Abstract
AbstractThe possibility of creating a chiral center directly from two C—H bonds, or from a C—H bond and an X—H bond (X = heteroatom), without any prior derivatization (e.g., the installation of a leaving group) opens up many new possibilities in synthesis. Many chiral ligands and organocatalysts have now been discovered to be compatible with the oxidizing conditions in which these transformations take place. Furthermore, as reactions that can be performed under milder conditions are found, such as those that involve the use of molecular oxygen or even air to accept the two hydrogen atoms lost, or that can be run at lower temperatures, the repertoire of cross-dehydrogenative coupling (CDC) methodologies has become even bigger. Ligands such as mono- and bisoxazolines, bisphosphines, axially chiral binaphthols and bi-2-naphthylamine derivatives, and salens, as well as organocatalysts such as amino acids, chiral amines and diamines, cinchona alkaloids, axially chiral phosphoric acids, imidodiphosphoric acids, imidazolinones, and thioureas, amongst others, have been found to be robust and to perform well under CDC reaction conditions, providing high asymmetric induction and good yields of products. Some of these catalysts also work well in synergy with another catalyst. Recent developments in this area include the use of light energy for activation in combination with photocatalysts, as well as methods based on the use of electrochemistry. In this review, methods involving CDC that have been developed for the synthesis of molecules with one or more chiral centers, including compounds with axial or planar chirality, are presented, and their scope and limitations are discussed. The organization is based firstly on the type of catalysis used, and then divided further according to the type of bond being formed.