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A Reusable Polymer-anchored Palladium Catalyst for Reduction of Nitroorganics, Alkenes,...
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A Reusable Polymer-anchored Palladium Catalyst for Reduction of Nitroorganics, Alkenes, Alkynes and Schiff Bases†
作者:
Monirul Islam,
期刊:
Journal of Chemical Research, Synopses
(RSC Available online 1998)
卷期:
Volume 0,
issue 1
页码: 44-45
ISSN:0308-2342
年代: 1998
DOI:10.1039/a703426a
出版商: RSC
数据来源: RSC
摘要:
NH2 P N P PhCOR R = H, Me, Ph C(R)Ph N P C Pd R OAc/2 OAc/2 Pd(OAc)2 AcOH N P C Pd H DMF 2 1 H2, DMF, 80 °C H R 44 J. CHEM. RESEARCH (S), 1998 J. Chem. Research (S), 1998, 44–45† A Reusable Polymer-anchored Palladium Catalyst for Reduction of Nitroorganics, Alkenes, Alkynes and Schiff Bases† Monirul Islam, Asish Bose, Dipakranjan Mal and Chitta R. Saha* Department of Chemistry, Indian Institute of Technology, Kharagpur - 721 302, India The preparation and utility of a new polymer-anchored PdII catalyst for the hydrogenation of a wide range of organic substrates is described.Catalytic reduction1 of organic compounds is an important process both in the laboratory and in industry, and continues to be the subject of active research. Methods based2 on catalytic hydrogenation in homogeneous and heterogeneous media are commonly adopted for the hydrogenation of organic compounds. However, the reproducibility and selectivity of these methods, particularly of noble-metal-catalysed hydrogenation, has restricted their popularity.To improve upon the selectivity of a catalyst, we considered immobilization of a catalytic system. Immobilization of catalysts or reagents to polymer supports often offers many advantages for carrying out organic transformations.3 Ease of work-up, higher yields, product selectivity and re-usability of the catalysts make them more attractive than their homogeneous counterparts. In the area of hydrogenation of organic substrates, extensive work has been performed on the development of Rh-based polymer supported catalysts.4,5 Sometime ago we and others reported the use of a PdII homogeneous catalyst6–9 for the reduction of a host of nitro-aromatics and -aliphatics.To develop its polymer-bound version, we have now prepared a synthetically useful PdII catalyst for reduction of functional groups like •NO2, \ /C‚C / \ , \ /C‚N•, •C‚C•, etc. in excellent yields. Catalyst 1 was readily accessible in two steps from aminopolystyrene10 (sP-NH2).Treatment of the polymer with PhCOR (R=H, Me, Ph) provided the corresponding Schiff bases, which were then treated with PdII acetate in acetic acid to yield catalysts 1 as dark brown solids (Scheme 1). Characterization of 1 was performed by IR and ESCA. The ESCA peaks11 at 338.25 eV (Pd 3d5/2) and 343.75 eV (Pd 3d3/2) and IR signals6,12,13 at 1585, 1420 and 722 cmµ1 indicate the presence of acetato-bridged orthometallated palladium( II) in catalyst 1.Catalysts 1 are activated by stirring them under H2 (1 atm) at 80 °C for 1 h to produce the active species 2. Chemical analysis indicates the presence of 112.25% of Pd in catalyst 2. Characteristic ESCA signals, and IR signals at 722, 1985 (vPd-H)14,6 and 1655 cmµ1 (vco, DMF), confirm the structure 2. Comparable ESCA and IR signals were also observed for the used catalyst. Moreover, the Pd-content of the catalyst, as measured by gravimetric analysis, remained unchanged even after several cycles.Exposure of an organic substrate in DMF–ethyl acetate medium containing catalyst 2 to hydrogen (1 atm) at room temperature resulted in rapid reduction of the substrate. As shown in Table 1, the reduction of nitroaromatics to aminoaromatics proceeds in excellent yields. In contrast, the reduction of nitroaliphatics (Table 3) requires higher temperatures (170 °C) and pressures (110.5Å103 kN mµ2). However, the yields of the products were consistently excellent.Hydrogenation of double bonds of alkenes, alkynes and Schiff bases to their corresponding saturated products under normal conditions is more facile, w-nitrostyrene being reducible to w-nitroethylbenzene in excellent yield. Catalyst 2 when R=H is the most active. The results described in Tables 1–3 involve the use of this catalyst. It offers a high degree of chemoselectivity. Hydrogenation of halonitroaromatics to the corresponding haloanilines is often accompanied by dehalogenation, and entails extensive optimization experiments.1 This problem could be greatly obviated by the use of catalyst 1.For example, a chloro substituent ortho or para to a nitro group remains intact. Similarly, a lactone (Tables 1 and 3, entry 7) group is not affected under the conditions employed. The superiority of catalyst 2 is clearly discernible from entry 8 of Table 1, in which a methoxy substituted nitroester is shown to be reducible to its amine in high yield.It may be noted that *To receive any correspondence. †This is a Short Paper as defined in the Instructions for Authors, Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is therefore no corresponding material in J. Chem. Research (M). Table 1 Atmospheric pressure reduction of nitroaromatics Initial turnover Entry Substrate Time t/h no. minµ1 Product % Yield 12345678 Nitrobenzene o-Nitrotoluene o-Chloronitrobenzene p-Chloronitrobenzene m-Dinitrobenzene 1-Nitronaphthalene 6-Nitrophthalide Methyl 4,5-dimethoxy-2-nitrobenzoate 3.5 6.25 5.37 4.9 7.0 8.0 8.5 8.5 2.30 1.22 1.40 1.70 1.00 0.86 0.80 0.79 Aniline o-Toluidine o-Chloroaniline p-Chloroaniline m-Phenylenediamine 1-Aminonaphthalene 6-Aminophthalide Methyl 4,5-dimethoxy-2-aminobenzoate 97 90 92 94 92 93 94b 92b a[sub] =0.50 mol dmµ3, total vol.=10 ml, medium=DMF; yields refer to GC analysis. bIsolated yield, [cat] =Pd content=1.50Å10µ3 mol.Scheme 1J.CHEM. RESEARCH (S), 1998 45 conventional hydrogenation of nitroesters over Pd or Pd/C provides variable yields of the corresponding amine.15 Although the described use of the catalyst 2 in the reduction of various organic substrates is quite general, it involves a restricted choice of reaction media. It appears that DMF is the best solvent for such reactions though hydrogenation occurs in ethyl acetate at a slower rate. The remarkable advantages with the use of the catalyst 2 are the ready accessibility of the catalysts, their reusability and storage. Even after recycling 7 or 8 times, the catalyst retains its original activity. Furthermore, the used catalysts are free from fire hazards or explosions. They can be used for selective reduction of an aromatic nitro group in the presence of an aliphalic nitro group, under atmospheric pressure, as evident from Tables 1 and 3.Further studies on product selectivity are under way. Experimental In a typical procedure, a solution of a substrate (5.0 mmol) in DMF (10 ml) containing the catalyst 2 (14.0 mg) was subjected to hydrogenation under hydrogen (1.0 atm) in a magnetically stirred glass reactor.The rate of hydrogen consumption was measured using a glass manometric apparatus. The detailed experimental setup and hydrogenation procedure have been described earlier.8 After the completion of the reaction, the catalyst was filtered off and the filtrate analysed by GC. In certain cases the products were isolated by usual work-up followed by preparative tlc.Received, 19th May 1997; Accepted, 4th September 1997 Paper E/7/03426A References 1 A. M. Tafesh and J. Weiguny, Chem. Rev., 1996, 96, 2035. 2 R. A. W. Johnstone, A. H. Wilby and I. D. Entwistle, Chem. Rev., 1985, 85, 129. 3 M. E. Wright and S. R. Pulley, J. Org. Chem., 1987, 52, 5036. 4 Z. Jaworska, S. Gobas, W. Mistra and J. Wrzysz, J. Mol. Catal., 1994, 88, 13. 5 D. T. Gokak and R. N. Ram, J.Mol. Catal., 1989, 49, 285. 6 A. Bose and C. R. Saha, J. Mol. Catal., 1989, 49, 281. 7 P. K. Santra and C. R. Saha, J. Mol. Catal., 1987, 39, 279. 8 D. K. Mukherjee, B. K. Palit and C. R. Saha, Indian J. Chem., 1992, 31A, 243. 9 A. M. Tafesh and M. Beller, Tetrahedron Lett., 1995, 36, 9305. 10 R. B. King and M. E. M. Sweet, J. Org. Chem., 1979, 44, 385. 11 B. M. Choudary, K. Ravikumar and M. Lakshmi Kantam, J. Catal., 1991, 41, 130. 12 T. A. Stephenson and G. Wilkinson, J. Inorg.Nucl. Chem., 1967, 29, 2122. 13 H. Onoue and I. Moritani, J. Organomet. Chem., 1972, 43, 431. 14 (a) J. V. Kingston and G. R. Scollary, Chem. Commun., 1969, 455; (b) E. H. Brooks and F. Glocking, J. Chem. Soc. A, 1966, 1241. 15 C. A. Fetscher and M. T. Bogert, J. Org. Chem., 1939, 4, 71. Table 2 Atmospheric pressure reduction of miscellaneous substratesa Initial turnover Entry Substrate Time/h no. minµ1 Product % Yield 12345678 Hex-1-ene Styrene w-Nitrostyrene Fumaric acid Isoprene Phenylacetylene Benzylideneaniline N-Methylbenzaldimine 1.2 0.70 1.4 2.8 0.72 0.81 1.96 1.5 7.0 14.70 5.80 3.00 14.70 13.10 4.30 5.60 Hexane, hex-2-ene Ethylbenzene w-Nitroethylbenzene Succinic acid 2-Methylbutane Ethylbenzene N-Phenylbenzylamine N-Methylbenzylamine 86, 12 98 95 92 97 97 100 100 a[sub] =0.50 mol dmµ3, reaction mixture vol.=10 ml, medium=DMF. Table 3 High pressure reduction of nitroaliphaticsa Initial turnover Entry Substrate Time (t/h) no. minµ1 Products % Yield 1234567 Nitroethane 1-Nitropropane 2-Nitropropane 1-Nitroheptane w-Nitroethylbenzene Acetonitrile Phthalic anhydride 6.2 6.5 7.2 6.8 7.0 8.6 8.5 3.56 3.33 2.93 3.11 2.99 2.12 1.58 Ethylamine 1-Aminopropane 2-Aminopropane 1-Aminoheptane w-Aminoethylbenzene Diethylamine Phthalide 98 97 96 94 95 93 86b aMedium=DMF, [sub] =1.50 mol dmµ3, total vol.=10 ml; bisolated yield, [cat] =1.70Å10µ3 g.atom dmµ3, pH2=10.5Å10µ3 kN mµ2, T=70 °C.
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