Control of Selectivity in Palladium(II)-Catalyzed Oxidative Transformations of Allenes

Indium-Mediated Preparation of Bis(α-hydroxyallenes) or α,α'-Dihydroxyallenynes and Further Gold-Catalyzed Cyclizations

We present the regio- and diastereoselective Barbier-type allenylation reaction of glyoxals mediated by indium to furnish highly valuable syn-bis(α-hydroxyallenes) and syn-α,α'-dihydroxyallenynes. The gold-catalyzed controlled cyclization of these unsaturated diols enables the divergent preparation of three types of oxacycles.

γ-CF3-Allenamides versus 3-CF3-Cyclopentenylamines: Substituent-Controlled Divergent Reaction of β-CF3-1,3-Enynamides with β-Dicarbonyl Compounds

A distinctive N-substituent-controlled regioselective 1,4-hydrocarbonation or 1,4-hydrocarbonation/5-endo-trig cyclization cascade reaction of β-CF3-1,3-enynamides with β-dicarbonyl compounds in the presence of a simple base is reported. β-CF3-1,3-enynamides having a Ts group N-substituent produce γ-CF3-allenamides via a 1,4-hydrocarbonation process, whereas β-CF3-1,3-enynamides bearing a Ms group N-substituent lead to 3-CF3-cyclopentenylamines through a 1,4-hydrocarbonation/5-endo-trig cyclization cascade process.

Rh(III)-catalyzed regioselective C(sp2)-H alkenylation of isoquinolones with methoxyallene: A facile access to aldehyde-bearing isoquinolones

A simple and rapid access to isoquinolone aldehyde scaffolds has been established by a rhodium-catalyzed reaction between isoquinolone and methoxyallene that forges alkenylation in an explicit regioselective manner. Herein, methoxyallene serving as an acrolein equivalent results in execution of this unique functionalization. Furthermore, the compatibility with complex molecules underscores the significance of this developed protocol. The mechanistic proposal for this regioselective transformation was consistent with kinetic studies and several control reactions.

Catalytic (4+2) Annulation via Regio- and Enantioselective Interception of in-situ Generated Alkylgold Intermediate

A regio- and stereoselective stepwise (4+2) annulation of N-propargylamides and α,β-unsaturated imines/ketones has been accomplished with synergetic catalysis by a combination of a gold-complex and a chiral quinine-derived squaramide (QN-SQA), leading to highly functionalized chiral tetrahydropyridines/dihydropyrans in good to high yields with generally excellent enantioselectivity. Mechanistic studies and DFT calculations indicate that the in situ formed alkylgold species is the key intermediate in this transformation, and the amide group served as a traceless directing group in this highly selective transformation. This method complements the enantioselective (4+2) annulation of allene reagents, providing the formal internal C-C π-bond cycloaddition products, which is challenging and remains elusive.

Dual Nickel/Photoredox-Catalyzed Arylsulfonylation of Allenes

The nickel/photoredox dual catalysis system is an efficient conversion platform for the difunctionalization of unsaturated hydrocarbons. Herein, we disclose the first dual nickel/photoredox-catalyzed intramolecular 1,2-arylsulfonylation of allenes, which can accurately construct a C(sp2)-C(sp2) bond and a C(sp3)-S bond. The reaction exhibits excellent chemoselectivity and regioselectivity, allowing modular conformations of a diverse series of 3-sulfonylmethylbenzofuran derivatives. Control experiments showed that the bipyridine ligand is crucial for the formation of a stable σ-alkyl nickel intermediate, providing the possibility for sulfonyl radical insertion. Meanwhile, the electrophilic sulfonyl radical facilitates further oxidative addition of the σ-alkyl nickel intermediate and inhibits addition with allenes. In addition, control experiments, cyclic voltammetry tests, Stern-Volmer experiments, and density functional theory calculations afford evidence for the Ni(0)/Ni(I)/Ni(II)/Ni(III) pathway in this 1,2-arylsulfonylation.

Total Synthesis of Strigolactones via Palladium-Catalyzed Cascade Carbonylative Carbocyclization of Enallenes

Here we report an efficient route for synthesizing strigolactones (SLs) and their derivatives. Our method relies on a palladium-catalyzed oxidative carbonylation/carbocyclization/carbonylation/alkoxylation cascade reaction, which involves the formation of three new C-C bonds and a new C-O bond while cleaving one C(sp3)-H bond in a single step. With our versatile synthetic strategy, both naturally occurring and artificial SLs were prepared.

Environmentally Friendly Synthesis of Highly Substituted Phenols Using Enallenoates and Grignard Reagents

We developed an efficient and environmentally friendly methodology for selectively synthesizing highly substituted phenols using readily available enallenoates and Grignard reagents. This method consistently yields good to excellent results across over 60 examples, demonstrating the substrate scope and the exploration of phenol product derivatization, further extending the method's utility.

Challenging Task of Ni-Catalyzed Highly Regio-/Enantioselective Semihydrogenation of Racemic Tetrasubstituted Allenes via a Kinetic Resolution Process

The first earth-abundant transition metal Ni-catalyzed highly regio- and enantioselective semihydrogenation of racemic tetrasubstituted allenes via a kinetic resolution process as a challenging task was well established. This protocol furnishes expedient access to a diversity of structurally important enantioenriched tetrasubstituted allenes and chiral allylic molecules with high regio-, enantio-, and Z/E-selectivity. Remarkably, this semihydrogenation proceeded with one carbon-carbon double bond of allenes, which was regioselective complementary to the Rh-catalyzed asymmetric version. Deuterium labeling experiments and density functional theory (DFT) calculations were carried out to reveal the reasonable reaction mechanism and explain the regio-/stereoselectivity.

Palladium-Catalyzed Dehydrogenative Carbonylative Esterification of Allenoic Acids for the Synthesis of γ-Butyrolactone Derivatives

A highly efficient dehydrogenative carbonylative esterification of allenoic acids using Pd-catalysis was developed, providing a novel approach to synthesizing esterified γ-butyrolactone derivatives with consistently good to excellent results demonstrated across over 50 examples. Additionally, we used a heterogeneous catalyst known as Pd-AmP-MCF and harnessed biomimetic-aerobic-oxidation conditions to facilitate the practical execution of this reaction. Furthermore, our detailed study of γ-butyrolactone products highlighted their potential in synthesizing bioactive compounds.

Pd-Catalyzed Enantioselective Creation of All-Carbon Quaternary Center with 2,3-Allenylic Carbonates

Enantioselective construction of all-carbon quaternary centers has been achieved via the palladium-catalyzed highly enantioselective allenylation of oxindoles with 2,3-allenylic carbonates to afford a variety of optically active allene products, which contain oxindole units with different functional groups, in high ee. The corresponding synthetic applications have also been demonstrated.

Electrochemical C-H/C-C Bond Oxygenation: A Potential Technology for Plastic Depolymerization

Herein, we provide eco-friendly and safely operated electrocatalytic methods for the selective oxidation directly or with water, air, light, metal catalyst or other mediators serving as the only oxygen supply. Heavy metals, stoichiometric chemical oxidants, or harsh conditions were drawbacks of earlier oxidative cleavage techniques. It has recently come to light that a crucial stage in the deconstruction of plastic waste and the utilization of biomass is the selective activation of inert C(sp3 )-C/H(sp3 ) bonds, which continues to be a significant obstacle in the chemical upcycling of resistant polyolefin waste. An appealing alternative to chemical oxidations using oxygen and catalysts is direct or indirect electrochemical conversion. An essential transition in the chemical and pharmaceutical industries is the electrochemical oxidation of C-H/C-C bonds. In this review, we discuss cutting-edge approaches to chemically recycle commercial plastics and feasible C-C/C-H bonds oxygenation routes for industrial scale-up.

A highly diastereoselective (5+1) annulation of allenoates and pyrazolones catalyzed by CH3OK

The first (5+1) annulation of allenoates and pyrazolones catalyzed by CH3OK has been reported. A series of spiro-pyrazolone derivatives were obtained as single diastereomers in high yields (≤95%) under mild conditions. The synthetic utility was demonstrated by a scale-up reaction and various transformations of the products. The proposed mechanism suggests that the allenoate works as a 1,5-biselectrophilic 5C synthon for the first time and controlled experiments disclose that K+ plays an important role in the diastereoselectivity-determining step through an eight-membered ring transition state. Also, this 1,5-biselectrophilic allenoate will be able to act as a 5C synthon for (5+n) annulation.

Palladium-Catalyzed Cyclization/Alkenylation of Ynone Oximes with Vinylsilanes for the Assembly of Isoxazolyl Vinylsilanes

A palladium-catalyzed cascade cyclization/alkenylation for the assembly of synthetically valuable isoxazolyl vinylsilane derivative has been accomplished. Easily accessible ynone oximes, and available vinylsilane agents were used as the reaction starting materials This protocol features broad substrate scope, good functional group tolerance, and good step- and atom-economy. Remarkably, this approach provides a new approach for the construction of structurally diverse isoxazolyl-containing vinylsilanes with high molecular complexity, showing a promising application in synthetic and pharmaceutical chemistry.

Tandem Isomerization/α,β-Site-Selective and Enantioselective Addition Reactions of N-(3-Butynoyl)-3,5-dimethylpyrazole Induced by Chiral π-Cu(II) Catalysts

Allenes are important building blocks, and derivatization of products via cycloadditions of allenes could become a powerful strategy for constructing carbocyclic and heterocyclic rings. However, the development of catalytic site-selective and enantioselective cycloaddition reactions of allenes still presents significant challenges. Here, we report chiral π-Cu(II)-complex-catalyzed isomerization of N-(3-butynoyl)-3,5-dimethyl-1H-pyrazole to generate N-allenoylpyrazole in situ and subsequent α,β-site-selective and enantioselective [3 + 2], [4 + 2], or [2 + 2] cycloaddition or conjugate addition reactions. The asymmetric environment created by the intramolecular π-Cu(II) interactions provides the corresponding adducts in moderate to high yield with excellent enantioselectivity. To the best of our knowledge, this is the first successful method for chiral-Lewis-acid-catalyzed tandem isomerization/α,β-site-selective and enantioselective cycloaddition or conjugate addition reactions of latent non-γ-substituted allenoyl derivative.

Heterogeneous Copper-Catalyzed Cross-Coupling for Sustainable Synthesis of Chiral Allenes: Application to the Synthesis of Allenic Natural Products

Classical Crabbé type SN 2' substitutions of propargylic substrates has served as one of the standard methods for the synthesis of allenes. However, the stereospecific version of this transformation often requires either stoichiometric amounts of organocopper reagents or special functional groups on the substrates, and the chirality transfer efficiency is also capricious. Herein, we report a sustainable methodology for the synthesis of diverse 1,3-di and tri-substituted allenes by using a simple and cheap cellulose supported heterogeneous nanocopper catalyst (MCC-Amp-Cu(I/II)). This approach represents the first example of heterogeneous catalysis for the synthesis of chiral allenes. High yields and excellent enantiospecificity (up to 97 % yield, 99 % ee) were achieved for a wide range of di- and tri-substituted allenes bearing various functional groups. It is worth noting that the applied heterogeneous catalyst could be recycled at least 5 times without any reduced reactivity. To demonstrate the synthetic utility of the developed protocol, we have applied it to the total synthesis of several chiral allenic natural products.

Synthesis of gem-Difluorinated Isoxazoles via Palladium-Catalyzed Oxylallylation of Alkynone Oxime Ethers

A novel and reliable palladium-catalyzed oxylallylation of alkynone oxime ethers with fluorine-containing alkenes was accomplished. Using the bulk industrial chemical 3-bromo-3,3-difluoroprop-1-ene as the coupling partner, this synthetic methodology offers the first example for the assembly of structurally diverse gem-difluorinated isoxazole derivatives in moderate to good yields with high atom- and step-economy and excellent functional group compatibility. More importantly, this strategy allows for the direct combination of the isoxazole motifs and gem-difluoroalkene unit, which is not easy to obtain through a general synthetic strategy.

δ-Acetoxy Allenoate as a 5C-Synthon in Domino-Annulation with Sulfamidate Imines: Ready Access to Coumarins

A DMAP-catalyzed sequential benzannulation and lactonization strategy in which δ-acetoxy allenoate functions as a 5C-synthon in its reaction with cyclic sulfamidate imines is reported. This platform delivers π-extended coumarin frameworks under metal-free conditions via allylic elimination followed by Mannich coupling, proton shifts, C-N bond cleavage, and lactonization as key steps. The driving force for this domino reaction is the formation of the diene-ammonium intermediate and O-S bond cleavage. ESI-HRMS has been useful in gaining insights into the reaction pathway.

Palladium-catalyzed asymmetric allenylic alkylation: construction of multiple chiral thiochromanone derivatives

The development of a new strategy for the construction of chiral cyclic sulfide-containing multiple stereogenic centers is highly desirable. Herein, by the combination of base-promoted retro-sulfa-Michael addition and palladium-catalyzed asymmetric allenylic alkylation, the streamlined synthesis of chiral thiochromanones containing two central chiralities (including a quaternary stereogenic center) and an axial chirality (allene unit) was successfully realized with up to 98% yield, 49.0 : 1 dr and >99% ee.

DBU-Catalyzed Ring Expansion or Ene-amine Formation Involving δ-Acetoxy Allenoates and N-Sulfonyl Hydrazides

DBU-catalyzed spiro-annulation and concomitant ring expansion/domino reaction of δ-acetoxy allenoates with cycl-2-ene-N-sulfonyl hydrazides afford ring-expanded (5 → 6, 6 → 7, and 7 → 8) products. By contrast, cycl-3-ene/ane-N-sulfonyl hydrazones under similar conditions deliver pyrazole cores with the same allenoate that involves allylic elimination in which δ-acetoxy allenoate serves as 3C-synthon. The key spirocyclic intermediates, as well as dienyl-amine intermediates, are isolated and characterized. An extension to (R)-(-)-carvone-derived sulfonyl hydrazide also led to ring expansion and gave pyrazoloazepine.

Stereodivergent Synthesis of Allenes with α,β-Adjacent Central Chiralities Empowered by Synergistic Pd/Cu Catalysis

The stereodivergent synthesis of allene compounds bearing α,β-adjacent central chiralities has been realized via the Pd/Cu-catalyzed dynamic kinetic asymmetric alkylation of racemic allenylic esters. The matched reactivity of bimetallic catalytic system enables the challenging reaction of racemic aryl-substituted allenylic acetates with sterically crowded aldimine esters smoothly under mild reaction conditions. Various chiral non-natural amino acids bearing a terminal allenyl group are easily synthesized in high yields and with excellent diastereo- and enantioselectivities (up to >20 : 1 dr, >99 % ee). Importantly, all four stereoisomers of the product can be readily accessed by switching the configurations of the two chiral metal catalysts. Furthermore, the easy interconversion between the uncommon η³ -butadienyl palladium intermediate featuring a weak C=C/Pd coordination bond and a stable Csp² -Pd bond is beneficial for the dynamic kinetic asymmetric transformation process (DyKAT).

Palladium-Catalyzed [6+2] Double Allene Annulation for Benzazocines Synthesis

An efficient double allene protocol for the formation of benzazocines has been developed. The reaction constitutes a highly regioselective palladium-catalyzed formal [6+2] annulation of allenyl benzoxazinanones with terminal allenes forming the challenging 8-membered cycles. Decent yields and excellent regioselectivity have been observed under mild conditions with a remarkable Z-stereoselectivity for the exo-cyclic C=C bonds. The synthetic potentials of benzazocine products have been demonstrated.

Organocatalytic synthesis of axially chiral tetrasubstituted allenes

Asymmetric organocatalysis is a growing method for the synthesis of axially chiral tetrasubstituted allenes, the most challenging one among allene syntheses. In this method, chiral organocatalysts such as phase-transfer catalysts, peptides, disulfonimides, and binaphthyl/bispiro phosphoric acids have displayed remote control of regio- and stereoselectivity. Highly functionalized enantiopure allenes including those with an adjacent tertiary or quaternary stereocenter have been efficiently prepared with high levels of regio-, diastereo-, and enantioselectivity using this method. Several mechanistic pathways, including electrophilic addition to cumulenolate or zwitterionic enolate intermediates, alkynylogous Mukaiyama aldol reaction, nucleophilic addition to quinone methides, and dearomative addition to imino esters, were proposed. The method is necessary for providing access to axially chiral tetrasubstituted allenes, which can be utilized for the preparation of novel ligands, natural products, and organic materials, particularly those having complex structures. This review covers the enantioselective organocatalytic synthesis of these tetrasubstituted allenes and the mechanistic insights into the formation of the chiral axis up to July 2022.

Nickel-catalyzed switchable 1,3-dienylation and enantioselective allenylation of phosphine oxides

The development of general catalytic methods for the regio- and stereoselective construction of phosphoryl derivatives from identical substrates remains a formidable challenge in organic synthesis. Enabled by the newly developed BDPP-type ligands, we disclosed a nickel-catalyzed allenylation of phosphine oxides rationally and predictably, allowing the construction of versatile chiral allenylphosphoryl derivatives with high enantiopurity (up to 94% e.e.). Alternatively, using an achiral phosphine ligand dcypbz under acidic conditions, we achieved a regiochemical switch of the 1,3-dienylation to afford functionalized phosphinoyl 1,3-butadienes (up to 93% yield). The salient features of this method include switchable reactivity, broad substrate scope, readily available feedstock, single-step preparation, and high asymmetric induction.

Regioselective Annulation of Allenylphosphine Oxides with Aromatic Amides under Ruthenium(II) Catalysis

Engaging allenes in transition-metal-catalyzed C-H bond activation strategy is immensely promising to access high-value scaffolds. However, such a reaction manifold remains largely elusive using cheap and sustainable ruthenium catalysis. We disclose an unprecedented ruthenium-catalyzed (4 + 2) annulation between aromatic amides and allenylphosphine oxides, furnishing NH-free isoquinolinones in high yields. This operationally simple methodology leverages weak coordination assistance, displays high selectivity, and is amenable to the late-stage functionalization of several biologically relevant motifs.

Insights into the gold(I)-catalyzed intermolecular annulations of alkynes with N-allenamides: a mechanistic DFT study

The mechanism of Au(I)-catalyzed intermolecular annulation of 2-(1-alkynyl)-2-alken-1-one with N-allenamide was carefully elucidated using density functional theory (DFT). The reaction is initiated by the binding of the Au(I) catalyst with 2-(1-alkynyl)-2-alken-1-one rather than with N-allenamide. The key intermediate, a gold all-carbon 1,3-dipole species, is revealed to be more reactive than the gold allylic carbocation. The influence of ligands and substituents was rationally analyzed. We believe that our study will provide deeper mechanistic insights into the chemoselective reactions of alkynes with N-allenamide.

Molecular insights into chirality transfer from double axially chiral phosphoric acid in a synergistic enantioselective intramolecular amination

In the most general practice of asymmetric catalysis, a chiral catalyst, typically bearing a center or an axis of chirality, is employed as the chiral source for imparting enantiocontrol over the developing product. Given the current interest toward optically pure compounds, various forms of chiral induction enabled by diverse chiral sources as well as the use of multiple catalysts under one-pot conditions have been in focus. In one such promising development, an achiral N-sulfonamide protected 1,6-amino allyl alcohol (NaphSO₂NHCH₂C(Ph)₂CH₂CH Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHCH₂OH) was subjected to Tsuji–Trost activation and an intramolecular amination to form important chiral pyrrolidine frameworks. A dual catalytic system comprising Pd(PPh₃)₄ and DAPCy (β-cyclohexyl substituted double axially chiral phosphoric acid derived from two homocoupled BINOL backbones with a dynamic central chiral axis) under mild conditions was reported to offer quantitative conversion with an ee of 95%. Here, we provide molecular insights into the origin of chiral induction by DAPCy, as obtained through a comprehensive density functional theory (SMD_(toluene)/B3LYP-D3/6-31G**,Pd(SDD)) investigation. Two key steps in the mechanism are identified to involve a cooperative mode of activation of the Pd-bound allyl alcohol in the form of a Pd-π-allyl moiety at one end of the substrate, followed by an intramolecular nucleophilic addition of N-sulfonamide from the other end to yield a pyrrolidine derivative bearing an α-vinyl stereogenic center. (S,R,S)-DAPCy is found to steer the dehydroxylation to yield a Pd-π-allyl intermediate with a suitably poised si prochiral face for the nucleophilic addition. In the enantiocontrolled (as well as the turn-over determining step) nucleophilic addition, the chiral catalyst is identified to serve as a chiral phosphate counterion. The chiral induction is facilitated by a series of N–H⋯O, C–H⋯O, C–H⋯π, lone pair (lp)⋯π, O–H⋯O, O–H⋯π, and π⋯π noncovalent interactions, which is noted as more effective in the lower energy C–N bond formation transition state through the si prochiral face of the Pd-π-allyl moiety. These insights into the novel dynamic axially double chiral catalyst could be valuable toward exploiting such modes of stereoinduction.

The origin of enantiocontrol in an intramolecular amination involving Pd(PPh₃)₄ and a double axially chiral phosphoric acid (DAPCy) dual catalytic system is traced to a more effective series of noncovalent interactions in the lower energy C–N bond formation transition state.

Copper-Catalysed Synthesis of Trifluoromethyl Allenes via Fluoro-carboalkynylation of Alkenes

Allenes and trifluoromethyl motifs are considered as important building blocks in materials and pharmaceuticals. A copper-catalysed synthesis of trifluoromethyl allenes utilizing readily available feedstocks under mild and environmentlly friendly conditions...

Selective 1,4-arylsulfonation of 1,3-enynes via photoredox/nickel dual catalysis

A photoredox/nickel-catalyzed selective 1,4-arylsulfonation of 1,3-enynes to access structurally diverse sulfone-containing allenes has been established with low catalyst loading.

Room Temperature Allenation of Terminal Alkynes with Aldehydes

A gold-catalyzed room temperature allenation of terminal alkynes (ATA) with aldehydes affording 1,3-disubstituted allenes with diverse functional groups has been developed by identifying a gold(I) catalyst and an amine. The practicality of this reaction has been demonstrated by a ten gram-scale synthesis and the synthetic potentials have been demonstrated via various transformations and formal total synthesis of (-)-centrolobine. Mechanistic studies revealed that the gold catalyst, the aldehyde effect, the fluoroalkyl hydroxyl solvent (TFE or HFIP) and the structure of amine are vital in this room temperature ATA reaction.

Catalytic Electrophilic Thiocarbocyclization of Allenes

An efficient approach via catalytic electrophilic thiocarbocyclization of allenes to construct indene-based sulfides with excellent regioselectivities is disclosed. The reactions were carried out at low temperatures by selenide catalysis in the presence of TMSOTf. Not only electrophilic arylthio reagents but also electrophilic alkylthio reagents worked well under these conditions. Furthermore, the method could be applied to intermolecular azidothiolation of allenes.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

EATA Reaction Catalyzed by Copper Halides of Mixed Oxidation States and Its Application to Total Synthesis of Scorodonin

Naturally occurring conjugated allenynes are of general interest to the scientific community for their potent and various biological activities. The 1,5-H transfer of alka-1,4-diyn-3-yl amines would be one of the most straightforward yet challenging approach to this class of compounds since it may, in principle, form two regioisomeric products involving two different C-C triple bonds. Herein, a catalytic recipe of copper halides with mixed oxidation states, i.e., CuCl/CuBr 2 , has been identified to address the issues of the side reaction of conjugate addition and the selectivity of 1,5-H transfer of the key intermediate, alka-1,4-diyn-3-yl amines, in EATA (Enantioselective Allenation of Terminal Alkynes) reaction involving the conjugated 2-alkynals. This protocol could accommodate a wide range of functional groups providing a series of allenynes with a very high enantioselectivity (up to >99% ee). In addition, the enantioenriched allenynes can be readily transformed into various building blocks and applied to the highly enantioselective total synthesis of linear allenic natural product scorodonin for the first time. Mechanistic studies and DFT calculations elucidated the high regioselectivity for observed 1,5-H transfer within the intermediate of 1,4-diyn-3-yl amines. The calculated energy difference between two of the most stable transition states of 3.4 kcal/mol accounts for a selectivity of over 99:1, which is in perfect agreement with the experimental results.

Iron-Catalyzed Cross-Coupling of Propargyl Ethers with Grignard Reagents for the Synthesis of Functionalized Allenes and Allenols

Herein we disclose an iron‐catalyzed cross‐coupling reaction of propargyl ethers with Grignard reagents. The reaction was demonstrated to be stereospecific and allows for a facile preparation of optically active allenes via efficient chirality transfer. Various tri‐ and tetrasubstituted fluoroalkyl allenes can be obtained in good to excellent yields. In addition, an iron‐catalyzed cross‐coupling of Grignard reagents with α‐alkynyl oxetanes and tetrahydrofurans is disclosed herein, which constitutes a straightforward approach towards fully substituted β‐ or γ‐allenols, respectively.

HFIP-Catalyzed Difluoroalkylation of Propargylic Alcohols to Access Tetrasubstituted Difluoroalkyl Allenes

A hexafluoroisopropanol (HFIP)-catalyzed difluoroalkylation of propargylic alcohols with difluoroenoxysilanes to access structurally diverse tetrasubstituted difluoroalkyl allenes has been developed. This convenient procedure enables the rapid construction of highly functionalized multisubstituted fluorinated allenes in a mild and straightforward way. Furthermore, the synthetic potential of this methodology has been demonstrated by the facile synthesis of various structurally interesting fluorine-containing molecules such as gem-difluorosubstituted dihydropyran, tetrasubstituted CF₂H-allene, and multisubstituted fluorinated cyclopentanone derivatives.

Divergent Reactivity of δ- and β'-Acetoxy Allenoates with 2-Sulfonamidoindoles via Phosphine Catalysis: Entry to Dihydro-α-carboline, α-Carboline, and Spiro-cyclopentene Motifs

The reactivity of 2-sulfonamidoindoles with acetoxy allenoates under phosphine catalysis depends on the disposition of the acetoxy (OAc) group on the allenoate. In the temperature-controlled [3 + 3] annulations, δ-acetoxy allenoates afforded dihydrocarboline and carboline scaffolds with carbon-nitrogen nucleophilic 2-sulfonamidoindoles, in which allenoate serves as a β-, γ-, and δ-carbon donor. At room temperature (25 °C), dihydro-α-carboline motifs were obtained exclusively through Michael addition, 1,4-proton shift, isomerization, 1,2-proton transfer, phosphine elimination, and aza-Michael addition. The higher temperature (80 °C) cascade protocol using Ph₃P-Cs₂CO₃ combination involves addition-elimination, aza-Claisen rearrangement, tosyl migration, and aromatization as key steps to give α-carbolines containing tosyl functionality at the γ-carbon. In contrast, with β'-acetoxy allenoate, 2-sulfonamidoindole acts only as a carbo-nucleophile in (p-tolyl)₃P-directed [4 + 1] spiro-annulation, leading to five-membered spiro-carbocyclic motifs essentially as single diastereomers (dr >20:1) via chemoselective carbo-annulation.

Cu/Pd-catalyzed borocarbonylative trifunctionalization of alkynes and allenes: synthesis of β-geminal-diboryl ketones

Functionalized bisboryl compounds have recently emerged as a new class of synthetically useful building blocks in organic synthesis. Herein, we report an efficient strategy to synthesize β-geminal-diboryl ketones enabled by a Cu/Pd-catalyzed borocarbonylative trifunctionalization of readily available alkynes and allenes. This reaction promises to be a useful method for the synthesis of functionalized β-geminal-diboryl ketones with broad functional group tolerance. Mechanistic studies suggest that the reaction proceeds through borocarbonylation/hydroboration cascade of both alkynes and allenes.