Phencyclidine

I've wanted to try my hand at the Grignard reaction for quite a while now by preparing something relatively easy. As I had a few grams of piperidine hydrochloride sitting around that I had derived from peppercorns, I decided I would attempt to synthesize another antiquated pharmaceutical, the rabid dodo of dissociatives commonly known as PCP. An advantage here was the use of phenylmagnesium bromide, which has a reputation of being among the easier Grignard reagents to produce.

After preparing bromobenzene and the nitrile intermediate PCC, I sort of just forgot about the whole project for months until last week I finally decided to spring clean it off the old backlog.

The procedure I was following can be viewed here.


Preparations

The reaction setup was simple. On top of a 50 mL flat-bottomed RBF were attached, respectively, a straight vacuum take-off/gas inlet adapter, a 300 mm coil condenser and finally a calcium chloride drying tube. Because of the relatively small scale, I had decided against using the 250 mL addition funnel, my only one, which could potentially contain several tens of milligrams of water vapor. In this sense, a shorter condenser would also have sufficed, but I had managed to destroy my Liebig by using it outside, unsupervised, when it was too cold. A lesson was learned, and I digress.
Instead of the addition funnel, additions of bromobenzene and PCC would be made through the vacuum port utilizing a disposable syringe fitted with a hypodermic needle.

Needless to say, everything was thoroughly dried beforehand. Relative humidity in the workspace was 21%.


Reaction

Chopped magnesium ribbon (0.85 grams, 35 mmol) was lightly mauled with a mortar and pestle and transferred to a 50 mL flat-bottomed RBF. The vacuum port was sealed using cling film and the apparatus was lightly flame dried. Bromobenzene (5.12 grams, 32.6 mmol) in 10 mL THF was added in two portions. Liberation of hydrogen was observed almost immediately and the reaction was placed in a cold water bath which was gradually heated to 60°C. Once the reaction had died down, a further 500 milligrams of neat bromobenzene was added in one portion and heating was continued until all of the remaining magnesium had been consumed.

Snow was added to the water bath to cool it down and 1-piperidinocyclohexanecarbonitrile (2.44 grams, 12.69 mmol) in 5 mL THF was added in small portions. The apparatus was removed from the water bath and on warming to room temperature the reaction picked up and moderate effervescence was observed. Once the reaction slowed down, heating was commenced and the mixture was kept at 65-66°C for three hours.

Left to right: PhBr in THF; magnesium in reaction vessel; PCC in THF


Formation of the Grignard reagent


Addition of PCC


Reaction mixture following addition of PCC


Before completion of the Grignard reaction


After completion of the Grignard reaction



Workup

To the cooled reaction mixture was carefully added dilute hydrobromic acid (26.22%, 11.50 mL). The mixture was refrigerated at 5°C for eight hours after which no more formation of crystals was observed. After filtration and air drying, 3.03 grams of solid material was obtained. This was dissolved in ethanol, gravity filtered through tightly packed cotton to get rid of a fine suspension of supposed magnesium salts, and combined with a solution of sodium hydroxide (1.02 g) in ethanol. Separation of product by filtration was impossible, so the mixture was evaporated to a thick paste, dissolved in toluene and gravity filtered through tightly packed cotton. To the clear toluene solution was added concentrated hydrochloric acid (1.10 g) followed by azeotropic distillation to remove water and excess acid. As the solution heated up, it went from straw yellow to orange. After the removal of water, the solids were filtered, washed with heptanes and air dried resulting in 1.19 grams of tan phencyclidine hydrochloride powder.

Addition of HBr


Following addition of HBr


Precipitation of crude phencyclidine HBr


Basified ethanol solution


Gravity filtration of toluene solution


Toluene solution heating up after addition of HCl


Distillation of toluene to remove water


Formation of precipitate following removal of water


Tan phencyclidine hydrochloride


From the original reaction mixture, 1.54 grams of waxy yellow solids had precipitated on standing in open air for about one day. These were filtered off, washed with heptanes, air dried and dissolved in ethanol. The remaining reaction mixture was washed with heptanes and evaporated to dryness. The white residue was washed with ethanol. The ethanolic solutions were combined and sodium hydroxide (0.99 g) was added, followed by evaporation of the ethanol. The residue was dissolved in toluene and gravity filtered through tightly packed cotton.

The tan product obtained earlier was deprotonated using NaOH and taken up in toluene. At this point the two toluene solutions were separately treated with activated charcoal and vacuum filtered through qualitative filter paper. Both solutions ended up similarly yellow and so were combined. Like before, hydrochloric acid was added and toluene distilled off until the distillate came over at 110°C. The final yield of off-white phencyclidine hydrochloride powder was 1.84 grams (51.83%). Melts with decomposition at 215-230°C. (The test was conducted in a test tube heated directly on a hotplate, so it's not as accurate as it should be.)

Activated charcoal treatment


Following addition of HCl to combined decolorized toluene solutions


Off-white phencyclidine hydrochloride


Finally, the product was recrystallized. A solution in isopropanol was gravity filtered through diatomite to get rid of a small amount of suspended white solid that refused to dissolve. The solution was concentrated on a 125°C hotplate, maintaining its volume at 10 milliliters with small additions of heptane. When approximately 10 mL of heptane in total was added, the beaker was covered and set to cool, first at room temperature overnight and then at -22°C for some hours. The first crop of crystals was distinctly yellowish and weighed 1.14 grams.

Recrystallized product



Discussion

This was an interesting and gratifying reaction. I was worried that I wouldn't be able to ensure sufficient dryness but based on this experience, it is not nearly as easy to mess up as I had originally thought. Granted, the low ambient humidity as well as the choice of Grignard reagent, I believe, were in my favor.

While I don't particularly enjoy working with potent toxins, which I would classify phencyclidine as owing to its powerful and unpredictable pharmacological profile, I appreciate having to pay extra attention to safety every now and then.

The workup was, somewhat unsurprisingly, pretty tedious. Compared to cleaning up methaqualone previously this was a breeze, but full of lengthy gravity filtrations which I absolutely detest. This time I have decided to settle for a slightly discolored end product.

Using a vast molar excess of phenylmagnesium bromide can supposedly result in yields up to 70%. I tried my darndest to minimize mechanical losses, but obviously some occurred anyway. I find it hard to believe, but not impossible, that such a loss occurred during the workup. In any case, I don't think that 50% outright sucks and therefore I'm quite happy.

Thank you for your time! As always, questions, remarks and all manner of feedback and input is appreciated.




[Edited on 17-3-2021 by Benignium]

That's very interesting, zed! Thanks for sharing! PCP is certainly deserving of its infamy. There's an awe-inspiring contrast between PCP and its successor ketamine, another arylcyclohexylamine dissociative that is not only a very promising treatment for depression and a better anesthetic, but could also ameliorate PCP-induced cognitive deficits, as discussed here.

I believe the most famous example of such primitive bulk production is the "bucket method". The full paper can be read here, and here's the diagram contained therein:




[Edited on 25-3-2021 by Benignium]