Asymmetric cyclopropanation of olefins catalysed by Cu(i) complexes of chiral pyridine-containing macrocyclic ligands (Pc-L*)

Abstract

The synthesis and characterisation of copper(I) complexes of chiral pyridine-containing macrocyclic ligands (Pc-L*) and their use as catalysts in asymmetric cyclopropanation reactions are reported.All ligands and metal complexes were fully characterised, including crystal structures of some species determined by X-ray diffraction on single crystals. This allowed characterising the very different conformations of the macrocycles which could be induced by different substituents or by metal complexation.The strategy adopted for the ligand synthesis is very flexible allowing several structural modifications. A small library of macrocyclic ligands possessing the same donor properties but with either C 1 or C 2 symmetry was synthesized. Cyclopropane products with both aromatic and aliphatic olefins were obtained in good yields and enantiomeric excesses up to 99%.

Organometallic Reactivity of [Silver(I)­(Pyridine‐Containing Ligand)] Complexes Relevant to Catalysis

“…As previously reported for copper(I) complexes, [13] the effect of the metal complexation to the ligand was clearly disclosed by 1 H and 13 C NMR spectroscopy. The partial desymmetrization of the ligand backbone is more evident in the complexes with a pendant naphthyl group on the amine nitrogen (2c-e).…”

“…We evaluated the effect of the counterion on the NMR spectra of silver complexes 2, choosing 2a 1-3 as case studies and, as expected, we observed the same signals in the 1 H, 13 C, and 15 N NMR spectra without any dependence on the coordinating features of the counterion. These complexes showed an apparent Cs symmetry of the structure in solution, with two signals for each couple of equivalent methylene groups.…”

“…The silver(I) complexes, 2, of Pc-Ls, 1, the syntheses of which have been previously reported, [13] have been obtained in almost quantitative yields from different sources; respectively, silver tetrafluoroborate (X 1 ), silver triflate (X 2 ), and silver bis-triflimidate (X Each silver(I) salt was added at room temperature to a solution of the ligand 1 in DCE (DCE = 1,2-dichloroethane) and the reaction mixture was stirred for one hour, keeping the flask in the dark and under a protective atmosphere until isolation of the final product. Silver complexes 2 were easily obtained as white crystals in yields up to 91 % by precipitation, concentrating the solvent, and adding nhexane.…”

“…Complexes 2a 1-3 and 2c 1,2 have been already described, [17,18] and experimental details for all others complexes can be found in the Experimental Section and in the Supporting Information. All the obtained silver complexes 2 have been fully characterized by 1 H and 13 C NMR spectroscopy, ESI-MS, and IR and UV/Vis spectroscopy.…”

“…[12] The synthetic methodology employed is simple and allows us to differently functionalize the macrocyclic framework to increase the molecular diversity and create different stereocenters on the backbone. [13] The copper(I) complexes of these Pc-L* ligands have been fully characterized and successfully employed as catalysts in the Henry reaction [14] and in the enantioselective cyclopropanation of alkenes. [15] Compared with the more extensively studied copper and gold catalysis, reports on the catalytic activity of silver complexes are relatively sparse.…”