Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

Palladium-Catalyzed Linear Hydrothiocarbonylation of Unactivated Terminal Alkenes: Synthesis of Aliphatic Thioesters

A Pd-catalyzed hydrothiocarbonylation of unactivated terminal alkenes is presented. According to this protocol, aliphatic thioesters were synthesized with exclusive linear selectivity under mild reaction conditions. Good to excellent yields (up to 91% yield), broad substrate scope, broad functional group tolerance, and utility of the method demonstrated the advantages of this protocol.

Copper-Catalyzed Asymmetric Borylacylation of Styrene and Indene Derivatives

The enantioselective copper-catalyzed borylacylation of aryl olefins with acyl chlorides and bis-(pinacolato)diboron is reported. This three-component reaction involves an enantioselective syn-borylcupration of the aryl olefin, followed by a nucleophilic attack on the acyl chloride. This reaction proceeds with a 2 mol % catalyst loading and is generally completed within 30 min at room temperature. Because the boron moiety can be converted into versatile functional groups and the carbonyl group is a ubiquitous functional group, the resulting chiral β-borylated ketones are versatile intermediates in organic synthesis.

Transition metal catalyzed asymmetric multicomponent reactions of unsaturated compounds using organoboron reagents

Asymmetric multicomponent reactions allow stitching several functional groups in an enantioselective and atom economical manner. The introduction of boron-based reagents as a multicomponent coupling partner has its own merits. In addition to being non-toxic and highly stable, organoboron compounds can be easily converted to other functional groups in a stereoselective manner. In the last decade several transition metal catalyzed asymmetric multicomponent strategies have been evolved using boron based reagents. This review will discuss the merits and scope of multicomponent strategies based on their difference in the reaction mechanism and transition metals involved.

Halogenated salt assisted Cu-catalyzed asymmetric 1,4-borylstannation of 1,3-enynes: enantioselective synthesis of allenylstannes

An enantioselective 1,4-borylstannation of 1,3-enynes employed a chiral sulfoxide phosphine (SOP)/Cu complex as a catalyst, and the desired products, chiral allenylstannes, were first synthesized by asymmetric catalysis with satisfactory yields and enantioselectivies. In this protocol, a catalytic amount of additive, a halogenated salt, plays a crucial role in the success. Control experiments and theoretical studies disclosed that the four-membered ring transmetallation transition states which were stabilized by a halide anion are the key to yields and stereochemical outcomes.

Copper-Catalyzed Asymmetric Coupling of Allenyl Radicals with Terminal Alkynes to Access Tetrasubstituted Allenes

In contrast to the wealth of asymmetric transformations for generating central chirality from alkyl radicals, the enantiocontrol over the allenyl radicals for forging axial chirality represents an uncharted domain. The challenge arises from the unique elongated linear configuration of the allenyl radicals that necessitates the stereo-differentiation of remote motifs away from the radical reaction site. We herein describe a copper-catalyzed asymmetric radical 1,4-carboalkynylation of 1,3-enynes via the coupling of allenyl radicals with terminal alkynes, providing diverse synthetically challenging tetrasubstituted chiral allenes. A chiral N,N,P-ligand is crucial for both the reaction initiation and the enantiocontrol over the highly reactive allenyl radicals. The reaction features a broad substrate scope, covering a variety of (hetero)aryl and alkyl alkynes and 1,3-enynes as well as radical precursors with excellent functional group tolerance.

Copper-Catalyzed Enantioselective Radical 1,4-Difunctionalization of 1,3-Enynes

Chiral allenes are important structural motifs frequently found in natural products, pharmaceuticals, and other organic compounds. Asymmetric 1,4-difunctionalization of 1,3-enynes is a promising strategy to construct axial chirality and produce substituted chiral allenes from achiral substrates. However, the previous state of the art in 1,4-difunctionalization of 1,3-enynes focused on the allenyl anion pathway. Because of this, only electrophiles can be introduced into the allene backbones in the second functionalization step, consequently limiting the reaction and allene product types. The development of asymmetric 1,4-difunctionalization of 1,3-enynes via a radical pathway would complement previous methods and support expansion of the toolbox for the synthesis of asymmetric allenes. Herein, we report the first radical enantioselective allene formation via a group transfer pathway in the context of copper-catalyzed radical 1,4-difunctionalization of 1,3-enynes. This method addresses a longstanding unsolved problem in asymmetric radical chemistry, provides an important strategy for stereocontrol with free allenyl radicals, and offers a novel approach to the valuable, but previously inaccessible, chiral allenes. This work should shed light on asymmetric radical reactions and may lead to other enantioselective group transfer reactions.

Copper-catalyzed functionalization of enynes

The copper-catalyzed functionalization of enyne derivatives has recently emerged as a powerful approach in contemporary synthesis. Enynes are versatile and readily accessible substrates that can undergo a variety of reactions to yield densely functionalized, enantioenriched products. In this perspective, we review copper-catalyzed transformations of enynes, such as boro- and hydrofunctionalizations, copper-mediated radical difunctionalizations, and cyclizations. Particular attention is given to the regiodivergent functionalization of 1,3-enynes, and the current mechanistic understanding of such processes.

Cu(I)-Catalyzed addition-cycloisomerization difunctionalization reaction of 1,3-enyne-alkylidenecyclopropanes (ACPs)

A copper(i)-catalyzed three-component addition-cycloisomerization difunctionalization reaction of 1,3-enyne-ACPs with Togni I reagent and TMSCN under mild reaction conditions has been developed, affording 3-trifluoroethylcyclopenta[b]naphthalene-4-carbonitrile derivatives. The reaction proceeded through a copper(i)-catalyzed 1,4-addition of conjugated 1,3-enynes via a radical relay process and aromatic cycloisomerization of allene-ACP intermediates.

Beyond Carbon: Enantioselective and Enantiospecific Reactions with Catalytically Generated Boryl- and Silylcopper Intermediates

Catalytic asymmetric C-C bond formation with alkylcopper intermediates as carbon nucleophiles is now textbook chemistry. Related chemistry with boron and silicon nucleophiles where the boryl- and accordingly silylcopper intermediates are catalytically regenerated from bench-stable pronucleophiles had been underdeveloped for years or did not even exist until recently. Over the past decade, asymmetric copper catalysis employing those main-group elements as nucleophiles rapidly transformed into a huge field in its own right with an impressive breadth of enantioselective C-B and C-Si bond-forming reactions, respectively. Its current state of the art does not have to shy away from comparison with that of boron's and silicon's common neighbor in the periodic table, carbon. This Outlook is not meant to be a detailed summary of those manifold advances. It rather aims at providing a brief conceptual summary of what forms the basis of the latest exciting progress, especially in the area of three-component reactions and cross-coupling reactions.

Palladium-Catalyzed Fluoroarylation of gem-Difluoroenynes to Access Trisubstituted Trifluoromethyl Allenes

Various new transformations of gem-difluoroalkenes leading to trifluoromethyl substituted compounds have been well established in the past years. However, the development of new transformations of gem-difluoroenynes lags much behind. Herein is reported the fluoroarylation of 1,1-difluoro-1,3-enynes with aryl halides in the presence of silver fluoride affording trisubstituted trifluoromethyl allenes under the catalysis of palladium. The reaction features mild conditions, high functional-group tolerance, and high regioselectivity.

Cu-Catalyzed Synthesis of Tetrasubstituted 2,3-Allenols through Decarboxylative Silylation of Alkyne-Substituted Cyclic Carbonates

An efficient and mild Cu-catalyzed protocol has been developed for the decarboxylative silylation of alkyne-functionalized cyclic carbonate substrates affording 2,3-allenols featuring four different substituents. This practical methodology gives access to a wide scope of tetrasubstituted functionalized allenes in excellent yields.

Kinetic Resolution of Propargylic Ethers via [2,3]-Wittig Rearrangement to Synthesize Chiral α-Hydroxyallenes

An efficient kinetic resolution of propargyloxy dicarbonyl compounds via asymmetric [2,3]-Wittig rearrangement was achieved by using a chiral N,N'-dioxide/Ni^II complex catalyst. Various chiral α-allenyl alcohols were obtained in high enantioselectivities under mild conditions. The utility of this method was readily demonstrated in the asymmetric synthesis of the chiral 2,5-dihydrofuran derivative.

Ligand-controlled copper-catalyzed 1,2 or 1,4-protoborylation of 2-trifluoromethyl-1,3-conjugated enynes

A novel ligand-controlled highly regioselective synthesis of CF₃-substituted homopropargylboronates and homoallenylboronates via copper-catalyzed 1,2 or 1,4-protoborylation of 2-trifluoromethyl-1,3-conjugated enynes with B₂pin₂ was developed.