Asymmetric Catalysis with Organic Azides and Diazo Compounds Initiated by Photoinduced Electron Transfer

Cyclopentene Annulations of Alkene Radical Cations with Vinyl Diazo Species Using Photocatalysis

A direct (3+2) cycloaddition between alkenes and vinyl diazo reagents using either Cr or Ru photocatalysis is described. The intermediacy of a radical cation species enables a nucleophilic interception by vinyl diazo compounds, a departure from their traditional electrophilic behavior. A variety of cyclopentenes are synthesized using this method, and experimental insights implicate a direct cycloaddition instead of a cyclopropanation/rearrangement process.

Organocatalytic Asymmetric Cascade Aerobic Oxidation and Semipinacol Rearrangement Reaction: A Visible Light-Induced Approach to Access Chiral 2,2-Disubstituted Indolin-3-ones

An enantioselective cascade aerobic oxidation and semipinacol rearrangement reaction of 2-aryl-3-alkyl-substituted indoles via visible-light-driven cooperative organophotoredox and H-bonding catalysis is reported. The current method provides an expedient and sustainable approach to furnish a variety of valuable chiral 2-aryl-2-alkyl-substituted indolin-3-ones in 64-90 % yield with 58-94 % ee. Preliminary control experiments present important insights into the stereochemistry.

Conjugate Addition-Enantioselective Protonation of N-Aryl Glycines to α-Branched 2-Vinylazaarenes via Cooperative Photoredox and Asymmetric Catalysis

An enantioselective protonation strategy has been successfully applied to the synthesis of chiral α-tertiary azaarenes. With a dual catalytic system involving a chiral phosphoric acid and a dicyanopyrazine-derived chromophore (DPZ) photosensitizer that is mediated by visible light, a variety of α-branched 2-vinylpyridines and 2-vinylquinolines with N-aryl glycines underwent a redox-neutral, radical conjugate addition-protonation process and provided valuable chiral 3-(2-pyridine/quinoline)-3-substituted amines in high yields with good to excellent enantioselectivities (up to >99% ee). An application of this methodology to a two-step synthesis of the enantiomerically pure medicinal compound pheniramine (Avil) is also presented.

Enantioselective counter-anions in photoredox catalysis: the asymmetric cation radical Diels-Alder reaction

Control of absolute stereochemistry in radical and ion radical transformations is a major challenge in synthetic chemistry. Herein, we report the design of a photoredox catalyst system comprised of an oxidizing pyrilium salt bearing a chiral N-triflyl phosphoramide anion. This class of chiral organic photoredox catalysts is able to catalyze the formation of cation radical-mediated Diels-Alder transformations in up to 75:25 e.r. in both intramolecular and intermolecular examples.

A general protocol for radical anion [3 + 2] cycloaddition enabled by tandem Lewis acid photoredox catalysis

We report herein a method for intermolecular [3 + 2] cycloaddition between aryl cyclopropyl ketones and alkenes involving the combination of Lewis acid and photoredox catalysis. In contrast to other more common methods for [3 + 2] cycloaddition, these conditions operate using a broad range of both electron-rich and electron-deficient reaction partners. The critical factors predicting the success of these reactions is the redox potential of the cyclopropyl ketone and the ability of the alkene to stabilize a key radical intermediate.

Chiral-at-Metal Rh(III) Complex-Catalyzed Michael Addition of Pyrazolones with α,β-Unsaturated 2-Acyl Imidazoles

An efficient enantioselective conjugate addition of pyrazolones with α,β-unsaturated 2-acyl imidazoles catalyzed by chiral-at-metal rhodium complex is reported. The corresponding adducts were obtained in good yields (85%-96%) with excellent enantioselectivities (up to >99%). This protocol exhibits extraordinary reactivity, because of the fact that a complex with as little as 0.05 mol % Rh(III) can catalyze the title reaction on a gram-scale with excellent enantioselectivity.

Rhodium(iii)/amine synergistically catalyzed enantioselective Michael addition of cyclic ketones with α,β-unsaturated 2-acyl imidazoles

An enantioselective Michael addition of α,β-unsaturated 2-acyl imidazoles with cyclic ketones catalyzed synergistically by a chiral-at-metal Rh(iii) complex and a secondary amine has been developed, affording the corresponding adducts with up to 98% yield and 99% ee.

Sulfonylative and Azidosulfonylative Cyclizations by Visible-Light-Photosensitization of Sulfonyl Azides in THF

The generation of sulfonyl radicals from sulfonyl azides using visible light and a photoactive iridium complex in THF is described. This process was used to promote sulfonylative and azidosulfonylative cyclizations of enynes to give several classes of highly functionalized heterocycles. The use of THF as the solvent is critical for successful reactions. The proposed mechanism of radical initiation involves the photosensitized formation of a triplet sulfonyl nitrene, which abstracts a hydrogen atom from THF to give a tetrahydrofuran-2-yl radical, which then reacts with the sulfonyl azide to generate the sulfonyl radical.

Organocatalytic Enantioselective Protonation for Photoreduction of Activated Ketones and Ketimines Induced by Visible Light

The first catalytic asymmetric photoreduction of 1,2-diketones and α-keto ketimines under visible light irradiation is reported. A transition-metal-free synergistic catalysis platform harnessing dicyanopyrazine-derived chromophore (DPZ) as the photoredox catalyst and a non-covalent chiral organocatalyst is effective for these transformations. With the flexible use of a chiral Brønsted acid or base in H⁺ transfer interchange to control the elusive enantioselective protonation, a variety of chiral α-hydroxy ketones and α-amino ketones were obtained with high yields and enantioselectivities.

Combining the catalytic enantioselective reaction of visible-light-generated radicals with a by-product utilization system

We report an unusual reaction design in which a chiral bis-cyclometalated rhodium(iii) complex enables the stereocontrolled chemistry of photo-generated carbon-centered radicals and at the same time catalyzes an enantioselective sulfonyl radical addition to an alkene.

We report an unusual reaction design in which a chiral bis-cyclometalated rhodium(iii) complex enables the stereocontrolled chemistry of photo-generated carbon-centered radicals and at the same time catalyzes an enantioselective sulfonyl radical addition to an alkene. Specifically, employing inexpensive and readily available Hantzsch esters as the photoredox mediator, Rh-coordinated prochiral radicals generated by a selective photoinduced single electron reduction are trapped by allyl sulfones in a highly stereocontrolled fashion, providing radical allylation products with up to 97% ee. The hereby formed fragmented sulfonyl radicals are utilized via an enantioselective radical addition to form chiral sulfones, which minimizes waste generation.

Enantioselective catalytic β-amination through proton-coupled electron transfer followed by stereocontrolled radical-radical coupling

A new mechanistic approach for the catalytic, enantioselective conjugate addition of nitrogen-based nucleophiles to acceptor-substituted alkenes is reported, which is based on a visible light induced and phosphate base promoted transfer of a single electron from a nitrogen nucleophile to a catalyst-bound acceptor-substituted alkene, followed by a stereocontrolled C-N bond formation through stereocontrolled radical-radical coupling. Specifically, N-aryl carbamates are added to the β-position of α,β-unsaturated 2-acyl imidazoles using a visible light activated photoredox mediator in combination with a chiral-at-rhodium Lewis acid catalyst and a weak phosphate base, affording new C-N bonds in a highly enantioselective fashion with enantioselectivities reaching up to 99% ee and >99 : 1 dr for a menthol-derived carbamate. As an application, the straightforward synthesis of a chiral β-amino acid ester derivative is demonstrated.

Direct Asymmetric Alkylation of Ketones: Still Unconquered

The alkylation of ketones is taught at basic undergraduate level. In many cases this transformation leads to the formation of a new stereogenic center. However, the apparent simplicity of the transformation is belied by a number of problems. So much so, that a general method for the direct asymmetric alkylation of ketones remains an unmet target. Despite the advancement of organocatalysis and transition-metal catalysis, neither field has provided an adequate solution. Indeed, even use of an efficient and general stoichiometric chiral reagent has yet to be reported. Herein we describe the state-of-the-art in terms of direct alkylation reactions of some carbonyl groups. We outline the limited progress that has been made with ketones, and potential routes towards ultimately achieving a widely applicable methodology for the asymmetric alkylation of ketones.

Recent advances in visible-light-driven organic reactions

In recent years, visible-light-driven organic reactions have been experiencing a significant renaissance in response to topical interest in environmentally friendly green chemical synthesis. The transformations using inexpensive, readily available visible-light sources have come to the forefront in organic chemistry as a powerful strategy for the activation of small molecules. In this review, we focus on recent advances in the development of visible-light-driven organic reactions, including aerobic oxidation, hydrogen-evolution reactions, energy-transfer reactions and asymmetric reactions. These key research topics represent a promising strategy towards the development of practical, scalable industrial processes with great environmental benefits.

Steering Asymmetric Lewis Acid Catalysis Exclusively with Octahedral Metal-Centered Chirality

Catalysts for asymmetric synthesis must be chiral. Metal-based asymmetric catalysts are typically constructed by assembling chiral ligands around a central metal. In this Account, a new class of effective chiral Lewis acid catalysts is introduced in which the octahedral metal center constitutes the exclusive source of chirality. Specifically, the here discussed class of catalysts are composed of configurationally stable, chiral-at-metal Λ-configured (left-handed propeller) or Δ-configured (right-handed propeller) iridium(III) or rhodium(III) complexes containing two bidentate cyclometalating 5-tert-butyl-2-phenylbenzoxazole (dubbed IrO and RhO) or 5-tert-butyl-2-phenylbenzothiazole (dubbed IrS and RhS) ligands in addition to two exchange-labile acetonitriles. They are synthetically accessible in an enantiomerically pure fashion through a convenient auxiliary-mediated synthesis. Such catalysts are of interest due to their intrinsic structural simplicity (only achiral ligands) and the prospect of an especially effective asymmetric induction due to the intimate contact between the chiral metal center and the metal-coordinated substrates or reagents. With respect to chiral Lewis acid catalysis, the bis-cyclometalated iridium and rhodium complexes provide excellent catalytic activities and asymmetric inductions for a variety of reactions including Michael additions, Friedel-Crafts reactions, cycloadditions, α-aminations, α-fluorinations, Mannich reactions, and a cross-dehydrogenative coupling. Mechanistically, substrates such as 2-acyl imidazoles are usually activated by two-point binding. Exceptions exist as for example for an efficient iridium-catalyzed enantioselective transfer hydrogenation of arylketones with ammonium formate, which putatively proceeds through an iridium-hydride intermediate. The bis-cyclometalated iridium complexes catalyze visible-light-induced asymmetric reactions by intertwining asymmetric catalysis and photoredox catalysis in a unique fashion. This has been applied to the visible-light-induced α-alkylation of 2-acyl imidazoles (and in some instances 2-acylpyridines) with acceptor-substituted benzyl, phenacyl, trifluoromethyl, perfluoroalkyl, and trichloromethyl groups, in addition to photoinduced oxidative α-aminoalkylations and a photoinduced stereocontrolled radical-radical coupling, each employing a single iridium complex. In all photoinduced reaction schemes, the iridium complex serves as a chiral Lewis acid catalyst and at the same time as precursor of in situ assembled photoactive species. The nature of these photoactive intermediates then determines their photochemical properties and thereby the course of the asymmetric photoredox reactions. The bis-cyclometalated rhodium complexes are also very useful for asymmetric photoredox catalysis. Less efficient photochemical properties are compensated with a more rapid ligand exchange kinetics, which permits higher turnover frequencies of the catalytic cycle. This has been applied to a visible-light-induced enantioselective radical α-amination of 2-acyl imidazoles. In this reaction, an intermediate rhodium enolate is supposed to function as a photoactivatable smart initiator to initiate and reinitiate an efficient radical chain process. If a more efficient photoactivation is required, a rhodium-based Lewis acid can be complemented with a photoredox cocatalyst, and this has been applied to efficient catalytic asymmetric alkyl radical additions to acceptor-substituted alkenes. We believe that this class of chiral-only-at-metal Lewis acid catalysts will be of significant value in the field of asymmetric synthesis, in particular in combination with visible-light-induced redox chemistry, which has already resulted in novel strategies for asymmetric synthesis of chiral molecules. Hopefully, this work will also pave the way for the development of other asymmetric catalysts featuring exclusively octahedral centrochirality.

Enantioselective conjugate addition of hydroxylamines to α,β-unsaturated 2-acyl imidazoles catalyzed by a chiral-at-metal Rh(iii) complex

A highly efficient chiral-at-metal Rh(iii) complex catalyzed asymmetric aza-Michael addition was developed, affordingN-protected β-amino acid derivatives with excellent enantioselectivity.

Asymmetric alkylation of remote C(sp3)–H bonds by combining proton-coupled electron transfer with chiral Lewis acid catalysis

The catalytic asymmetric alkylation of the remote, unactivated δ-position of N-alkyl amides was enabled by the combination of visible-light-induced proton-coupled electron transfer, 1,5-hydrogen atom transfer, and chiral Lewis acid catalysis.

Visible-light-induced thiotrifluoromethylation of terminal alkenes with sodium triflinate and benzenesulfonothioates

An unconventional reductive quenching cycle was developed to realize the visible-light-induced thiotrifluoromethylation of terminal alkenes.