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Electrochemical Reduction of Nitroalkenes to Oximes and Amines

Chem.Ber. 124, 2303-2306 (1991)

ASCII by Vitus Verdegast, HTML by Rhodium

Abstract

1-Nitroalkenes are reduced in high yields at -0.3 to -0.5 V (vs. SCE) at a mercury or graphite cathode to oximes. At higher cathodic reduction potentials (-1.1 V) primary amines are selectively obtained in fair yields. Nitroalkadienes are selectively reduced at the double bond conjugated with the nitro group to either the oxime or amine.

The 1-nitro-2-arylalkenes are prepared by the condensation of arylaldehydes and nitroalkanes. They exhibit reduction potentials between -0.17 and -0.33 V vs. SCE. A second more cathodic reduction potential appears at -1.1 V, which can be attributed to the reduction of the oxime to the primary amine, as the oxime shows a reduction potential at -1.02 V.

The reduction of the nitrostyrene/P2NP to the corresponding amines is not complete due to an increasing competing hydrogen evolution at the cathode with the progress of the electrolysis. This seems to be ascribed to the presence of hydroxylamine originating from the hydrolysis of the intermediate oxime, which decreases the hydrogen overvoltage at the cathode. This leads to recovered nitrostyrene/P2NP, the formation of oxime and the corresponding aldehyde/ketone as well as only fair yields of the amine.

Experimental

Electrolysis: Divided standard beaker-type cell (150 ml), cathode = Hg pool (19.5 cm2) or graphite P 127 (9 cm2, Sigri), anode = Pt foil (2 cm2, ref. electrode = SCE, Luggin cappilary, diaphragm = glass frit (G4), current source = Wenking HP 88 (Bank Electronic).

General procedure for the reduction of 1-nitroalkenes to oximes

The cathodic compartement of the cell was charged with 30 ml of 0.1M sulfuric acid in IPA/water (3:2, v/v), subsequently by 3 mmol of substrate. Then the anodic chamber was filled with 8 ml of supporting electrolyte and introduced into the catholyte properly in order to minimize the space between the electrodes. With stirring and cooling (10-15C) controlled potential electrolysis was carried out at -0.25 to -0.55 V (SCE), depending on the substrate (for the graphite cathode the potentials were about 400 mA more cathodic). After 4.0-4.2 F*mol-1 had been consumed (i < 10 mA) the catholyte was treated with 10 mmol of hydroxylammonium chloride in 25 ml of water and with sat'd NaHCO3 until pH was approx. 5. After 1 h of stirring, 100 ml of water was added and the solution extracted with 3 portions of ether. The combined org. phases were washed with brine and water, dried (MgSO4) and concentrated to afford essentially pure oximes. Final chromatographic purification (petr.ether/ether 4:1 to 2:1) afforded the oximes in the following yields:

General procedure for the reduction of the 1-nitroalkenes to the amines

The cathodic compartement was charged with 70 ml of 0.3M sulfuric acid in IPA/water (2:1, v/v), subsequently by 6 mmol of the 1-nitro olefin. After introducing the anodic chamber filled with 20 ml of supporting electrolyte, controlled potential electrolysis (-1.1 to -1.3 V) was performed at 10-15C with stirring. After 7 to 8 F*mol-1 gas evolution increased, and the reduction was terminated. the acidic catholyte was partially evaporated at reduced pressure and the residue extracted twice with ether. To the remaining aqueous phase 2M NaOH was added (pH 13) and the resulting emulsion was extracted with five portions of ether; the combined extracts were dried (MgSO4) and concentrated to afford the amines: