Studies on the rhodium-catalyzed organic reactions using n-heterocyclic carbene ligands

Abstract

$\Bf{Part 1. Rhodium-Catalyzed Conversion of Aldoximes to Amides Using N-Heterocyclic Carbene Ligands in Mild Reaction Conditions}$ It was found that the catalytic activity of rhodium complexes was highly sensitive to the type of $\It{N}$-heterocyclic carbene (NHC) ligands in the conversion of aldoximes to amides. Among those species examined, the (cyclooctadiene)rhodium chloride-carbene complex Rh(cod)(IMes)Cl exhibited the highest reactivity when it was employed in combination with a Brønsted acid, thus allowing mild reaction conditions. A significant rate acceleration effect resulting from the addition of nitrile additives was also observed. With the new protocol, the substrate scope of aldoximes has been widely expanded to include sterically congested and electronically varied derivatives. On the basis of detailed mechanistic studies, it is proposed that the reaction proceeds mainly via intramolecular electrophilic addition of aldoxime to rhodium-bound nitrile, which is different from the generally postulated two-step route: dehydration of aldoxime to nitrile followed by hydration of the latter intermediate. $\Bf{Rhodium(NHC)-Catalyzed C-H Functionalization of Arenes via the Proton Abstraction Mechanism}$ A new catalyst system of rhodium was developed for the chelation-assisted direct intermolecular arylation with the help of electron-rich $\It{N}$-heterocyclic carbene (NHC) and phosphine ligands. The reaction is operationally simple, and proceeds smoothly under mild conditions to afford mono- or di- arylated products in excellent yields. The present protocol could be readily applied for the arylation at the $sp^3$ or $sp^2$ C-H bonds of 2-pyridyl-containing aryl, vinyl, or alkyl moieties. $\Bf{Part 3. Rhodium(NHC)-Catalyzed C-N Bond Formation: Amination of Aryl Halides}$ The Rhodium-catalyzed amination reaction of aryl halides with amines has been developed with the use of $\It{N}$-heterocyclic carbene (NHC) ligand (IiPr = 1,3-diisopropylimidazol...