Usually in PTC etherification of diols, much more diether is formed than monoether, even when the amount of alkylating agent is limited to enough to alkylate just one hydroxyl. The reason is that the quaternary ammonium cation prefers to pair with more organophilic anions. Since the monoether is usually more organophilic than the starting diol, then the ion pair [Q+ R’ORO-] is preferred over [Q+ HORO-]. Since the quat delivers the alkoxide to the alkylating agent (propargyl chloride in this case) in the organic phase and the quat delivers more monoether anion than diol anion, we expect more diether. We teach this in our 2-day course “Industrial Phase-Transfer Catalysis.”
In the reaction shown, the inventor increased the probability of monoetherification by adding the alkylating agent slowly in a controlled manner that kept a high ratio of free diol to monoether in the early stages of the reaction. Performing the reaction at room temperature likely helped selectivity for controlled etherification (avoiding chaotic collisions that are not statistical). The reason that PTC was chosen for this reaction may have been to reduce the energy of activation enough to be able to perform the reaction at room temperature instead of at higher temperatures often used in etherifications of secondary alcohols.
However, it is possible that diether dominated in the reaction shown and that the inventor separated out and isolated the monoether formed in a lesser amount by chromatography. That would be consistent with what we usually observe in PTC etherification of diols, even when only 1 equiv of alkylating agent is used (1.2 equiv were used in this reaction). Since the amount of isolated product was not reported, we don’t know the actual selectivity of the etherification of this diol.
This monoetherification was performed with other reactants including a polyglycol.
The question mark in the title of this post is due to concern that the monooether may not be the dominant product but was successfully isolated for the synthetic purposes of the inventor. We think it is important to highlight the thought process that PTC etherification of diols favors diether formation over monoether formation, so when we see a PTC monoetherification of a diol, we should not conclude that PTC favors monoetherification.
Contact Marc Halpern of PTC Organics to achieve low-cost high-performance green processes for strong base reactions, usually using inexpensive inorganic bases and phase-transfer catalysis.
