Actual Process Impurity or an Analytical Artifact? A Case Study Involving Conversion of Amide to Nitrile to Uncover the Truth and Mitigate

During the in-process monitoring of a few synthetic intermediates containing an amide group using reversed-phase liquid chromatography, an unknown impurity was observed. The mass data suggested it to be the nitrile analogue of the corresponding intermediate, which could not be explained by the established reaction pathway(s). A study was undertaken to track the source of this impurity to enable the development of an appropriate control strategy. Subsequent investigations revealed that this impurity was generated in the sample from the amide intermediate in the presence of residual amounts of a heavy-metal catalyst carried forward from the previous step. Additionally, under similar conditions, alternative catalysts with different heavy metals such as rhodium(II), silver(II), nickel(II), platinum(II), zinc(II), copper(II), and ruthenium(II) did not produce this impurity. Analysis of structurally different amide compounds under similar conditions revealed that amides with nitrogen at the α or β position in the structure were less prone to generate this impurity. Also, the nature of aromatic substituents and the composition of the diluent were found to exert a substantial influence on the level of formation of this impurity. A strategy was developed to mitigate or reduce the generation of this impurity during analytical sample preparation by implementing measures such as adding a chelating agent, ethylenediaminetetraacetic acid (EDTA), into the sample or substituting acetonitrile with methanol as the diluent. To gain further insights into the factors contributing to the formation of this impurity, a full-factorial experimental design was performed, examining the effects of temperature, analyte concentration, and Pd content. The outcomes of the modeling experiments indicated that adjusting the sample dilution could serve as an additional control strategy to eliminate the in situ generation of this impurity. A comprehensive exploration of the specific details pertaining to this work is presented in this article.