Chiral-at-Ru Catalyst with Cyclometalated Imidazo[1,5-a]pyridin­ylidene for Enantioselective Intramolecular Cyclopropanations

Abstract

A chiral ruthenium catalyst is introduced which contains a cyclometalated N-(3-nitrophenyl)-imidazo[1,5-a]pyridinylidene ligand in addition to a bidentate 4-mesityl-2-(pyridin-2-yl)thiazole and two acetonitriles to complement the octahedral coordination sphere of the monocationic complex. Tetrafluoroborate serves as the counterion. Since all coordinated ligands are achiral, the overall chirality is formally due to a stereogenic metal center generating either a left-handed (Λ) or right-handed (Δ) helical topology of this chiral-at-metal complex. Nonracemic Λ and Δ complexes were synthesized using (R)- and (S)-N-benzoyl-tert-butanesulfinamide as chiral auxiliary ligands, respectively. The position of the nitro group in the metalated phenyl moiety is of crucial importance for the generation of enantiomerically pure complexes. The catalytic activity of the cycloruthenated chiral-at-metal catalyst was demonstrated for the enantioselective intramolecular cyclopropanation of trans-cinnamyl diazoacetate and an alkenyl diazoketone to generate bicyclic cyclopropanes in high yields (96–97%) and with satisfactory enantioselectivity (93% ee).

Chiral-at-Ruthenium Catalysts for Nitrene-Mediated Asymmetric C–H Functionalizations

“…Finally, the electronics and sterics of this scaffold can be conveniently modulated by introducing substituents into the pyridyl moiety of the PyNHC ligands, thereby providing a variety of catalysts optimized for different types of asymmetric nitrene-mediated reactions. Thus, the robustness of the scaffold [Ru­(PyNHC) 2 (MeCN) 2 ]­(PF 6 ) 2 , together with its ability to efficiently catalyze a wide variety of stereocontrolled nitrene-mediated C­(sp 3 )–H aminations, − ,− renders this catalyst scaffold of particular interest, while some other chiral-at-ruthenium catalysts developed in our group suffer from limited configuration stability and/or lower catalytic activity. − …”