Copper-Catalyzed Borylcupration of Allenylsilanes

Palladium-catalyzed selective C-C bond cleavage of keto-vinylidenecyclopropanes: construction of structurally rich dihydrofurans and tetrahydrofurans

Palladium-catalyzed selective cleavage of the distal C-C bond and proximal C-C bond of keto-vinylidenecyclopropanes by altering the sterically bulky phosphine ligands has been realized. The proximal C-C bond cleavage can be achieved by using dtbpf as a phosphine ligand, affording bicyclic products containing dihydrofuran skeletons in good yields along with broad substrate scope. In proximal C-C bond cleavage reactions, the eight-membered cyclic palladium intermediate plays a key role in the reaction. The [3 + 2] cycloaddition of keto-vinylidenecyclopropanes through the distal C-C bond cleavage can be effectively accomplished with t BuXPhos as a phosphine ligand and ZnCl2 as an additive, delivering bicyclic products containing tetrahydrofuran skeletons in good yields. The further transformation of these bicyclic products has been demonstrated, and the reaction mechanisms of two different C-C bond cleavage reactions have been investigated by control experiments and DFT calculations.

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.

Ligand-Controlled Regiodivergent Cyanoboration of Internal Allenes by Copper Catalysis

The first copper-catalyzed regiodivergent cyanoboration of internal allenes with B₂ pin₂ (bis(pinacolato)diboron) and NCTS (N-cyano-N-phenyl-p-toluenesulfonamide) derivatives is reported. The β,γ- and α,β-cyanoborylated products were synthesized with high regio- and stereo-selectivity. Computational studies revealed that nucleophilic addition of allylcopper or related intermediates on cyanation reagent is the regio- and stereo-determining step, while transmetalation with B₂ pin₂ is the rate-determining step. The nucleophilic addition step proceeds via inner-sphere mechanism in the Cu^I /P(o-tol)₃ and Cu^I /Xantphos (P(o-tol)₃ =tris(o-methylphenyl)phosphine, Xantphos=4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) catalytic systems and via outer-sphere mechanism in the Cu^II /Xantphos catalytic system, respectively.

Copper catalysed protoboration of allenyl-Bdans: efficient synthesis of β-boryl allyl-Bdans

A novel strategy for the selective synthesis of β-boryl allyl-Bdan derivatives by employing Cu-catalyzed protoboration of allenyl-Bdan derivatives was developed.

Cu-catalyzed coupling of vinylidene cyclopropanes with allyl and allenyl boronates

The development of Cu-catalyzed coupling of vinylidene cyclopropanes with allyl or allenyl boronates is reported. The reaction forms a C-C bond at the terminal carbon atom of the allene moiety of vinylidene cyclopropanes with concurrent opening of the cyclopropane ring. In addition, the resulting Cu-enolate intermediate can be intercepted by external electrophiles.

Regio- and Stereoselective Synthesis of Multi-Alkylated Allylic Boronates through Three-Component Coupling Reactions between Allenes, Alkyl Halides, and a Diboron Reagent

Multisubstituted allylic boronates are attractive and valuable precursors for the rapid and stereoselective construction of densely substituted carbon skeletons. Herein, we report the first synthetic approach for differentially 2,3,3-trialkyl-substituted allylic boronates that contain a stereodefined tetrasubstituted alkene structure. Copper(I)-catalyzed regio- and stereoselective three-component coupling reactions between gem-dialkylallenes, alkyl halides, and a diboron reagent afforded sterically congested allylic boronates. The allylboration of aldehydes diastereoselectively furnished the corresponding homoallylic alcohols that bear a quaternary carbon. A computational study revealed that the selectivity-determining mechanism was correlated to the coordination of a boryl copper(I) species to the allene substrate as well as the borylcupration step.

Regio-divergent hydroboration of terminal allenes controlled by nickel and cobalt catalysts

A highly efficient regioselective method for allene hydroboration has been developed, with the regioselectivity governed primarily by the appropriate choice of the metal.

Copper-catalyzed boroacylation of allenes to access tetrasubstituted vinylboronates

A distinct copper-catalyzed boroacylation of allenes with acyl chlorides and bis(pinacolato)diboron is developed. For aromatic acyl chlorides, 1,2-boroacylation of allenes readily takes place, leading to the formation of tetrasubstituted vinylboronates with exclusive (E)-stereoselectivity. In comparison, the employment of alkyl acyl chlorides as electrophiles alters the selectivity to 2,3-boroacylated products. Additionally, the product can easily undergo Suzuki-Miyaura cross-coupling to afford tetrasubstituted alkene with complete retention of the configuration.

Catalytic enantioselective allene-anhydride approach to β,γ-unsaturated enones bearing an α-all-carbon-quarternary center

A protocol of highly regio- and enantioselective copper-catalyzed hydroacylation of the non-terminal C[double bond, length as m-dash]C bond in 1,1-disubstituted terminal allenes with anhydrides has been developed. Both aromatic and aliphatic carboxylic anhydrides are applicable to the efficient construction of all carbon quarternary centers connected with a versatile C[double bond, length as m-dash]C bond and a useful ketone functionality. The synthetic potentials of the enantioenriched products have also been demonstrated. Density functional theory (DFT) calculations were performed to explain the steric outcome of the products: the hydroacylation proceeds through a six-membered transition state and the ligand-substrate steric interactions account for the observed enantioselectivity although the chiral ligand is far away from the to-be-genetated chiral center.

Cu-catalyzed C-C bond formation of vinylidene cyclopropanes with carbon nucleophiles

The development of Cu-catalyzed addition of carbon nucleophiles to vinylidene cyclopropanes was reported. The reactions with 1,1-bisborylmethane provided homopropargylic boronate products by forming a C-C bond at the terminal carbon atom of the allene moiety of vinylidene cyclopropanes. Alkynyl boronates are also suitable nucleophile precursors in reactions with vinylidene cyclopropanes, and skipped diynes were obtained in high yields. In addition, the Cu-enolate generated from the initial addition of nucleophilic copper species to vinylidene cyclopropanes can be intercepted by an external electrophile. As such, vinylidene cyclopropane serves as a linchpin to connect a nucleophile and an electrophile by forming two carbon-carbon bonds sequentially.

Access to Stereodefined (E)-2-Silylallylboronates via Regioselective Chloroboration of Allenylsilanes

A catalyst-free method for the highly regioselective chloroboration of allenylsilanes is described. In the presence of BCl₃ and 2,6-lutidine, chloroboration of allenylsilanes proceeds smoothly without any catalyst, and the product could be treated with pinacol to afford the corresponding pinacol borates in one-pot reaction. This reaction provides a direct approach to construct valuable 2-silylallylboronate frameworks with operational simplicity and high atom-economy.

An Efficient Approach to Regio- and Stereodefined Fully-Substituted Alkenylsilanes by Pd-Catalyzed Allenic C(sp3 )-H Oxidation

A highly efficient palladium-catalyzed functionalization of allenylsilanes to give regio- and stereodefined fully-substituted alkenylsilanes has been developed. This oxidative coupling reaction showed good functional group compatibility with exclusive regio- and stereoselectivity. The pending olefin on the silyl group was shown to be an indispensable element for the initial allenic C(sp³ )-H bond cleavage, and performs as the directing group to control the overall selectivity. The addition of substoichiometric amounts of Et₃ N was found to increase the reaction rate leading to a higher reaction yield. The reaction can be easily scaled up and applied for the late-stage functionalization of natural products and pharmaceutical compounds, including amino acids and steroid derivatives. The newly introduced functional groups include aryl, alkynyl, and boryl groups. The highly strained four-membered ring, silacyclobutene was obtained when B₂ pin₂ was employed as the coupling partner. Mechanistic studies, including kinetic isotope effects, showed that the allenic C(sp³ )-H bond cleavage is the rate-limiting step.

Cu-Catalyzed SN2' Substitution of Propargylic Phosphates with Vinylarene-Derived Chiral Nucleophiles: Synthesis of Chiral Allenes

A new Cu-catalyzed enantioselective three-component (i.e., styrenes, B₂pin₂, and propargylic phosphates) allenylation via an S_N2' substitution of propargylic electrophiles with vinylarene-derived chiral nucleophiles is presented. This method provides an efficient and enantioselective approach to access a range of optically pure di-(1,1-), tri-, and tetra-substituted allenes with α-central chirality and axial chirality in excellent chemo-, regio-, diastereo-, and enantioselectivities.

Highly Selective Palladium-Catalyzed Hydroborylative Carbocyclization of Bisallenes to Seven-Membered Rings

A highly selective palladium-catalyzed hydroborylative carbocyclization of bisallenes to afford seven-membered rings has been established. This ring-closing coupling reaction showed good functional group compatibility with high chemo- and regioselectivity, as seven-membered ring 3 was the only product obtained. The extensive use of different linkers, including nitrogen, oxygen, malononitrile, and malonate, showed a broad substrate scope for this approach. A one-pot cascade reaction was realized by trapping the primary allylboron compound with an aldehyde, affording a diastereomerically pure alcohol and a quaternary carbon center by formation of a new C-C bond. A comprehensive mechanistic DFT investigation is also presented. The calculations suggest that the reaction proceeds via a concerted hydropalladation pathway from a Pd(0)-olefin complex rather than via a pathway involving a defined palladium hydride species. The reaction was significantly accelerated by the coordination of the pendant olefin, as well as the introduction of suitable substituents in the bridge, due to the Thorpe-Ingold effect.

Featuring Xantphos

This review highlights the use of the bisphosphine ligand group in homogeneous catalysis.

Palladium-catalyzed intermolecular dearomatic allenylation of hydrocycloalk[b]indoles with 2,3-allenyl carbonates

A palladium-catalyzed allenylation of hydrocycloalk[b]indoles with 2,3-allenyl carbonates provides functionalized allenes containing an indoline.

Enantioselective copper catalysed, direct functionalisation of allenes via allyl copper intermediates

We explore the breadth of the copper catalysed functionalisation of allenes, which enables efficient access to enantioenriched, densely functionalised molecules.

The direct functionalisation of allenes under copper catalysis enables efficient access to enantioenriched, densely functionalised molecules. In this review we explore the breadth and depth of a versatile reaction manifold, which involves the element-cupration of allenes to generate allyl copper intermediates that are subsequently coupled with diverse arrays of electrophiles.

Copper-catalyzed highly selective approach to 2-boroallylic silanes from allenylsilanes

A highly selective copper-catalyzed borylcupration of 1,2-allenylsilanes affords 2-borylallylsilanes with many synthetically attractive functional groups by applying BIPHEP as the ligand has been developed.

Mechanistic study on ligand-controlled copper-catalyzed regiodivergent silacarboxylation of allenes with carbon dioxide and silylborane

The origin of ligand-controlled copper-catalyzed regiodivergent silacarboxylation was rationalized.

Enantioselective Generation of Adjacent Stereocenters in a Copper-Catalyzed Three-Component Coupling of Imines, Allenes, and Diboranes

A highly enantio- and diastereoselective copper-catalyzed three-component coupling affords the first general synthesis of homoallylic amines bearing adjacent stereocenters from achiral starting materials. The method utilizes a commercially available NHC ligand and copper source, operates at ambient temperature, couples readily available simple imines, allenes, and diboranes, and yields high-value homoallylic amines that exhibit versatile amino, alkenyl, and boryl units.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B-C bonds and other processes.