepared according to literature ^[8-10]. Ru₃(CO)₉(PETPP)₃ was prepared according to the method for the preparation of Ru₃(CO)₉(TPPTS)₃ ^[11,12]. The complex was characterized by IR and ³¹P-NMR(36.20MHz, internal standard 85%H₃PO₄, 45ºC, D₂O). The results were C=O(cm^-1):2036(m),1972(m),1965(s); ³¹P-NMR:d=34.5ppm.
2.3 CO reduction
CO selective reductions of aromatic nitro compounds to aromatic amines were carried out in a 75ml stainless steel autoclave, which was placed in a thermostatic oil bath. To this were added toluene, water, aromatic nitro compounds, Ru₃(CO)₉(PETPP)₃ and the internal standard n-decane. Then the autoclave was flushed five times with CO up to the reaction pressure and held at the designated temperature with magnetic stirring for a fixed time. After the completion of the reaction, the reactor was cooled and discharged and the reaction solution was siphoned into a separatory funnel. After phase separation, the organic layer was washed with distilled water, dried over anhydrous magnesium sulfate and subjected to GC analysis.
3. RESULTS AND DISCUSSION
3.1 CO selective reduction of o-ClC₆H₄NO₂
The data in Table 1 indicate that the rate of CO selective reduction increases with the increase of reaction temperature and CO pressure. Higher than 99% of conversion of o-ClC₆H₄NO₂ and 99% of the yield of desired amine were obtained when the reaction was carried out at 140ºC. The recycling effect of the catalyst was also examined. Ru₃(CO)₉(PETPP)₃ catalyst retained in aqueous phase after phase separation was used for 4 times with little loss of the catalyst activity (entries 6-9).

Table 1 Two-phase CO selective reduction of o-ClC₆H₄NO₂ catalyzed by Ru₃(CO)₉(PETPP)₃ catalysts ^a

a: Reaction conditions:Ru₃(CO)₉(PETPP)₃ =0.01mmol, V(toluene)/V(water)=4/4(ml),
   o-ClC₆H₄NO₂=10mmol.; b: Number of catalyst recycles.

3.2 The CO selective reduction of aromatic nitro compounds carrying other functional groups
Aromatic nitro compounds bearing other functional groups in CO selective reduction catalyzed by Ru₃(CO)₉(PETPP)₃ in water/organic biphasic system were tested and the results were listed in Table2. The results showed that both the chemoselective reduction of the nitro group and the yields of corresponding aromatic amines are all higher than 98%.

Table 2 The CO selective reduction of some aromatic nitro compounds and aromatic compounds

Reaction conditions:T=140ºC, P(CO)=4Mpa,t=10hr,all other conditions are the same as in Table 1.

    In order to affirm that the reduction reaction by CO is chemoselective toward nitro group only, compounds with other reducible functional group are chosen as substrate. Under the same reaction conditions, benzenenitrile, bromobenzene and styrene do not react, and very little benzylalcohol from benzaldehyde was detected. These data support that Ru₃(CO)₉(PETPP)₃ is highly active and chemoselective for nitro group as well. Such a high chemoselectivity can be explained that the nitro group is not reduced by hydrogen from water-gas shift reaction. Instead it has been postulated that the reduction aromatic nitro compounds involves a probable ruthenium nitrene complex intermediate through successive oxygen transfer reaction between the nitro compound and coordinated metal-carbonyl ligand ^[4]. A possible mechanism is illustrated in Fig1.

Fig.1 Mechanism of the CO selective reduction of aromatic nitro compounds

3.3 CO selective reduction of o-ClC₆H₄NO₂ catalyzed by different Ru/P complexes
Four catalysts possessed different water-solubilities are tested for the two-phase CO selective reduction of o-ClC₆H₄NO₂ and the results are listed in Table 3. The data in Table 3 show that the catalyst activity decreases as water-solubility of the ligand increases from TPP to TPPTS. Although the water-solubility of the PETPP is nearly the same as that of the TPPTS, the catalytic activity of the PETPP/Ru is the same as that of the water insoluble TPP/Ru. This phenomenon could be explained from the thermoregulated phase-transfer function of Ru₃(CO)₉(PETPP)₃ catalyst. The Ru₃(CO)₉(PETPP)₃ used has a cloud point of Tp = 82.5ºC and thus possesses a property of inverse temperature-dependent water-solubility. That is, the Ru₃(CO)₉(PETPP)₃ catalyst soluble in the aqueous phase at lower temperature can transfer into organic phase to catalyze the CO selective reduction of aromatic nitro compounds at temperature higher than Tp. Therefore, CO selective reduction of o-ClC₆H₄NO₂ occurs in the organic phase. The experimental results revealed that there was a 'thermoregulated phase-transfer catalysis (TRPTC)' process present in the reaction.

Table 3 The activities of different phosphine/Ruthenium on the CO reduction of o-ClC₆H₄NO₂

Reaction conditions: catalyst = 0.01mmol,T=140ºC, P(CO) = 4 Mpa, t = 10h. Other conditions are the same as Table 1.

REFERENCES
[1] Mestroni G, Camus A. Zassinovich in Aspects of Homogeneous Catalysis, 1981, 4: 71.
[2] Jenner G, Taleb B A. J. Mol. Catal., 1991, 68: 257.
[3] Nomura K. J. Mol. Catal. A. Chemical, 1998, 130: 14.
[4] Nomura K. J. Mol. Catal. A. Chemical, 1995, 95: 203.
[5] Tafesh A M, Weigun Y J. Chem. Rev., 1996, 96: 2035.
[6] Tafesh A, Beller M. Tetrhedron Lett., 1995, 36: 9305.
[7] Jin Z L, Zheng X L, Fell B. J. Mol. Catal., 1997, 116: 55.
[8] Zheng X L, Jiang J Y, Liu X Z, Jin Z L. Catalysis Today, 1998, 44:175.
[9] Chen R F, Liu X Z, Jin Z L. J.Organomet. Chem., 1998, 571: 201.
[10] Eady C R, Jackson P F, Johnson F G et al. J. Chem. Soc., Dalton Trans., 1980, 383.
[11] Fontal B, Orlewski J, Santini C C et al. Inorg. Chem., 1986, 25: 4320.
[12] Jin Z L, Zheng X L. Aqueous Phase Organometallic Catalysis -Concepts and Application. Weinheim: Wiley-VCH, 1998: 233-240.

[ Back ] [ Home ] [ Up ] [ Next ] Mirror Site in USA  Europe  China  CSTNet ChinaNet