A Highly Efficient Dimeric Manganese-Catalyzed Selective Hydroarylation of Internal Alkynes

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Manganese(I)-catalyzed nucleophilic addition of C(sp3)-H bonds to aldehydes

The C-H bond activation catalyzed by a manganese(I) complex has achieved significant development but is limited to C(sp2)-H bonds. In this work, an efficient manganese(I)-catalyzed direct nucleophilic addition reaction of C(sp3)-H bonds to aromatic aldehydes has been developed. This is the first example of manganese(I)-catalyzed C(sp3)-H bond transformation. A manganacycle complex was isolated and proved to be the key active intermediate in the catalytic cycle.

Manganese(I)-Catalyzed Enantioselective C(sp2)-C(sp3) Bond-Forming for the Synthesis of Skipped Dienes with Synergistic Aminocatalysis

Mn(I)-catalyzed enantioselective C-C bond-forming reactions represent a great challenge in homogeneous catalysis primarily due to a limited understanding of its mechanistic principles. Herein, we have developed an interesting catalytic strategy that leverages a synergistic combination of a dimeric manganese(I) catalyst and a chiral aminocatalyst to address this issue. A range of conjugated dienals and trienals can exclusively proceed 1,4-hydroalkenylation by using readily available aromatic and aliphatic alkenyl boronic acids as coupling partners, producing a rich library of skipped diene aldehydes in synthetically useful yields and high levels of enantioselectivities. Notably, downstream transformations of these products can not only afford a concise approach to construct enantioenriched skipped trienes but also realize enantioselective total synthesis of analogues to (-)-Blepharocalyxin D in four steps. DFT calculations suggest the 1,4-hydroalkenylation is kinetically more favorable than 1,6-hydroalkenylation.

Leveraging cascade alkynyl Prins cyclization towards the stereoselective synthesis of spiro-furan quinazolinone scaffolds

A TfOH-promoted, metal-free protocol has been unveiled for the synthesis of spiro-furan quinazolinones employing alkynol urea derivatives utilizing alkynyl Prins cyclization reaction. This methodology produces highly functionalized spiro-heterocycles in excellent yields with exclusive E-selectivity under ambient conditions. Furthermore, late-stage modifications incorporate bromide and acetyl functionalities into the synthesized spiro-heterocycles.

Atroposelective Synthesis of C-N Vinylindole Atropisomers by Palladium-Catalyzed Asymmetric Hydroarylation of 1-Alkynylindoles

Transition-metal-catalyzed hydroarylation of unsymmetrical internal alkynes remains challenging because of the difficulty in controlling regioselectivity and stereoselectivity. Moreover, the enantioselective hydroarylation of alkynes using organoboron reagents has not been reported. Herein, we report for the first time that palladium compounds can catalyze the hydroarylation of 1-alkynylindoles with organoborons for the synthesis of chiral C-N atropisomers. A series of rarely reported vinylindole atropisomers was synthesized with excellent regio-, stereo- (Z-selectivity), and enantioselectivity under mild reaction conditions. The ready availability of organoborons and alkynes and the simplicity, high stereoselectivity, and good functional group tolerance of this catalytic system make it highly attractive.

Merging Manganese and Iminium Catalysis: Selective Hydroalkenylation of Unsaturated Aldehydes and Ketones

The use of synergistic catalytic strategy can usually circumvent the intrinsic limitations of one catalytic system. In this communication, we disclose a cooperative catalysis strategy of manganese and iminium catalysis to realize selective hydroalkenylation of unsaturated aldehydes and ketones. Its success stems from the LUMO activation of unsaturated carbonyl compounds with secondary amines as the organocatalyst and the synergistic HOMO activation of alkenylboronic acids with Mn2 (CO)8 Br2 . This protocol exhibits several synthetic advances, e.g., simple operation, good functional group compatibility and good regioselectivity. The theoretical calculation indicates the migratory insertion followed by demetallation-isomerization process is kinetically more favorable than Michael-like nucleophilic addition. The use of proline-derived organocatalyst can deliver the desired products in moderate enantioselectivity.

Dimeric Manganese-Catalyzed Hydroalkenylation of Alkynes with a Versatile Silicon-Based Directing Group

Herein, we present a manganese-catalyzed, branched-selective hydroalkenylation of terminal alkynes, under mild conditions through facile installation of a versatile silanol as a removable directing group. With an alkenyl boronic acid as the coupling partner, this reaction produces stereodefined (E,E)-1,3-dienes with high regio-, chemo- and stereoselectivity. The protocol features mild reaction conditions such as room temperature and an air atmosphere, while maintaining excellent functional group compatibility. The resulting 1,3-dienesilanol products serve as versatile building blocks, as the removal of the silanol group allows for the synthesis of both branched terminal 1,3-dienes for downstream coupling reactions, as well as stereoselective construction of linear (E,E)-1,3-dienes and (E,E,E)- or (E,E,Z)-1,3,5-trienes. In addition, a Diels-Alder cycloaddition can smoothly and selectively deliver silicon-containing pentasubstituted cyclohexene derivatives. Mechanistic investigations, in conjunction with DFT calculations, suggest a bimetallic synergistic activation model to account for the observed enhanced catalytic efficiency and good regioselectivity.

Gold-catalyzed four-component multifunctionalization of alkynes

The alkyne unit is a versatile building block in organic synthesis and the development of selective multifunctionalization of alkynes is an important object of research in this field. Herein, we report an interesting gold-catalyzed, four-component reaction that achieves the oxo-arylfluorination or oxo-arylalkenylation of internal aromatic or aliphatic alkynes, efficiently breaking a carbon-carbon triple bond and forming four new chemical bonds. The reaction divergence can be controlled by site-directing functional groups in the alkynes; the presence of a phosphonate unit favors the oxo-arylfluorination, while the carboxylate motif benefits oxo-arylalkenylation. This reaction is enabled by an Au(I)/Au(III) redox coupling process using Selectfluor as both an oxidant and a fluorinating reagent. A wide range of structurally diverse α,α-disubstituted ketones, and tri- or tetra-substituted unsaturated ketones have been prepared in synthetically valuable yields and with excellent chemo-, regio- and stereoselectivity. The gram-scale preparation and late-stage application of complex alkynes have further enhanced their synthetic value.

Palladium-catalyzed regioselective decarboxylative hydroarylation of alkynyl carboxylic acids with arylboronic acids

The synthesis of (deuterated) 1,1-disubstituted alkenes via Pd-catalyzed decarboxylative hydroarylation of alkynyl carboxylic acids with arylboronic acids has been developed. The reaction features excellent regioselectivity, a broad substrate scope and gram-scale synthetic ability and offers a general synthetic method to synthesize 1,1-dideuterio olefins. Preliminary mechanism investigations indicate that 1,1-disubstituted alkenes are formed by hydroarylation of terminal alkynes generated by in situ decarboxylation of alkynyl carboxylic acids.

Syn-Selective Chlorosulfonylation of Alkynes via a Copper-Powder-Initiated Atom Transfer Radical Addition Reaction and Mechanistic Studies

Copper-powder-catalyzed syn-selective chlorosulfonylation of readily available alkynes by an atom transfer radical addition (ATAR) process has been developed, providing straightforward access to a broad range of (Z)-β-chlorovinylsulfones in good yields under mild conditions. In addition, this method is ligand-free and features excellent stereoselectivity and high atom economy. Moreover, the product was obtained without an apparent loss of yield when the reaction was performed on the gram scale at a low catalyst loading. In this reaction, the copper powder not only acts as a sulfone radical initiator but also produces the catalytically active CuCl species. Mechanistic investigations and DFT calculation studies revealed that the stereoselectivity is controlled by the thermodynamic stabilities of the in situ-generated cyclic alkenyl Cu^II complex intermediate, which can serve as a chlorine atom transfer agent.

Homogeneous Manganese-Catalyzed Hydrofunctionalizations of Alkenes and Alkynes: Catalytic and Mechanistic Tendencies

In recent years, many manganese-based homogeneous catalytic precursors have been developed as powerful alternatives in organic synthesis. Among these, the hydrofunctionalizations of unsaturated C-C bonds correspond to outstanding ways to afford compounds with more versatile functional groups, which are commonly used as building blocks in the production of fine chemicals and feedstock for the industrial field. Herein, we present an account of the Mn-catalyzed homogeneous hydrofunctionalizations of alkenes and alkynes with the main objective of finding catalytic and mechanistic tendencies that could serve as a platform for the works to come.

Dimeric Manganese-Catalyzed Direct Nucleophilic Addition of C(sp2)-H Bonds to Inert Aldehydes

An efficient direct nucleophilic addition reaction of C(sp²)-H bonds to aldehydes catalyzed by a dimeric manganese has been developed. This reaction has a broad range of substrates, and high yields were also obtained with inert aliphatic aldehydes as substrates. A dimeric Mn₂(CO)₈Br₂ was proven to be a more efficient catalyst precursor than the monomeric Mn(CO)₅Br.

Carboxylic Acid-Directed Manganese(I)-Catalyzed Regioselective Hydroarylation of Unactivated Alkenes

A carboxylic acid-directed regioselective hydroarylation reaction of unactivated alkenes with aryl boronic acids was reported. This transformation was enabled by homogeneous manganese catalyst MnBr(CO)₅ in the presence of KOH and H₂O in the m-xylene reaction medium. Both internal and terminal alkenes worked well in this transformation, and a series of functional groups were tolerated. This reaction not only provided an expeditious method to prepare γ-aryl carboxylic acids from simple starting materials but also would inspire further studies in employing homogeneous manganese catalysis in organic synthesis.

Nickel-catalyzed Thioester Transfer Reaction with sp2-Hybridized Electrophiles

We report a thioacylation transfer reaction based on nickel-catalyzed C-C bond cleavage of thioesters with sp²-hybridized electrophiles. Aryl bromides, iodides, and alkenyl triflates can participate in thioester transfer reaction of aryl thioesters, affording a wide range of structurally diverse new thioesters in yields of up to 98% under mild reaction conditions. With this protocol, it is possible to construct alkenyl thioesters from the corresponding ketones through the generation of alkenyl triflates.

Regioselective Cycloaddition and Substitution Reaction of Tertiary Propargylic Alcohols and Heteroareneboronic Acids via Acid Catalysis

We report an acid-catalyzed formal cycloaddition and dehydrative substitution reaction of tertiary propargylic alcohols and heteroareneboronic acids. The properties of the substituents on the alkynyl moiety determines the regioselectivity of the reaction, which could selectively construct fused heterocycles, tetrasubstituted allenes, or 1,3-dienes. This reaction proceeds efficiently with a wide array of substrate scope in up to 89% yield. A significant advantage of this protocol is the transition-metal-free and mild conditions needed.

Rhodium-Catalyzed Chemodivergent Pyridylation of Alkynes with Pyridylboronic Acids

The pyridylation of alkynes with pyridylboronic acids is realized under rhodium catalysis. Chemodivergent pyridylation products, including alkenylpyridines produced via the hydropyridylation pathway and cyclopenta[c]pyridines produced via the pyridylation/cyclization pathway, were selectively produced by fine-tuning the reaction conditions. A mechanistic study revealed that 1,4-rhodium migration to the pyridine ring was involved as the key step in the chemodivergent synthesis.

Interplay between the Directing Group and Multifunctional Acetate Ligand in Pd-Catalyzed anti-Acetoxylation of Unsymmetrical Dialkyl-Substituted Alkynes

The cooperative action of the acetate ligand, the 2-pyridyl sulfonyl (SO₂Py) directing group on the alkyne substrate, and the palladium catalyst has been shown to be crucial for controlling reactivity, regioselectivity, and stereoselectivity in the acetoxylation of unsymmetrical internal alkynes under mild reaction conditions. The corresponding alkenyl acetates were obtained in good yields with complete levels of β-regioselectivity and anti-acetoxypalladation stereocontrol. Experimental and computational analyses provide insight into the reasons behind this delicate interplay between the ligand, directing group, and the metal in the reaction mechanism. In fact, these studies unveil the multiple important roles of the acetate ligand in the coordination sphere at the Pd center: (i) it brings the acetic acid reagent into close proximity to the metal to allow the simultaneous activation of the alkyne and the acetic acid, (ii) it serves as an inner-sphere base while enhancing the nucleophilicity of the acid, and (iii) it acts as an intramolecular acid to facilitate protodemetalation and regeneration of the catalyst. Further insight into the origin of the observed regiocontrol is provided by the mapping of potential energy profiles and distortion–interaction analysis.

Lewis Acid-Driven Meyer-Schuster-Type Rearrangement of Yne-Dienone

Developed herein is a Cu(II)-catalyzed Meyer-Schuster-type rearrangement of alkyne-tethered cyclohexadienone for the construction of m-enone-substituted phenols. The reaction involves an uncommon 5-exo-trig 1,6-enyne cyclization of alkyne-tethered-cyclohexadienone, aromatization-triggered C-O bond cleavage, and an electrocyclic 4π-ring-opening of oxetene intermediate. This atom-efficient transformation provides access to a wide range of synthetically important α-(m-substituted phenol)-α,β-unsaturated ketones, featuring a broad scope with labile functional group tolerance. The gram-scale demonstration makes this transformation synthetically viable. The synthetic application of α,β-unsaturated ketones is also showcased.

Dinuclear gold catalysis

This review summarizes the recent achievements of dinuclear gold-catalyzed redox coupling, asymmetric catalysis and photocatalysis. The dinuclear gold catalysts show a better catalytic performance than the mononuclear gold catalysts in certain cases.

Recent advances of dinuclear nickel- and palladium-complexes in homogeneous catalysis

In this highlight, we provide a current perspective of synthetic methodology development catalyzed by dinuclear Ni- and Pd-complexes in the past decade. The new catalytic reactivities of dinuclear Ni- and Pd-complexes are discussed.