Use of N,N-Coordinating Ligands in Catalytic Asymmetric C--C Bond Formations: Example of Cyclopropanation, Diels--Alder Reaction, Nucleophilic Allylic Substitution

Abstract

Chiral N,N-coordinating ligands have been particularly synthesized for their use as metal chelates catalyzing asymmetric C-C bond formations. Bis(oxazoline) structures have been the most studied, probably due to their straightforward synthesis from the chiral pool, their easy handling, and their high stability. Efficient methods have been reported for the intermolecular cyclopropanation of olefins (mainly styrene derivatives) with alkyldiazoacetates involving various bis(oxazolines)-Cu(I) catalysts. Bis(oxazolinyl)-pyridine ruthenium complexes proved also highly active and selective catalysts for this reaction. Associated to copper(II) pre-catalysts, bis(oxazolines) also allowed the asymmetric Diels--Alder and hetero Diels--Alder transformations to be achieved in nearly quantitative yield and high diastereo- and enantioselectivities. Optically active sulfoximines, with their nitrogen-coordinating site located at close proximity to the stereogenic sulfur atom, have also proven their efficiency as copper ligands for these asymmetric cycloadditions. Other precursors for this Lewis acid-catalyzed transformation have been described (e.g., zinc salts, ruthenium derivatives, or rare earth complexes) which, when associated to bis(oxazolines), pyridine-oxazolines or pyridine-bis(oxazolines), led to efficient catalysts. Chiral pyridine-based ligands were, among various N,N-coordinating ligands, more efficient associated to palladium for asymmetric nucleophilic allylic substitution. Asymmetric molybdenum-catalyzed alkylations, especially of non-symmetric allylic derivatives as substrates, have been very efficiently performed with bis(pyridylamide) ligands. The heterogeneous versions of most of these efficient nitrogen-containing complexes have been described via immobilization on organic or inorganic supports by covalent or non-covalent interactions. Such catalysts have been used, leading to activities and selectivities comparable to their homogeneous counterparts, and some recycling experiments have been successfully performed. Nevertheless, these methodologies need to be further optimized in order to enhance the activity and stability of the solid catalysts.