Publication

Chiral N-Heterocyclic Carbene Ligands for Asymmetric Catalysis

Johannes Diesel
2019
EPFL thesis
Abstract

N-Heterocyclic carbenes (NHCs) are the ligands of choice in a large variety of transformations entailing different transition metals. However, the number of chiral NHCs suitable as stereocontrolling ligands in asymmetric catalysis remains limited. In particular, a chiral version of the widely applied IPr may be of use for a large variety of asymmetric transformations. This thesis focuses on the introduction of a modular NHC ligand family, resembling a chiral version of IPr, and their application in nickel catalyzed enantioselective C-H functionalizations of N-heterocycles. Nickel-NHC catalysis enabled the C-H annulation of 2- and 4-pyridones, delivering fused bicyclic compounds found in many biologically active compounds. Crucial to the transformation was the introduction of a sterically hindered and tuneable chiral NHC. Applying this bulky, yet flexible ligand scaffold enabled the highly enantioselective C-H functionalization of pyridones under mild conditions. The introduction of a bulky chiral SIPr analogue enabled the nickel catalyzed enantioselective C-H functionalization of indoles, yielding valuable tetrahydropyridoindoles. The process is characterized by a clear endo-cyclization preference to form the sought-after six-membered-ring annulated N-heterocycles without the need for the typically applied Lewis basic directing groups. Additionally, pyrrolopyridines, pyrrolopyrimidines and pyrroles were efficiently functionalized, delivering chiral annulated azoles.

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Related concepts (31)
Transition metal carbene complex
A transition metal carbene complex is an organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and reactions utilizing them have been reported. The term carbene ligand is a formalism since many are not derived from carbenes and almost none exhibit the reactivity characteristic of carbenes.
Persistent carbene
A persistent carbene (also known as stable carbene) is a type of carbene demonstrating particular stability. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC) (sometimes called Arduengo carbenes), for example diaminocarbenes with the general formula (R2N)2C:, where the four R moieties are typically alkyl and aryl groups. The groups can be linked to give heterocyclic carbenes, such as those derived from imidazole, imidazoline, thiazole or triazole.
Transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinide elements (the f-block) are called inner transition metals and are sometimes considered to be transition metals as well. Since they are metals, they are lustrous and have good electrical and thermal conductivity.
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