Byun, I.; Han, K.; Ga, H.; (Aminologics) US Patent 9,598,353, 21-Mar-2017
As always, the economics of an organic chemical process depend on many factors. In the case of producing optically pure alpha-amino acids by resolution of a racemic mixture, half of the mass balance and economic value is tied up in the off-isomer. Racemization of the off-isomer enables recycle and reprocessing by resolution.
An effective racemization catalyst for alpha-amino acids that works at mild temperatures was found to be a complex between copper (II), nitrosalicylic acid and the amino acid itself used under basic conditions. At the end of the racemization process, the racemized amino acids are located in an aqueous phase and must be acidified. The acidification decomposes the copper complex and makes catalyst recovery and recycle not feasible, in addition to causing problems of separation from the amino acids.
The use of an organophilic phase-transfer catalyst, such as Aliquat 336 that has an average of 27 carbons atoms, solves these problems by assuring that the copper complex remains in the organic phase, if an organic solvent is chosen that forms two phases with water, such as dichloromethane and solubilizes the Aliquat 336-copper complex.
At the end of the racemization step, the catalyst located in the organic phase is easily separated from the amino acids in the aqueous phase. This enables recovery and recycle of the copper complex while at the same time enables acidification of the aqueous phase that does not come in contact with the acid-sensitive copper complex.
The patent notes that the organic phase containing the Aliquat 336-copper complex was recycled five times with no loss of activity!
The major lesson from this patent and other patents that combine PTC with transition metal catalysis is that PTC often improves cost by making more efficient use of the transition metal by assuring that the transition metal catalyst is located in the proper phase by adjusting the overall organophilicity of the combined catalyst.
Now contact Marc Halpern of PTC Organics to explore how your company can achieve lower-cost higher performance green chemistry by integrating phase-transfer catalysis with transition metal catalysis.
