In a recent patent application, US 2025/0313540 A1, the authors describe the synthesis of substituted 1,4,5-oxadiazepines using diacetyldihydrazine (DAH) and a variety of acylating agents under basic biphasic conditions. The stated advantage of this process is avoiding the use of DMSO that suffers from multiple disadvantages of handling, solvent swap and toxicity (higher risk of skin penetration when used with dichlorodiethylether alkylating agent).
Several of the examples (6 through 9) incorporate tetrabutylphosphonium chloride or tetrabutylphosphonium hydroxide as a phase-transfer catalyst (PTC), using a mixture of KOH and potassium carbonate as base with n-butanol as the solvent, and achieve yields of 45%-60%. Other examples (10 through 12) proceed without any PTC at all using butanol or methoxyethanol as the solvent and use only potassium carbonate as the base and achieve yields of 55%-58%. Examples 1-5 use amines such as DABCO, trimethylamine and quinuclidine at 2 mole% with KOH/K2CO3 as base and n-butanol as solvent and achieve yields of 46%-54%.
At first glance, it may be surprising that the yields for PTC and non-PTC examples are in a similar range. Reaction conditions such as temperature, use of Dean-Stark traps and controlled reagent addition rates are rather consistent across all examples.
Given the highly managed reaction environment, it appears that phase-transfer catalysis is not strictly necessary for this transformation—at least not under the exact conditions described. This may lead some to question whether the inclusion of a PTC here adds value.
However, as we teach in the 2-day course “Industrial Phase-Transfer Catalysis,” tetrabutylphosphonium is one of the least stable quats in the presence of hydroxide, about 50 times less stable than the corresponding tetrabutylammonium in the presence of 50% NaOH at room temperature.
So, it is possible that the phosphonium quat decomposed early in the reaction and could not act as a phase-transfer catalyst. It would have been useful to examine the use of quaternary ammonium salts as phase-transfer catalysts or even thermally stable phase-transfer catalysts. We find it very strange that the inventors chose the least stable phase-transfer catalyst in the presence of hydroxide to perform this reaction.
If you are developing a commercial PTC-hydroxide reaction, do not waste precious R&D resources performing the wrong experiments (e.g., using tetrabutylphosphonium for a PTC-OH reaction!). Now contact Marc Halpern of PTC Organics, who has more than 49 years of experience with PTC-OH reactions, to improve your company process R&D efficiency and process performance through PTC Process Consulting.
