In this patent, the inventors performed a simple esterification using a quat acetate, either tetrabutylammonium or tetramethylammonium. The solvent is acetonitrile. The yield is high at 95%-97%.
There are several interesting thoughts when reading this patent. First, they used stoichiometric quat acetate. They should have used catalytic quat salt and inexpensive sodium acetate. One would assume that the economics of recycling a full equivalent of tetrabutylammonium salt or tetramethylammonium salt would be expensive and required.
Secondly, they used acetonitrile as the solvent. If already one is using stoichiometric quat acetate, one would expect the reaction to proceed well using almost any organic solvent, preferably one that makes two phases with water for easy work up using water washes to separate the product from the waste salts. In fact, the inventors explicitly cite the use of water-immiscible solvents for the extraction step during workup. Performing a solvent exchange after reaction before isolation of product would add unnecessary and cost and complexity to the process.
The inventors do show that using ammonium acetate instead of a quaternary ammonium acetate gives no yield. This hints that true phase-transfer catalysis should work.
It should also be noted that an imidazolium ionic liquid was screened for this reaction and did not perform nearly as well as the classical much less expensive quaternary ammonium phase-transfer catalyst.
We would probably use stoichiometric sodium acetate and a few mole% tetrabutylammonium bromide that would form the bromomethyl furfural in-situ from the chloromethyl furfural to achieve bromide co-catalysis to enhance the phase-transfer catalysis by the tetrabutylammonium cation. Then again, the reaction works well in 5 minutes at room temperature homogenously in acetonitrile, so rate enhancement may not be needed. In any case, this process begs to be performed with catalytic phase-transfer catalyst in a properly chosen water-immiscible solvent.
Now contact Marc Halpern of PTC Organics to improve process performance and reduce cost if your company is developing or performing commercial specialty esterifications that must be irreversible, have very high conversion (i.e., nucleophilic esterifications using alkyl halides instead of dehydration using alcohols) and be performed in short reaction times. PTC Organics can show you how to achieve low-cost high-performance green chemistry using phase-transfer catalysis, including effective isolation of the ester product.
