CO₂ Offgas as a Mechanistic Probe and Scale-up Tool in N-Acylations Using Mixed Anhydrides from Amino Acids and Isobutyl Chloroformate

The reaction pathways of an N-acylation reaction, involving a mixed anhydride intermediate produced from reaction of an amino acid with isobutyl chloroformate, were analyzed using a novel technique involving the quantitation of CO₂ evolved during various parts of the process. Under the normal mode of addition, in which chloroformate is added to a carboxylate anion solution, the dominant mode of yield loss to undesired product was shown to be the formation of a symmetric anhydride, implying that byproduct urethane formation was almost exclusively due to reaction of unreacted chloroformate with amine. This result suggests that inferred high concentrations of carboxylate anion present with the normal addition mode should be minimized to decrease yield loss. This hypothesis was confirmed by demonstrating that symmetric anhydride formation is almost eliminated when the reverse addition (viz., addition of carboxylate anion solution to chloroformate) was carried out. These results, in conjunction with the relative temperature insensitivity of the pathways occurring after amine addition, were used to deduce that the higher amounts of remaining starting acid observed on scale-up were due to slower mixing at larger scales, resulting in greater symmetric anhydride yields. The reverse addition, although forming much lower symmetric anhydride amounts, is nevertheless a rapid reaction whose selectivity could be impacted adversely on scale-up due to slower mixing. The offgas methodology presented here can be used to quantify reaction selectivity as a function of scale and thus serve as a diagnostic tool for assessing mixing efficiency at larger scales.