Opportunities and challenges for direct C-H functionalization of piperazines

Piperazine ranks within the top three most utilized N-heterocyclic moieties in FDA-approved small-molecule pharmaceuticals. Herein we summarize the current synthetic methods available to perform C-H functionalization on piperazines in order to lend structural diversity to this privileged drug scaffold. Multiple approaches such as those involving alpha-lithiation trapping, transitionmetal-catalyzed alpha-C-H functionalizations, and photoredox catalysis are discussed. We also highlight the difficulties experienced when successful methods for alpha-C-H functionalization of acyclic amines and saturated mono-nitrogen heterocyclic compounds (such as piperidines and pyrrolidines) were applied to piperazine substrates.