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Phenyl-2-Propanones from Phenyl-2-Nitropropenes
Using Tin(II)chloride and Hydrochloric Acid

Written by Barium

HTML by Rhodium

p2p.p2np.sncl2-hcl.html

This is a variation of the Varma and Kabalka method1,2, but they stopped at the oxime stage and said that it was just to extract the oxime with ether. Well the extraction of the oxime from the tin crap/sludge is a nightmare. So I thought way not render the whole thing acidic and hydrolyse the oxime to the ketone. At the same time the problem with the tin is no more, since both Sn(II)chloride and Sn(IV)chloride will stay in the water phase.

Tin(II)chloride is quite non-toxic and generally very nice. If desired, it's possible for the tin(II)chloride to be regenerated by electrolysis of the formed tin(IV)chloride.

When the reduction/hydrolysis (reflux) is over there is actually no need to separate the layers. Just steam distill it as is. Toluene, ketone and water will distill, not the tin salts. One step less - this variation has been exemplified in the synthesis of 1-(2,4,5-trimethoxyphenyl)-2-propanone (2,4,5-TMP2P - a TMA-2 precursor).

First Trial: o-Fluoro-P2P

18.1g (100mmol) 1-(2-fluorophenyl)-2-nitropropene was added dry in portions to 49.5g (220mmol) SnCl2·2H2O suspended in 75ml EtOAc while the reaction temperature was kept between 20-40C by a cold water bath. When all nitropropene was added and the color had changed to white (5 min) the EtOAc solution was transferred to a RB flask containing 250ml water and 50 ml hydrochloric acid. The EtOAc was removed by distillation under diminished pressure and the aqueous suspension of oxime and tin salts was allowed to stir at 80C for 1 hour. The water phase was now steam distilled to remove the ketone. When no more oily drops came over the distillate was extracted with DCM. The DCM extracts was dried over MgSO4 and the DCM removed by distillation leaving a quite pure ketone as a colorless oil.

Yield: 13,5g (89%) 1-(2-fluorophenyl)-2-propanone
Purity: 98% (HPLC)
Properties: bp 47°C/0.05 mmHg, d 1.077g/ml (according to the Aldrich Catalog).

It's a clear oily liquid with a aromatic smell, quite nice. I have not distilled it yet. I rarely do so with the ketones since I get them pure enough anyway.


Further Trials: Preparation of P2P and MDP2P

This nitroalkene → ketoxime → ketone procedure was tried with two other phenyl-2-nitropropenes:

Phenyl-2-Nitropropene to P2P

Yield: 30.82g (230mmol, 92%) 1-phenyl-2-propanone (P2P)

1-(3,4-Methylenedioxyphenyl)-2-Nitropropene to MDP2P

Yield: 41.83g (235mmol, 94%) 1-(3,4-methylenedioxyphenyl)-2-propanone (MDP2P)

General Procedure

This is a very general procedure and it has so far worked with every single phenylnitropropene I've tried. The yields has been in the range of 85-92%. The lower yields are very likely a result of my impatience. Longer time for hydrolysis should have given higher yields.

Procedure:

Dissolve the tin(II)chloride in water mixed with the hydrochloric acid. Add the substrate to a rb flask and rinse it all down with the toluene. Now add the aqueous solution to the rb flask in one portion. With good stirring bring the mixture to a gentle reflux for 2 hours. Chill the flask to room temp and saturate the aqueous phase with NaCl, separate the two layers and transfer the top organic layer to a 1L flask containing 500ml water. Steam distill until no more oily drops comes over. This means between 500ml to 1.5L distillate is to be collected depending on the volatility of the ketone. When the steam distillation is over the distillate consits of two layers, one top layer of touene and the major part of the ketone. Separate the layers in a separatory funnel and extract the aqueous phase with 2x100ml toluene. Combine the toluene extracts, dry them with MgSO4 and remove the toluene in a rotovap. This leaves the ketone as a yellow oil ready for whatever you might have in mind for it.

2,4,5-TMP2P Using Direct Steam Distillation

Tin(II)chloride was dissolved in 15ml water and 15ml conc hydrochloric acid and the resulting solution was added to a 200ml RB flask containing 20ml toluene and 5g substrate. The mixture was heated to 80°C on a water bath with good stirring for 1.5 hours. During this time the color changed from a deep yellow to dark orange and then to light yellow.

After 1.5 hours 200ml water was added and the toluene was removed by steam distillation. The now bright yellow aqueous phase containing dark yellow oily droplets was saturated with NaCl and extracted with 2x50ml EtOAc which removed all color. The ethyl acetate was washed once with 50ml saturated NaHCO3 solution, once with 50ml water, once with brine and then dried with MgSO4. The solvent was removed in a rotovap leaving 3.7g of a yellow oil which slowly partially solidified to something reminding of honey.

Yield: 3.7g (83.8%) 1-(2,4,5-trimethoxyphenyl)-2-propanone

Notes on the process by GC_MS:

I was a long time ago since I tested the procedure on Rh's page, and I am currently in the need of some phenyl-2-propanone analogues, I wanted to re-evaluate the applicability of the method.

Substances: 32.6 g 1-phenyl-2-nitropropene (recrystallized from toluene), 100 g tin(II)chloride dihydrate, water and hydrochloric acid.

I allowed hydrolysis of the ketoxime for ca 1.5 h at 85-90C. The reaction mixture was steam distilled to yield a yellow oil. The distillate was extracted with DCM and the combined organic phases washed with water. It is claimed that this procedure yields a reasonably pure product. On this, a little bit of comment...

For the most amongst us, this oil indeed is pure enough. However, when washing the organic phase with aqueous NaOH instead of water, a discoloured aqueous phase is noticed. Some chromatography showed me that there was some benzyl alcohol present. Also, the "purified" oil contained some benzaldehyde (ca 5%). Depending on what the purposes are of the P2P, benzaldehydes are an interfering factor (reductive amination).

References

  1. Rajender S. Varma, Manju Varma & George W. Kabalka, Tetrahedron Letters 26(49), 6013-14 (1985)
  2. George W. Kabalka & N. M. Goudgaon, Synthetic Communications 18(7), 693-697 (1988)