Experiences with commercial production scale operation of dissolving metal reduction using lithium metal and liquid ammonia

The final step which generates free base in the synthesis of Sumanirole Maleate (PNU-95666E) consists of a cryogenic dissolving metal reduction using lithium metal and liquid ammonia. This chemistry was new to the Pfizer API production plant. Due to the hazards associated with the handling of lithium metal and ammonia gas at cryogenic reaction temperature, special challenges were encountered related to the design of the equipment, choice and handling of materials, operations, waste treatment, and both safety and economic issues. The topics discussed in this article include the use of Li instead of Na or K, impact of the choice of physical form of lithium metal, design of the lithium addition apparatus, and problems experienced during the addition. We also discuss techniques for addition of ammonia to the reactor, evaporation of ammonia from the reaction mixture, options for ammonia disposal, and internal reuse of ammonia. Comments on hazards for this reaction are also provided. It is hoped that this document will be of benefit to other professionals who may want to develop and scale-up dissolving metal reduction processes.