Asymmetric Photocatalysis with Bis-cyclometalated Rhodium Complexes.

Abstract

Aspects of sustainability are playing an increasingly important role for the development of new synthetic methods. In this context, the combination of asymmetric catalysis, which is considered one of the most economic strategies to generate nonracemic chiral compounds, and visible light as an abundant source of energy to induce or activate chemical reactions has recently gained much attention. Furthermore, the combination of photochemistry with asymmetric catalysis provides new opportunity for the development of mechanistically unique reaction schemes. However, the development of such asymmetric photocatalysis is very challenging and two main problems can be pinpointed to undesirable photochemical background reactions and to difficulties in controlling the stereochemistry with photochemically generated highly reactive intermediates. In this Account, we present and discuss asymmetric photocatalysis using one of the currently most versatile photoactivatable asymmetric catalysts, namely, reactive bis-cyclometalated rhodium(III) complexes. The catalysts contain two inert cyclometalating 5-( tert-butyl)-2-phenyl benzoxazole or benzothiazole ligands together with two labile acetonitriles, and the overall chirality is due to a stereogenic metal center. The bis-cyclometalated rhodium complexes serve as excellent chiral Lewis acids for substrates such as 2-acyl imidazoles and N-acyl pyrazoles, which, upon replacement of the two labile acetonitrile ligands, coordinate to the rhodium center in a 2-point fashion. These rhodium-substrate intermediates display unique photophysical and photochemical properties and are often the photoactive intermediates in the developed asymmetric photocatalysis reaction schemes. This combination of visible light excitation to generate long-lived photoexcited states and intrinsic Lewis acid reactivity opens the door for a multitude of visible-light-induced asymmetric conversions. In a first mode of reactivity, bis-cyclometalated rhodium complexes function as chiral Lewis acids to control asymmetric radical reactions of rhodium enolates with electron-deficient radicals, rhodium-coordinated enones with electron-rich radicals, or rhodium-bound radicals generated by photoinduced single electron transfer. The rhodium-substrate complexes in their ground states are key intermediates of the asymmetric catalysis, while separate photoredox cycles initiate radical generations via single electron transfer with either the rhodium-substrate complexes or additional photoactive compounds serving as the photoredox catalyst (secondary asymmetric photocatalysis). In a second mode of reactivity, the rhodium-substrate complexes serve as photoexcited intermediates within the asymmetric catalysis cycle (primary asymmetric photocatalysis) and undergo stereocontrolled chemistry either upon single electron transfer or by direct bond forming reactions out of the excited state. These multiple modes of intertwining photochemistry with asymmetric catalysis have been applied to asymmetric α- and β-alkylations, α- and β-aminations, β-C-H functionalization of carbonyl compounds, [3 + 2] photocycloadditions between cyclopropanes and alkenes or alkynes, [2 + 2] photocycloadditions of enones with alkenes, dearomative [2 + 2] photocycloadditions, and [2 + 3] photocycloadditions of enones with vinyl azides. We anticipate that these reaction schemes of chiral bis-cyclometalated rhodium complexes as (photoactive) chiral Lewis acids will spur the development of new photocatalysts for visible-light-induced asymmetric catalysis.