Finally, after many years, the nut of all nuts has been cracked. Here is a simple way to selectivly reduce the C=C bond in phenylnitropropenes. The catalyst has so far been reused six times without any loss of activity. This makes this method by far the most economical of all the other available methods. The trick is to use neutral conditions. Any traces of acids in the solvent and the result will be simultaneous oxime formation and subsequent carbonyl formation due to hydrolysis. Remember that the catalyst is very likely to contain acidic residues.
Experimental
A 300 ml hydrogenation reactor containing 100 ml water was charged with 16.3 gram (100 mmol) twice recrystallised phenyl-2-nitropropene crushed to a fine dust and mixed with 320 mg 5% Pd/C (2% w/w of the substrate). The reacor was pressurised to 3 bar with argon and then vented back to atmospheric pressure. This was repeated six times to effectively reduce the oxygen content to a non-dangerous level. Then the reactor was filled with hydrogen to 3 bar and the stirrer started using a stirring rate of 1500 rpm. After a induction period of 2 minutes hydrogen consumption started and continued at the same rate for about 40 minutes. At that point exactly 0.1 mol hydrogen had been consumed and the hydrogen uptake ceased abruptly.
The reactor was ventilated back to atmospheric pressure and was then flushed with argon for 10 seconds. 30 ml DCM was added to the opened reactor, now containing a oil floating on the water surface, the reactor was closed again and stirring started for 30 seconds. This was done to get all the oil from the reactor walls into the DCM-phase. The reactor contents was filtered through a pad of celite to remove the catalyst and the DCM removed in a rotovap leaving 16 g phenyl-2-nitropropane as almost colorless oil. On HPLC analysis the nitroalkane contained a just a trace of unreacted substrate, no oxime and no phenylacetone.
Edit:
The same method has been applied on the following substrates:
1-(4-Fluorophenyl)-2-nitropropene, 96% yield.
1-(3,4-dimethoxyphenyl)-2-nitropropene, 99% yield.
1-(4-hydroxy-3-methoxyphenyl)-2-nitropropene, 99% yield.
1-(3,4-methylenedioxyphenyl)-2-nitropropene, 97% yield.
1-(2,4,5-trimethoxyphenyl)-2-nitropropene, 99% yield.
1-(3,4,5-trimethoxyphenyl)-2-nitropropene, 99% yield.