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Palladium-Sodium Hypophosphite CTH
Reduction of Nitroalkenes to Oximes

Rajender S. Varma, Manju Varma and George W. Kabalka
Synthetic Communications 16(1), 91-96 (1986)

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Abstract

α,β-Unsaturated nitroalkenes are readily reduced by sodium hypophosphite to the corresponding oximes in the presence of palladium.

Catalytic transfer hydrogenation has found widespread use in the reduction of a variety of compounds1. Procedures involving relatively inexpensive inorganic hydrogen donors such as hypophosphorous acid and its salts2-5 are especially attractive. Although the reduction of nitroarenes2,3,6 has been extensively studied using transfer reduction techniques, the reduction of α,β-unsaturated nitroalkenes to the corresponding oximes has not been reported7,8.

Table I. Reduction of Nitroalkenes to Oximes

Nitroalkene Producta Time Yieldb Ref.
0.7 h
62%
23
2 h
56%
13
2.5 h
66%
13
0.5 h
40%c
13
0.7 h
72%
13
13 h
61%d
13
  1. All products were oily mixtures of syn and anti isomers.
  2. Isolated and unoptimized yields.
  3. 35% of phenylacetone oxime was also obtained.
  4. 40% by weight of 5% Palladium charcoal was used.

We decided to explore the use of transfer hydrogenation methodology for the reduction of α,β-unsaturated nitroalkenes as part of our ongoing investigation of the reductions of these readily accessible precursors9-16. We wish to report that α,β-unsaturated nitroalkenes are readily reduced by sodium hypophosphite to the corresponding oximes in the presence of 5% palladium at room temperature17; only minor amounts of saturated nitroalkanes and carbonyl compounds are formed. The reaction appears to be general as evidenced by the formation of phenylacetaldehyde oxime from β-nitrostyrene and cyclohexanone oxime from 1-nitro-1-cyclohexene (Table I) which are not obtainable under acidic conditions13. However, dehalogenation, a typical transfer reduction side reaction for halogenated aromatic compounds18 was observed in the reduction of p-bromo-β-methyl-β-nitrostyrene; almost equal amounts p-bromophenylacetone oxime and phenylacetone oxime were obtained. Our results are summarized in Table I.

Experimental

Commercially available samples of 1-nitro-1-cyclohexene, β-nitrostyrene, sodium hypophosphite and 5% palladium on charcoal (Aldrich) were used as received. Other nitroalkenes were prepared via published procedures10,19.

Synthesis of oximes. General Procedure.

The synthesis of phenylacetone oxime is representative of the procedure employed. To a solution of β-methyl-β-nitrostyrene (2 mmol, 0.326g in 7 mL THF) at room temperature was added palladium on charcoal (5%, 30% w/w of nitroalkene20) and an aqueous solution of sodium hypophosphite (0.7g in 5 mL). After stirring for 3 h, the mixture was filtered, saturated brine solution was added to the filtrate, and the product extracted into ether (3x30 mL). The combined ethereal extracts were dried (MgSO4) and the solvent removed under reduced pressure. The crude product (0.28g) was chromatographed over silica gel; elution with ether/petroleum ether (1:10) afforded 0.2g (67%) of phenylacetone oxime21 as an oil.

References

  1. For recent leading reviews see: M. Hudlicky, "Reductions in Organic Chemistry", Ellis Horwood Ltd., Chichester, 1984; pp. 13; R. A. W. Johnstone, A. H. Wilby and I. D. Entwistle, Chem. Rev., 85, 129 (1985) and references cited therein.
  2. I. D. Entwistle, T. Gilkerson, R. A. W. Johnstone and R. P. Telford, Tetrahedron, 34, 213 (1978)
  3. I. D. Entwistle, A. E. Jackson, R. A. W. Johnstone and R. P. Telford, J. Chem. Soc. Perkin 1, 443 (1977)
  4. R. Sala, G. Doria and C. Passarotti, Tetrahedron Lett., 25, 4565 (1984)
  5. D. Monti, P. Gramatica, G. Speranza and P. Manitto, Tetrahedron Lett., 24, 417 (1983)
  6. S. Ram and R. E. Ehrenkaufer, Tetrahedron Lett., 25, 3415 (1984)
  7. The formation of carbonyl compounds from nitroolefins has been reported using Raney nickel and sodium hypophosphite5.
  8. The conversion of β-nitrostyrene under catalytic transfer reduction conditions (palladium/formic acid) to phenylacetaldehyde oxime was casually mentioned without any experimental details3.
  9. R. S. Varma and G. W. Kabalka, Synth. Commun., 14, 1093 (1984);
    R. S. Varma and G. W. Kabalka, Synth. Commun., 15, 151 (1985)
  10. M. S. Mourad, R. S. Varma and G. W. Kabalka, J. Org. Chem., 50, 133 (1985)
  11. M. S. Mourad, R. S. Varma and G. W. Kabalka, Synth. Commun., 14, 1099 (1984)
  12. M. S. Mourad, R. S. Varma, G. W. Kabalka, Synthesis, 654-656 (1985); R. S. Varma, M. Varma and G. W. Kabalka, Tetrahedron Lett., 26, ??? (1985)
  13. R. S. Varma, M. Varma and G. W. Kabalka, Synth. Commun., 15, ??? (1985)
  14. R. S. Varma and G. W. Kabalka, Heterocycles, 23, 139 (1985)
  15. R. S. Varma and G. W. Kabalka, Chem. Lett., 243 (1985)
  16. R. S. Varma and G. W. Kabalka, Synth. Commun., 15, 443 (1985)
  17. At elevated temperatures (>50C), concomitant formation generation of the corresponding carbonyl compounds was observed as the common feature.
  18. P. N. Rylander, "Catalytic Hydrogenations over Platinum Metals", Academic Press, New York, 1967; pp. 22 and 181-186.
  19. C. B. Gairaud and G. R. Lappin, J. Org. Chem., 18, 1 (1953)
  20. Using catalytic amounts (10% palladium-charcoal, by weight) reactions were incomplete even after 24 hrs. at room temperature.
  21. Further confirmation of the structure of the product was provided by the hydrolysis22 to phenylacetone.
  22. C. H. DePuy and B. W. Ponder, J. Am. Chem. Soc., 81, 4629 (1959)
  23. Standard Spectra Collection, Sadtler Research Laboratory, 6160M (1969) and 4007C (1978).