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Co. and collaborators recently published work on Swern oxidations, reactions used extensively in making pharmaceutical intermediates. The oxidation using dimethyl sulfoxide (DMSO) is a versatile and reliable method for oxidizing alcohols to carbonyl compounds. Until now, however, the reaction using DMSO activated by trifluoroacetic anhydride (TFAA) has seen limited industrial use since it has to be conducted at or below -50 ºC to avoid the decomposition of an unstable cationic intermediate.
The partners--Co. & Partners developed a microscale tube reactor system for conducting Swern oxidations between -20 and 20 ºC. They used an IMM micromixer to combine the DMSO and TFAA; the solution was then passed through a stainless steel tube reactor and mixed with a solution of the alcohol in a second micromixer. This solution passed into a second reactor, was mixed with a triethylamine solution, and flowed through two final reactors before the product was collected.
The combination of precise temperature control, extremely fast and efficient mixing, and short residence times--a combined eight to 11 seconds for all four reactors--allows for the reactive intermediate to move to the next stage and react with the alcohol substrate before decomposing in 0.01 second. The system, the partners say, has allowed them to achieve equivalent or often significantly better conversions and yield for primary, secondary, cyclic, and benzylic alcohols at -20, 0, and 20 ºC, compared with lower temperature conditions.
To test for durability, the partners ran a cyclohexanol oxidation reaction for three hours at 20 ºC and saw consistent high conversion and product selectivity. "Our goal is to apply this technology to the process on a commercial scale," says, Co. s director for fine chemicals and active pharmaceutical ingredients (APIs). But first, "we will work on the pilot scale in the near future." The challenge, he adds, will be to improve microreactor performance and durability.
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