Radical Carbonyl Propargylation by Dual Catalysis

1,3-Butadiene Dicarbofunctionalization Enabled by the Dual Role of Diaryl Ketone in Photo-HAT/Chromium Catalysis

We report a three-component Nozaki-Hiyama-Kishi type reaction of 1,3-dioxolane, 1,3-butadienes, and aldehydes to access masked aldehyde-incorporated homoallylic alcohols, facilitated by photo-hydrogen atom transfer (HAT)/chromium dual catalysis. The diaryl ketone serves dual roles both in the HAT process and in facilitating the turnover of the chromium catalyst. A range of functional groups are tolerated owing to the mild conditions. Both aromatic and aliphatic aldehydes are suitable substrates for coupling with several 1,3-butadienes and 1,3-dioxolane.

Highly Selective 1,4-Diacylation/Cycloisomerization of 1,3-Enynes: De Novo Synthetic Strategy to Polysubstituted Furans

The development of a de novo synthetic strategy for rapid assembly of biologically relevant multisubstituted furans is an appealing but challenging task. Herein, we disclose NHC and organophotocatalysis cocatalyzed three-component radical 1,4-diacylation/cycloisomerization cascade process of readily available 1,3-enynes, which provides an efficient and straightforward entry to a wide range of polysubstituted furans with good yields and excellent regio- and chemoselectivities. The reaction features mild conditions, broad substrate scopes, and good functional group compatibilities.

Catalyst-Free Propargylboration of Ketones with Allenyl-Bpins: Highly Stereoselective Synthesis of tert-Homopropargyl Alcohols Bearing Vicinal Stereocenters

A practical and efficient propargylboration of ketones is presented using general allenylboronic acid pinacol esters (allenyl-Bpins) without a catalyst. This reaction is triggered by in-situ activation of stable allenyl-Bpins through the sequential addition of 1.25 equiv. of n BuLi and the prerequisite 2.0 equiv. of TFAA. Under the optimized reaction conditions, the versatile trisubstituted allenyl-Bpins react with various ketones smoothly to afford a wide range of tert-homopropargyl alcohols bearing vicinal stereocenters in high yields with good to excellent diastereoselectivities. Furthermore, propargylboration of ketones with chiral trisubstituted allenyl-Bpins allows for the asymmetric synthesis of chiral tert-homopropargyl alcohols with a full chirality transfer.

Photoredox-Catalyzed Multicomponent Synthesis of Functionalized γ-Amino Butyric Acids via Reductive Radical Polar Crossover

Multicomponent radical polar crossover (RPC) reactions are useful for leveraging both radical and polar bond-forming steps to rapidly build molecular complexity in a single transformation. However, multicomponent RPC reactions that utilize carbonyl π-bond electrophiles are underrepresented in the literature. Herein, we describe a mild, photoredox-catalyzed decarboxylative multicomponent RPC reaction that couples carboxylic acids, Michael acceptors, and carbonyl electrophiles for the formation of diversely functionalized γ-amino butyric acid derivatives. This transformation also facilitates the synthesis of complex and biologically relevant γ-lactam compounds.

Unlocking the Nucleophilicity of Strong Alkyl C-H Bonds via Cu/Cr Catalysis

Direct functionalization of inert C-H bonds is one of the most attractive yet challenging strategies for constructing molecules in organic chemistry. Herein, we disclose an unprecedented and Earth abundant Cu/Cr catalytic system in which unreactive alkyl C-H bonds are transformed into nucleophilic alkyl-Cr(III) species at room temperature, enabling carbonyl addition reactions with strong alkyl C-H bonds. Various aryl alkyl alcohols are furnished under mild reaction conditions even on a gram scale. Moreover, this new radical-to-polar crossover approach is further applied to the 1,1-difunctionalization of aldehydes with alkanes and different nucleophiles. Mechanistic investigations reveal that the aldehyde not only acts as a reactant but also serves as a photosensitizer to recycle the Cu and Cr catalysts.

Recent Advances in the Synthesis of Propargyl Derivatives, and Their Application as Synthetic Intermediates and Building Blocks

The propargyl group is a highly versatile moiety whose introduction into small-molecule building blocks opens up new synthetic pathways for further elaboration. The last decade has witnessed remarkable progress in both the synthesis of propargylation agents and their application in the synthesis and functionalization of more elaborate/complex building blocks and intermediates. The goal of this review is to highlight these exciting advances and to underscore their impact.

Iron-Catalyzed Intermolecular Oxyamination of Terminal Alkenes Promoted by HFIP Using Hydroxylamine Derivatives

An atom-economical intermolecular iron-catalyzed oxyamination of alkenes is described herein. The insertion of oxygenated and nitrogenated moieties from the hydroxylamine substrate was observed with full regio- and chemo-selectivity for terminal alkenes in good yields. HFIP as a solvent appeared to have a synergistic effect with the iron catalyst to promote the formation of the oxyaminated products. Preliminary mechanistic studies suggest a pathway going through an aziridination reaction followed by an in situ ring opening.

Enantioselective and Diastereodivergent Allylation of Propargylic C-H Bonds

An iridium-catalyzed stereoselective coupling of allylic ethers and alkynes to generate 3,4-substituted 1,5-enynes is reported. Under optimized conditions, the coupling products are formed with excellent regio-, diastereo-, and enantioselectivities, and the protocol is functional group tolerant. Moreover, we report conditions that allow the reaction to proceed with complete reversal of diastereoselectivity. Mechanistic studies are consistent with an unprecedented dual role for the iridium catalyst, enabling the propargylic deprotonation of the alkyne through π-coordination, as well as the generation of a π-allyl species from the allylic ether starting material.

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.

Dihydroquinazolinones as adaptative C(sp3) handles in arylations and alkylations via dual catalytic C-C bond-functionalization

C–C bond forming cross-couplings are convenient technologies for the construction of functional molecules. Consequently, there is continual interest in approaches that can render traditionally inert functionality as cross-coupling partners, included in this are ketones which are widely-available commodity chemicals and easy to install synthetic handles. Herein, we describe a dual catalytic strategy that utilizes dihydroquinazolinones derived from ketone congeners as adaptative one-electron handles for forging C(sp³) architectures via α C–C cleavage with aryl and alkyl bromides. Our approach is achieved by combining the flexibility and modularity of nickel catalysis with the propensity of photoredox events for generating open-shell reaction intermediates. This method is distinguished by its wide scope and broad application profile––including chemical diversification of advanced intermediates––, providing a catalytic technique complementary to existing C(sp³) cross-coupling reactions that operates within the C–C bond-functionalization arena.

Although derived from feedstock chemicals and therefore in principle abundant, ketones are not widely used as cross-coupling partners in organic synthesis. Herein, the authors use ketone derivatives as one-electron handles for forging C(sp3) architectures via dual photo- and nickel catalysis.

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.

Tethered photocatalyst-directed palladium-catalysed C-H allenylation of N-aryl tetrahydroisoquinolines

Harnessing radical intermediates in regioselective reactions presents a substantial challenge. Here, we report a novel control strategy through engineering covalently tethered transition metal-photocatalysts that conjoin Pd-phosphine and Ru/Ir photoredox units. This strategy allows us to override the innate regioselectivity of the Pd-catalysed C-H allenylation of N-aryl tetrahydroisoquinolines.

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

NHC and visible light-mediated photoredox co-catalyzed 1,4-sulfonylacylation of 1,3-enynes for tetrasubstituted allenyl ketones

The modulation of selectivity of highly reactive carbon radical cross-coupling for the construction of C-C bonds represents a challenging task in organic chemistry. N-Heterocyclic carbene (NHC) catalyzed radical transformations have opened a new avenue for acyl radical cross-coupling chemistry. With this method, highly selective cross-coupling of an acyl radical with an alkyl radical for efficient construction of C-C bonds was successfully realized. However, the cross-coupling reaction of acyl radicals with vinyl radicals has been much less investigated. We herein describe NHC and visible light-mediated photoredox co-catalyzed radical 1,4-sulfonylacylation of 1,3-enynes, providing structurally diversified valuable tetrasubstituted allenyl ketones. Mechanistic studies indicated that ketyl radicals are formed from aroyl fluorides via the oxidative quenching of the photocatalyst excited state, allenyl radicals are generated from chemo-specific sulfonyl radical addition to the 1,3-enynes, and finally, the key allenyl and ketyl radical cross-coupling provides tetrasubstituted allenyl ketones.

Radical modulated regioselective difunctionalization of vinyl enynes: tunable access to naphthalen-1(2H)-ones and allenic alcohols

A novel and efficient radical-modulated difunctionalization of vinyl enynes has been disclosed using TEMPO as a radical regulator. Facile access to structurally diverse 3-bromo-naphthalen-1(2H)-ones and 4-bromo-allenic alcohols was realized via 1,2-addition/1,2-migration or 1,4-addition, respectively. This protocol represents the first example of radical-modulated metal-free difunctionalization of 1,3-enynes with high regioselectivity.

Employing Visible-Light Photoredox Catalysis in Multicomponent-Multicatalyst Reactions: One-Pot Synthesis of Spiroquinazolin-2-(thi)ones

The multicomponent-multicatalyst reaction ((MC)²R) and visible-light catalysis have emerged as green and powerful strategies for achieving ideal syntheses. Here, we report the first example of a visible-light-induced approach toward spiroquinazolin(thi)ones. This (MC)²R features an eco-friendly energy source and solvent, metal-free catalysts, step- and atom-economy, a relay catalysis strategy, air as green oxidant, mild conditions, and easily accessible starting materials.

Metal-free photocleavage of C(non-acyl)-S bond of thioesters for regioselective pyridylthioesterification of styrenes

Transformation of thioesters via transition-metal-mediated C(acyl)−S bond cleavage is an emerging method to forge C-C and C-heteroatom bonds. Herein, we report the first activation of stronger C(non-acyl)–S bond of thioesters...

Transition-Metal-Free Alkylation Strategy: A Facile Access of Alkylated Oxindoles via Alkyl Transfer

An efficient transition-metal-free alkylation/cyclization of activated alkenes using Hantzsch ester derivatives as effective alkyl reagents was described. A wide variety of valuable oxindoles were constructed in a single step with...

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.

N-Heterocyclic Carbene-Catalyzed 1,4-Alkylacylation of 1,3-Enynes

The radical relay coupling reaction recently emerged as a powerful synthetic strategy for producing tetrasubstituted allenes. However, bond-forming processes involving the allenyl radical intermediate are mostly limited to those promoted by transition metals. In this report, we describe that a ketyl radical generated from single-electron oxidation of the Breslow intermediate is an excellent coupling partner of allenyl radicals. An organocatalytic 1,4-alkylacylation of 1,3-enynes occurred smoothly in the presence of an aldehyde, a radical precursor, and an N-heterocyclic carbene catalyst. This transformation showed remarkable tolerance to both aromatic and aliphatic aldehydes, enyne substitution, and diversified radical precursors.

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.

Regiodivergent sulfonylarylation of 1,3-enynes via nickel/photoredox dual catalysis

Catalytic difunctionalization of 1,3-enynes represents an efficient and versatile approach to rapidly assemble multifunctional propargylic compounds, allenes and 1,3-dienes. Controlling selectivity in such addition reactions has been a long-standing challenging task due to multiple reactive centers resulting from the conjugated structure of 1,3-enynes. Herein, we present a straightforward method for regiodivergent sulfonylarylation of 1,3-enynes via dual nickel and photoredox catalysis. Hinging on the nature of 1,3-enynes, diverse reaction pathways are feasible: synthesis of α-allenyl sulfones via 1,4-sulfonylarylation, or preparation of (E)-1,3-dienyl sulfones with high chemo-, regio- and stereoselectivity through 3,4-sulfonylarylation. Notably, this is the first example that nickel and photoredox catalysis are merged to achieve efficient and versatile difunctionalization of 1,3-enynes.

Cooperative Photoredox- and Nickel-Catalyzed Alkylative Cyclization Reactions of Alkynes with 4-Alkyl-1,4-dihydropyridines

Cooperative photoredox- and nickel-catalyzed alkylative cyclization reactions of iodoalkynes with 4-alkyl-1,4-dihydropyridines as alkylation reagents under visible light irradiation have been achieved to afford the corresponding alkylated cyclopentylidenes in good to high yields. Introduction of substituents at the propargylic position of iodoalkynes has led to the stereoselective formation of E-isomers. The present reaction system provides a novel synthetic method for alkylative cyclization reactions of both terminal and internal alkynes with cooperative photoredox and nickel catalysis.

Decarboxylative 1,4-carbocyanation of 1,3-enynes to access tetra-substituted allenes via copper/photoredox dual catalysis

We disclose herein the first example of merging photoredox catalysis and copper catalysis for radical 1,4-carbocyanations of 1,3-enynes. Alkyl N-hydroxyphthalimide esters are utilized as radical precursors, and the reported mild and redox-neutral protocol has broad substrate scope and remarkable functional group tolerance. This strategy allows for the synthesis of diverse multi-substituted allenes with high chemo- and regio-selectivities, also permitting late stage allenylation of natural products and drug molecules.

On the application of 3d metals for C-H activation toward bioactive compounds: The key step for the synthesis of silver bullets

Several valuable biologically active molecules can be obtained through C-H activation processes. However, the use of expensive and not readily accessible catalysts complicates the process of pharmacological application of these compounds. A plausible way to overcome this issue is developing and using cheaper, more accessible, and equally effective catalysts. First-row transition (3d) metals have shown to be important catalysts in this matter. This review summarizes the use of 3d metal catalysts in C-H activation processes to obtain potentially (or proved) biologically active compounds.

Photoredox Propargylation of Aldehydes Catalytic in Titanium

A practical and effective photoredox propargylation of aldehydes promoted by 10 mol % of [Cp₂TiCl₂] is presented. No stoichiometric metals or scavengers are used for the process. A catalytic amount of the cheap and simply prepared organic dye 3DPAFIPN is used as the reductant for titanium. The reaction displayed a broad scope, and no traces of allenyl isomers were detected for simple propargyl bromide, whereas mixtures of propargyl and allenyl isomers were observed for substituted propargyl bromides.