Chiral-at-Rhodium Catalyst Containing Two Different Cyclometalating Ligands

A method for the synthesis of a bis-cyclometalated rhodium complex containing two different cyclometalating ligands is reported and applied to asymmetric catalysis. The preparation of this previously inaccessible class of tris-heteroleptic bis-cyclometalated rhodium(III) complexes was achieved by a stepwise protocol that relies on the formation of an isolable mono-cyclometalated rhodium(III) species in the first step, providing the opportunity to introduce a different second ligand in a subsequent additional cyclometalation step. The obtained racemic complex was resolved into its single enantiomers using an established chiral auxiliary ligand approach. The final Lambda- and Delta-configured chiral-at-metal rhodium complexes contain a cyclometalated 5-tert-butyl-1-methyl-2-phenylbenzimidazole, a cyclometalated 5-tert-butyl-2-phenylbenzothiazole, and two acetonitrile ligands, complemented by a hexafluorophosphate counterion, and proved to be highly efficient for asymmetric [2 + 2] photocycloadditions.