Asymmetric Lewis acid catalysis directed by octahedral rhodium centrochirality

Abstract

A chiral-at-metal octahedral rhodium(iii) complex serves as an effective asymmetric catalyst for Michael additions (electrophile activation) and α-aminations (nucleophile activation).

Merger of Visible Light Induced Oxidation and Enantioselective Alkylation with a Chiral Iridium Catalyst

“…The involvement of an enolate complex in the catalytic cycle is further supported by a reaction of 1 a with the electrophile dibenzyl diazodicarboxylate catalyzed by D-IrO which afforded the a-amination product 4 in 87 % yield and 89 % ee, apparently through the intermediate formation of a nucleophilic iridium enolate complex (Figure 4 b). Thus, D-IrO is capable of catalyzing enolate chemistry as has been recently also demonstrated for a related iridium [7] and rhodium [14] complex and the observed enantioselective CÀC bond formation can be explained with the stereoselective reaction between the chiral iridium enolate B and an intermediate iminium ion. The formation of the electrophile through chemical oxidation-replacing the photosensitized oxidation-also provides the desired CÀC bond formation product in an enantioselective fashion as shown for the oxidant tBuOOH (Figure 4 b).…”

“…[16] The replacement of iridium in the catalyst D-IrO with rhodium (D-RhO) therefore allows us to dissect the catalytic and photoredox activity of D-IrO, with the rhodium congener D-RhO only displaying a recently demonstrated activity for asymmetric enolate catalysis but presumably lacking photoactivity. [14] And indeed, the reaction of imidazole 1 a with amine 2 a in the presence of D-RhO (2 mol %) under irradiation with visible light provided the CÀC bond formation product 3 a only in very low yield (6 % after an elongated reaction time, compare entries 1 and 2 of Table 2). Revealingly, when we next complemented D-RhO with the established photosensitizers [Ir(ppy) 2 (dtbbpy)]PF 6 (1.0 mol %) [1,17] or [Ru(bpy) 3 ]Cl 2 ·6H 2 O (0.5 mol %), [1,18] the reaction provided the product 3 a with good conversions and high enantioselectivities.…”

Exploiting Octahedral Stereocenters: From Enzyme Inhibition to Asymmetric Photoredox Catalysis

“…Two achiral 5-tert-butyl-2-phenylbenzoxazole or analogous phenylbenzothiazole ligands cyclometalate a central iridium or rhodium center in a propeller-type and configurationally inert fashion; these are complemented by two additional, labile acetonitrile ligands ( Figure 5). [27][28][29] In this design, the metal serves as both the reactive center and the only stereogenic center. A related "chiral-only-at-metal" polypyridine complex of ruthenium was reported by Fontecave and co-workers but provided only very low enantioselectivities in the oxidation of organic sulfides to sulfoxides.…”

“…[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).…”

Asymmetric Friedel–Crafts alkylation of indoles with 2-nitro-3-arylacrylates catalyzed by a metal-templated hydrogen bonding catalyst

“…We recently introduced a convenient auxiliary-mediated methodology for the synthesis of chiral octahedral metal complexes which only contain achiral ligands and therefore draw their chirality exclusively from the metal center, [4][5][6][7][8] and we demonstrated that such chiral-only-at-metal complexes could serve as highly powerful catalysts for a variety of asymmetric transformations. [9][10][11][12][13][14][15] As an example, we developed a hydrogen-bond-mediated chiral iridium(III) complex as a very efficient catalyst for the Friedel-Crafts alkylation of indoles with β-nitroacrylates, thereby implementing all-carbon quaternary stereocenters with excellent enantioselectivities. 10 We were then wondering if related catalysts would be suitable for α-nitroacrylate substrates, thereby creating two consecutive stereocenters.…”