Electroreduction of 2,5-dimethoxy-beta-nitrostyrene
Of all substrates I have used for electroreduction this is by far the most problematic one, as there seems to be a lot of tarry by-product formed during the course of the reduction, and more specifically during the course of the last stage of the reduction (when all nitrostyrene has gone into solution and the intermediate oxime has formed). This might be due to the sensitivity of the intermediate aldoxime to the conditions used.
The nitrostyrene was obtained from condensation of 2,5-dimethoxybenzaldehyde with nitromethane in ethanol using ethylenediamine dipropionate as the Henry catalyst, which resulted into an almost quantitative yield and spontaneous crystallization of the product. This was washed twice with and recrystallized once from ethanol and dried to constant weight.
The catholyte consisted of a mixture of 50 ml 29% HCl (technical), 140 ml denatured ethanol and 100 ml ethyl acetate. The anolyte was 10% H2SO4. Diaphragm was a flowerpot (diameter 14 cm, height 10 cm, thickness 1 cm) purchased at the local Turkish bazaar and immersed into dilute H2SO4 overnight prior to the first use.
Anode and cathode were 12 cm x 5 cm lead sheets (technical roofing type grade), of which a 12 cm x 3 cm surface area were immersed in the anolyte/catholyte. In the cathode were punched 6 holes (about 0.5 cm diameter) evenly distributed amongst the immersed surface area, and two 2 cm cuts were made at every 4 cm interval, this to ensure maximum contact of the catholyte with the cathode surface. The cathode was also pre-treated with a PbO2 layer according to the known art.
The setup consisted of the catholyte placed in the flowerpot, which was placed in the anolyte, which was placed in a cooling bath (water), placed on a magnetic stirrer. As there is considerable evaporation of the catholyte during the reduction process, this experiment should only be conducted in a fume hood or a well-ventilated area.
9.5 grams of 2,5-dimethoxy-beta-nitrostyrene were suspended into the stirring catholyte, and current was applied from a car battery charger. 12V was applied for 45 min which brought the current to 1.5 A (current density 41 mA/cm²). The power was raised to 24V and the current level rose to 8A over 30 minutes (current density 222 mA/cm²). This was kept at this level for 2.5 hours, after which all nitrostyrene went into solution (which became quite hot).
After this time the power was switched back to 12V, which delivered a steady 4A (current density 111 mA/cm²). At this time 2 ice-cubes were added to the catholyte, followed by 50 ml distilled water. The intermediate oxime precipitated as a fine yellow suspension, which was brought back into solution after addition of 70 ml ethanol.
After 1 hour a clear solution with a light greenish hue was obtained. 30 ml distilled water was added and the precipitation of the oxime was again witnessed, which was brought back into solution with the addition of 40 ml ethanol. The formation of a tarry substance on the electrode and on the stirring magnet was noticed.
After an additional hour the current dropped to 3A (current density 83 mA/cm²) and the catholyte was clear with a slight yellowish hue, into which some tarry particle matter was noticed. Addition of 50 ml 2% HCl caused a yellowish fine precipitate at this point, which was brought into solution by addition of 30 ml ethanol. The catholyte was kept at 3A for an additional 2 hours.
To the catholyte was added 100 ml 6% HCl and this was washed once with toluene and once with ethyl acetate. The washed catholyte was basified with ice-cold concentrated NaOH-solution and extracted with toluene. This caused a gigantic emulsion which did not clear after 2 days of standing.
Normally vacuum filtration over a pad of Celite would have solved this issue, but as this was temporarily unavailable different methods were applied to break this emulsion, which consisted of vibrating the separatory funnel, heating, waiting, cursing and praying to God, Jesus, the virgin Mary, and the prophet Elias, all which were unsuccessful. After standing for one week, separating the organic extract as much as possible, re-extracting everything with more toluene, crying, cursing, praying, playing with a knife and my fingers etc.. and applying heat, followed by gravity filtration through 2 coffee filters (and repeating this) I was able to separate most organic extract. The emulsion is caused by a decent amount of brownish tarry substance of unknown identity, which was dried and stored as a moment to remember.
The pooled extracts were washed twice with distilled water and twice with brine. The toluene was distilled off in vacuo without additional drying (as the azeotrope with water boils first and takes care of the little remaining water after 2 brine washings). 1 gram of 2C-H remains as a light-brown oil with a fishy amine-like smell, yield 12%
Most of the losses are due to the tar formation and emulsion fighting, which took over a week. Maximum yield ever obtained with this substrate using this method is about 30%, although the literature gives a higher yield for this substrate (around 50% if I remember correctly).
Ideas to improve this process include formation of the oxime using dithionite, as oxime electroreductions seem cleaner when pure oxime is used compared to the in situ formation from nitroalkenes. Reduction to the nitropropane prior to electroreduction also seems a viable option. Perhaps addition of some Zn(Hg) to the catholyte when the oxime stage is reached? Or what about addition of the Zn/Ni couple à la Urushibara at that point (using mechanical stirring of course)? If the tar formation doesn’t interfere too much, this could take advantage of the competing and increasing hydrogen evolution and turn it into a electroreduction/catalytic hydrogenation combo? (just thinking out loud here).
Included are some pictures of the process:
Beginning of the process. Nitroalkene is suspended. Stirring magnet is too small but it doesn't really matter as the nitroalkene will go into solution anyway:
24 V and near to 8 Amps:
Oxime formation and tar formation:
At the end of the reduction the catholyte becomes clear:
Acidified catholyte after washing with toluene:
Unfortunately I have not taken pics of the dreaded emulsion that comes after basifying. But I'm sure most of you have encountered such a phenomenon at least once in your chemist lives...
The end results from the effort applied 
1265402.pdf