Stable cyclic (alkyl)(amino)carbenes as rigid or flexible, bulky, electron-rich ligands for transition-metal catalysts: a quaternary carbon atom makes the difference.

Abstract

The availability of catalysts to perform specific transformations is critical for both industry and academia. Over the years, the success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of a variety of metal-containing systems. Advances in ligand design have allowed not only for improvements of known processes in terms of scope, mildness, and catalyst loadings, but also for the discovery of new selective reactions. A good illustration is given by palladium-catalyzed coupling reactions, which are applied to a wide area of endeavors ranging from synthetic organic chemistry to materials science.[1] For these catalytic processes, which represent some of the most powerful and versatile tools available for synthetic chemists, major advances have recently been reported thanks to the use of bulky, electron-rich, phosphines A and cyclic diaminocarbenes (N-heterocyclic carbenes (NHCs)) B (Figure 1).[2] These ligands stabilize the active catalytic species, and accelerate the important catalytic steps, namely oxidative addition, transmetallation, and reductive elimination. On the other hand, excessive steric hindrance can present some drawbacks for the coupling of bulky reactants.[3] To overcome this problem Glorius and co-workers have successfully developed ligands with “flexible steric bulk” using the conformational flexibility of cyclohexane.[4]