Stohler, R: Asymmetric Metal-Catalyzed [3+2] Cycloadditions

Cycloadditions of azomethine ylides with olefins provide a short, attractive route to pyrrolidine units with the potential to control the relative and absolute configuration by means of a chiral catalyst. Grigg and co-workers have pioneered the use of chiral transition metal complexes to induce enantioselective cycloadditions of this type. However, stoichiometric amounts of metal complexes were employed in this work.39a,41

Paying regard to Griggs investigations it was intended to develop a system which gives asymmetric induction using catalytic amounts of chiral transition metal complexes. A first screening of different transition metal sources showed promising results for Cu(I) and Ag(I) species. The application of their complexes with several members of different ligand classes to the 1,3-dipolar cycloaddition reaction directed the interest towards phosphinooxazoline (PHOX) ligands. The catalysts derived from the two different metals and PHOX ligand 39 induced enantioselectivity in both cases (Scheme 46, Table 28). Subsequent investigations demonstrated that also the use of the analogous Au(I) catalyst led to the formation of enantioenriched pyrrolidines.