Enantioselective Decarboxylative Arylation of α-Amino Acids via the Merger of Photoredox and Nickel Catalysis

The Asymmetric Synthesis of Amines via Nickel-Catalyzed Enantioconvergent Substitution Reactions

Chiral dialkyl carbinamines are important in fields such as organic chemistry, pharmaceutical chemistry, and biochemistry, serving for example as bioactive molecules, chiral ligands, and chiral catalysts. Unfortunately, most catalytic asymmetric methods for synthesizing dialkyl carbinamines do not provide general access to amines wherein the two alkyl groups are of similar size (e.g., CH2R versus CH2R1). Herein, we report two mild methods for the catalytic enantioconvergent synthesis of protected dialkyl carbinamines, both of which use a chiral nickel catalyst to couple an alkylzinc reagent (1.1-1.2 equiv) with a racemic partner, specifically, an α-phthalimido alkyl chloride or an N-hydroxyphthalimide (NHP) ester of a protected α-amino acid. The methods are versatile, providing dialkyl carbinamine derivatives that bear an array of functional groups. For couplings of NHP esters, we further describe a one-pot variant wherein the NHP ester is generated in situ, allowing the generation of enantioenriched protected dialkyl carbinamines in one step from commercially available amino acid derivatives; we demonstrate the utility of this method by applying it to the efficient catalytic enantioselective synthesis of a range of interesting target molecules.

Synthesis of Chiral Amines by C–C Bond Formation with Photoredox Catalysis

Chiral amines are key substructures of biologically active natural products and drug candidates. The advent of photoredox catalysis has changed the way synthetic chemists think about building these substructures, opening new pathways that were previously unavailable. New developments in this area are reviewed, with an emphasis on C–C bond constructions involving radical intermediates generated through photoredox processes.

1 Introduction

2 Radical–Radical Coupling of α-Amino Radicals

2.1 Radical–Radical Coupling Involving Amine Oxidation

2.2 Radical–Radical Coupling Involving Imine Reduction

2.3 Couplings Involving both Amine Oxidation and Imine Reduction

3 Addition Reactions of α-Amino Radicals

3.1 Conjugate Additions of α-Amino Radicals

3.2 Addition of α-Amino Radicals to Heteroaromatic Systems

3.3 Cross Coupling via Additions to Transition Metal Complexes

4 Radical Addition to C=N Bonds Using Photoredox Catalysis

4.1 Intramolecular Radical Addition to C=N Bonds

4.2 Intermolecular Radical Addition to C=N Bonds

5 Conclusion

Chichibabin pyridinium synthesis via oxidative decarboxylation of photoexcited α-enamine acids

A visible light-induced decarboxylative Chichibabin pyridinium synthesis between α-amino acids and aldehydes was developed. When the in situ generated α-enamine acids were photoexcited, they were oxidized by aerobic oxygen to give radical cation species. After decarboxylation and further oxidation, the generated iminium undergoes Chichibabin cyclization to afford pyridiniums. This photochemical protocol enables the synthesis of various tetra-substituted pyridiniums and related natural products in one-step.

NiH-catalyzed asymmetric hydroarylation of N-acyl enamines to chiral benzylamines

Enantiomerically pure chiral amines and related amide derivatives are privilege motifs in many pharmacologically active molecules. In comparison to the well-established hydroamination, the transition metal-catalysed asymmetric hydrofunctionalization of enamines provides a complementary approach for their construction. Here we report a NiH-catalysed enantio- and regioselective reductive hydroarylation of N-acyl enamines, allowing for the practical access to a broad range of structurally diverse, enantioenriched benzylamines under mild, operationally simple reaction conditions.

Photoredox-Catalyzed Decarboxylative Cross-Coupling of α-Amino Acids with Nitrones

A decarboxylative cross-coupling reaction of α-amino acids with nitrones via visible-light-induced photoredox catalysis has been established for easy access to β-amino hydroxylamines and vicinal diamines with structural diversity, which is featured with simple operation, mild conditions, readily available α-amino acids, and a broad scope of nitrone substrates. The application of this protocol can furnish efficient synthetic strategies for some valuable vicinal diamine-containing molecules.

Intermolecular alkene arylcyanation using BnSCN as a cyanide source via a reductive strategy: access to 3,3-disubstituted oxindoles

Herein, a nickel-catalyzed two-component reductive arylcyanation of aryl (pseudo)halide tethered alkenes using benzyl thiocyanate as a cyanide source via C–S bond activation is developed.

Metallaphotoredox catalysis with organic dyes

Here…comes the fun…Combination of metals and organic photocatalysts allows the practical invention of new methodologies!

Visible-Light-Promoted Asymmetric Catalysis by Chiral Complexes of First-Row Transition Metals

This short review presents an overview of visible-light-driven asymmetric catalysis by chiral complexes of first-row transition metals. The processes described here include dual catalysis by a chiral complex of copper, nickel, cobalt, or chromium and an additional photoredox or energy-transfer catalyst, and bifunctional catalysis by a single chiral copper or nickel catalyst. These methods allow valuable transformations with high functional group compatibility. They provide stereoselective construction of carbon–carbon or carbon–heteroatom bonds under mild conditions, and produce a diverse range of previously unknown enantioenriched compounds.

1 Introduction

2 Nickel-Based Photocatalytic Asymmetric Catalysis

3 Copper-Based Photocatalytic Asymmetric Catalysis

4 Photocatalytic Asymmetric Catalysis by Chiral Complexes of Cobalt or Chromium

5 Conclusion

General Method for Enantioselective Three-Component Carboarylation of Alkenes Enabled by Visible-Light Dual Photoredox/Nickel Catalysis

A visible-light-promoted photoredox/nickel protocol for the enantioselective three-component carboarylation of alkenes with tertiary and secondary alkyltrifluoroborates and aryl bromides is described. This redox-neutral protocol allows for facile and divergent access to a wide array of enantioenriched β-alkyl-α-arylated carbonyls, phosphonates, and sulfones in high yields and excellent enantioselectivities from readily available starting materials. We also report a modular and enantioselective synthesis of flurbiprofen analogs and piragliatin lead compound to demonstrate synthetic utility. Experimental and computational mechanistic studies were performed to gain insights into the mechanism and origin of chemo- and enantioselectivity.

Direct Enantioselective C(sp3)-H Acylation for the Synthesis of α-Amino Ketones

A direct enantioselective acylation of α-amino C(sp³)-H bonds with carboxylic acids has been achieved via the merger of transition metal and photoredox catalysis. This straightforward protocol enables cross-coupling of a wide range of carboxylic acids, one class of feedstock chemicals, with readily available N-alkyl benzamides to produce highly valuable α-amino ketones in high enantioselectivities under mild conditions. The synthetic utility of this method is further demonstrated by gram scale synthesis and application to late-stage functionalization. This method provides an unprecedented solution to address the challenging stereocontrol in metallaphotoredox catalysis and C(sp³)-H functionalization. Mechanistic studies suggest the α-C(sp³)-H bond of the benzamide coupling partner is cleavage by photocatalytically generated bromine radicals to form α-amino alkyl radicals, which subsequently engages in nickel-catalyzed asymmetric acylation.

Copper-Catalyzed Enantioconvergent Cross-Coupling of Racemic Alkyl Bromides with Azole C(sp2 )-H Bonds

The development of enantioconvergent cross-coupling of racemic alkyl halides directly with heteroarene C(sp² )-H bonds has been impeded by the use of a base at elevated temperature that leads to racemization. We herein report a copper(I)/cinchona-alkaloid-derived N,N,P-ligand catalytic system that enables oxidative addition with racemic alkyl bromides under mild conditions. Thus, coupling with azole C(sp² )-H bonds has been achieved in high enantioselectivity, affording a number of potentially useful α-chiral alkylated azoles, such as 1,3,4-oxadiazoles, oxazoles, and benzo[d]oxazoles as well as 1,3,4-triazoles, for drug discovery. Mechanistic experiments indicated facile deprotonation of an azole C(sp² )-H bond and the involvement of alkyl radical species under the reaction conditions.

Ni-Catalyzed Reductive and Merged Photocatalytic Cross-Coupling Reactions toward sp3/sp2-Functionalized Isoquinolones: Creating Diversity at C-6 and C-7 to Address Bioactive Analogues

Naturally occurring isoquinolones have gained considerable attention over the years for their bioactive properties. While the late-stage introduction of various functionalities at certain positions, namely, C-3, C-4, and C-8, has been widely documented, the straightforward introduction of challenging sp3 carbon-linked acyclic aminoalkyl or aza- and oxacyclic appendages at C-6 and C-7 remains largely underexplored. Interest in 6-substituted azacyclic analogues has recently garnered attention in connection with derivatives exhibiting anticancer activity. Reported here is the first application of the versatile and recently emerging field of Ni-catalyzed reductive cross-coupling reactions to the synthesis of 6- and 7- hetero(cyclo)alkyl-substituted isoquinolones. In a second and complementary approach, a new set of C-6- and C-7-substituted positional isomers of hetero(cyclo)alkyl appendages were obtained from the merging of photocatalytic and Ni-catalyzed coupling reactions. In both cases, 6- and 7-bromo isoquinolones served as dual-purpose reacting partners with readily available tosylates and carboxylic acids, respectively.

Copper-Catalyzed Enantioselective Radical 1,4-Difunctionalization of 1,3-Enynes

Chiral allenes are important structural motifs frequently found in natural products, pharmaceuticals, and other organic compounds. Asymmetric 1,4-difunctionalization of 1,3-enynes is a promising strategy to construct axial chirality and produce substituted chiral allenes from achiral substrates. However, the previous state of the art in 1,4-difunctionalization of 1,3-enynes focused on the allenyl anion pathway. Because of this, only electrophiles can be introduced into the allene backbones in the second functionalization step, consequently limiting the reaction and allene product types. The development of asymmetric 1,4-difunctionalization of 1,3-enynes via a radical pathway would complement previous methods and support expansion of the toolbox for the synthesis of asymmetric allenes. Herein, we report the first radical enantioselective allene formation via a group transfer pathway in the context of copper-catalyzed radical 1,4-difunctionalization of 1,3-enynes. This method addresses a longstanding unsolved problem in asymmetric radical chemistry, provides an important strategy for stereocontrol with free allenyl radicals, and offers a novel approach to the valuable, but previously inaccessible, chiral allenes. This work should shed light on asymmetric radical reactions and may lead to other enantioselective group transfer reactions.

Recent developments in enantioselective photocatalysis

Enantioselective photocatalysis has rapidly grown into a powerful tool for synthetic chemists. This review describes the various strategies for creating enantioenriched products through merging enantioselective catalysis and photocatalysis, with a focus on the most recent developments and a particular interest in the proposed mechanisms for each. With the aim of understanding the scope of each strategy, to help guide and inspire further innovation in this field.

Enantio- and Regioselective NiH-Catalyzed Reductive Hydroarylation of Vinylarenes with Aryl Iodides

A highly enantio- and regioselective hydroarylation process of vinylarenes with aryl halides has been developed using a NiH catalyst and a new chiral bis imidazoline ligand. A broad range of structurally diverse, enantioenriched 1,1-diarylalkanes, a structure found in a number of biologically active molecules, have been obtained with excellent yields and enantioselectivities under extremely mild conditions.

Stereoinduction in Metallaphotoredox Catalysis

Metallaphotoredox catalysis has evolved into an enabling platform to construct C(sp³ )-hybridized centers under remarkably mild reaction conditions. The cultivation of abundant radical precursor feedstocks has significantly increased the scope of transition-metal-catalyzed cross-couplings, especially with respect to C(sp² )-C(sp³ ) linkages. In recent years, considerable effort has been devoted to understanding the origin of stereoinduction in dual catalytic processes. In this context, Ni- and Cu-catalyzed transformations have played a predominant role exploiting this mode of catalysis. Herein, we provide a critical overview on recent progress in enantioselective bond formations enabled by Ni- and Cu-catalyzed manifolds. Furthermore, selected stereochemical control elements within the realm of diastereoselective transformations are discussed.

Stereochemistry of Transition Metal Complexes Controlled by the Metallo-Anomeric Effect

The use of stereoelectronic interactions to control reactivity and selectivity has a long history in chemistry. The anomeric effect, one of the fundamental concepts in organic chemistry, describes the preferences of a substituent at the anomeric carbon in glycosides to adopt axial configuration when the anomeric group is an electronegative element such as oxygen or a halogen. The origin of the anomeric effect has been the subject of intense debate. Explanations capitalizing on either the delocalization of the endocyclic oxygen lone pair into the antibonding σ*_(C-X) orbital or the minimization of the dipole-dipole interactions are currently the two leading theoretical models. Although the majority of experimental and theoretical studies have focused on the elements from groups 6 and 7, little is known about conformational preferences of tetrahydropyran rings substituted with a transition metal at the anomeric carbon and the role of these interactions in stereoselective synthesis. Here, we report studies on conformational and configurational preferences of organometallic complexes stabilized by vicinal heteroatoms. We provide computational evidence that late transition metals adopt the axial position in heterocycles or synclinal geometry in acyclic systems. Furthermore, the anomeric preferences of late transition metals correlate with the oxidation state of the metal and can be explained by hyperconjugative interactions between endocyclic heteroatom and the σ* acceptor orbitals of the C-M bond. In a broader context, this discovery provides insight into the role of previously unanticipated stereoelectronic effects that can be harnessed in the design of stereoselective reactions, including chemical glycosylation and enantioselective catalysis.

Nickel-Catalyzed Enantioselective Reductive Cross-Coupling Reactions

Nickel-catalyzed reductive cross-coupling reactions have emerged as powerful methods to join two electrophiles. These reactions have proven particularly useful for the coupling of sec-alkyl electrophiles to form stereogenic centers; however, the development of enantioselective variants remains challenging. In this Perspective, we summarize the progress that has been made toward Ni-catalyzed enantioselective reductive cross-coupling reactions.

Catalytic Enantioselective Radical Transformations Enabled by Visible Light

Visible light has been recognized as an economical and environmentally benign source of energy that enables chemoselective molecular activation of chemical reactions and hence reveal a new horizon for the design and discovery of novel chemical transformations. On the other hand, asymmetric catalysis represents an economic method to satisfy the increasing need for enantioenriched compounds in the chemical and pharmaceutical industries. Therefore, combining visible light photocatalysis with asymmetric catalysis creates a wider range of opportunities for the development of mechanistically unique reaction schemes. However, there arise two main problems like undesirable photochemical background reactions and difficulties in controlling the stereochemistry with highly reactive photochemical intermediates which can pose a serious challenge to the development of asymmetric visible light photocatalysis. In recent years, several methods have been developed to overcome these challenges. This review summarizes the recent advances in visible light-induced enantioselective reactions. We divide our discussion into four categories: Asymmetric photoredox organocatalysis, asymmetric transition metal photoredox catalysis, asymmetric photoredox Lewis acid catalysis and asymmetric photoinduced energy transfer catalysis. Special emphasis has been given to different catalytic activation modes that enable the construction of challenging carbon-carbon and carbon-heteroatom bond in an enantioselective fashion. A brief analysis of substrate scope and limitation as well as reaction mechanism of these reactions has been included.

Enantioselective synthesis enabled by visible light photocatalysis

Enantioselective photoreaction has been a synthetic challenge for decades. With the continuous development of modern visible light photocatalysis and asymmetric catalysis, remarkable advances have been achieved through the synergistic action of these catalytic reactions, allowing the construction of various enantiomerically enriched molecules that were once inaccessible using photocatalytic reactions. This review presents some of the contemporary developments in enantioselective visible-light photocatalysis reactions, covering the period from 2008 to March 2020, with the contents classified by catalysis type.

Recent advances in photoredox and nickel dual-catalyzed cascade reactions: pushing the boundaries of complexity

Cascade reactions that produce multiple chemical bonds in one synthetic operation are important in the efficient construction of complex molecules. In addition, photoredox and nickel dual catalysis opens a new and powerful avenue for transition-metal-catalyzed cross-coupling reactions. By combining these two concepts, photoredox and nickel dual-catalyzed cascade reactions have been recently established, and they provide an efficient and mild method for accessing a series of valuable organic compounds.

Highly Diastereoselective Functionalization of Piperidines by Photoredox-Catalyzed α-Amino C-H Arylation and Epimerization

We report a photoredox-catalyzed α-amino C-H arylation reaction of highly substituted piperidine derivatives with electron-deficient cyano(hetero)arenes. The scope and limitations of the reaction were explored, with piperidines bearing multiple substitution patterns providing the arylated products in good yields and with high diastereoselectivity. To probe the mechanism of the overall transformation, optical and fluorescent spectroscopic methods were used to investigate the reaction. By employing flash-quench transient absorption spectroscopy, we were able to observe electron transfer processes associated with radical formation beyond the initial excited-state Ir(ppy)₃ oxidation. Following the rapid and unselective C-H arylation reaction, a slower epimerization occurs to provide the high diastereomer ratio observed for a majority of the products. Several stereoisomerically pure products were resubjected to the reaction conditions, each of which converged to the experimentally observed diastereomer ratios. The observed distribution of diastereomers corresponds to a thermodynamic ratio of isomers based upon their calculated relative energies using density functional theory (DFT).