1,2-Amino Alcohols via Cr/Photoredox Dual-Catalyzed Addition of α-Amino Carbanion Equivalents to Carbonyls

Metal-carbene-guided twofold cross-coupling of ethers with chromium catalysis

Coupling by metal-carbene transfer enables the formation of several different bonds at the carbenoid site, enabling prochiral Csp3 centers that are fundamental three-dimensional substructures for medicines to be forged with increased efficiency. However, strategies using bulk chemicals are rare because of the challenge of breaking two unactivated geminal bonds. Herein, we report the reactivity of ethers to form metal-carbene intermediate by cleavage of α-Csp3-H/Csp3-O bonds, which achieve selective coupling with arylmagnesium bromides and chlorosilanes. These couplings are catalysed by cyclic (alkyl)(amino)carbene-chromium complex and enable the one-step formation of 1,n-arylsilyl alcohols and α-arylated silanes. Mechanistic studies indicate that the in-situ formed low-valent Cr might react with iodobenzene to form phenyl radical species, which abstracts the α-H atom of ether in giving α-oxy radical. The latter combines with Cr by breaking α-Csp3-O bond to afford metal-carbene intermediate, which couples with aryl Grignard and chlorosilane to form two σ-bonds.

Mild ketyl radical generation and coupling with alkynes enabled by Cr catalysis: stereoselective access to E-exocyclic allyl alcohols

The mild catalytic generation of ketyl radicals for organic transformations remains an unsolved issue, although it facilitates the discovery of metal-catalyzed reactions with the features of high functional group tolerance. Here, we report the generation of the ketyl radicals and coupling with alkynes that was enabled by cost-effective chromium catalysis, allowing for the formation of valuable E-exocyclic allyl alcohols with high stereo- and chemoselectivity. A broad range of synthetically useful functional groups that are sensitive to strong reductants are compatible with the catalytic system, providing access to diverse substituted E-exocyclic allyl alcohols under mild conditions. Appended hydroxyl groups in products are facilely late-stage functionalized in accessing numerous derivatives, as well as the enantio-enrichment of exocyclic allyl alcohol using chiral ligands. Mechanistic studies suggest that bipyridine-ligated Cr(ii) complex serves as a reactive catalyst enabling the generation of the ketyl radical for coupling, giving vinyl radical, followed by the combination of Cr and transmetalation with Cp2ZrCl moiety in affording oxazirconiumacycle. This reaction provides a new opportunity for the mild formation of transient ketyl radicals from widely accessible aliphatic aldehydes for coupling with Earth-abundant metal catalysis.

Regioselective Brønsted acid catalyzed ring opening of aziridines by phenols and thiophenols; a gateway to access functionalized indolines, indoles, benzothiazines, dihydrobenzo-thiazines, benzo-oxazines and benzochromenes

Brønsted acid catalyzed regioselective ring opening of aziridines by phenols and thiophenols have been reported. Involvement of a series of aziridines with a range of phenols and thiophenols offer the generality of the reported protocol. Completion of the reaction at room temperature within very short time brings the uniqueness of the developed technique. To emphasis on the application of the developed methodology, the products have been used for the further synthesis of a range of useful and novel heterocyclic molecules such as indolines, indoles, benzothiazines, dihydrobenzothiazines, benzo-oxazines and benzochromenes.

Cr-Catalyzed Intramolecular Arylative Cross-Coupling of Unactivated C-H Bonds with C-Halide Bonds

We report here a chromium-catalyzed intramolecular arylation of unactivated C-H bonds with C-halide bonds under mild conditions. This reaction was enabled by using a low-cost CrCl2 salt as the precatalyst in combination with allylmagnesium bromide and E/Z-mixed 1-halo-2-styrylarenes as substates, providing a strategy for the construction of functionalized phenanthrene compounds without using external ligands.

Cr-catalyzed borylation of C(aryl)-F bonds using a terpyridine ligand

The defluoroborylation of fluoroarenes by chromium-catalyzed cleavage of unactivated C-F bonds is described. The reaction uses HBpin as the boron source, low-cost and commercially available chromium salt as the precatalyst, and terpyridine as a crucial ligand, providing a protocol with atom-efficient benefits and a wide range of applicable substrates for the functionalization of aryl C-F bonds. Preliminary mechanistic studies indicate that an unprecedented Cr-catalyzed magnesiation of the unactivated C-F bond occurred. The generated arylmagnesium intermediates then participated in the subsequent borylation reaction. The application of the strategy in the preparation of valuable derivatives is demonstrated by the late-stage functionalization of boronate ester groups.

γ-Amino Alcohols via Energy Transfer Enabled Brook Rearrangement

In the long-standing quest to synthesize fundamental building blocks with key functional group motifs, photochemistry in the recent past has comprehensively established its attractiveness. Amino alcohols are not only functionally diverse but are ubiquitous in the biologically active realm of compounds. We developed bench-stable bifunctional reagents that could then access the sparsely reported γ-amino alcohols directly from feedstock alkenes through energy transfer (EnT) photocatalysis. A designed 1,3-linkage across alkenes is made possible by the intervention of a radical Brook rearrangement that takes place downstream to the EnT-mediated homolysis of our reagent(s). A combination of experimental mechanistic investigations and detailed computational studies (DFT) indicates a radical chain propagated reaction pathway.

Chemoselective Three-Component Geminal Cross Couplings of Dihaloalkanes with Cr Catalysis: Rapid Access to Tertiary and Quaternary Alkanes via a Metal-Carbene Intermediate

Geminal cross couplings using multiple components enable the formation of several different bonds at one site in the building of tertiary and quaternary alkanes. Nevertheless, there are remaining issues of concern-cleavage of two geminal bonds and control of selectivity present challenges. We report here the geminal cross couplings of three components by reactions of dihaloalkanes with organomagnesium and chlorosilanes or alkyl tosylates by Cr catalysis, affording the formation of geminal C-C/C-Si or C-C/C-C bonds in the creation of tertiary and quaternary alkanes. The geminal couplings are catalyzed by low-cost CrCl2 , enabling the sluggishness of competitive Kumada-type side couplings and homocouplings of Grignard reagents, in achieving high chemoselectivity. Experimental and theoretical studies indicate that two geminal C-halide bonds are continuously cleaved by Cr to afford a metal carbene intermediate, which couples with a Grignard reagent, followed by silylation, in the formation of geminal C-C and C-Si bonds via a novel inner-sphere radical coupling mechanism. These three-component geminal cross couplings are value-addition to the synthesis of commercial drugs and bioactive molecules in medicinal chemistry.

Direct conversion of amino acids to oxetanol bioisosteres via photoredox catalysis

Carboxylic acids are an important structural feature in many drugs, but are associated with a number of unfavorable pharmacological properties. To address this problem, carboxylic acids can be replaced with bioisosteric mimics that interact similarly with biological targets but avoid these liabilities. Recently, 3-oxetanols have been identified as useful carboxylic acid bioisosteres that maintain similar hydrogen-bonding capacity while decreasing acidity and increasing lipophilicity. However, the installation of 3-oxetanols generally requires multistep de novo synthesis, presenting an obstacle to investigation of these promising bioisosteres. Herein, we report a new synthetic approach involving direct conversion of carboxylic acids to 3-oxetanols using a photoredox-catalyzed decarboxylative addition to 3-oxetanone. Two versions of the transformation have been developed, in the presence or absence of CrCl3 and TMSCl cocatalysts. The reactions are effective for a variety of N-aryl α-amino acids and have excellent functional group tolerance. The Cr-free conditions generally provide higher yields and avoid the use of chromium reagents. Further, the Cr-free conditions were extended to a series of N,N-dialkyl α-amino acid substrates. Mechanistic studies suggest that the Cr-mediated reaction proceeds predominantly via in situ formation of an alkyl-Cr intermediate while the Cr-free reaction proceeds largely via radical addition to a Brønsted acid-activated ketone. Chain propagation processes provide quantum yields of 5 and 10, respectively.

Unveiling catalyst-free electro-photochemical reactivity of aryl diazoesters and facile synthesis of oxazoles, imide-fused pyrroles and tetrahydro-epoxy-pyridines via carbene radical anions

Herein, we report a reagent-less (devoid of catalyst, supporting electrolyte, oxidant and reductant) electro-photochemical (EPC) reaction [electricity (50 μA) and blue LED (5 W)] of aryl diazoesters to generate radical anions which are subsequently reacted with acetonitrile or propionitrile and maleimides to generate diversely substituted oxazoles, diastereo-selective imide-fused pyrroles and tetrahydroepoxy-pyridines in good to excellent yield. Thorough mechanistic investigation including a 'biphasic e-cell' experiment supports the reaction mechanism involving a carbene radical anion. The tetrahydroepoxy-pyridines could be fluently converted to fused pyridines resembling vitamin B₆ derivatives. The source of the electric current in the EPC reaction could be a simple cell phone charger. The reaction was efficiently scaled up to the gram level. Crystal structure, 1D, 2D NMRs and HRMS data confirmed the product structures. This report demonstrates a unique generation of radical anions via electro-photochemistry and their direct applications in the synthesis of important heterocycles.

Room Temperature Construction of Vicinal Amino Alcohols via Electroreductive Cross-Coupling of N-Heteroarenes and Carbonyls

Despite the widespread applications of α-hydroxyalkyl cyclic amines, direct and diverse access to such a class of unique vicinal amino alcohols still remains, to date, a challenge. Here, through a strategy of electroreductive α-hydroxyalkylation of inactive N-heteroarenes with ketones or electron-rich arylaldehydes, we describe a room temperature approach for the direct construction of α-hydroxyalkyl cyclic amines, which features a broad substrate scope, operational simplicity, high chemoselectivity, and no need for pressurized H₂ gas and transition metal catalysts. The zinc ion generated from anode oxidation plays a crucial role in the activation of both reactants by decreasing their reduction potentials. The strategy of electroreduction in combination with substrate activation by Lewis acids in this work is anticipated to develop more useful transformations.

Chromium-catalyzed stereodivergent E- and Z-selective alkyne hydrogenation controlled by cyclic (alkyl)(amino)carbene ligands

The hydrogenation of alkynes allows the synthesis of olefins, which are important feedstock for the materials, pharmaceutical, and petrochemical industry. Thus, methods that enable this transformation via low-cost metal catalysis are desirable. However, achieving stereochemical control in this reaction is a long-standing challenge. Here, we report on the chromium-catalyzed E- and Z-selective olefin synthesis via hydrogenation of alkynes, controlled by two carbene ligands. A cyclic (alkyl)(amino)carbene ligand that contains a phosphino anchor enables the hydrogenation of alkynes in a trans-addition manner, selectively forming E-olefins. With an imino anchor-incorporated carbene ligand, the stereoselectivity can be switched, giving mainly Z-isomers. This ligand-enabled geometrical stereoinversion strategy by one metal catalysis overrides common methods in control of the E- and Z-selectivity with two different metal catalysis, allowing for highly efficient and on-demand access to both E- and Z-olefins in a stereo-complementary fashion. Mechanistic studies indicate that the different steric effect between these two carbene ligands may mainly dominate the selective forming E- or Z-olefins in control of the stereochemistry.

Photoredox-Catalyzed Synthesis of β-Amino Alcohols: Hydroxymethylation of Imines with α-Silyl Ether as Hydroxymethyl Radical Precursor

Carbon-carbon bond formation is an efficient approach for the synthesis of amino alcohols using two simple starting materials. Herein, we present a novel method for a divergent synthesis of β-amino ethers and β-amino alcohols in a sequential one-pot protocol under high-efficiency, mild, and metal- or metal-free conditions. Especially, TMSCH₂OPMP was developed as a synthetic equivalent of α-hydroxymethyl radical in an in situ photocatalyzed oxidative PMP group deprotection strategy under air. A preliminary mechanistic investigation provides evidence for reaction mechanism involving a photoinduced α-alkoxy methyl radical and superoxide.

α-Acylation of Alkenes by a Single Photocatalyst

A direct strategy for the difunctionalization of alkenes, with acylation occurring at the more substituted alkene position, would be attractive for complex ketone synthesis. We report herein a reaction driven by a single photocatalyst that enables α-acylation in this way with the introduction of a fluoromethyl, alkyl, sulfonyl or thioether group at the β-position of the alkene with high chemo- and regioselectivity under extremely mild conditions. Crucial to the success of this method are rate differences in the kinetics of radical generation through single-electron transfer (SET) between different radical precursors and the excited photocatalyst (PC*). Thus, the β-position of the alkene is first occupied by the group derived from the radical precursor that can be generated most readily, and α-keto acids could be used as an electrophilic reagent for the α-acylation of alkenes.

Asymmetric 1,4-functionalization of 1,3-enynes via dual photoredox and chromium catalysis

The merger of photoredox and transition-metal catalysis has evolved as a robust platform in organic synthesis over the past decade. The stereoselective 1,4-functionalization of 1,3-enynes, a prevalent synthon in synthetic chemistry, could afford valuable chiral allene derivatives. However, tremendous efforts have been focused on the ionic reaction pathway. The radical-involved asymmetric 1,4-functionalization of 1,3-enynes remains a prominent challenge. Herein, we describe the asymmetric three-component 1,4-dialkylation of 1,3-enynes via dual photoredox and chromium catalysis to provide chiral allenols. This method features readily available starting materials, broad substrate scope, good functional group compatibility, high regioselectivity, and simultaneous control of axial and central chiralities. Mechanistic studies suggest that this reaction proceeds through a radical-involved redox-neutral pathway.

Visible Light-Mediated Enantioselective Addition of α-Aminoalkyl Radicals to Ketones Catalyzed by Chiral Oxazaborolidinium Ion

The development of a visible light-mediated synthetic method for chiral 1,2-amino tertiary alcohols is described. In the presence of a chiral oxazaborolidinium ion catalyst and photosensitizer, the enantioselective addition of an α-aminoalkyl radical to aryl methyl ketones under visible light provides chiral 1,2-amino tertiary alcohol derivatives in high yields (up to 88%) with excellent enantioselectivities (up to 98% ee). With mechanistic studies such as radical trapping analysis, radical clock analysis, and the measurement of quantum yield, a plausible catalytic cycle is proposed.

Chromium-catalyzed couplings of C(aryl)-SMe bonds for accessing arylated and alkylated benzaldehyde derivatives

Described here is the chromium-catalyzed cleavage of C(aryl)-SMe bonds leading to coupling with organomagnesium to give functionalized benzaldehydes under mild conditions. This reaction was promoted specifically by a low-cost and simple CrCl₂ salt used as a precatalyst, enabling synchronous activations of ortho-C(aryl)-SMe and ortho'-C(aryl)-H bonds to achieve difunctionalization of benzaldimines. This work provided a strategy for accessing arylated, alkylated, and diarylated benzaldehyde derivatives as a result of the couplings of C(aryl)-SMe and C(aryl)-SMe/C(aryl)-H bonds promoted with cost-effective Cr catalysis.

Palladium-Catalyzed trans-Hydroalkoxylation: Counterintuitive Use of an Aryl Iodide Additive to Promote C–H Bond Formation

We report an enantioselective palladium-catalyzed trans-hydroalkoxylation of propargylic amines with a trifluoroacetaldehyde-derived tether to build chiral oxazolidines. Diastereoselective hydrogenation using a heterogeneous palladium catalyst then gave access to protected benzylic amino alcohols in 45–87% yields and 84–94% ee values. Hydroalkoxylation of the alkynes required a catalytic amount of aryl iodide, highlighting the counterintuitive key role played by a putative Pd(II)/ArI oxidative addition complex to promote oxypalladation/protodemetalation.

Merging Carbonyl Addition with Photocatalysis

The carbonyl group stands as a fundamental scaffold and plays a ubiquitous role in synthetically important chemical reactions in both academic and industrial contexts. Venerable transformations, including the aldol reaction, Grignard reaction, Wittig reaction, and Nozaki-Hiyama-Kishi reaction, constitute a vast and empowering synthetic arsenal. Notwithstanding, two-electron mechanisms inherently confine the breadth of accessible reactivity and topological patterns.Fostered by the rapid development of photoredox catalysis, combing well-entrenched carbonyl addition and radicals can harness several unique and increasingly sustainable transformations. In particular, unusual carbon-carbon and carbon-heteroatom disconnections, which are out of reach of two-electron carbonyl chemistry, can be conceived. To meet this end, a novel strategy toward the utilization of simple carbonyl compounds as intermolecular radical acceptors was developed. The reaction is enabled by visible-light photoredox-initiated hole catalysis. In situ Brønsted acid activation of the carbonyl moiety prevents β-scission from occurring. Furthermore, this regioselective alkyl radical addition reaction obviates the use of metals, ligands, or additives, thus offering a high degree of atom economy under mild conditions. On the basis of the same concept and the work of Schindler and co-workers, carbonyl-olefin cross-metathesis, induced by visible light, has also been achieved, leveraging a radical Prins-elimination sequence.Recently, dual chromium and photoredox catalysis has been developed by us and Kanai, offering a complementary approach to the revered Nozaki-Hiyama-Kishi reaction. Leveraging the intertwined synergy between light and metal, several radical-to-polar crossover transformations toward eminent molecular motifs have been developed. Reactions such as the redox-neutral allylation of aldehydes and radical carbonyl alkylation can harvest the power of light and enable the use of catalytic chromium metal. Overall, exquisite levels of diastereoselectivity can be enforced via highly compact transition states. Other examples, such as the dialkylation of 1,3-dienes and radical carbonyl propargylation portray the versatile combination of radicals and carbonyl addition in multicomponent coupling endeavors. Highly valuable motifs, which commonly occur in complex drug and natural product architectures, can now be accessed in a single operational step. Going beyond carbonyl addition, seminal contributions from Fagnoni and MacMillan preconized photocatalytic HAT-based acyl radical formation as a key aldehyde valorization strategy. Our group articulated this concept, leveraging carboxy radicals as hydrogen atom abstractors in high regio- and chemoselective carbonyl alkynylation and aldehyde trifluoromethylthiolation.This Account, in addition to the narrative of our group and others' contributions at the interface between carbonyl addition and radical-based photochemistry, aims to provide core guiding foundations toward novel disruptive synthetic developments. We envisage that extending radical-to-polar crossovers beyond Nozaki-Hiyama-Kishi manifolds, taming less-activated carbonyls, leveraging multicomponent processes, and merging single electron steps with energy-transfer events will propel eminent breakthroughs in the near future.

Photochemical Nozaki-Hiyama-Kishi Coupling Enabled by Excited Hantzsch Ester

This work reports the first photochemical Nozaki-Hiyama-Kishi coupling enabled by bioinspired Hantzsch ester. The salient feature of this process is that commercially available and low-cost organic photoactive Hantzsch ester can serve as both an electron and a proton donor to reduce Cr/Ni to low-valent species and hydrolyze the Cr^III-alkoxy bond, thus bypassing the use of stoichiometric metallic reductants and additives such as TMSCl and Cp₂ZrCl₂. The mild conditions and operationally easy method showed broad compatibility with various alkenyl triflates and aldehydes, including electron-poor pentafluorobenzaldehyde which failed under previous conditions.

Recent Progress in Chromium-Mediated Carbonyl Addition Reactions

Organochromium(III) species are versatile nucleophiles in complex molecule synthesis due to their high functional group tolerance and chemoselectivity for aldehydes. Traditionally, carbonyl addition reactions of organochromium(III) species were performed through reduction of organohalides either using stoichiometric chromium(II) salts or catalytic chromium salts in the presence of stoichiometric reductants [such as Mn(0)]. Recently, alternative methods emerged involving organoradical formation from readily available starting materials (e.g., N-hydroxyphthalimide esters, alkenes, and alkanes), followed by trapping the radical with stoichiometric or catalytic chromium(II) salts. Such methods, especially using catalytic chromium(II) salts, will lead to the development of sustainable chemical processes minimizing salt wastes and number of synthetic steps. In this review, methods for generation of organochromium(III) species for addition reactions to carbonyl compounds, classified by nucleophiles are described.

1 Introduction

2 Alkylation

2.1 Branch-Selective Reductive Alkylation of Aldehydes Using Unactivated Alkenes

2.2 Linear-Selective Alkylation of Aldehydes

2.2.1 Catalytic Decarboxylative Alkylation of Aldehydes Using NHPI ­Esters

2.2.2 Catalytic Reductive Alkylation of Aldehydes Using Unactivated Alkenes

2.2.3 Alkylation of Aldehydes via C(sp3)–H Bond Functionalization of Unactivated Alkanes

2.3 Catalytic α-Aminoalkylation of Carbonyl Compounds

3 Allylation

3.1 Catalytic Allylation of Aldehydes via Three-Component Coupling

3.2 Catalytic Allylation of Aldehydes via C(sp3)–H Bond Functionalization of Alkenes

4 Propargylation: Catalytic Propargylation of Aldehydes via Three-Component Coupling

5 Conclusion

Catalytic Cleavage of Unactivated C(aryl)-P Bonds by Chromium

We describe here the coupling to transform aryl phosphine derivatives by the cleavage of unactivated C(aryl)-P bonds with chromium catalysis, allowing us to achieve the reaction with alkyl bromides and arylmagnesium reagents under mild conditions. Mechanistic studies indicate that catalytic cleavage of unactivated C(aryl)-P bonds is due to the in situ formed reactive Cr, followed by transmetalation and coupling with alkyl bromides.

Nickel Mediated Enantioselective Photoredox Allylation of Aldehydes with Visible Light

Here we report a practical, highly enantioselective photoredox allylation of aldehydes mediated by chiral nickel complexes with commercially available allyl acetate as the allylating agent. The methodology allows the clean stereoselective allylation of aldehydes in good to excellent yields and up to 93% e.e. using a catalytic amount of NiCl 2 (glyme) in the presence of the chiral aminoindanol-derived bis(oxazoline) as the chiral ligand. The photoredox system is constituted by the organic dye 3DPAFIPN and a Hantzsch's ester as the sacrificial reductant. The reaction proceeds under visible light irradiation (blue LEDs, 456 nm) at 8-12 °C with excellent stereoselectivities. Compared to other published procedures, no metal reductants (such as Zn or Mn), additives (e.g. CuI) or air-sensitive Ni(COD) 2 are necessary for this reaction. Accurate DFT calculations and photophysical experiments have clarified the mechanistic picture of this stereoselective allylation reaction showing a key role played by Hantzsch's ester for the turnover of the catalyst.

Photo-Induced Ruthenium-Catalyzed C-H Benzylations and Allylations at Room Temperature

The ruthenium-catalyzed synthesis of diarylmethane compounds was realized under exceedingly mild photoredox conditions without the use of exogenous photocatalysts. The versatility and robustness of the ruthenium-catalyzed C-H benzylation was reflected by an ample scope, including multifold C-H functionalizations, as well as transformable pyrazoles, imidates and sensitive nucleosides. Mechanistic studies were indicative of a photoactive cyclometalated ruthenium complex, which also enabled versatile C-H allylations.

Metallaphotoredox: The Merger of Photoredox and Transition Metal Catalysis

The merger of photoredox catalysis with transition metal catalysis, termed metallaphotoredox catalysis, has become a mainstay in synthetic methodology over the past decade. Metallaphotoredox catalysis has combined the unparalleled capacity of transition metal catalysis for bond formation with the broad utility of photoinduced electron- and energy-transfer processes. Photocatalytic substrate activation has allowed the engagement of simple starting materials in metal-mediated bond-forming processes. Moreover, electron or energy transfer directly with key organometallic intermediates has provided novel activation modes entirely complementary to traditional catalytic platforms. This Review details and contextualizes the advancements in molecule construction brought forth by metallaphotocatalysis.

Metal-Free Aminomethylation of Aromatic Sulfones Promoted by Eosin Y

A metal-free α-aminomethylation of heteroaryls promoted by eosin Y under green light irradiation is reported. A large variety of α-trimethylsilylamines as precursor of α-aminomethyl radical species were engaged to functionalize sulfonyl-heteroaryls following a Homolytic Aromatic Substitution (HAS) pathway. This method has provided a range of α-aminoheteroaryl compounds including a functionalized natural product. The mechanism of this late-stage functionalization of aryls was investigated and suggests the formation of a sulfonyl radical intermediate over a reductive quenching cycle.

Synthesis of 1,2-Aminoalcohols through Enantioselective Aminoallylation of Ketones by Cu-Catalyzed Reductive Coupling

Herein, we report the development of a catalytic enantioselective addition of N-substituted allyl equivalents to ketone electrophiles through use of Cu-catalyzed reductive coupling to access important chiral 1,2-aminoalcohol synthons in high levels of regio-, diastereo-, and enantioselectivity. Factors affecting enantioinduction are discussed including the identification of a reversible ketone allylation step that has not been previously reported in Cu-catalyzed reductive coupling.

High Regio- and Stereoselective Multi-enzymatic Synthesis of All Phenylpropanolamine Stereoisomers from β-Methylstyrene

We present a one-pot cascade for the synthesis of phenylpropanolamines (PPAs) in high optical purities (er and dr up to >99.5 %) and analytical yields (up to 95 %) by using 1-phenylpropane-1,2-diols as key intermediates. This bioamination entails the combination of an alcohol dehydrogenase (ADH), an ω-transaminase (ωTA) and an alanine dehydrogenase to create a redox-neutral network, which harnesses the exquisite and complementary regio- and stereo-selectivities of the selected ADHs and ωTAs. The requisite 1-phenylpropane-1,2-diol intermediates were obtained from trans- or cis-β-methylstyrene by combining a styrene monooxygenase with epoxide hydrolases. Furthermore, in selected cases, the envisioned cascade enabled to obtain the structural isomer (1S,2R)-1-amino-1-phenylpropan-2-ol in high optical purity (er and dr >99.5 %). This is the first report on an enzymatic method that enables to obtain all of the four possible PPA stereoisomers in great enantio- and diastereo-selectivity.

Accessing Aliphatic Amines in C-C Cross-Couplings by Visible Light/Nickel Dual Catalysis

A general aminoalkylation of aryl halides was developed, overcoming intolerance of free amines in nickel-mediated C-C coupling. This transformation features broad functional group tolerance and high efficiency. Taking advantage of the fast desilylation of α-silylamines upon single-electron transfer (SET) facilitated by carbonate, α-amino radicals are generated regioselectively, which then engage in nickel-mediated C-C coupling. The reaction displays high chemoselectivity for C-C over C-N bond formation. Highly functionalized pharmacophores and peptides are also amenable.

N-Alkylation/aldol reaction of α-aldimino thioesters: a facile three-component coupling reaction

Theoretical calculation of the reactivity of α-imino thioesters indicates that they are very reactive substrates for Umpolung N-alkylation. In fact, treatment of α-aldimino thioesters with dialkylzinc reagents in the presence of aldehydes or imines gives three-component coupling products in good yields.

Developments in Photoredox-Mediated Alkylation for DNA-Encoded Libraries

Recently, DNA-encoded library (DEL) technology has emerged as an innovative screening modality for the rapid discovery of therapeutic candidates in pharmaceutical settings. This platform enables a cost-effective, time-efficient, and large-scale assembly and interrogation of billions of small organic ligands against a biological target in a single experiment. An outstanding challenge in DEL synthesis is the necessity for water-compatible transformations under ambient conditions. To access uncharted chemical space, the adoption of photoredox catalysis in DELs, including Ni-catalyzed manifolds and radical/polar crossover reactions, has enabled the construction of novel structural scaffolds through regulated odd-electron intermediates. Herein, a critical discussion of the validation of photoredox-mediated alkylation in DEL environments is presented. Current synthetic gaps are highlighted and opportunities for further development are speculated upon.

Asymmetric Transfer Hydrogenation of α-Substituted-β-Keto Carbonitriles via Dynamic Kinetic Resolution

A catalytic protocol for the enantio- and diastereoselective reduction of α-substituted-β-keto carbonitriles is described. The reaction involves a DKR-ATH process with the simultaneous construction of β-hydroxy carbonitrile scaffolds with two contiguous stereogenic centers. A wide range of α-substituted-β-keto carbonitriles were obtained in high yields (94%-98%) and excellent enantio- and diastereoselectivities (up to >99% ee, up to >99:1 dr). The origin of the diastereoselectivity was also rationalized by DFT calculations. Furthermore, this methodology offers rapid access to the pharmaceutical intermediates of Ipenoxazone and Tapentadol.

Visible-Light-Promoted Metal-Free Synthesis of (Hetero)Aromatic Nitriles from C(sp3 )-H Bonds*

The metal-free activation of C(sp3 )-H bonds to value-added products is of paramount importance in organic synthesis. We report the use of the commercially available organic dye 2,4,6-triphenylpyrylium tetrafluoroborate (TPP) for the conversion of methylarenes to the corresponding aryl nitriles via a photocatalytic process. Applying this methodology, a variety of cyanobenzenes have been synthesized in good to excellent yield under metal- and cyanide-free conditions. We demonstrate the scope of the method with over 50 examples including late-stage functionalization of drug molecules (celecoxib) and complex structures such as l-menthol, amino acids, and cholesterol derivatives. Furthermore, the presented synthetic protocol is applicable for gram-scale reactions. In addition to methylarenes, selected examples for the cyanation of aldehydes, alcohols and oximes are demonstrated as well. Detailed mechanistic investigations have been carried out using time-resolved luminescence quenching studies, control experiments, and NMR spectroscopy as well as kinetic studies, all supporting the proposed catalytic cycle.

Photoredox-enabled 1,2-dialkylation of α-substituted acrylates via Ireland-Claisen rearrangement

Herein, we report the 1,2-dialkylation of simple feedstock acrylates for the synthesis of valuable tertiary carboxylic acids by merging Giese-type radical addition with an Ireland-Claisen rearrangement. Key to success is the utilization of the reductive radical-polar crossover concept under photocatalytic reaction conditions to force the [3,3]-sigmatropic rearrangement after alkyl radical addition to allyl acrylates. Using readily available alkyl boronic acids as radical progenitors, this redox-neutral, transition-metal-free protocol allows the mild formation of two C(sp³)-C(sp³) bonds, thus providing rapid access to complex tertiary carboxylic acids in a single step. Moreover, this strategy enables the efficient synthesis of highly attractive α,α-dialkylated γ-amino butyric acids (GABAs) when α-silyl amines are used as radical precursors - a structural motif that was still inaccessible in related transformations. Depending on the nature of the radical precursors and their inherent oxidation potentials, either a photoredox-induced radical chain or a solely photoredox mechanism is proposed to be operative.