[3+2] cycloadditions of aryl cyclopropyl ketones by visible light photocatalysis

Catalytic Asymmetric Redox-Neutral [3+2] Photocycloadditions of Cyclopropyl Ketones with Vinylazaarenes Enabled by Consecutive Photoinduced Electron Transfer

Consecutive photoinduced electron transfer (ConPET) is a powerful and atom-economical protocol to overcome the limitations of the intrinsic redox potential of visible light-absorbing photosensitizers, thereby considerably improving the substrate and reaction types. Likely because such an exothermic single-electron transfer (SET) process usually does not require the aid of chiral catalysts, resulting in an inevitable racemic background reaction, notably, no enantioselective manifolds have been reported. Herein, we report on the viability of cooperative ConPET and chiral hydrogen-bonding catalysis for the [3+2] photocycloaddition of cyclopropyl ketones with vinylazaarenes. In addition to enabling the first use of olefins that preferentially interact with chiral catalysts, this catalysis platform paves the way for the efficient synthesis of pharmaceutically and synthetically important cyclopentyl ketones functionalized by azaarenes with high yields, ees and dr. The robust capacity of the method can be further highlighted by the low loading of the chiral catalyst (1.0 mol %), the good compatibility of both 2-azaarene and 3-pyridine-based olefins, and the successful concurrent construction of three stereocenters on cyclopentane rings involving an elusive but important all-carbon quaternary.

Mild Photochemical Reduction of Alkenes and Heterocycles via Thiol-Mediated Formate Activation

The reduction of alkenes to their respective alkanes is one of the most important transformations in organic chemistry, given the abundance of natural and commercial olefins. Metal-catalyzed hydrogenation is the most common way to reduce alkenes; however, the use of H2 gas in combination with the precious metals required for these conditions can be impractical, dangerous, and expensive. More complex substrates often require extremely high pressures of H2, further emphasizing the safety concerns associated with these hydrogenation reactions. Here we report a safe, cheap, and practical photochemical alkene reduction using a readily available organophotocatalyst, catalytic thiol, and formate. These conditions reduce a variety of di-, tri-, and tetra-substituted alkenes in good yield as well as dearomatize pharmaceutically relevant heterocycles to generate sp3-rich isosteres of benzofurans and indoles. These formal-hydrogenation conditions tolerate a broad range of functionalities that would otherwise be sensitive to typical hydrogenations and are likely to be important for industry applications.

Cooperative Photoredox and Cobalt-Catalyzed Acceptorless Dehydrogenative Functionalization of Cyclopropylamides towards Allylic N,O-Acyl-acetal Derivatives

We disclose herein a novel photoredox and cobalt co-catalyzed ring-opening/acceptorless dehydrogenative functionalization of mono-donor cyclopropanes. This sustainable and atom-economic approach allows the rapid assembly of a wide range of allylic N,O-acyl-acetal derivatives. The starting materials are readily available and the reaction features mild conditions, broad substrate scope, and excellent functional group compatibility. The optimized conditions accommodate assorted cycloalkylamides and primary, secondary, and tertiary alcohols, with applications in late-stage functionalization of pharmaceutically relevant compounds, stimulating further utility in medicinal chemistry. Moreover, selective nucleophilic substitutions with various carbon nucleophiles were achieved in a one-pot fashion, offering a reliable avenue to access some cyclic and acyclic derivatives.

Photoinduced [3 + 2] Cycloadditions of Aryl Cyclopropyl Ketones with Alkynes and Alkenes

The five-membered ring skeleton is one of the most pivotal in the area of pharmaceutical and natural products. [3 + 2] cycloadditions of cyclopropyl and unsaturated compounds are a highly efficient and atom-economical way to build a five-member compound. The previous works about the kind of [3 + 2] cycloadditions usually utilized metal or organic small molecule catalysts. However, an ideal [3 + 2] cycloaddition reaction that smoothly happens without any additives and catalysts under mild conditions is underdeveloped. Hence, we report [3 + 2] cycloadditions of aryl cyclopropyl without any additives and catalysts under purple LED. In this method, a broad scope of cyclopropyl, alkyne, and alkene was very compatible, especially drug derivatives ibuprofen and Ioxoprofen, to obtain the corresponding cycloaddition product with a good yield up to 93%.

Alkyl Cyclopropyl Ketones in Catalytic Formal [3 + 2] Cycloadditions: The Role of SmI2 Catalyst Stabilization

Alkyl cyclopropyl ketones are introduced as versatile substrates for catalytic formal [3 + 2] cycloadditions with alkenes and alkynes and previously unexplored enyne partners, efficiently delivering complex, sp3-rich products. The key to effectively engaging this relatively unreactive new substrate class is the use of SmI2 as a catalyst in combination with substoichiometric amounts of Sm0; the latter likely acting to prevent catalyst deactivation by returning SmIII to the catalytic cycle. In the absence of Sm0, background degradation of the SmI2 catalyst can outrun product formation. For the most recalcitrant alkyl cyclopropyl ketones, catalysis is "switched-on" using these new robust conditions, and otherwise unattainable products are delivered. Combined experimental and computational studies have been used to identify and probe reactivity trends among alkyl cyclopropyl ketones, including more complex bicyclic alkyl cyclopropyl ketones, which react quickly with various partners to give complex products. In addition to establishing alkyl cyclopropyl ketones as a new substrate class in a burgeoning field of catalysis, our study provides vital mechanistic insight and robust, practical approaches for the nascent field of catalysis with SmI2.

Visible-Light-Induced Defluorinative α-C(sp3)-H Alkylation for the Synthesis of gem-Difluoroallylated α-Trifluoromethylamines

Herein, we describe a novel and efficient photoredox catalytic Cα radical addition/defluoroalkylation coupling reaction between α-trifluoromethyl alkenes and N-trifluoroethyl hydroxylamine. A series of gem-difluoroallylated α-trifluoromethylamines were synthesized by the Cα radical addition enabled by a 1,2-H shift of the in situ-generated N-trifluoroethyl radical. Notably, this protocol is distinguished by its mild conditions, easy operation, and excellent functional group tolerability.

Visible Light Promoted [3+2]-Cycloaddition for the Synthesis of Cyclopenta[b]chromenocarbonitrile Derivatives

In the manuscript, a novel method for the preparation of cyclopenta[b]chromenocarbonitrile derivatives via [3+2] cycloaddition reaction of substituted 3-cyanochromones and N-cyclopropyloamines initiated by visible light catalysis has been described. The reaction was performed in the presence of Eosin Y as a photocatalyst. The key parameters responsible for the success of the described strategy are visible light, a small amount of photoredox catalyst, an anhydrous solvent, and an inert atmosphere.

Spirocyclization and Michael addition of 3-benzylidene succinimides: route to spirocyclopentapyrrolidine-tetraones and benzylidene N-arylpyrrolidine-diones

Reactions of 3-benzylidene succinimides with 2-substituted 2-hydroxy-indane-1,3-diones and unsaturated pyrazolones are carried out under basic conditions to afford spirocyclized derivatives and Michael adducts, respectively, with high regio- and stereo-selectivities. The most notable aspect of the reaction is the ability of highly reactive benzylidene succinimide to act as both an electrophile and a nucleophile causing spirocyclization. The reaction proceeded under mild and metal-free conditions and products were isolated in good to high yields. The current strategy utilizes simple and easily accessible precursors, and provides functionally rich products of medicinal interest with two to four contiguous stereogenic centres and complete regioselectivity with excellent diastereoselectivity.

Photocatalyzed [2σ + 2σ] and [2σ + 2π] Cycloadditions for the Synthesis of Bicyclo[3.1.1]heptanes and 5- or 6-Membered Carbocycles

We report the use of photocatalysis for the homolytic ring-opening of carbonyl cyclopropanes. In contrast to previous studies, our approach does not require a metal cocatalyst or a strong reductant. The carbonyl cyclopropanes can be employed for both [2σ + 2σ] and [2σ + 2π] annulation with either alkenes/alkynes or bicyclo[1.1.0]butanes, yielding cyclopent-anes/-enes and bicyclo[3.1.1]heptanes (BCHs), respectively. BCHs are promising bioisosteres for 1,2,4,5 tetra-substituted aromatic rings. Mechanistic studies, including density functional theory computation and a trapping experiment with DMPO, support a 1,3-biradical generated from cyclopropane as a key intermediate for these transformations.

Stereoselective Photoredox Catalyzed (3+3) Dipolar Cycloaddition of Nitrone with Aryl Cyclopropane

By resorting to the principle of remote activation, we herein demonstrate the first photoredox catalyzed (3+3) dipolar cycloaddition of nitrones with aryl cyclopropanes. Key to the fidelity of the reaction resides in a facile manner of substrate activation by single-electron transfer (SET) oxidation with photoredox catalysis, and the reaction takes place through a stepwise cascade encompassing a three-electron-type nucleophilic substitution triggered cyclopropane ring-opening and a diastereoselective 6-endo-trig radical cyclization manifold. The reaction proceeds under mild conditions with excellent regio- and stereoselectivity, nicely complementing the well-developed Lewis acid catalyzed cycloaddition of donor-acceptor cyclopropanes. Other merits of the protocol include wide scope of aryl cyclopropanes with diversified substitution patterns and good functional-group compatibility. A mechanism involving an aryl radical cation promoted remote activation mode was also proposed and supported by mechanistic experiments.

Recent Advances in the Catalytic Synthesis of the Cyclopentene Core

Five-membered carbocycles are ubiquitously found in natural products, pharmaceuticals, and other classes of organic compounds. Within this category, cyclopentenes deserve special attention due to their prevalence as targets and as well as key intermediates for synthesizing more complex molecules. Herein, we offer an overview summarizing some significant recent advances in the catalytic assembly of this structural motif. A great variety of synthetic methodologies and strategies are covered, including transition metal-catalyzed or organocatalyzed processes. Both inter- and intramolecular transformations are documented. On this ground, our expertise in the application of C-H functionalization reactions oriented towards the formation of this ring and its subsequent selective functionalization is embedded.

Visible-light-induced [3+2] cycloadditions of donor/donor diazo intermediates with alkenes to achieve (spiro)-pyrazolines and pyrazoles

To date, [3 + 2] cycloadditions of diazo esters with alkynes or alkenes have been a robust tool to generate pyrazoles and pyrazolines. However, methods capable of generating donor/donor diazo species from readily available N-tosylhydrazones to furnish [3 + 2] cycloadditions, remain elusive. Herein, we describe the first visible-light-induced [3 + 2] cycloadditions of donor/donor diazo precursors with alkenes to afford pyrazoles and novel (spiro)pyrazolines bearing a quaternary center. This protocol shows a tolerable substrate scope covering versatile carbonyl compounds and alkenes. Late-stage functionalization of bioactive molecules, one-pot approach, and gram-scale synthesis have also been introduced successfully to prove the practicability. At last, mechanistic experiments and DFT studies suggested the formation of non-covalent interactions enabling the activation of N-tosylhydrazones and the formation of the donor/donor diazo intermediates.

Strain-Release Photocatalysis

The concept of strain in organic compounds is as old as modern organic chemistry and was initially introduced to justify the synthetic setbacks along the synthesis of small ring systems (pars construens of strain). In the last decades, chemists have developed an arsenal of strain-release reactions (pars destruens of strain) which can generate─with significant driving force─rigid aliphatic systems that can act as three-dimensional alternatives to (hetero)arenes. Photocatalysis added an additional dimension to strain-release processes by leveraging the energy of photons to create chemical complexity under mild conditions. This perspective presents the latest advancements in strain-release photocatalysis─with emphases on mechanisms, catalytic cycles, and current limitations─the unique chemical architectures that can be produced, and possible future directions.

Enantioselective radical cascade cyclization via Ti-catalyzed redox relay

Radical cascade cyclization reactions provide an efficient method for the construction of polycyclic architectures with multiple stereogenic centers. However, achieving enantioselectivity control of this type of reaction is a challenging task. Here, we report an enantioselective cyclization of polyfunctional aryl cyclopropyl ketone and alkyne units, wherein the stereochemical outcome is directed by a chiral Ti(salen) catalyst. This transformation was proposed to proceed via a radical cascade process involving the reductive ring-opening of the cyclopropyl ketone followed by two annulation events entailing cyclization of the ensuing alkyl radical onto the alkyne and subsequent addition of the incipient vinyl radical to the Ti(IV)-enolate.

Iron-Catalyzed Reductive Ring Opening/gem-Difluoroallylation of Cyclopropyl Ketones

By merging C-C and C-F bond cleavage, we developed a regioselective ring opening/gem-difluoroallylation of cyclopropyl ketones with α-trifluoromethylstyrenes, which proceeds under the catalysis of iron with the combination of manganese and TMSCl as the reducing agents, providing a new entry to the synthesis of carbonyl-containing gem-difluoroalkenes. Remarkably, the ketyl radical-induced selective C-C bond cleavage and the following generation of more-stable carbon-centered radicals enable complete regiocontrol of this ring opening reaction for various substitution patterns of the cyclopropane ring.

Visible Light Induced C-H/N-H and C-X Bonds Reactions

Herein, we report efficient visible light-induced photoredox reactions of C–H/N–H and C–X Bonds. These methods have provided access to varied portfolio of synthetically important γ-ketoesters, azaspirocyclic cyclohexadienones spirocyclohexadienones, multisubstituted benzimidazole derivatives, substituted N,2-diarylacetamide, 2-arylpyridines and 2-arylquinolines in good yields and under mild conditions. Moreover, we have successfully discussed the construction through visible light-induction by an intermolecular radical addition, dearomative cyclization, aryl migration and desulfonylation. Similarly, we also spotlight the visible light-catalyzed aerobic C–N bond activation from well-known building blocks through cyclization, elimination and aromatization. The potential use of a wide portfolio of simple ketones and available primary amines has made this transformation very attractive.

Z-Trisubstituted α,β-Unsaturated Esters and Acid Fluorides through Stereocontrolled Catalytic Cross-Metathesis

Catalytic cross-metathesis (CM) reactions that can generate trisubstituted alkenes in high stereoisomeric purity are important but remain limited in scope. Here, CM reactions are introduced that generate Z-trisubstituted α-methyl, α,β-unsaturated, alkyl and aryl esters, thiol esters, and acid fluorides. Transformations are promoted by a Mo bis-aryloxide, a monoaryloxide pyrrolide, or a monoaryloxide chloride complex; air-stable and commercially available paraffin tablets containing a Mo complex may also be used. Alkyl, aryl, and silyl carboxylic esters as well as thiol esters and acid fluoride reagents are either purchasable or can be prepared in one step. Products were obtained in 55-95% yield and in 88:12->98:2 Z/E ratio (typically >95:5). The applicability of the approach is highlighted by a two-step conversion of citronellol to an isomintlactone precursor (1.7 g, 73% yield, and 97:3 Z/E) and a single-step transformation of lanosterol acetate to 3-epi-anwuweizic acid (72% yield and 94:6 Z/E). Included are the outcomes of DFT studies, regarding several initially puzzling catalyst activity trends, providing the following information: (1) it is key that a disubstituted Mo alkylidene, generated by a competing homo-metathesis (HM) pathway, can re-enter the productive CM cycle. (2) Whereas in a CM cycle the formation of a molybdacyclobutane is likely turnover-limiting, the collapse of related metallacycles in a HM cycle is probably rate-determining. It is therefore the relative energy barrier required for these steps that determines whether CM or HM is dominant with a particular complex.

Formal Cycloadditions Driven by the Homolytic Opening of Strained, Saturated Ring Systems

The field of strain-driven, radical formal cycloadditions is experiencing a surge in activity motivated by a renaissance in free radical chemistry and growing demand for sp³ -rich ring systems. The former has been driven in large part by the rise of photoredox catalysis, and the latter by adoption of the "Escape from Flatland" concept in medicinal chemistry. In the years since these broader trends emerged, dozens of formal cycloadditions, including catalytic, asymmetric variants, have been developed that operate via radical mechanisms. While cyclopropanes have been studied most extensively, a variety of strained ring systems are amenable to the design of analogous reactions. Many of these processes generate lucrative, functionally decorated sp³ -rich ring systems that are difficult to access by other means. Herein, we summarize recent efforts in this area and analyze the state of the field.

Diboron(4)-Catalyzed Remote [3+2] Cycloaddition of Cyclopropanes via Dearomative/Rearomative Radical Transmission through Pyridine

Ring structures such as pyridine, cyclopentane or their combinations are important motifs in bioactive molecules. In contrast to previous cycloaddition reactions that necessitated a directly bonded initiating functional group, this work demonstrated a novel through-(hetero)arene radical transmission concept for selective activation of a remote bond. An efficient, metal-free and atom-economical [3+2] cycloaddition between 4-pyridinyl cyclopropanes and alkenes or alkynes has been developed for modular synthesis of pyridine-substituted cyclopentanes, cyclopentenes and bicyclo[2.1.1]hexanes that are difficult to access using known methods. This complexity-building reaction was catalyzed by a very simple and inexpensive diboron(4) compound and took place via dearomative/rearomative processes. The substrate scope was broad and more than 100 new compounds were prepared in generally high yields. Mechanistic experiments and density function theory (DFT) investigation supported a radical relay catalytic cycle involving alkylidene dihydropyridine radical intermediates and boronyl radical transfer.

Development of Carbazole-Cored Organo-Photocatalyst for Visible Light-Driven Reductive Pinacol/Imino-Pinacol Coupling

Benzoperylenocarbazole (BPC), a unique carbazole-based organophotocatalyst, is reported herein as a potent organo-photoreductant. Lower excited state oxidation potential (-2.0 V vs SCE) and reasonable excited state lifetime (4.61 ns) render BPC an effective photosensitizer. Under irradiation of blue light employing low catalyst loading (0.5 mol %), a plethora of vicinal diols and diamines were synthesized in excellent yields through reductive coupling of carbonyls and imines, respectively. Insight about the electronic structure of BPC was obtained by DFT calculations.

Gem-Difluoroallylation of Aryl Sulfonium Salts

The unprecedented photochemical late-stage defluorinative gem-difluoroallylation of aryl sulfonium salts, which are formed site-selectively by direct C(sp²)─H functionalization, is herein disclosed. This method is distinguished by its mild reaction conditions, wide scope, and excellent site-selectivity. As showcase examples, a Flurbiprofen and Pyriproxyfen derivatives could be late stage C(sp²)─H gem-difluoroallylated with high yields. Experimental and computational investigations were conducted.

Reversible Homolysis of a Carbon-Carbon σ-Bond Enabled by Complexation-Induced Bond-Weakening

A case study of catalytic carbon-carbon σ-bond homolysis is presented. The coordination of a redox-active Lewis acid catalyst reduces the bond-dissociation free energies of adjacent carbon-carbon σ-bonds, and this complexation-induced bond-weakening is used to effect reversible carbon-carbon bond homolysis. Stereochemical isomerization of 1,2-disubstituted cyclopropanes was investigated as a model reaction with a ruthenium (III/II) redox couple adopted for bond weakening. Results from our mechanistic investigation into the stereospecificity of the isomerization reaction are consistent with selective complexation-induced carbon-carbon bond homolysis. The ΔG‡ of catalyzed and uncatalyzed reactions were estimated to be 14.4 and 40.0 kcal/mol, respectively with the computational method, (U)PBE0-D3/def2-TZVPP-SMD(toluene)//(U)B3LYP-D3/def2-SVP. We report this work as the first catalytic example where the complexation-induced bond-weakening effect is quantified through transition state analysis.

CdS Quantum Dots as Potent Photoreductants for Organic Chemistry Enabled by Auger Processes

Strong reducing agents (<-2.0 V vs saturated calomel electrode (SCE)) enable a wide array of useful organic chemistry, but suffer from a variety of limitations. Stoichiometric metallic reductants such as alkali metals and SmI₂ are commonly employed for these reactions; however, considerations including expense, ease of use, safety, and waste generation limit the practicality of these methods. Recent approaches utilizing energy from multiple photons or electron-primed photoredox catalysis have accessed reduction potentials equivalent to Li⁰ and shown how this enables selective transformations of aryl chlorides via aryl radicals. However, in some cases, low stability of catalytic intermediates can limit turnover numbers. Herein, we report the ability of CdS nanocrystal quantum dots (QDs) to function as strong photoreductants and present evidence that a highly reducing electron is generated from two consecutive photoexcitations of CdS QDs with intermediate reductive quenching. Mechanistic experiments suggest that Auger recombination, a photophysical phenomenon known to occur in photoexcited anionic QDs, generates transient thermally excited electrons to enable the observed reductions. Using blue light-emitting diodes (LEDs) and sacrificial amine reductants, aryl chlorides and phosphate esters with reduction potentials up to -3.4 V vs SCE are photoreductively cleaved to afford hydrodefunctionalized or functionalized products. In contrast to small-molecule catalysts, QDs are stable under these conditions and turnover numbers up to 47 500 have been achieved. These conditions can also effect other challenging reductions, such as tosylate protecting group removal from amines, debenzylation of benzyl-protected alcohols, and reductive ring opening of cyclopropane carboxylic acid derivatives.

Nickel-Catalyzed Reductive Coupling of γ-Metalated Ketones with Unactivated Alkyl Bromides

A nickel-catalyzed reductive cross-coupling reaction of aryl cyclopropyl ketones with easily accessible unactivated alkyl bromides to access aryl alkyl ketones has been developed. This strategy facilitates access to various of γ-alkyl-substituted ketones via ring opening of cyclopropyl ketones (26 examples, 50-90% yield). Initial mechanistic studies revealed that the reaction proceeds via radical cleavage of the alkyl bromide.

Catalysis with Diboron(4)/Pyridine: Application to the Broad-Scope [3 + 2] Cycloaddition of Cyclopropanes and Alkenes

In contrast to the extensive but non-recyclable use of tetraalkoxydiboron(4) compounds as stoichiometric reagents in diverse reactions, this article reports an atom-economical reaction using a commercial diboron(4) as the catalyst. The key to success was designing a catalytic cycle for radical [3 + 2] cycloaddition involving a pyridine cocatalyst to generate from the diboron(4) catalyst and reversibly mediate the transfer of boronyl radicals. In comparison with known [3 + 2] cycloaddition with transition metal-based catalysts, the current reaction features not only metal-free conditions, inexpensive and stable catalysts, and simple operation but also remarkably broadened substrate scope. In particular, previously unusable cyclopropyl ketones without an activating group and/or alkenes with 1,2-disubstitution and 1,1,2-trisubstitution patterns were successfully used for the first time. Consequently, challenging cyclopentane compounds with various levels of substitution (65 examples, 57 new products, up to six substituents at all five ring atoms) were readily prepared in generally high to excellent yield and diastereoselectivity. The reaction was also successfully applied in concise formal synthesis of an anti-obesity drug and building natural product-like complex bridged or spirocyclic compounds. Mechanistic experiments and computational investigation support the proposed radical relay catalysis featuring a pyridine-assisted boronyl radical catalyst. Overall, this work demonstrates the first approach to use tetraalkoxydiboron(4) compounds as catalysts and may lead to the development of new, green, and efficient transition metal-like boron-catalyzed organic reactions.

Asymmetric [3 + 2] photocycloadditions of cyclopropylamines with electron-rich and electron-neutral olefins

Radical addition to olefins is a common and useful chemical transformation. In the context of offering enantioenriched three-dimensional molecules via such a highly reactive process, chiral hydrogen-bonding (H-bonding) catalysis has been widely used to provide enantiocontrol. The current strategies for operating H-bonding induction are confined to following that are prevalent in ionic-type manifolds. Here, we report a novel protocol towards electron-rich olefins based on converting these species from acting as H-bonding donors to acceptors. It facilitates the first development of asymmetric [3 + 2] photocycloadditions with cyclopropylamines. The method is also effective for electron-neutral olefins, in which the successful construction of all-carbon quaternary stereocentres from 1,1-diaryl ethylenes that feature two structurally similar aryl substituents demonstrates the versatility of this new chiral H-bonding catalytic strategy. Furthermore, the importance of the obtained six kinds of products in pharmaceuticals and asymmetric catalysis underscores the practicability of this work.

Radical addition to olefins is a common and useful chemical transformation.

Vinylcyclopropane [3+2] Cycloaddition with Acetylenic Sulfones Based on Visible Light Photocatalysis

The first intermolecular visible light [3+2] cycloaddition reaction performed on a meta photocycloadduct employing acetylenic sulfones is described. The developed methodology exploits the advantages of combining UV and visible-light in a two-step sequence that provides a photogenerated cyclopropane which, through a strain-release process, generates a new cyclopentane ring while significantly increasing the molecular complexity. Mechanistic studies and DFT calculations indicate an energy transfer pathway for the visible light-driven reaction step. This strategy could be extended to simpler vinylcyclopropanes.

Radical Carbonyl Umpolung Arylation via Dual Nickel Catalysis

The formation of carbon-carbon bonds lies at the heart of synthetic organic chemistry and is widely applied to construct complex drugs, polymers, and materials. Despite its importance, catalytic carbonyl arylation remains comparatively underdeveloped, due to limited scope and functional group tolerance. Herein we disclose an umpolung strategy to achieve radical carbonyl arylation via dual catalysis. This redox-neutral approach provides a complementary method to construct Grignard-type products from (hetero)aryl bromides and aliphatic aldehydes, without the need for pre-functionalization. A sequential activation, hydrogen-atom transfer, and halogen atom transfer process could directly convert aldehydes to the corresponding ketyl-type radicals, which further react with aryl-nickel intermediates in an overall polarity-reversal process. This radical strategy tolerates─among others─acidic functional groups, heteroaryl motifs, and sterically hindered substrates and has been applied in the late-stage modification of drugs and natural products.

Visible-light-mediated regioselective ring-opening hydrogenolysis of donor–acceptor cyclopropanes with DIPEA and H2O

A visible-light-mediated regioselective ring-opening hydrogenolysis of donor–acceptor cyclopropanes has been developed for the rapid construction of alkylated aryl ketones in good yields with excellent functional group compatibility.

Recent Applications of Rare Earth Complexes in Photoredox Catalysis for Organic Synthesis

:

In recent years, photoredox catalysis has appeared as a new paradigm for forging a wide range of chemical bonds under mild conditions using abundant reagents. This approach allows many organic transformations through the generation of various radical species, enabling the valorization of non-traditional partners. A continuing interest has been devoted to the discovery of novel radical-generating procedures. Over the last ten years, strategies using rare-earth complexes as either redox-active centers or as redox-neutral Lewis acids have emerged. This review provides an overview of the recent accomplishments made in this field. It especially aims to demonstrate the utility of rare-earth complexes for ensuring photocatalytic transformations and to inspire future developments.

Radical umpolung chemistry enabled by dual catalysis: concept and recent advances

We present a perspective on recent advances in radical umpolung chemistry; some selected examples in this area have been highlighted.

Visible-Light-Induced Photoannulation of α-Naphthyl Cyclopropane Carboxylic Esters to Functionalized Dihydrophenalenes

A general synthetic entry to functionalized dihydrophenalenes has been found using naphthyl-cyclopropane esters as starting materials. The desired annulation was possible with visible light, Ir(Fppy)₃ as photocatalyst, BnNMe₂ or DABCO as electron donor, HAT-catalyst, and proton source. A broad scope of substituted naphthyl and azanaphthyl derivatives provided the photoannulation products in high yield. Deuteration studies support a photoredox mechanism involving the photoreductive cyclopropane opening to an enolate radical followed by an aryl radical trapping.

Thiol-Mediated α-Amino Radical Formation via Visible-Light-Activated Ion-Pair Charge-Transfer Complexes

Visible-light-activated electron donor-acceptor complexes offer distinct reaction pathways for the synthesis of complex molecules under mild conditions. Herein, we report a method for the reductive generation of α-amino radicals via the reaction of a visible-light-activated ion-pair charge-transfer complex formed between an in situ-generated alkyl-iminium ion and a thiophenolate. This distinct activation mode is demonstrated through the development of a multicomponent coupling reaction to form substituted aminomethyl-cyclopentanes from secondary amines, cyclopropyl aldehydes, and alkenes. The operationally straightforward transformation displays broad scope and provides a means to generate cyclic amine-containing scaffolds from readily available feedstocks.

Photo-induced copper-catalyzed sequential 1,n-HAT enabling the formation of cyclobutanols

Cyclobutanols are privileged cyclic skeletons in natural products and synthetic building blocks. C(sp3)-H functionalization is a prolonged challenge in organic synthesis. The synthesis of cyclobutanols through double C(sp3)-H bond functionalization remains elusive. Here we report the efficient synthesis of cyclobutanols through intermolecular radical [3 + 1] cascade cyclization, involving the functionalization of two C - H bonds through sequential hydrogen atom transfer. The copper complex reduces the iodomethylsilyl alcohols efficiently under blue-light irradiation to initiate the tandem transformation. The mild reaction tolerates a broad range of functional groups and allows for the facile generation of elaborate polycyclic structures.

Recent advances in ring-opening of donor acceptor cyclopropanes using C-nucleophiles

Ring-opening transformations of donor-acceptor cyclopropanes (DAC) with carbon-centered nucleophiles is a simple, straight-forward approach to 1,3-bifunctional compounds that has witnessed remarkable progress over the past several years. To date, different reactivity patterns of DACs have been successfully exploited in racemic/stereoselective syntheses of various acyclic compounds or carbocycles with an impressive structural diversity. The thriving strategies have been successfully utilized in multistep synthesis of complex target molecules. Herein, the recent advances (2015-present) in the ring-opening of DAC involving electron rich arenes and indoles, active methylene compounds, various dipolarophiles, organoborates/boronates, vinyl ethers etc. following Friedel-Crafts alkylation, annulation/formal cycloaddition reaction, organocatalytic reaction, Nazarov cyclisation etc. are presented.

SmI2-Catalyzed Intermolecular Coupling of Cyclopropyl Ketones and Alkynes: A Link between Ketone Conformation and Reactivity

The archetypal single electron transfer reductant, samarium(II) diiodide (SmI₂, Kagan's reagent), remains one of the most important reducing agents and mediators of radical chemistry after four decades of widespread use in synthesis. While the chemistry of SmI₂ is very often unique, and thus the reagent is indispensable, it is almost invariably used in superstoichiometric amounts, thus raising issues of cost and waste. Of the few reports of the use of catalytic SmI₂, all require the use of superstoichiometric amounts of a metal coreductant to regenerate Sm(II). Here, we describe a SmI₂-catalyzed intermolecular radical coupling of aryl cyclopropyl ketones and alkynes. The process shows broad substrate scope and delivers a library of decorated cyclopentenes with loadings of SmI₂ as low as 15 mol %. The radical relay strategy negates the need for a superstoichiometric coreductant and additives to regenerate SmI₂. Crucially, our study uncovers an intriguing link between ketone conformation and efficient cross-coupling and thus provides an insight into the mechanism of radical relays involving SmI₂. The study lays further groundwork for the future use of the classical reagent SmI₂ in contemporary radical catalysis.

Tunable photochemical 6π heterocyclization reactions mediated by a boron Lewis acid

The regulation effect of boron Lewis acid catalyst on the photoinduced 6π heterocyclization was investigated by using multi-configurational ab initio calculations.

Asymmetric Catalysis Mediated by Synthetic Peptides, Version 2.0: Expansion of Scope and Mechanisms

Low molecular weight synthetic peptides have been demonstrated to be effective catalysts for an increasingly wide array of asymmetric transformations. In many cases, these peptide-based catalysts have enabled novel multifunctional substrate activation modes and unprecedented selectivity manifolds. These features, along with their ease of preparation, modular and tunable structures, and often biomimetic attributes make peptides well-suited as chiral catalysts and of broad interest. Many examples of peptide-catalyzed asymmetric reactions have appeared in the literature since the last survey of this broad field in Chemical Reviews (Chem. Rev. 2007, 107, 5759-5812). The overarching goal of this new Review is to provide a comprehensive account of the numerous advances in the field. As a corollary to this goal, we survey the many different types of catalytic reactions, ranging from acylation to C-C bond formation, in which peptides have been successfully employed. In so doing, we devote significant discussion to the structural and mechanistic aspects of these reactions that are perhaps specific to peptide-based catalysts and their interactions with substrates and/or reagents.

Radical coupling of β-ketoesters and amides promoted by Brønsted/Lewis acids

Recent advances in photocatalysis have enabled radical methods with complementary chemoselectivity to established two electron bond forming approaches. While this radical strategy has previously been limited to substrates with favorable redox potentials, Brønsted/Lewis acid activation has emerged as a means of facilitating otherwise difficult reductions. We report herein our investigations into the Lewis acid-promoted redox activation of β-ketocarbonyls in a model photocatalytic radical alkylation reaction. Rapid evaluation of substrates and reactions conditions was achieved by high throughput experimentation using 96-well plate photoreactors.