[Babcock_Hall]

Recently I made two batches of this compound, which is in the class of H-phosphonates using the method of H Goldwhite and  BC Saunders (J Chemical Society 1975 2409).  The synthesis uses PCl₃ and three equivalents each of tert-butyl alcohol and pyridine in diethyl ether.  After I filtered the pyridinium chloride using celite, I distilled the crude product under reduced pressure using a short path apparatus and a cooled receiving flask.  I discarded the first cut, which smelled like pyridine, and I took the second cut, which did not have a well-defined boiling point.  I treated each batch separately, except I combined the distillant pots of both and distilled to obtain a third fraction.  Particularly the first synthesis had a good deal of white precipitate even after distillation.  I wonder whether or not this might be pyridinium chloride, but I have not analyzed it.  I don't have NMR data on the products yet but will acquire them shortly.

I am wondering what to do about the precipitate.  I have performed this synthesis several times over the years, and I seem to recall the same issue.  The liquid is viscous, and if I filter, it may be that I will lose material in the filtration.  I could make a solution, filter, and remove the solvent.  Ideas?

[wildfyr]

Can it handle aqueous workup?

[Babcock_Hall]

Good question.  I never tried an aqueous workup, but I don't think that it would be a problem.  In retrospect, would it make more sense to do that first, then the distillation?  By the way, the H-1 NMR spectra look as if I was successful in the synthesis, but there could be an impurity, based on some small peaks in the aromatic region.  I have not checked the values yet.

[wildfyr]

If you have pyridine as an impurity an aq copper sulfate wash is the magic brew to remove it. works like a charm, easy as pie.

[Babcock_Hall]

A few weeks ago I found some old, degraded stocks of this compound and pooled them.  I sampled what appeared to be the most impure one (it had precipitate, which I discarded) for NMR, but regrettably I did not sample after pooling.  I used a copper sulfate extraction, followed by a reduced-pressure distillation using a short path set-up.  The one stock that I sampled did not have any of the desired phosphite.  The product did have the phosphite, although it was impure.  A I have previously observed, the distillation was over a relatively broad range, roughly 30-50 °C.

Just now I found a protocol I was given many years ago for this synthesis (I had forgotten about it, and I am not sure whether or not it is published).  The relative amounts of PCl₃, pyridine, and tert-butanol look the same as the ones from the 1958 paper.  The protocol used an aqueous wash with 2% NaOH; the ether layer was dried, the solvent was removed, and the product stripped with toluene to remove remaining tert-butanol and pyridine.  Apparently, no distillation was needed.  Next time I will wash with NaOH, then copper sulfate, and I will then perform the toluene strip.  I will distill if needed.