The Azomethine Ylide Route to Amine C−H Functionalization: Redox-Versions of Classic Reactions and a Pathway to New Transformations

CONSPECTUS: Redox-neutral methods for the functionalization of amine α-C−H bonds are inherently efficient because they avoid external oxidants and reductants and often do not generate unwanted byproducts. However, most of the current methods for amine α-C−H bond functionalization are oxidative in nature. While the most efficient variants utilize atmospheric oxygen as the terminal oxidant, many such transformations require the use of expensive or toxic oxidants, often coupled with the need for transition metal catalysts. Redox-neutral amine α-functionalizations that involve intramolecular hydride transfer steps provide viable alternatives to certain oxidative reactions. These processes have been known for some time and are particularly well suited for tertiary amine substrates. A mechanistically distinct strategy for secondary amines has emerged only recently, despite sharing common features with a range of classic organic transformations. Among those are such widely used reactions as the Strecker, Mannich, Pictet−Spengler, and Kabachnik−Fields reactions, Friedel−Crafts alkylations, and iminium alkynylations. In these classic processes