Selective Carbon–Carbon Bond Cleavage of Cyclopropylamine Derivatives

Modular Synthesis of Dehydroprolines by an Energy-Transfer Enabled Cloke-Wilson Rearrangement

A one-pot photocatalytic method is reported for the generation of dehydroprolines, valuable precursors to 5-aryl prolines. Imines, obtained via simple condensation of aminocyclopropane carboxylates (ACPC) with a broad range of aldehydes, were employed in this transformation without purification. We demonstrate this energy-transfer (EnT) enabled Cloke-Wilson-type rearrangement affords both the rare 1,2-dehydroprolines or the more thermodynamically favored 1,5-isomers with up to >20 : 1 selectivity in both directions, using an identical catalytic system from the same starting material. Syntheses of intermediates of bioactive molecules in high yields and selectivity highlight this enabling transformation. Mechanistic studies support the proposed triplet-triplet EnT mode of activation, distinct from the previously developed singlet excited states accessed via UV excitation.

Photoinduced ligand-to-metal charge transfer (LMCT) in organic synthesis: reaction modes and research advances

In recent years, visible light-induced ligand-to-metal charge transfer (LMCT) has emerged as an attractive approach for synthesizing a range of functionalized molecules. Compared to conventional photoredox reactions, photoinduced LMCT activation does not depend on redox potential and offers diverse reaction pathways, making it particularly suitable for the activation of inert bonds and the functional modification of complex organic molecules. This review highlights the indispensable role of photoinduced LMCT in synthetic chemistry, with a focus on recent advancements in LMCT-mediated hydrogen atom transfer (HAT), C-C bond cleavage, decarboxylative transformations, and radical ligand transfer (RLT) reactions.

A Photochemical Strategy towards Michael Addition Reactions of Cyclopropenes

The development of Michael addition reactions to conjugated cyclopropenes is a challenge in organic synthesis due to the fleeting and reactive nature of such strained Michael acceptor systems. Herein, the development of a photochemical approach towards such conjugated cyclopropenes is reported that serves as a strategic entry point to densely functionalized cyclopropanes in a diastereoselective fashion. The process involves the light-mediated generation of transient cyclopropenyl α,β-unsaturated esters from vinyl diazo esters, followed by an organic base catalyzed nucleophilic addition of N-heterocycles to directly access β-N-heterocyclic cyclopropanoic esters. With this synergistic approach, various trisubstituted cyclopropanes bearing N-heteroaryl and N-heterocyclic rings such as indole, pyrrole, benzimidazole, isatin, pyridinone, and quinolinone were accessed efficiently in good yield and decent to good diastereoselectivities. Further, β-indolyl cyclopropanoic acids have been synthesized and were successfully evaluated as FABP-4 inhibitors. Theoretical calculations have been performed to elucidate the mechanism, which was further supported by experimental findings.

Two-step continuous flow-driven synthesis of 1,1-cyclopropane aminoketones

The continuous flow telescoped synthesis of 1,1-cyclopropane aminoketones was achieved by optimizing the photocyclization of 1,2-diketones to 2-hydroxycylobutanones (HCBs) and their reaction with aryl- and alkylamines, via tandem condensation C4-C3-ring contraction reaction. With the achieved operational conditions, we were able to obtain a library of cyclopropylamines with good chemical yields, high productivity, and short residence times.

Bicyclo[2.1.1]hexanes via Intramolecular Formal (3+2)-Cycloaddition

We report a synthesis of bicyclo[2.1.1]hexanes via an intramolecular formal (3+2) cycloaddition of allylated cyclopropanes bearing a 4-nitrobenzimine substituent. Both activated and unactivated alkenes are tolerated in the transformation. The bicyclic imine products are prone to photo-induced ring opening, allowing for the epimerization of C5-stereogenic compounds.

Selective Hydroboration of C-C Single Bonds without Transition-Metal Catalysis

Selective hydroboration of C-C single bonds presents a fundamental challenge in the chemical industry. Previously, only catalytic systems utilizing precious metals Ir and Rh, in conjunction with N- and P- ligands, could achieve this, ensuring bond cleavage and selectivity. In sharp contrast, we discovered an unprecedented and general transition-metal-free system for the hydroboration of C-C single bonds. This methodology is transition-metal and ligand-free and surpasses the transition-metal systems regarding chemo- and regioselectivities, substrate versatility, or yields. In addition, our system tolerates various functional groups such as Ar-X (X=halides), heterocyclic rings, ketones, esters, amides, nitro, nitriles, and C=C double bonds, which are typically susceptible to hydroboration in the presence of transition metals. As a result, a diverse range of γ-boronated amines with varied structures and functions has been readily obtained. Experimental mechanistic studies, density functional theory (DFT), and intrinsic bond orbital (IBO) calculations unveiled a hydroborane-promoted C-C bond cleavage and hydride-shift reaction pathway. The carbonyl group of the amide suppresses dehydrogenation between the free N-H and hydroborane. The lone pair on the nitrogen of the amide facilitates the cleavage of C-C bonds in cyclopropanes.

Stereoselective Hydroxyallylation of Cyclopropenes with Cyclopropanols via NHC Catalysis of Transient Organozinc Species

A stereoselective hydroxyallylation reaction of cyclopropenes with cyclopropanols is achieved under zinc-mediated conditions, affording densely functionalized cyclopropanes with excellent diastereocontrol over three contiguous stereocenters within and outside the cyclopropane ring. A racemic variant of the reaction is synergistically promoted by catalytic N-heterocyclic carbene (NHC) and organic base, whereas chiral amino alcohol-derived bifunctional NHC enables a catalytic enantioselective variant. The reaction likely involves the generation of enolized zinc homoenolate via ring-opening of zinc cyclopropoxide and enolization of the resulting homoenolate, followed by its addition to the cyclopropene as a prochiral allylzinc nucleophile. Our mechanistic investigations highlighted the transient nature of enolized homoenolate, which, once generated from thermodynamically predominant cyclopropoxide, immediately proceeds to allylzincation with cyclopropene. The NHC not only promotes the rate-determining generation of enolized homoenolate but also engages in the allylzincation process. The resulting cyclopropylzinc species undergoes partial in situ protonation while partially remaining intact, thereby leaving an opportunity for trapping with an external electrophile.

Iron(II)-Catalyzed 1,2-Diamination of Styrenes Installing a Terminal NH2 Group Alongside Unprotected Amines

1,2-Diamination of alkenes represents an attractive way to generate differentiated vicinal diamines, which are prevalent motifs in biologically active compounds and catalysts. However, existing methods are usually limited in scope and produce diamines where one or both nitrogens are protected, adding synthetic steps for deprotection and further N-functionalization to reach a desired target. Furthermore, the range of amino groups that can be introduced at the internal position is fairly limited. Here we describe a 1,2-diamination of styrenes that directly installs a free amino group at the terminal position and a wide variety of unprotected nitrogen nucleophiles (primary or secondary alkyl or aromatic amines, sulfoximines, N-heterocycles, and ammonia surrogate) at the internal position. Two complementary sets of conditions encompass electronically activated and deactivated styrenes with diverse substitution patterns and functional groups. Moreover, this strategy can be extended to the 1,2-aminothiolation of styrenes.

Radical 6-Endo Addition Enables Pyridine Synthesis under Metal-Free Conditions

Metal-free synthesis of heterocycles is highly sought after in the pharmaceutical industry and has garnered widespread attention due to eliminating the need to remove trace metal catalysts from the reaction. We report a radical 6-endo addition method for pyridine synthesis from cyclopropylamides and alkynes under metal-free conditions. Various terminal and substituted alkynes are inserted as C2 units into cyclopropylamides to synthesize versatile pyridines with 57 examples. Mechanistic investigations and computational studies indicate the unprecedented 6-endo-trig addition of vinyl radicals to the imine nitrogen atom rather than the conventional 5-exo-trig addition to the imine carbon atom, in which the hypervalent iodine(III) plays a critical role. This reaction easily scales up with excellent functional group compatibility and suits the late-stage pyridine installation on complex molecules.

Transforming cyclopropanes to enamides via σ-C-C bond eliminative borylation

Recent strides in C-H borylation have significantly expanded our toolkit for the preparation of organoboronates. Nevertheless, avenues alternative to obtain these compounds via σ-C-C cleavage, thereby facilitating molecular scaffold editing, remain scarce. Several methodologies have been proposed for hydroboration of cyclopropanes by activating C-C bonds, conventionally relying on noble and hazardous metal catalysts to control reaction outcomes. Here, we present a strategy for crafting stereochemically precise γ-borylenamides through ring-opening of cyclopropanes avoiding any metallic entities. Boryl species, generated through a ternary reaction with BCl3, cyclopropanes, and a tertiary amine, selectively undergo C-C bond eliminative borylation under the directing of N-acyl group, thereby ensuring enhanced selectivity and efficiency along the reaction pathway. Such inherently stereoconvergent approach accommodates precursors of diverse geometries, including cis/trans isomeric blends.

Synthesis of Acylation Polycyclic Derivatives via Regioselective Acylation/Cyclization of 1,7-Dienes with Acyl Oxime Esters

A visible-light-induced radical cascade regioselective acylation/cyclization of 1,7-dienes with acyl oxime esters for the preparation of acylation polycyclic compounds via NCR-mediated C-C σ-bond cleavage is established. The transformation involves the cleavage of the C-C σ-bond in acyl oxime esters and selective addition of the electron neutral C═C bonds in 1,7-dienes for the synthesis of acyl polycyclic quinolinone derivatives, not the traditional seven-membered ring products. The strategy offers several advantages, including broad substrate tolerance, no need for bases, hyperstoichiometric radical initiators, and other auxiliaries.

Rearrangement, Elimination, and Ring-Opening Reactions of Cyclopropyl-Substituted Nitrenium Ions: A Computational and Experimental Investigation

N-(4-Biphenylyl)-N-cyclopropyl nitrenium ion 5 and N-benzyl-N-cyclopropyl nitrenium ion (6) were generated through photolysis of their corresponding N-aminopyridinium ion photoprecursors. In the case of 5, stable products result from a combination of cyclopropyl ring expansion (N-biphenylazetium ion) and ethylene elimination (biphenylisonitrilium ion). When present in high concentrations, methanol can add to the cyclopropyl ring-forming N-3-methoxypropyl-N-biphenyl iminium ion. In contrast, the only detectable product from the N-benzyl-N-cyclopropyl nitrenium ion (6) is benzylisonitrile, resulting from the elimination of ethylene. Density functional theory (DFT) calculations predict the product distributions from the more stable biphenyl system 5 with reasonable accuracy. However, product distributions from the less stable benzyl system 6 are forecast with less accuracy.

Metal-free, photoredox-catalyzed aromatization-driven deconstructive functionalization of spiro-dihydroquinazolinones with α-CF3 alkenes

Metal-free, photoredox-catalyzed aromatization-driven deconstructive functionalization of spiro-dihydroquinazolinones with α-CF3 alkenes is presented. The readily available spiro-dihydroquinazolinones reacted efficiently with α-CF3 alkenes during photocatalysis to give the gem-difluoroallylated and the CF3-containing quinazolin-4(3H)-ones in good yields with excellent chemoselectivity. The selectivity depends on the electron effect of substituents in α-CF3 alkenes. A wide range of four-, five-, six-, seven-, eight- and twelve-membered spiro-dihydroquinazolinones were compatible with this transformation. The protocol is also characterized by the mild and redox-neutral reaction conditions, good functional group compatibility and excellent atom economy. Mechanistic studies revealed that the reaction proceeds via a radical pathway.

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.

Palladium-catalyzed C-C bond cleavage of N-cyclopropyl acylhydrazones

Despite their utility as directing groups, the C-C bond cleavage of cyclopropanes utilizing hydrazones has not been explored. Herein, Pd-catalyzed C-C bond cleavage reaction of N-cyclopropyl acylhydrazones, followed by cycloisomerization to yield pyrazoles, has been developed. The protocol enables the synthesis of various α-pyrazole carbonyl compounds, which have a potential of biological activity. Control experiments and DFT calculations suggest that β-carbon elimination of a stable 6-membered chelate palladium complex occurs, generating a conjugated azine as a reaction intermediate for the following cycloisomerization.

Photocatalytic O- to S-Rearrangement of Tertiary Cyclopropanols

Despite the importance of heteroatom-substituted cyclopropane derivatives in drug design and organic synthesis, cyclopropanethiols remain critically underexplored. Inspired by the wide use of the Newman-Kwart rearrangement to access valuable thiophenols from phenol feedstocks, we report the development of a photocatalytic approach for efficient ambient temperature aliphatic O- to S-rearrangement on tertiary cyclopropanol derivatives. After demonstrating that a range of cyclopropanethiols-that are difficult to access by other methods-can be obtained with this strategy, we show that these rearranged products can be easily hydrolyzed and further derivatized. We conclude this study with mechanistic findings that enabled an initial extension of this approach toward other classes of aliphatic alcohols.

Iron(II)-Catalyzed Radical [3 + 2] Cyclization of N-Aryl Cyclopropylamines for the Synthesis of Polyfunctionalized Cyclopentylamines

A facile iron(II)-catalyzed radical [3 + 2] cyclization of N-aryl cyclopropylamines with various alkenes to access the structurally polyfunctionalized cyclopentylamine scaffolds has been developed. Using low-cost FeCl2·4H2O as catalyst, N-aryl cyclopropylamines could be utilized to react with a wide range of alkenes including exocyclic/acyclic terminal alkenes, cycloalkenes, alkenes from the natural-occurring compounds (Alantolactone, Costunolide), and known drugs (Ibuprofen, l-phenylalanine, Flurbiprofen) to obtain a variety of cyclopentylamines fused with different useful motifs in generally good yields and diastereoselectivities. The highlight of this protocol is also featured by no extra oxidant, no base, EtOH as the solvent, gram-scale synthesis, and further diverse transformations of the synthetic products. More importantly, an iron(II)-mediated hydrogen radical dissociation pathway was proposed based on the mechanism research experiments.

Electrochemical Ring-Opening of Cyclopropylamides with Alcohols toward the Synthesis of 1,3-Oxazines

An electrochemical method is presented to construct 1,3-oxazines by the oxidative ring-opening of cyclopropylamides with alcohols. This method avoids the use of external oxidants and thus shows good functional group tolerance. The substrate scope covers primary, secondary, and tertiary alcohols as well as (hetero)aryl amide-substituted cyclopropanes.

Stereoselective Cyclopropanation of Multisubstituted Enesulfinamides: Asymmetric Construction of α-Tertiary Cyclopropylamine Derivatives Containing β-Quaternary Stereocenters

Enesulfinamides with α,β,β-trisubstitution undergo a Simmons-Smith reaction to yield multisubstituted cyclopropylamine derivatives with high stereocontrol. The resulting α-tertiary cyclopropylamine derivatives, which feature β-quaternary stereocenters bearing two electronically and sterically similar substituents (e.g., methyl and ethyl), are seldom achieved by using conventional methods. By adjusting the stereochemistry of the carbon-carbon double bond and/or sulfinyl group within the enesulfinamides, it is feasible to selectively produce four stereoisomers of the cyclopropylamines, each with different absolute configurations at the α- and β-carbons.

Diastereo- and Enantioselective Synthesis of Tetracyclic Cycloheptanols through (4+3) Annulation via C-C/C-H Activation Cascade

Transition metal-catalyzed annulations of four-membered rings via C-C activation are powerful tools to construct complex fused and bridged ring systems. Despite significant progress in (4+1), (4+2) and (4+4) annulations, the (4+3) annulation remains unexplored. Herein, we develop an asymmetric Rh-catalyzed intramolecular (4+3) annulation of α-arylalkene-tethered benzocyclobutenols for the synthesis of dihydrofuran-annulated dibenzocycloheptanols with two discontinuous chiral carbon centers via a C-C and C-H activation cascade. The reaction features excellent diastereo- and enantioselectivities and 100 % atom economy, and is applicable to late-stage modification of complex molecules.

Visible-light-induced synthesis of 2,4-disubstituted quinolines from o-vinylaryl isocyanides and oxime esters

A visible-light-induced radical cyclization reaction of o-vinylaryl isocyanides and oxime esters to access various 2,4-disubstituted quinolines was disclosed. Oxime esters were employed as acyl radical precursors via the carbon-carbon bond cleavage. It provided an effective way for the synthesis of 2-acyl-4-arlysubstituted quinolines under mild conditions and exhibited good functional group tolerance and substrate applicability.

Room-Temperature CuI-Catalyzed N-Arylation of Cyclopropylamine

A general and efficient CuI/N-carbazolyl-1H-pyrrole-2-carbohydrazide catalyst system was developed for the N-arylation of cyclopropylamine using aryl bromides at room temperature. Herein, 5 mol % CuI and 5 mol % of the ligand were used to synthesize N-aryl cyclopropylamines in moderate to excellent yields. This protocol was scaled up to produce the desired product at gram levels and has been generalized for C-N coupling between aryl bromides and amines at room temperature.

Carbon-Carbon Bond Cleavage for Late-Stage Functionalization

Late-stage functionalization (LSF) introduces functional group or structural modification at the final stage of the synthesis of natural products, drugs, and complex compounds. It is anticipated that late-stage functionalization would improve drug discovery's effectiveness and efficiency and hasten the creation of various chemical libraries. Consequently, late-stage functionalization of natural products is a productive technique to produce natural product derivatives, which significantly impacts chemical biology and drug development. Carbon-carbon bonds make up the fundamental framework of organic molecules. Compared with the carbon-carbon bond construction, the carbon-carbon bond activation can directly enable molecular editing (deletion, insertion, or modification of atoms or groups of atoms) and provide a more efficient and accurate synthetic strategy. However, the efficient and selective activation of unstrained carbon-carbon bonds is still one of the most challenging projects in organic synthesis. This review encompasses the strategies employed in recent years for carbon-carbon bond cleavage by explicitly focusing on their applicability in late-stage functionalization. This review expands the current discourse on carbon-carbon bond cleavage in late-stage functionalization reactions by providing a comprehensive overview of the selective cleavage of various types of carbon-carbon bonds. This includes C-C(sp), C-C(sp2), and C-C(sp3) single bonds; carbon-carbon double bonds; and carbon-carbon triple bonds, with a focus on catalysis by transition metals or organocatalysts. Additionally, specific topics, such as ring-opening processes involving carbon-carbon bond cleavage in three-, four-, five-, and six-membered rings, are discussed, and exemplar applications of these techniques are showcased in the context of complex bioactive molecules or drug discovery. This review aims to shed light on recent advancements in the field and propose potential avenues for future research in the realm of late-stage carbon-carbon bond functionalization.

Ligand-Metal Cooperation Enables Net Ring-Opening C-C Activation / Difunctionalization of Cyclopropyl Ketones

Reactions that cleave C-C bonds and enable functionalization at both carbon sites are powerful strategic tools in synthetic chemistry. Stereodefined cyclopropyl ketones have become readily available and would be an ideal source of 3-carbon fragments, but general approaches to net C-C activation / difunctionalization are unknown. Herein we demonstrate the cross-coupling of cyclopropyl ketones with organozinc reagents and chlorotrimethylsilane to form 1,3-difunctionalized, ring-opened products. A combination of experimental and theoretical studies rule out more established mechanisms and shed light on how cooperation between the redox-active terpyridine (tpy) ligand and the nickel atom enables the C-C bond activation step. The reduced (tpy•-)NiI species activates the C-C bond via a concerted asynchronous ring-opening transition state. The resulting alkylnickel(II) intermediate can then be engaged by aryl-, alkenyl-, and alkylzinc reagents to furnish cross-coupled products. This allows quick access to products that are difficult to make by conjugate addition methods, such as β-allylated and β -benzylated enol ethers. The utility of this approach is demonstrated in the synthesis of a key (±)-taiwaniaquinol B intermediate and the total synthesis of prostaglandin D1.

Rh-Catalyzed [4 + 1] Reaction of Cyclopropyl-Capped Dienes (but not Common Dienes) and Carbon Monoxide: Reaction Development and Mechanistic Study

Transition-metal-catalyzed [4 + 1] reaction of dienes and carbon monoxide (CO) is the most straightforward and easily envisioned cyclization for the synthesis of five-membered carbocycles, which are ubiquitously found in natural products and functional molecules. Unfortunately, no test of this reaction was reported, and consequently, chemists do not know whether such kind of reaction works or not. Herein, we report that the [4 + 1] reaction of common dienes and CO cannot work, at least under the catalysis of [Rh(cod)Cl]2. However, using cyclopropyl-capped dienes (also named allylidenecyclopropanes) as substrates, the corresponding [4 + 1] reaction with CO proceeds smoothly in the presence of [Rh(cod)Cl]2. This [4 + 1] reaction, with a broad scope, provides efficient access to five-membered carbocyclic compounds of spiro[2.4]hept-6-en-4-ones. The [4 + 1] cycloadducts can be further transformed into other molecules by using the unique chemistry of cyclopropyl groups present in these molecules. The mechanism of this [4 + 1] reaction has been investigated by quantum chemical calculations, uncovering that cyclopropyl-capped dienes are strained dienes and the oxidative cyclization step in the [4 + 1] catalytic cycle can release this (angular) strain both kinetically and thermodynamically. The strain release in this step then propagates to all followed CO coordination/CO insertion/reductive elimination steps in the [4 + 1] catalytic cycle, helping the realization of this cycloaddition reaction. In contrast, common dienes (including cyclobutyl-capped dienes) do not have such advantages and their [4 + 1] reaction suffers from energy penalty in all steps involved in the [4 + 1] catalytic cycle. The reactivity of ene-allenes for the [4 + 1] reaction with CO is also discussed.

Visible-light-induced reactions of methylenecyclopropanes (MCPs)

Diverse, visible-light-induced transformations of methylenecyclopropanes (MCPs) have been reported in recent years, attracting significant attention from synthetic chemists. As readily accessible strained molecules, MCPs have sufficient reactivity to selectively generate different target products, through reactions with various radical species upon visible-light irradiation under regulated reaction conditions. These transformations can be classified into three subcategories of reaction pathway, forming ring-opened products, cyclopropane derivatives, and alkynes. These products include pharmaceutical intermediates and polycyclic/heterocyclic compounds that are challenging to obtain using traditional methods. This review summarizes the recent advancements in this field.

P/N-Heteroleptic Cu(I)-Photosensitizer-Catalyzed [3 + 2] Regiospecific Annulation of Aminocyclopropanes and Functionalized Alkynes

We report here a regiospecific [3 + 2] annulation between aminocyclopropanes and various functionalized alkynes enabled by a P/N-heteroleptic Cu(I) photosensitizer under photoredox catalysis conditions. Thus, a divergent construction of 3-aminocyclopentene derivatives including methylsulfonyl-, arylsulfonyl-, chloro-, ester-, and trifluoromethyl-functionalized aminocyclopentenes could be achieved with advantages of high regioselectivity, broad substrate compatibility, and mild and environmentally benign reaction conditions.

Cyclopropane-Fused N-Heterocycles via Aza-Heck-Triggered C(sp3)-H Functionalization Cascades

Unique examples of aza-Heck-based C(sp³)–H functionalization cascades are described. Under Pd(0)-catalyzed conditions, the aza-Heck-type cyclization of N-(pentafluorobenzoyloxy)carbamates generates alkyl–Pd(II) intermediates that effect C(sp³)–H palladation en route to cyclopropanes. Key factors that control the site selectivity of the cyclopropanation process have been elucidated such that selective access to a wide range of ring- or spiro-fused systems can be achieved.

C-C Bond Activation of Cyclopropanes Enabled by Phosphine-Catalyzed In Situ Formation of High-Strain Methylenecycopropane Intermediate

An effective strategy for the ring-opening/elaboration of cyclopropanes by phosphine catalyst is documented, providing the 2,4-pentadiene sulfonamides and isoindolines in moderate to good yields. The key to the success of this reaction is phosphine-catalyzed introduction of a trigonal center into cyclopropanes, which results in the formation of higher ring strain cyclopropylidenemethyl phosphonium salt. Moreover, this methodology is employed as the key step for the synthesis of bioactive molecules.

Activation of aminocyclopropanes via radical intermediates

Aminocyclopropanes are versatile building blocks for accessing high value-added nitrogen-containing products. To control ring-opening promoted by ring strain, the Lewis acid activation of donor-acceptor substituted systems is now well established. Over the last decade, alternative approaches have emerged proceeding via the formation of radical intermediates, alleviating the need for double activation of the cyclopropanes. This tutorial review summarizes key concepts and recent progress in ring-opening transformations of aminocyclopropanes via radical intermediates, divided into formal cycloadditions and 1,3-difunctionalizations.

An endo-Directing-Group Strategy Unlocks Enantioselective (3+1+2) Carbonylative Cycloadditions of Aminocyclopropanes

An endo-directing group strategy enables enantioselective (3+1+2) cycloadditions that are triggered by carbonylative C-C bond activation of cyclopropanes. These processes are rare examples of cycloadditions where C-C bond oxidative addition is enantiodetermining, and the first where this is achieved within the context of a multicomponent (higher order) reaction design.

Visible-Light-Induced Decarboxylative Alkylation/Ring Opening and Esterification of Vinylcyclopropanes

A visible-light-induced four-component reaction of vinylcyclopropanes, N-(acyloxy)phthalimide esters, N,N-dimethylformamide (DMF), and H₂O through an oxidative ring opening of cyclopropane is presented. This procedure provides a new and effective way to construct formate esters. DMF is employed as both a solvent and the source of CHO. This difunctionalization of vinylcyclopropanes shows good functional group tolerance under room temperature. A radical pathway is involved, and carbonyl oxygen of ester originated from water in this transformation.

Why [4 + 2 + 1] but Not [2 + 2 + 1]? Why Allenes? A Mechanistic Study of the Rhodium-Catalyzed [4 + 2 + 1] Cycloaddition of In Situ Generated Ene-Ene-Allenes and Carbon Monoxide

Transition metal-catalyzed [4 + 2 + 1] cycloaddition of in situ generated ene/yne-ene-allenes (from ene/yne-ene propargyl esters) and carbon monoxide (CO) gives the [4 + 2 + 1] cycloadducts rather than [2 + 2 + 1] cycloadducts. Investigating the mechanism of this [4 + 2 + 1] reaction and understanding why the [2 + 2 + 1] reaction does not compete and the role of the allene moiety in the substrates are important. This is also helpful to guide the future design of new [4 + 2 + 1] cycloadditions. Reported here are the kinetic and computed studies of the [4 + 2 + 1] reactions of ene-ene propargyl esters and CO. A quantum chemical study (at the DLPNO-CCSD(T)//BMK level) revealed that the [4 + 2 + 1] reaction includes four key steps, which are 1,3-acyloxy migration (rate-determining step), oxidative cyclization, CO migratory insertion, and reductive elimination. The allene moiety in the substrates is critical for providing additional coordination to the rhodium center in the final step of the catalytic cycle, which in turn favors the reductive elimination transition state in the [4 + 2 + 1] rather than in the [2 + 2 + 1] pathway. The CO insertion step in the [4 + 2 + 1] reaction, which could occur through either the UP (favored here) or DOWN CO insertion pathway, has also been deeply scrutinized, and some guidance from this analysis has been provided to help the future design of new [4 + 2 + 1] reactions. Quantum chemical calculations have also been applied to explain why [4 + 2] and [4 + 1] cycloadditions do not happen and how trienes as side products for some substrates are generated.

Phosphine-Catalyzed Enantioselective (3+2) Annulation of Vinylcyclopropanes with Imines for the Synthesis of Chiral Pyrrolidines

A phosphine-catalyzed highly enantioselective and diastereoselective (up to 98 % ee and >20 : 1 dr) (3+2) annulation between vinylcyclopropanes and N-tosylaldimines has been developed, which allows facile access to a range of highly functionalized chiral pyrrolidines. Notably, this method makes use of vinylcyclopropanes as a synthon for phosphine-mediated asymmetric annulation reaction, which will offer new opportunities for potential applications of cyclopropanes substrates in phosphine-catalyzed organic transformations.

Palladium-Catalyzed Stagewise Strain-Release-Driven C-C Activation of Bicyclo[1.1.1]pentanyl Alcohols

A palladium-catalyzed chemoselective coupling of readily available bicyclo[1.1.1]pentanyl alcohols (BCP-OH) with various halides is reported, which offers expedient approaches to a wide range of cyclobutanone and β,γ-enone skeletons via single or double C-C activation. The chemistry shows a broad substrate scope in terms of both the range of BCP-OH and halides including a series of natural product derivatives. Moreover, dependency of reaction chemodivergence on base additive has been investigated through experimental and density functional theory (DFT) studies, which is expected to significantly enrich the reaction modes and increase the synthetic potential of BCP-OH in preparing more complex molecules.

Visible-Light-Accelerated Copper-Catalyzed [3 + 2] Cycloaddition of N-Tosylcyclopropylamines with Alkynes/Alkenes

Copper-catalyzed [3 + 2] cycloadditions of N-tosylcyclopropylamine with alkynes and alkenes have been accomplished under visible light irradiation. The developed approach is compatible with a range of functionalities and allows the synthesis of diversified aminated cyclopentene and cyclopentane derivatives being relevant for drug synthesis. The protocol is operationally simple and economically affordable as it does not require any ligand, base, or additives. As the key step, the one-electron oxidation of the N-tosyl moiety by visible light-induced homolysis of a transient Cu(II)-tosylamide complex is proposed, providing a facile entry for N-centered radicals.

Electricity mediated [3+2]-cycloaddition of N-sulfonylcyclopropanes with olefins via N-centered radical intermediates: access to cyclopentane analogs

An external oxidant free electrochemical strategy is designed towards the β-scission of strained C-C bonds in cyclopropylamine. Moreover, the mechanistic studies ascertained that the methodology encompasses the N-center radical (NCRs) route and provides access to di- or tri-substituted cyclopentane analogs.

Ti-Catalyzed Diastereoselective Cyclopropanation of Carboxylic Derivatives with Terminal Olefins

Cyclopropanols and cyclopropylamines not only serve as important structural motifs in medicinal chemistry but also show diverse reactivities in organic synthesis. Owing to the high ring strain energy, the development of a general protocol from stable and readily available starting materials to afford these cyclopropyl derivatives remains a compelling challenge. Herein, we describe that a Ti-based catalyst can effectively promote the diastereoselective syntheses of cyclopropanols and cyclopropylamines from widely accessible carboxylic derivatives (acids, esters, amides) with terminal olefins. To the best of our knowledge, this method represents the first example of direct converting alkyl carboxylic acids into cyclopropanols. Distinct from conventional studies in Ti-mediated cyclopropanations with reactive alkyl Grignard reagents as nucleophiles or reductants, this protocol utilizes Mg and Me₂SiCl₂ to turn over the Ti catalyst. Our method exhibits broad substrate scope with good functional group compatibility and is amenable to late-stage synthetic manipulations of natural products and biologically active molecules.