Catalytic, Enantioselective Addition of Alkyl Radicals to Alkenes via Visible-Light-Activated Photoredox Catalysis with a Chiral Rhodium Complex

Abstract

An efficient enantioselective addition of alkyl radicals, oxidatively generated from organotrifluoroborates, to acceptor-substituted alkenes is catalyzed by a bis-cyclometalated rhodium catalyst (4 mol %) under photoredox conditions. The practical method provides yields up to 97% with excellent enantioselectivities up to 99% ee and can be classified as a redox neutral, electron-transfer-catalyzed reaction.

Exploiting Octahedral Stereocenters: From Enzyme Inhibition to Asymmetric Photoredox Catalysis

“…[15a] In contrast, to our delight, these biscyclometalated iridium and rhodium complexes turned out to be highly versatile chiral catalysts for a variety of transformations, such as enantioselective Friedel-Crafts reactions, Michael reactions, cycloadditions, cross-dehydrogenative couplings, and even asymmetric radical addition reactions. [27][28][29][30][31][32] Most of these reactions can be classified as either conjugate additions or enolate reactions in which one substrate is activated by two-point binding to the metal center. However, other mechanisms have also been realized, for example, in asymmetric transfer hydrogenations, which supposedly proceed through an iridium-hydride intermediate with an ancillary pyrazole ligand forming a crucial hydrogen bond to the ketone substrate ( Figure 5).…”

“…[32,[49][50][51] Their limited photophysical properties are of no consequence if the photoredox reaction is an efficient chain process, as demonstrated recently for a rhodium-catalyzed enantioselective radical amination activated by visible light. [49] In such reactions, the photoredox catalyst is merely a "smart initiator", a terminology recently introduced by Studer and Curran.…”

“…We have applied the latter strategy to a highly efficient enantioselective RhS-catalyzed Giese-type radical addition to a,b-unsaturated 2-acyl imidazoles and a,bunsaturated N-acyl pyrazoles (Figure 6 b). [32,51] Figure 5. Simple octahedral chiral-only-at-metal iridium and rhodium complexes for asymmetric Lewis acid catalysis and asymmetric transfer hydrogenation.…”

Catalytic Asymmetric C−H Functionalization under Photoredox Conditions by Radical Translocation and Stereocontrolled Alkene Addition

“…[15] Thee nantioselectivity was improved to 79 % ee when D-RhS [16] (3 mol %) was used as the chiral Lewis acid (entry 3). [17] At ac atalyst loading of 8mol %, even 92 % ee was reached (entry 6). Other photosensitizers,s uch as [Ir(ppy) 2 (dtbbpy)]PF 6 and [Ru(bpy) 3 ](PF 6 ) 2 ,w ere inferior to fac-[Ir(ppy) 3 ]( entries 4a nd 5).…”

Light‐Driven Enantioselective Organocatalytic β‐Benzylation of Enals

“…From a synthetic perspective, the chemistry described in Figure 1 b is relevant since it provides a straightforward method for the direct β‐benzylation of enals ( 3 ), a transformation for which there are few effective catalytic enantioselective precedents 6. Metal‐catalyzed conjugate additions to enals are generally plagued7 by the intrinsic instability of the benzyl‐metallic reagents and the competing 1,2‐addition manifold, while organocatalytic iminium‐ion‐based strategies have been successful only for a specific class of highly activated nitro‐toluene substrates 8…”