Removing Residual Triphenylphosphine Oxide from Reaction Mixtures

Triphenylphosphine oxide is a common and annoying coproduct in the Wittig reaction, for example. Many ways have been proposed for the separation of this contaminant but most are not fast, cheap, rugged, or necessarily quantitative. It would be a valuable contribution to chemical science if someone demonstrated the following treatment.

It is known that triphenylphosphine oxide forms large blockish cocrystals with N-acetylglycine with a very strong hydrogen bond between amide and phosphine oxide. It can be imagined that these adducts further associate as dimers through the free carboxyl group producing an even high molecular weight dimeric adduct. Perhaps the addition of excess N-acetyl glycine into a solution of desired product and triphenylphosphine oxide impurity could precipitate the cocrystals and perhaps residual N-acetyl glycine. This has not been established. But, if it works filtration would give a purified solution of the desired product with just some residual dissolved N-acetyl glycine and so long as the desired product is not acidic, this residual N-acetylglycine will be cleanly back-extracted into aqueous base.

Further Data Needed for a New Reagent to Separate Aldehydes Cleanly

This subject provides a tremendous opportunity for an undergraduate to get publishable work.

For 40 years I have been thinking about writing something about an article published in  Chem. Pharm. Bull. In 1980. Shunsaku Ohta and Masao Okamoto, in that year, published a three-page communication that taught a method for extracting aldehydes selectively into an aqueous layer and then simply recovering them in pure form and high yield. I expected to find more complete details later along with experimentation to support a hypothesis for the mechanism of action and subsequently many applications of the method. Nothing could be further from reality. There does not seem to have been any further work or use!

What the authors taught in Chem. Pharm. Bull. 28(6) 1917-1919 (1980) was that 1.2 M 6-aminohexanoic acid sodium salt solution could quantitatively convey aldehydes from mixtures comprising at least one aldehyde in either diethyl ether or diisopropyl ether into an aqueous phase and, after separating the water and organic solvent layers, the aldehyde could be liberated by acidifying the aqueous phase to pH 4-6 and back extraction into an organic phase….. free of non-aldehydes (including ketones). If emulsions formed during the initial extraction the addition of a little isopropanol was taught to break the emulsions.

6-aminocaproic acid (6-aminohexanoic acid) is cheap since it is the monomer for making nylon! So this procedure seems very practical.

Of course, it may not work! Perhaps that is why nothing more has been written about it. But surely it is worth investigating.

The authors pictured this isolation as proceeding through the formation of the imine the covalent bond of which pulled the aldehydic moiety into water courtesy of the sodium carboxylate functionality on the other end of the reagent. The authors do not offer any explanation however of why the equilibrium so greatly favors the imine. 

How high can the molecular weight the aldehyde be and still have it successfully transferred to the aqueous phase? What organic solvents can be used besides diethyl ether or diisopropyl ether? All remain clouded.