Multigram Electrochemical Hofmann Rearrangement Using a Spinning Three-Dimensional Anode

A multigram electrochemical method for the Hofmann rearrangement of a primary amide into a methyl carbamate was developed using a spinning cylinder electrode cell. The method is based on the anodic oxidation of NaBr, an inexpensive and readily available bromide source, circumventing the use of stoichiometric amounts of oxidants. The multigram process relies on the use of a three-dimensional anode material, graphite felt, which has been implemented into the spinning cylinder electrode assembly. The felt material can efficiently oxidize the bromide salt by anodic oxidation, initiating the reaction. Additionally, the motion of the spinning three-dimensional electrode material provides excellent mass transfer to the system. The reaction was first optimized in a small-scale undivided beaker cell and then translated to a spinning electrode reactor. Improved productivity of the spinning cylinder technology was realized in a larger reactor with a 5-fold increase in the electrode surface area, providing analogous results to those obtained with the small cell. Under optimal conditions, high yields (95%) and productivity of 27 g of material per hour have been attained.