Copper-Catalyzed Functionalization of 1,3-Dienes: Hydrofunctionalization, Borofunctionalization, and Difunctionalization

Enantio- and Diastereoselective Copper-Catalyzed Borylative Coupling of Styrenes and Azadienes

Here we disclose a CuB-catalyzed reaction between aurone-derived α,β-unsaturated imines and styrenes to produce 2-substituted benzofuran derivatives bearing both the γ-boryl functionality and α,β-unsymmetric stereogenic centers. The reaction represents the first transition-metal-catalyzed unsymmetric 1,4-Michael additions of azadienes, which would enrich the arsenal of CuB catalysis in organic synthesis. In addition, the synthetically versatile boron-alkylated products can be elaborated by chemical transformations to useful optically active benzofuran heterocycles.

Ligand-Controlled Cobalt-Catalyzed Regiodivergent and Stereoselective Ring-Opening Isomerization of Vinyl Cyclopropanes

A ligand-controlled regiodivergent and stereoselective ring-opening isomerization of vinylcyclopropane was developed with cobalt catalysis. Employing the commercially available Xantphos ligand, the reactions afforded exclusively linear-type 1,3-dienes as the products. Interestingly, when switching the ligand to an amido-diphosphine ligand (PNP), branched-type 1,3-dienes were obtained with high regioselectivity and stereoselectivity. Preliminary mechanistic investigations suggested that a π-allyl metal and a metal-hydride species are involved as key intermediates in the two transformations, respectively.

Access to Cyclic Borates by Cu-Catalyzed Borylation of Unactivated Vinylcyclopropanes

We report the copper-catalyzed borylation of unactivated vinylcyclopropanes to form six-membered cyclic borate salts. A copper complex bearing an N-heterocyclic ligand in combination with bis(pinacolato)diboron and LiOtBu catalyzes the ring-opening of the substrate under mild reaction conditions. The protocol can be applied to aryl- and heteroaryl-substituted vinylcyclopropanes and can be conducted on a gram scale. The synthetic utility of the lithium salts of the cyclic borate has been demonstrated through regioselective ring-opening functionalizations.

Hydrogen Source Tuned Regiodivergent Asymmetric Hydroalkylations of 2-Substituted 1,3-Dienes with Aldehydes by Cobalt-Catalysis

Catalytic methods allowing for the reliable prediction and control of diverse regioselectivity along with the control of enantioselectivity to access different regio- and enantiomers by switching the least reaction parameters are one of the most attractive ways in organic synthesis, which provide access to diverse enantioenriched architectures from identical starting materials. Herein, a Co-catalyzed regiodivergent and enantioselective reductive hydroalkylation of 1,3-dienes with aldehydes has been achieved, furnishing different enantioenriched homoallylic alcohol architectures in good levels of enantioselectivity. The reaction features the switch of regioselectivity tuned by the selection of proton source. The use of an acid as proton source provided asymmetric 1,2-hydroalkylation products under reductive conditions, yet asymmetric 4,3-hydroalkylation products were obtained with silane as hydride source. This catalytic protocol allows for the access of homoallylic alcohols with two continuous saturated carbon centers in good levels of regio-, diastereo-, and enantioselectivity.

Understanding the Regiodivergence between Hydroarylation and Trifluoromethylarylation of 1,3-Dienes Using Anilines in HFIP

Conjugated dienes (1,3-dienes) are versatile and valuable chemical feedstocks that can be used as two-carbon or four-carbon synthons with vast applications across the chemical industry. However, the main challenge for their productive incorporation in synthetic routes is their chemo-, regio-, and stereoselective functionalization. Herein, we introduce a unified strategy for the 1,2-hydroarylation and 1,4-trifluoromethylarylation of 1,3-dienes using anilines in hexafluoroisopropanol. DFT calculations point toward a kinetically controlled process in both transformations, particularly in the trifluoromethylarylation, to explain the regiodivergent outcome. In addition, we perform an extensive program of functionalization and diversification of the products obtained, including hydrogenation, oxidation, cyclizations, or cross-coupling reactions, that allows access to a library of high-value species in a straightforward manner.

Enantioselective and Regiodivergent Gold and Chiral Brønsted Acid Catalyzed Cycloisomerization/Diels-Alder Reaction of 1,10-Dien-4-yn-3-yl Acetates: Synthesis of Norbornene-Embedded Tricarbocycles

A synthetic method for the enantioselective and regiodivergent synthesis of hexahydro-2H-2,4a-methanonaphthalen-4-yl and octahydro-2,4-methanoazulen-1-yl esters that relies on the gold(I)- and chiral Brønsted acid-catalyzed cycloisomerization/Diels-Alder (CDA) reaction of (E)-1,10-dien-4-yn-3-yl acetates is described.

Copper-Catalyzed 1,2-Sulfonyletherification of 1,3-Dienes

A selective three-component 1,2-sulfonyl etherification of aryl 1,3-dienes enabled by copper catalysis to afford biologically interesting alkenyl 1,2-sulfone ether derivatives through C-S and C-O bond formation is described. The protocol proceeds with the sulfonyl chloride and alcohols under simple, mild, and base-free conditions, providing a straightforward route to sulfonylated allyl ether compounds with broad functional group tolerance and excellent chemo- and regioselectivity. Mechanistic studies indicate that the selective alkene difunctionalization includes a key copper-mediated single-electron transfer process.

Theoretical Insight into the Palladium-Catalyzed Prenylation and Geranylation of Oxindoles with Isoprene

This work presents a comprehensive mechanistic study of the ligand-controlled palladium-catalyzed prenylation (with C5 added) and geranylation (with C10 added) reactions of oxindole with isoprene. The calculated results indicate that the prenylation with the bis-phosphine ligand and geranylation with the monophosphine ligand fundamentally share a common mechanism. This mechanism involves the formation of two crucial species: a η3-allyl-Pd(II) cation and an oxindole carbon anion. Furthermore, the reactions necessitate the assistance of a second oxindole molecule, which serves as a Brønsted acid, providing a proton to generate the oxindole nitrogen anion. The oxindole nitrogen anion then acts as a Brønsted base, abstracting a C-H proton from another oxindole molecule to form an oxindole carbon anion. These mechanistic details differ significantly from those proposed in the experimental work. The present calculations do not support the presence of the Pd-H species and the η3, η3-diallyl-Pd(II) intermediate, which were previously suggested in experiments. The theoretical results rationalize the experimental finding that the bis-phosphine ligand favors the prenylation of oxindole, while the monophosphine ligand enables the geranylation of oxindole.

Cobalt-Catalyzed Enantioselective Hydroboration of α-Substituted Acrylates

Even though metal-catalyzed enantioselective hydroborations of alkenes have attracted enormous attention, few preparatively useful reactions of α-alkyl acrylic acid derivatives are known, and most use rhodium catalysts. No examples of asymmetric hydroboration of the corresponding α-arylacrylic acid esters are known. In our continuing efforts to search for new applications of earth-abundant cobalt catalysts for broadly applicable organic transformations, we have identified 2-(2-diarylphosphinophenyl)oxazoline ligands and mild reaction conditions for efficient and highly regio- and enantioselective hydroboration of α-alkyl- and α-aryl- acrylates, giving β-borylated propionates. Since the C-B bonds in these compounds can be readily replaced by C-O, C-N, and C-C bonds, these intermediates could serve as valuable chiral synthons, some from feedstock carbon sources, for the synthesis of propionate-bearing motifs including polyketides and related molecules. Two-step syntheses of "Roche" ester from methyl methacrylate (79%; er 99:1), arguably the most widely used chiral fragment in polyketide synthesis, and tropic acid esters (∼80% yield; er ∼93:7), which are potential intermediates for several medicinally important classes of compounds, illustrate the power of the new methods. Mechanistic studies confirm the requirement of a cationic Co(I) species [(L)Co]+as the viable catalyst in these reactions and rule out the possibility of a [L]Co-H-initiated route, which has been well-established in related hydroborations of other classes of alkenes. A mechanism involving an oxidative migration of a boryl group to the β-carbon of an η4-coordinated acrylate-cobalt complex is proposed as a plausible route.

Regiodivergent Radical Termination for Intermolecular Biocatalytic C-C Bond Formation

Radical hydrofunctionalizations of electronically unbiased dienes are challenging to render regioselective, because the products are nearly identical in energy. Here, we report two engineered FMN-dependent "ene"-reductases (EREDs) that catalyze regiodivergent hydroalkylations of cyclic and linear dienes. While previous studies focused exclusively on the stereoselectivity of alkene hydroalkylation, this work highlights that EREDs can control the regioselectivity of hydrogen atom transfer, providing a method for selectively preparing constitutional isomers that would be challenging to prepare using traditional synthetic methods. Engineering the ERED from Gluconabacter sp. (GluER) furnished a variant that favors the γ,δ-unsaturated ketone, while an engineered variant from a commercial ERED panel favors the δ,ε-unsaturated ketone. The effect of beneficial mutations has been investigated using substrate docking studies and the mechanism probed by isotope labeling experiments. A variety of α-bromo ketones can be coupled with cyclic and linear dienes. These interesting building blocks can also be further modified to generate difficult-to-access heterocyclic compounds.

Cooperative Cu/Pd-Catalyzed 1,5-Boroacylation of Cyclopropyl-Substituted Alkylidenecyclopropanes

A Cu/Pd-cocatalyzed 1,5-boroacylation of cyclopropyl-substituted ACPs with B2pin2 and acid chlorides has been developed. Using cyclopropyl-substituted ACPs as the starting material, a broad range of 1,5-boroacylated products with multiple functional groups was prepared in good yields with excellent regio- and stereoselectively. Both aromatic and aliphatic acid chlorides were tolerated in this reaction.

Electrochemical Reductive Cross-Coupling of Vinyl Bromides for the Synthesis of 1,3-Dienes

An electroreductive cross-electrophile coupling protocol was developed for the construction of valuable 1,3-dienes from vinyl bromides. Furthermore, this scalable method can also be used to forge complex [4 + 2] cycloadducts in a one-pot manner. One of the most important advantages of this green and sustainable protocol is the in situ release of nickel catalyst from the inexpensive electrodes without the addition of extra harmful metal catalysts and reductant.

Synthesis of highly substituted 1,3-dienes through halonium promoted 1,2-sulfur migration of propargylic thioethers

Conjugated 1-bromo or 1-iodo-1,3-dienes bearing a sulfide substituent have been synthesized via 1,2-sulfur migration from propargylic thioethers upon activation with NIS or NBS. The reaction generally proceeds with high control over the regio- and diastereoselectivity. Highly substituted thiophenes and selenophenes are easily obtained from the generated dienes.

Cu-Catalyzed Asymmetric Dicarboxylation of 1,3-Dienes with CO2

Dicarboxylic acids and derivatives are important building blocks in organic synthesis, biochemistry, and the polymer industry. Although catalytic dicarboxylation with CO2 represents a straightforward and sustainable route to dicarboxylic acids, it is still highly challenging and limited to generation of achiral or racemic dicarboxylic acids. To date, catalytic asymmetric dicarboxylation with CO2 to give chiral dicarboxylic acids has not been reported. Herein, we report the first asymmetric dicarboxylation of 1,3-dienes with CO2 via Cu catalysis. This strategy provides an efficient and environmentally benign route to chiral dicarboxylic acids with high regio-, chemo-, and enantioselectivities. The copper self-relay catalysis, that is, Cu-catalyzed boracarboxylation of 1,3-dienes to give carboxylated allyl boronic ester intermediates and subsequent carboxylation of C-B bonds to give dicarboxylates, is key to the success of this dicarboxylation. Moreover, this protocol exhibits broad substrate scope, good functional group tolerance, easy product derivatizations, and facile synthesis of chiral liquid crystalline polyester and drug-like scaffolds.

Cu-Catalyzed Divergent Transformations of Allenylethylene Carbonates with Diboron Reagents

Divergent transformations of allenylethylene carbonates with diboron reagents catalyzed by copper are disclosed. By using CuCl/IPr·HCl as the catalyst, the allenylethylene carbonates react with B2hex2 to afford 2,4-dien-1-ols as the product in the presence of Cs2CO3 as the base, iPrOH as the additive, and 1,4-dioxane as the solvent. And they react with B2pin2 to form boronic half acids in the presence of NaOtBu as the base, water as the additive, and THF as the solvent. The reactions afford corresponding products in good stereoselectivities and yields, and further derivatizations of boronic half acids and study of the mechanism are also demonstrated.

Enantioenriched Allylesters via a Copper-Catalyzed Diene Carboesterification with Alkyltrifluoroborates and Carboxylic Acids

The rapid synthesis of a range of enantioenriched allylic esters is enabled by a new 3-component catalytic enantioselective 1,2-carboesterification of readily available dienes with carboxylic acids and potassium alkyltrifluoroborates. The chiral copper catalyst, formed in situ from Cu(OTf)2 and (4S,4'S)-2,2'-(cyclopentane-1,1-diyl)bis(4-phenyl-4,5-dihydrooxazole), is implicated in both the generation of alkyl radicals from the alkyltrifluoroborates as well as the enantioselective formation of C-O bonds. Potassium salts of primary and secondary alkyltrifluoroborates as well as several benzylic trifluoroborates, tert-butyltrifluoroborate, and phenyltrifluoroborate participate in the reaction. The regioselectivity and enantioselectivity are strongly impacted by variations in all of the reaction components, which in turn are thought to impact the C-O bond-forming reductive elimination from a [Cu(III)] intermediate.

Copper(I)-Catalyzed Asymmetric Allylation of Ketones with 2-Aza-1,4-Dienes

Catalytic asymmetric allylation of ketones under proton-transfer conditions is a challenging issue due to the limited pronucleophiles and the electrophilic inertness of ketones. Herein, a copper(I)-catalyzed asymmetric allylation of ketones with 2-aza-1,4-dienes (N-allyl-1,1-diphenylmethanimines) is disclosed, which affords a series of functionalized homoallyl tertiary alcohols in high to excellent enantioselectivity. Interestingly, N-allyl-1,1-diphenylmethanimines work as synthetic equivalents of propanals. Upon the acidic workup, a formal asymmetric β-addition of propanals to ketones is achieved. An investigation on KIE effect indicates that the deprotonation of N-allyl-1,1-diphenylmethanimines is the rate-determining step, which generates nucleophilic allyl copper(I) species. Finally, the synthetic utility of the present method is demonstrated by the asymmetric synthesis of (R)-boivinianin A and (R)-gossonorol.

Electrochemical Nickel-Catalyzed 1,2-Diarylation of 1,3-Dienes

Due to the presence of carbon-carbon double bonds, 1,3-dienes exhibit great reactivity. A protocol for the site-selective diarylation of terminal 1,3-dienes is reported here. The transformation is facilitated by the Ni catalyst without the need for additional ligands, utilizing an electrochemical setup. Preliminary results indicate that by introducing chiral ligands moderate enantioselective diarylation products can be obtained. This method affords diversely substituted diarylated products that occur as structural motifs in various natural products.

Copper-Catalyzed Umpolung Reactivity of Propargylic Carbonates in the Presence of Diboronates: One Stone Four Birds

Allylation and propargylation are two powerful synthetic strategies for making new substances that have been of significant importance in chemistry, medicine, and material fields. Conventional tactics employ various preformed allylation and propargylation reagents. In this study, a conceptually novel copper-catalyzed and B2pin2-mediated Umpolung reactivity of propargylic carbonates has been achieved for the first time, realizing both allylation and propargylation of aldehydes and ketones without additional reductants. Three types of allylation products and one type of propargylation product are generated efficiently, and all allylation products are formed with syn-configurations predominantly. The choice of ligands plays a vital role in modulating the Umpolung modes. The synthetic applications have been demonstrated in a myriad of further transformations including natural product synthesis, and systematic mechanistic studies have been conducted to reveal detailed insights into the Umpolung processes.

Palladium-Catalyzed Regiodivergent Three-Component Alkenylamination of 1,3-Dienes with Alkyl and Aryl Amines

We report a palladium-catalyzed method for 4,3- or 4,1-selective alkenylamination of terminal dienes. Three-component couplings proceed with alkenyl triflates and several amines, giving vicinal carboamination with a Xantphos-supported catalyst and distal difunctionalization with a phosphoramidite ligand. A number of constitutionally different disubstituted dienes also participate in regiodivergent carboaminations. Experimental evidence indicates that selectivity in the Xantphos reactions is largely influenced by the substrate, whereas the phosphoramidite-promoted process is catalyst controlled, orchestrated by a key π-stacking interaction among the ligand, solvent, and substrate.

Palladium-Catalyzed Aminoalkylative Cyclization Enables Modular Synthesis of Exocyclic 1,3-Dienes

A novel and efficient palladium-catalyzed regioselective and stereodivergent ring-closing reaction of aminoenynes with aldehydes and boronic acids or hydrosilane is developed. This three-component reaction allows for the modular synthesis of a series of exocyclic 1,3-dienes bearing 5- to 8-membered saturated N-heterocycles. The reactions utilize a simple Pd-catalyst and work with broad range of aminoenynes, aldehydes and organometallic reagents under mild reaction conditions. The products represent useful intermediates for chemical synthesis due to the versatility of the conjugated diene. Preliminary mechanistic details of the method are also revealed.

Photoinduced Cobalt Catalysis for the Reductive Coupling of Pyridines and Dienes Enabled by Paired Single-Electron Transfer

Selective hydroarylation of dienes has potential to provide swift access to useful building blocks. However, most existing methods rely on dienes stabilised by an aromatic group and transmetallation or nucleophilic attack steps require electron-rich aryl coupling partners. As such, there are few examples which tolerate wide-spread heteroarenes such as pyridine. Whilst allylic C-H functionalisation could be considered an alternative approach, the positional selectivity of unsymmetrical substrates is hard to control. Here, we report a general approach for selective hydropyridylation of dienes under mild conditions using metal catalysed hydrogen-atom transfer. Photoinduced, reductive conditions enable simultaneous formation of a cobalt-hydride catalyst and the persistent radical of easily-synthesised pyridyl phosphonium salts. This facilitates selective coupling of dienes in a traceless manner at the C4-position of a wide-range of pyridine substrates. The mildness of the method is underscored by its functional-group tolerance and demonstrated by applications in late-stage functionalisation. Based on a combination of experimental and computational studies, we propose a mechanistic pathway which proceeds through non-reversible hydrogen-atom transfer (HAT) from a cobalt hydride species which is uniquely selective for dienes in the presence of other olefins due to a much higher relative barrier associated with olefin HAT.

Switch in Selectivities by Dinuclear Nickel Catalysis: 1,4-Hydroarylation of 1,3-Dienes to Z-Olefins

One of the most challenging tasks in organic synthesis is to control selectivities, especially switching the well-known selectivity to obtain new isomers that were previously inaccessible. Inspired by biological catalysis involving multiple metal centers, catalysis enabled by binuclear metal complexes offers the potential to induce reactivity and selectivity that might not be available to mononuclear catalysts. Herein, we describe that using a macrocyclic bis pyridyl diimine dinickel complex as the catalyst, the commonly observed 4,3-regioselectivity of hydroarylation of 1,3-dienes is switched to 1,4-hydroarylation with thermodynamically less stable Z-stereoselectivity, offering challenging synthetic target Z-olefins. DFT calculations show that the activation of 1,3-diene proceeds through dinuclear Ni-diolefin coordination, and the synergistic effects of two Ni nuclei enable reactivity and selectivity of this binuclear catalysis substantially different from those of mononuclear nickel complexes in the current reaction.

Rh(II)/Pd(0) Dual-Catalyzed Regio-Divergent Three-Component Propargylic Substitution

Regio-divergent propargylic substitution to generate functionally diverse products from identical starting materials remains a formidable challenge, probably due to the unpredictable regiochemical complexity. In practically, the synthesis of α-quaternary propargylic-substituted products is still much less developed, and preprepared nucleophiles are generally applied in this type of reaction with propargylic substrates, which limits the reaction efficiency and diversity of the obtained products. Herein, we disclose unprecedented three-component propargylic substitution of α-diazo esters with amines and propargylic carbonates under dirhodium/palladium dual catalysis. The key to the success of this multicomponent propargylic substitution is to avoid two-component side reactions through a tandem process of dirhodium(II)-catalyzed carbene insertion and palladium-catalyzed regiodivergent propargylic substitution. The judicious selection of a diphosphine (dppf) or monophosphine (tBuBrettphos) as the ligand is crucial for the reaction to generate different products in a switchable way, α-quaternary 1,3-dienyl or propargylated products, with high regio- and chemoselectivities.

Copper-Catalyzed Asymmetric Acylboration of 1,3-Butadienylboronate with Acyl Fluorides

We report herein a Cu-catalyzed regio-, diastereo- and enantioselective acylboration of 1,3-butadienylboronate with acyl fluorides. Under the developed conditions, the reactions provide (Z)-β,γ-unsaturated ketones bearing an α-tertiary stereocenter with high Z-selectivity and excellent enantioselectivities. While direct access to highly enantioenriched E-isomers was not successful, we showed that such molecules can be synthesized with excellent E-selectivity and optical purities via Pd-catalyzed alkene isomerization from the corresponding Z-isomers. The orthogonal chemical reactivities of the functional groups embedded in the ketone products allow for diverse chemoselective transformations, which provides a valuable platform for further derivatization.

Asymmetric formal sp2-hydrocarbonations of dienes and alkynes via palladium hydride catalysis

Transition metal-catalyzed asymmetric hydrofunctionalizations of unsaturated bonds via π-ƞ3 substitution have emerged as a reliable method to construct stereogenic centers, and mainly rely on the use of heteroatom-based or carbon nucleophiles bearing acidic C-H bonds. In comparison, sp2 carbon nucleophiles are generally not under consideration because of enormous challenges in cleaving corresponding inert sp2 C-H bonds. Here, we report a protocol to achieve asymmetric formal sp2 hydrocarbonations, including hydroalkenylation, hydroallenylation and hydroketenimination of both 1,3-dienes and alkynes via hydroalkylation and Wittig reaction cascade. A series of unachievable motifs via hydrofunctionalizations, such as di-, tri- and tetra-substituted alkenes, di-, tri- and tetra-substituted allenes, and tri-substituted ketenimines in allyl skeletons are all facilely constructed in high regio-, diastereo- and enantioselectivities with this cascade design. Stereodivergent synthesis of all four stereoisomers of 1,4-diene bearing a stereocenter and Z/E-controllable olefin unit highlights the power of present protocol. An interesting mechanistic feature is revealed that alkyne actually undergoes hydrocarbonation via the formation of conjugated diene intermediate, different from conventional viewpoint that the hydrofunctionalization of alkynes only involves allene species.

Cu/Pd-catalyzed arylboration of a 1-silyl-1,3-cyclohexadiene for stereocontrolled and diverse cyclohexane/ene synthesis

The synthesis and Cu/Pd-catalyzed arylboration of 1-silyl-1,3-cyclohexadiene is described. This diene is significant as it allows for synthesis of polyfunctional cyclohexane/enes. To achieve high levels of diastereoselectivity, the use of a pyridylidene Cu-complex was employed. In addition, through the use of a chiral catalyst, an enantioselective reaction was possible. Due to the presence of the silyl and boron substituents, the products can be easily diversified into a range of valuable cyclohexane/ene products.

Iso-Pentadienyl Carbonate as a Five Carbon Synthon in Manganese(I)-Catalyzed Selective Linear 1,3-Dienylation

Selective linear 1,3-dienylations are essential transformations, and numerous synthetic efforts have been documented. However, a general method enabling access to electron-rich, -poor, and biologically relevant dienyl molecules is in high demand. Hence, we report a straightforward method of manganese(I)-catalyzed C-H dienylation of arenes by using iso-pentadienyl carbonate as a five carbon synthon. This is a highly unprecedented report for selective linear 1,3-dienylation using manganese C-H activation catalysis. Our method facilitates the synthesis of varieties of dienes, including those suitable for normal or inverse electron demand Diels-Alder reactions, dienyl glycoconjugates, and unnatural amino acids. Extensive mechanistic studies, including isolation of C-H activated organo-manganese complex and isotopic analyses, have supported the proposed mechanism of this dienylation. The synthetic applicability of this method eased to deliver a 6/6/5-fused tricyclic nagilactone scaffold.

Cu-Catalyzed Enantioselective Protoboration of 2,3-Disubstituted 1,3-Dienes

A Cu-catalyzed regio- and enantioselective protoboration of 2,3-disubstituted 1,3-dienes is described. The protocol operates under mild conditions and is applicable to symmetrically and unsymmetrically substituted dienes, providing access to homoallylic boronates in consistently high yield, regioselectivity, and enantiomeric ratio. Preliminary investigations point to a complex mechanism.

Conceptual advances in nucleophilic organophosphine-promoted transformations

Catalysis by trivalent nucleophilic organophosphines has emerged as an essential tool in organic synthesis. Several new organic transformations promoted by phosphines substantiate and complement the existing synthetic chemistry tools. Mere design of the substrate and reagent combinations has introduced new modes of reactivity patterns, which are otherwise difficult to achieve. These design considerations have led to the rapid build-up of complex molecular entities and laid a solid foundation to synthesise bioactive natural products and pharmaceuticals. This article presents an overview of some of the conceptual advances, including our contributions to nucleophilic organophosphine chemistry. The scope, limitations, mechanistic insights, and applications of these metal-free transformations are discussed elaborately.

Asymmetric Syntheses of (Z)- or (E)-β,γ-Unsaturated Ketones via Silane-Controlled Enantiodivergent Catalysis

Cu-catalyzed highly stereoselective and enantiodivergent syntheses of (Z)- or (E)-β,γ-unsaturated ketones from 1,3-butadienyl silanes are developed. The nature of the silyl group of the dienes has a significant impact on the stereo- and enantioselectivity of the reactions. Under the developed catalytic systems, the reactions of acyl fluorides with phenyldiemthylsilyl-substituted 1,3-diene gave (Z)-β,γ-unsaturated ketones bearing an α-tertiary stereogenic center with excellent enantioselectivities and high Z-selectivities, where the reactions with triisopropylsilyl-substituted 1,3-butadiene formed (E)-β,γ-unsaturated ketones with high optical purities and excellent E-selectivities. The products generated from the reactions contain three functional groups with orthogonal chemical reactivities, which can undergo a variety of transformations to afford synthetically valuable intermediates.

Rhodium-Catalyzed Enantioselective 1,4-Oxyamination of Conjugated gem-Difluorodienes via Coupling with Carboxylic Acids and Dioxazolones

The incorporation of fluorine atoms in organics improves their bioactivity and lipophilicity. Catalytic functionalization of gem-difluorodienes represents one of the most straightforward approaches to access fluorinated alkenes. In contrast to the regular 1,3-dienes that undergo diverse asymmetric di/hydrofunctionalizations, the regio- and enantioselective oxyamination of gem-difluorodienes remains untouched. Herein, we report asymmetric 1,4-oxyamination of gem-difluorodiene by chiral rhodium-catalyzed three-component coupling with readily available carboxylic acid and dioxazolone, affording gem-difluorinated 1,4-amino alcohol derivatives. Our asymmetric protocol exhibits high 1,4-regio- and enantioselectivity with utility in the late-stage modification of pharmaceuticals and natural products. Stoichiometric experiments provide evidences for the π-allylrhodium pathway. Related oxyamination was also realized when trifluoroethanol was used as an oxygen nucleophile.

Diastereodivergent and Enantioselective Synthesis of Homoallylic Alcohols via Nickel-Catalyzed Borylative Coupling of 1,3-Dienes with Aldehydes

We present the first enantioselective nickel-catalyzed borylative coupling of 1,3-dienes with aldehydes, providing an efficient route to highly valuable homoallylic alcohols in a single step. The reaction involves the 1,4-carboboration of dienes, leading to the formation of C-C and C-B bonds accompanied by the construction of two continuous stereogenic centers. Enabled by a chiral spiro phosphine-oxazoline nickel complex, this transformation yields products with exceptional diastereoselectivity, E-selectivity, and enantioselectivity. The diastereoselectivity of the reaction can be controlled by employing either (Z)-1,3-dienes or (E)-1,3-dienes.

Regiodivergent and Stereoselective Synthesis of Highly Substituted 1,3-Dienes via Arylative Acyloxy Migration of Propargyl Esters

We report the first catalyst-controlled regiodivergent method that enables the synthesis of structurally diverse 1,2,3,4-tetrasubstituted conjugated dienes with excellent regio- and stereochemical outcomes from the same set of readily available propargyl esters and diaryliodonium salts. In this diene chemistry, the in situ generated, highly electrophilic aryl-CuIII complex serves not only as a π-Lewis acid catalyst for alkyne activation/acyloxy migration but also as an aryl electrophile equivalent. The competitive arylative 1,2- and 1,3-acyloxy migration patterns are exquisitely dictated by Cu and Au/Cu relay catalyses, respectively, providing a modular and attractive approach to traditionally inaccessible tetrasubstituted 1,3-dienes in a regiodivergent manner. Finally, the synthetic utility of this method is demonstrated by further synthetic derivatization of 1,3-dienes into an array of useful compounds.

Copper-Catalyzed Regio- and Stereoselective Three-Component Coupling of Allenyl Ethers with gem-Dichlorocyclobutenones and B2pin2

The three-component coupling method for regio- and stereoselective difunctionalization of allenes with allenyl ethers, bis(pinacolato)diboron, and gem-dichlorocyclobutenones as electrophiles was reported, yielding a variety of highly functionalized cyclobutenone products tethering with an alkenylborate fragment. The polysubstituted cyclobutenone products also underwent diverse transformations.

Copper-catalyzed synthesis of β-boryl cyclopropanes via 1,2-borocyclopropanation of aryl olefins with CO as the C1 source

Cyclopropane represents one of the most critical rings and has been found present in various bioactive compounds, especially in clinical medicines. It can be synthesized by the reaction of olefins with diazo-derived carbenoids which are potentially hazardous. Carbonylation is a powerful tool for synthesizing carbonylated or carbon-extended compounds. In this communication, we describe a straightforward approach for synthesizing β-boryl cyclopropane derivatives catalyzed by an inexpensive copper catalyst with CO as the C1 source. This reaction was mediated by an in situ generated carbene intermediate and afforded a wide range of cyclopropane-containing organoboron compounds in moderate to good yields.

Palladium-Catalyzed 1,1-Alkynyloxygenation of 2-Vinylbenzoates with Alkynyl Bromides

The palladium-catalyzed reaction of alkyl 2-vinylbenzoates with silyl-protected alkynyl bromides leads to the selective production of 3-alkynylated isochroman-1-ones. The use of an alkyl ester group as an effective oxygen nucleophile is crucial for the efficient 1,1-alkynyloxygenation of alkenes.

Three-Component Electrochemical Aminoselenation of 1,3-Dienes

Azoles and organoselenium compounds are pharmacologically important scaffolds in medicinal chemistry and natural products. We developed an efficient regioselective electrochemical aminoselenation reaction of 1,3-dienes, azoles, and diselenide derivatives to access selenium-containing allylazoles skeletons. This protocol is more economical and environmentally friendly and features a broad substrate scope; pyrazole, triazole, and tetrazolium were all tolerated under the standard conditions, which could be applied to the expedient synthesis of bioactive molecules and in the pharmaceutical industry.

Ligand Control in Co-Catalyzed Regio- and Enantioselective Hydroboration: Homoallyl Secondary Boronates via Uncommon 4,3-Hydroboration of 1,3-Dienes

Enantiopure homoallylic boronate esters are versatile intermediates because the C-B bond in these compounds can be stereospecifically transformed into C-C, C-O, and C-N bonds. Regio- and enantioselective synthesis of these precursors from 1,3-dienes has few precedents in the literature. We have identified reaction conditions and ligands for the synthesis of nearly enantiopure (er >97:3 to >99:1) homoallylic boronate esters via a rarely seen cobalt-catalyzed [4,3]-hydroboration of 1,3-dienes. Monosubstituted or 2,4-disubstituted linear dienes undergo highly efficient regio- and enantioselective hydroboration with HBPin catalyzed by [(L*)Co]+[BARF]-, where L* is typically a chiral bis-phosphine ligand with a narrow bite angle. Several such ligands (e.g., i-PrDuPhos, QuinoxP*, Duanphos, and BenzP*) that give high enantioselectivities for the [4,3]-hydroboration product have been identified. In addition, the equally challenging problem of regioselectivity is uniquely solved with a dibenzooxaphosphole ligand, (R,R)-MeO-BIBOP. A cationic cobalt(I) complex of this ligand is a very efficient (TON >960) catalyst while also providing excellent regioselectivities (rr >98:2) and enantioselectivities (er >98:2) for a broad range of substrates. A detailed computational investigation of the reactions using Co complexes from two widely different ligands (BenzP* and MeO-BIBOP) employing the B3LYP-D3 density functional theory provides key insights into the mechanism and the origins of selectivities. The computational results are in full agreement with the experiments. For the complexes we have examined thus far, the relative stabilities of the diastereomeric diene-bound complexes [(L*)Co(η4-diene)]+ lead to the initial diastereofacial selectivity, which in turn is retained in the subsequent steps, providing exceptional enantioselectivity for the reactions.

Generation of Allylmagnesium Reagents by Hydromagnesiation of 2-Aryl-1,3-dienes

A protocol for the generation of allylmagnesium reagents from 2-aryl-1,3-dienes was developed using magnesium hydride (MgH₂ ) that is generated in situ by solvothermal treatment of sodium hydride (NaH) and magnesium iodide (MgI₂ ) in tetrahydrofuran (THF). Downstream functionalization of the resulting allylmagnesium reagents with carbonyl compounds or alkyl (pseudo)halides delivers branched products having an allylic quaternary carbon center, whereas that with chlorosilanes resulted in formation of linear allylsilanes in regio and stereoselective manners. Further derivatizations of the homoallylic alcohols and allylsilanes were also demonstrated.

Palladium-Catalyzed Chemo- and Regioselective C-H Bond Functionalization of Phenols with 1,3-Dienes

Chemo- and site-selective functionalization of phenols offers a rapid strategy for the synthesis of phenol derivatives with diverse structures. Herein, we report a Pd-catalyzed regioselective C-H bond allylic alkylation of phenols with 1,3-dienes, which has precision reactivity at the ortho C-H bond of 2-naphthols, 1-naphthols, and electron-rich phenols. The reaction is accelerated by a diphosphine ligand, does not need any other additive, and features broad substrate scope and good chemo- and regioselectivity. In addition, we have also investigated the asymmetric variant, and the product could be achieved in up to 55% ee.

Catalytic Asymmetric Hydroalkoxylation and Formal Hydration and Hydroaminoxylation of Conjugated Dienes

The straightforward construction of stereogenic centers bearing unprotected functional groups, as in nature, has been a persistent pursuit in synthetic chemistry. Abundant applications of free enantioenriched allyl alcohol and allyl hydroxylamine motifs have made the asymmetric hydration and hydroaminoxylation of conjugated dienes from water and hydroxylamine, respectively, intriguing and efficient routes that have, however, been unachievable thus far. A fundamental challenge is the failure to realize transition-metal-catalyzed enantioselective C-O bond constructions via hydrofunctionalization of conjugated dienes. Here, we perform a comprehensive study toward the stereoselective formal hydration and hydroaminoxylation of conjugated dienes by synthesizing a set of new P,N-ligands and identifying an aryl-derived oxime as a surrogate for both water and hydroxylamine. Asymmetric hydroalkoxylation with new P,N-ligands is also elucidated. Furthermore, versatile derivatizations following hydration provide indirect but concise routes to formal hydrophenoxylation, hydrofluoroalkoxylation, and hydrocarboxylation of conjugated dienes that have been unreported thus far. Finally, a ligand-to-ligand hydrogen transfer process is proposed based on the results of preliminary mechanistic experiments.

Asymmetric Assemblies of 1,3-Dienes, Sulfuric Diamide, and Aldehydes to Access 1,3-Diamines through Relay Amine/Palladium Catalysis

A relay catalytic protocol using pyrrolidine and palladium catalysis has been developed for asymmetric synthesis of 1,3-diamine derivatives from 3-substituted 1,3-dienes, sulfuric diamide, and aldehydes. This one-pot, three-component reaction features the advantages of a high atom step economy and operational simplicity, providing an efficient and straightforward access to valuable 1,3-diamines incorporating quaternary and tertiary stereogenic centers with moderate to good enantioselectivity.

Scalable Palladium-Catalyzed Alkoxycarbonylation of Conjugated Dienes

The Pd(cod)Cl₂-catalyzed alkoxycarbonylation of conjugated dienes to β,γ-unsaturated esters was approached by both intramolecular phosphinesulfonate L¹ and intermolecular PPh₃/PTSA in this study. However, the poor solubility of the Pd/L¹ complex and the labile monodentate Pd/PPh₃ structure restricts the system efficiency, especially for the scale-up application. By contrast, the stable and well-soluble bidentate Xantphos system allows for the quantitative formation of 3-pentenoate (96%) on a gram scale within 6 h in weakly alkaline N-methylpyrrolidone (NMP), which also functions as a basic site to promote the rate-limiting alcoholysis step while reducing the dosage of ligand to a theoretical value.

Skeleton-Reorganizing Coupling Reactions of Cycloheptatriene and Cycloalkenones with Amines

Skeletal reorganization reactions have emerged as an intriguing tool for converting readily available compounds into complicated molecules inaccessible by traditional methods. Herein, we report a unique skeleton-reorganizing coupling reaction of cycloheptatriene and cycloalkenones with amines. In the presence of Rh/acid catalysis, cycloheptatriene can selectively couple with anilines to deliver fused 1,2-dihydroquinoline products. Mechanistic studies indicate that the retro-Mannich type ring-opening and subsequent intramolecular Povarov reaction account for the ring reorganization. Our mechanistic studies also revealed that skeleton-reorganizing amination between anilines and cycloalkenones can be achieved with acid. The synthetic utilization of this skeleton-reorganizing coupling reaction was showcased with a gram-scale reaction, synthetic derivatizations, and the late-stage modification of commercial drugs.

Mechanistic Studies of Carbonyl Allylation Mediated by (NHC)CuH: Isoprene Insertion, Allylation, and β-Hydride Elimination

The ability of Cu-H complexes to undergo selective insertion of unsaturated hydrocarbons under mild conditions has rendered them valuable, versatile catalysts. The direct formation of Cu allyl intermediates from unfunctionalized 1,3-dienes and transient Cu hydrides is an appealing strategy for upgrading conjugated diene feedstocks. However, empirical mechanistic studies of the underlying elementary steps and characterization of key intermediates in Cu-H catalysis are sparse. Using [(NHC)CuH]₂ (NHC = N-heterocyclic carbene), we examined the steric effects of NHC ligands on two key elementary steps of CuH-catalyzed carbonyl allylation: the insertion of a diene into the Cu-H bond to produce a Cu-allyl complex, and the formation of C-C bonds from stoichiometric allylations of ketones and aldehydes. The resulting allyl and homoallylic alkoxide complexes have been characterized by NMR spectroscopy and single-crystal X-ray diffraction. Employing isolable (NHC)Cu-allyl complexes, we further evaluated the roles of the ligand size, electronic properties of carbonyl substrates, coordinating groups within the substrate, and solvent on the regioselectivity, diastereoselectivity, and relative rate of the C-C bond formation step. In contrast to the clean allylation of ketones, allylation of aldehydes provided a rare example of a formal β-hydride elimination reaction from a secondary homoallylic alkoxide species. Mechanistic studies of key elementary steps provide insights for a range of catalytic reactions of dienes mediated by hydride complexes.

Iron-Catalyzed Cross-Coupling of α-Allenyl Esters with Grignard Reagents for the Synthesis of 1,3-Dienes

Structurally diverse 1,3-dienes are valuable building blocks in organic synthesis. Herein we report the iron-catalyzed coupling between α-allenyl esters and Grignard reagents, which provides a fast and practical approach to a variety of complex substituted 1,3-dienes. The reaction involves an inexpensive iron catalyst, mild reaction conditions, and provides easy scale up.

Catalytic, Regioselective 1,4-Fluorodifunctionalization of Dienes

A catalysis-based regioselective 1,4-fluorofunctionalization of trifluoromethyl substituted 1,3-dienes has been developed to access compact, highly functionalized products. The process allows E,Z-mixed dienes to be processed to a single E-alkene isomer, and leverages an inexpensive and operationally convenient I(I)/I(III) catalysis platform. The first example of catalytic 1,4-difluorination is disclosed and subsequently evolved to enable 1,4-hetero-difunctionalization, which allows δ-fluoro-alcohol and amine derivatives to be forged in a single operation. The protocol is compatible with a variety of nucleophiles including fluoride, nitriles, carboxylic acids, alcohols and even water thereby allowing highly functionalized products, with a stereocenter bearing both C(sp³ )-F and C(sp³ )-CF₃ groups, to be generated rapidly. Scalability (up to 3 mmol), and facile post-reaction modifications are demonstrated to underscore the utility of the method in expanding organofluorine chemical space.