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Journal of the Chinese Chemical Society, 2003, 50, 267-271

267

First and Efficient Method for Reduction of Aliphatic and Aromatic Nitro Compounds with Zinc Borohydride as Pyridine Zinc Tetrahydroborato Complex: A New Stable Ligand-Metal Borohydride

Behzad Zeynizadeh* and Karam Zahmatkesh Chemistry Department, Faculty of Sciences, Urmia University, Urmia 57159, Iran

Pyridine zinc tetrahydroborate, [(Py)Zn(BH4)2], as a new stable ligand-metal borohydride, is prepared quantitatively by complexation of 1:1 zinc borohydride and pyridine at room temperature. This reagent efficiently reduces different aromatic and aliphatic nitro compounds to their primary amines in refluxing THF. In addition, the reduction shows chemoselectivity for aliphatic nitro compounds over the aromatic nitro compounds.

Keywords: Reduction; Zinc borohydride; Nitro compound; Chemoselectivity; Amine.

INTRODUCTION For over forty years, sodium borohydride has been widely recognized as the reagent of choice for the reduction of carbonyl compounds and acid chlorides in protic solvents.1 Over the past decades, the utility of sodium borohydride has been greatly expanded and different techniques and modifications have been developed for it to reduce organofunctional groups in various solvents. The preparation and application of modified borohydride agents in organic synthesis have been reviewed recently. 2 These modifications may involve replacement of one or more hydrides with other substituents,3 change of the sodium cation to other metal,4 quaternary ammonium 5 and phosphonium 6a,b cations, a concurrent cation and hydride exchange, 7 ligand-metal borohydrides 8 and finally combination of the hydride transferring agents with metals, metal salts, Lewis acids9 and solid supports.10 Zinc tetrahydroborate, Zn(BH 4 ) 2 , a stable transition metal borohydride in ethereal solvents, has found applications in organic synthesis.11 However, the requirement of its storage in a cold place puts some restrictions on its uses. Recently, new stable modifications of zinc borohydride in the form of tertiary amino or phosphino ligand complexes such as polytetrahydro[(1,4-h)pyrazine]boratozinc, 12 [(Pyz)Zn(BH4)2]n, (1,4-diazabicyclo[2.2.2]octane)tetrahydroboratozinc complex,2c,6a [(dabco)Zn(BH4)2] and mono or (bistriphenylphosphine)zinc tetrahydroborate,6c [(Ph3P)Zn(BH4)2], [(Ph3P)2Zn(BH4)2], have been introduced and their applications for the reduction of different functional groups have

* Corresponding author. E-mail: [email protected]

been reviewed. These successes prompted us to prepare [(Py)Zn(BH4)2] and investigate its reducing ability.13 In our study, we also observed that this reagent efficiently reduces aliphatic and aromatic nitro compounds to their corresponding primary amines in refluxing THF (Scheme I). Scheme I

(Py)Zn(BH4)2, 4 mols RNO2 ¾¾¾¾¾¾¾¾¾® RNH2 THF, Reflux, 80-98% R: Alkyl, Aryl, Heteroaryl

RESULTS AND DISCUSSION Our literature review has shown that the combination systems of MBH4 (M=Na, K, Li) and borohydride exchange resin (BER) in the presence of metals, metal halides or metal salts such as NaBH4/CoCl2,14 NaBH4/FeCl2,15 NaBH4/SnCl2,16 NaBH 4 /BiCl 3 or SbCl 3 , 17 NaBH 4 /CuSO 4 , 18 NaBH 4 /Pd/C, 19 NaBH 4 /Co(pyridyl), 20 NaBH 4 /copper acetylacetonate, 21 LiBH 4 (NaBH 4 )/Me 3 SiCl, 22 KBH 4 /Cu 2 Cl 2 , 23 Ni 2 B [NaBH 4 / Ni(OAc)2],24 Ni2B (NaBH4/NiCl2),25 NaBH4/TiCl4,26 borohydride exchange resin; BER/CuCl, CoCl2, PdCl2, Cu(OAc)227 and Ni(OAc)228 are effective for the reduction of aliphatic or aromatic nitro compounds. On the application of zinc borohydride or its modifications for this transformation we can't find any report. We feel it worthwhile to investigate our newly prepared reagent for the reduction of nitro compounds.

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Zeynizadeh and Zahmatkesh

(Py)Zn(BH4)2 is stable, easy to handle and is quantitatively prepared by complexation of 1:1 zinc borohydride and pyridine in dry ether at room temperature (Equations 1 & 2).

ZnCl2 + 2NaBH4 Zn(BH4)2 + Dry Ether Zn(BH4)2 + 2NaCl (1) Pyridine.Zn(BH4)2 (2)

Pyridine

This reducing agent could be stored for months without losing its activity. Characterization by atomic absorption and gravimetric techniques has been carried out. As shown in Table 1, a great variety of aliphatic, aromatic and heteroaryl nitro compounds are reduced to their corresponding amines with 4 molar equivalents of the reagent in refluxing THF. It is noted that carboxylic acid and amides were re-

Table 1. Reduction of Nitro Compounds to Their Amines with (Py)Zn(BH4)2a Entry 1 2 3 Substrate Product Molar Ratio Reag./Subs. 4 4 4 Time (h) 7.5 5 6 Yield (%)b 95 96 80

4 5 6 7

4 4 4 8

1.5 2 2.2 4.5

97 93 95 92

8 9

4 8

3.5 7

98 95

10c 11

4 8

2 3.1

91 94

12 13 14

4 4 4

2 0.25 0.8

96 94 94

15

a

4

0.67

96

All reactions were performed in THF under reflux condition. Yields referred to isolated products. c Ac-means acetyl group.

b

Reaction of Nitro Compounds with (Py)Zn(BH4)2

J. Chin. Chem. Soc., Vol. 50, No. 2, 2003

269

Table 2. Comparison Reduction of Nitro Compounds with (Py)Zn(BH4)2 and Some Other Reported Reagents Molar Ratio (Reag./Subs.)a/Time (h)/Yield (%) Entry 1 2 3 Substrate Product I 4(7.5)(95) 4(5)(96) 4(6)(80) II28 (5:0.1)(1)(94) ---(5:0.1)(1)(96) III17 (8:1.5)(2)(86) (8:1.5)(1.5)(85) (8:1.5)(1.5)(90) IV17 (5:2)(0.5)(90) (5:2)(0.33)(91) (5:2)(0.17)(85)

4

4(2)(93)

----

(8:1.5)(1.5)(95)

(5:2)(0.17)(88)

5

4(3.5)(98) 4(0.67)(96)

(5:0.1)(1)(96) (6:0.1)(1)(96)

-------

-------

6

I

a

(Py)Zn(BH4)2; IIBER/Ni(OAc)2; IIINaBH4/BiCl3; IVNaBH4/SbCl3. In the case of reported reagents, molar ratios are: NaBH4 or BER/halides or salts/substrate.

duced faster than the nitro group; therefore chemoselective reduction of such functional groups in the presence of nitro group is feasible. (Entries 6, 7, 10 & 11). On the other hand our attempts to selective reduction of this functional group in the presence of carboxylic acid or amido group were unsatisfactory under different conditions. The reducing ability of the reagent for the reduction of dinitro groups in the substrates is demonstrated by ready reduction of 2,5-dinitrochlorobezene to the corresponding amine with 8 molar equivalents in refluxing THF (95% yield, Entry 9). Heterocyclic compounds such as 2-chloro-4-nitropyridine that contain a nitro group could also be reduced to their corresponding amine faster than the aromatic ones (Entry 12). A comparison in Table 1, shows that with respect to the aromatic and heterocyclic nitro compounds, primary, secondary and tertiary aliphatic nitro compounds were rapidly reduced to their corresponding amines in excellent yields under reflux condition (Entries 13-15) whereas sodium borohydride-transition metal salts systems 18,19,25d for reduction of aliphatic nitro compounds usually give 60-80% yields. This advantage is shown by a comparison in the reduction of 2methyl-2-nitro butane with (Py)Zn(BH4)2 and BER/Ni(OAc)2 (Table 2, Entry 6). Since aliphatic nitro compounds are reduced faster than nitroarenes, we decided to investigate the chemoselectivity for the reduction of aliphatic and aromatic nitro compound

mixtures. In a typical example, the selectivity ratio for the reduction of aliphatic nitro compound with respect to the aromatic one is 100% (Scheme II). Scheme II

We also observed that less hindered nitro compounds are reduced faster, as demonstrated by a competitive reduction reaction between two aliphatic nitro compounds (Scheme III). Scheme III

To highlight the limitations and advantages of our methods, we list some of our results against those of reported reagents (Table 2). In conclusion, we have shown that (Py)Zn(BH4)2 is a

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J. Chin. Chem. Soc., Vol. 50, No. 2, 2003

Zeynizadeh and Zahmatkesh

new stable ligand-metal borohydride which easily and efficiently reduces aliphatic, aromatic and heteroaryl nitro compounds to their corresponding amines. Excellent chemoselectivity was observed for the reduction of aliphatic nitro compounds in the presence of the aromatic compounds. Easy preparation of the reagent, a new potentiality of zinc borohydride for the reduction of nitro compounds, mild reaction condition, high yield of the products and easy work-up of the reaction mixture, make (Py)Zn(BH4)2 a synthetically useful reagent to the present methodologies for the reduction of a variety of nitro compounds.

ACKNOWLEDGEMENT The authors are grateful for the partial support of this work by Urmia University Research Council.

Received June 4, 2002.

REFERENCES

1. (a) Seyden-Penne, J. Reductions by the Alumino and Borohydrides in Organic Synthesis; 2th ed., Wiley-VCH, 1997. (b) Hudlicky, M. Reductions in Organic Chemistry; Ellis Horwood Ltd., Chichester, 1984. (c) House, H. O. Modern Synthetic Reactions; 2th ed., Benjamine, Menlo Park, CA, 1972. 2. (a) Firouzabadi, H.; Zeynizadah, B. Iranian J. Sci. Tech. Trans. A 1995, 19, 103. (b) Firouzabadi, H. The Alembic 1998, 58. (c) Firouzabadi, H.; Zeynizadah, B. Bull. Chem. Soc. Jpn. 1997, 70, 155. 3. (a) Nutaitis, C. F.; Bernardo, J. J. Org. Chem. 1989, 54, 5629 and the references cited therein. (b) Narayana, C.; Periasamy, M. Tetrahedron Lett. 1985, 6361. 4. (a) Arase, A.; Nunokawa, Y.; Masuda, Y.; Hoshi, M. J. Chem. Soc. Chem. Commun. 1991, 205. (b) Fujii, H.; Oshima, K.; Utimato, K. Chem. Lett. 1991, 1847. 5. (a) Firouzabadi, H.; Afsharifar, G. R. Synth. Commun. 1992, 22, 497. (b) Firouzabadi, H.; Afsharifar, G. R. Bull. Chem. Soc. Jpn. 1995, 68, 2595 and the references cited therein. 6. (a) Firouzabadi, H.; Adibi, M.; Zeynizadeh, B. Synth. Commun. 1998, 28, 1257. (b) Firouzabadi, H.; Adibi, M. Phosphorus, Sulfur Silicon Relat. Elem. 1998, 142, 125. (c) Firouzabadi, H.; Adibi, M.; Ghadami, M. Phosphorus, Sulfur Silicon Relat. Elem. 1998, 142, 191. 7. (a) Blough, B. E.; Carroll, F. I. Tetrahedron Lett. 1993, 7239. (b) Fuller, J. C.; Stangeland, E. L.; Goralski, C. T.; Singaram, B. Tetrahedron Lett. 1993, 257. 8. Hutchins, R. O.; Markowitz, M. Tetrahedron Lett. 1980, 813. 9. Ganem, B.; Osbey, J. O. Chem. Rev. 1986, 86, 763. 10. Sherrington, D. C.; Hodge, P. Synthesis and Separations Using Functional Polymers; John Wiley: NY, 1988. 11. (a) Ranu, B. C. Synlett 1993, 885 and the references cited therein. (b) Narasimhan, S.; Balakumar, A. Aldrichimica Acta 1998, 31, 19. 12. Tamami, B.; Lakouraj, M. M. Synth. Commun. 1995, 25, 3089. 13. Faraji, F. M.Sc. Thesis, Urmia University, March 2002, Iran. 14. Satoh, T.; Suzuki, S.; Suzuki, Y.; Miyaji, Y.; Imai, Z. Tetrahedron Lett. 1969, 4555. 15. Ono, A.; Sasaki, H.; Yaginuma, F. Chem. Ind. 1983, 480.

EXPERIMENTAL SECTION General All products were characterized by a comparison of their physical data with those of authentic samples (IR, 1 H NMR and mp). All yields referred to isolated products. TLC accomplished the purity determination of the substrates, products and reactions monitoring over silica gel PolyGram SILG/UV 254 plates. Preparation of Pyridine Tetrahydroborato Zinc Complex; [(Py)Zn(BH4)2] An ethereal solution of Zn(BH4)2 (0.16 M, 250 mL) was prepared from ZnCl2 (5.452 g, 0.04 mol) and NaBH4 (3.177 g, 0.084 mol) according to an established procedure in the literature.29 Then, pyridine (3.164 g, 0.04 mol) in ether (50 mL) was added dropwise to the ethereal solution of Zn(BH4)2 and stirred for 30 min. Careful evaporation of the solvent under vacuum at room temperature gave [(Py)Zn(BH4)2] in almost quantitative yield (6.83 g, 98%).13 Reduction of Nitrobenzene with [(Py)Zn(BH4)2]: A Typical Procedure In a round-bottom flask equipped with magnetic stirrer and condenser, was placed nitrobenzene (0.123 g, l mmol) and THF (8 mL), also the reducing agent (0.7 g, 4 mmol) was added. The reaction mixture was stirred magnetically under reflux condition for 7.5 h with TLC monitor of the progress of the reaction. After completion of the reaction, methanol (3 mL) was added to the reaction mixture and magnetically stirred for 15 min. The solvent was evaporated and the resulting crude material was purified by a silica gel column chromatography with appropriate eluent. Evaporation of the solvent affords aniline as a pure liquid compound (0.088 g, 95% yield).

Reaction of Nitro Compounds with (Py)Zn(BH4)2

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16. Satoh, T.; Mitsuo, N.; Nishiki, M. Chem. Pharm. Bull. 1981, 29, 1443. 17. Ren, P. D.; Pan, S. F.; Dong, T. W.; Wu, S. H. Synth. Commun. 1995, 25, 3799. 18. Yoo, S. E.; Lee, S. H. Synlett 1990, 419. 19. Petrini, M.; Ballini, R.; Rosini, G. Synthesis 1987, 713. 20. Vlcek, A. A.; Rusina, A. Proc. Chem. Soc. (London) 1961, 161. 21. Hanaya, K.; Chow, Y. L. J. Chem. Soc. Perkin Trans. 1 1979, 2409. 22. Giannis, A.; Sandhoff, K. Angew. Chem. Int. Ed. Eng. 1989, 28, 218. 23. He, Y.; Zhao, H.; Pan, X.; Wang, S. Synth. Commun. 1989, 19, 3047. 24. Seltzman, H. H.; Berrang, B. D. Tetrahedron Lett. 1993, 34,

3083. 25. (a) Osby, J. O.; Ganem, B. Tetrahedron Lett. 1985, 26, 6413. (b) Nose, A.; Kudo, T. Chem. Pharm. Bull. 1981, 29, 1159. (c) Sarma, J. C.; Borbaruah, M.; Sharma, R. P. Tetrahedron Lett. 1985, 26, 4657. (d) Sarma, D. N.; Sharma, R. P. Tetrahedron Lett. 1985, 26, 2581. 26. Kano, S.; Tanaka, Y.; Sugino, E.; Hibino, S. Synthesis 1980, 695. 27. Chen, J. W.; Qin, C. Q. Reactive Polymers 1992, 16, 287. 28. Yoon, N. M.; Choi, J. Synlett 1993, 135 and the references cited therein. 29. (a) Gensler, W. J.; Johnson, F.; Sloan, A. D. B. J. Am. Chem. Soc. 1960, 82, 6074. (b) Crabbe, P.; Garcia, G. A.; Rius, C. J. Chem. Soc, Perkin Trans. 1 1973, 810.

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