Skeletal and Mechanistic Diversity in Ir-Catalyzed Cycloisomerizations of Allene-Tethered Pyrroles and Indoles

Pyrroles and indoles bearing N-allenyl tethers participate in a variety of iridium-catalyzed cycloisomerization processes initiated by a C-H activation step, to deliver a diversity of synthetically relevant azaheterocyclic products. By appropriate selection of the ancillary ligand and the substitution pattern of the allene, the reactions can diverge from simple intramolecular hydrocarbonations to tandem processes involving intriguing mechanistic issues. Accordingly, a wide range of heterocyclic structures ranging from dihydro-indolizines and pyridoindoles to tetrahydroindolizines, as well as cyclopropane-fused tetrahydroindolizines can be obtained. Moreover, by using chiral ligands, these cascade processes can be carried out in an enantioselective manner. DFT studies provide insights into the underlying mechanisms and justify the observed chemo- regio- and stereoselectivities.

Rhodium-Catalyzed Remote Borylation of Alkynes and Vinylboronates

Remote functionalization involving a fascinating chain-walking process has emerged as a powerful strategy for the rapid access to value-added functional molecules from readily available feedstocks. However, the scope of current methods is predominantly limited to mono- and di-substituted alkenes. The remote functionalization of multi- and heteroatom-substituted alkenes is challenging, and the use of alkynes in the chain walking is unexplored. We herein report a rhodium catalyzed remote borylation of internal alkynes, offering an unprecedented reaction mode of alkynes for the preparation of synthetically valuable 1,n-diboronates. The regioselective distal migratory hydroboration of sterically hindered tri- and tetra-substituted vinylboronates is also demonstrated to furnish various multi-boronic esters. Synthetic utilities are highlighted through the selective manipulation of the two boryl groups in products such as the regioselective cross coupling, oxidation, and amination.

Ligand-Controlled NiH-Catalyzed Regiodivergent Chain-Walking Hydroalkylation of Alkenes

A NiH-catalyzed migratory hydroalkylation of alkenyl amines with predictable and switchable regioselectivity is reported. By utilizing a ligand-controlled, directing group-assisted strategy, various alkyl units are site-selectively installed at inert sp³ C-H sites far away from the original C=C bonds. A range of structurally diverse α- and β-branched protected amines are conveniently synthesized via stabilization of 5- and 6-membered nickelacycles respectively. This method exhibits broad scope and high functional group tolerance, and can be applied to late-stage modification of medicinally relevant molecules.

Cobalt-Catalyzed Asymmetric Alkylation of (Hetero)Arenes with Styrenes

An efficient and general intermolecular Cobalt(II)-catalyzed asymmetric alkylation of styrenes with (hetero)arenes including indoles, thiophene and electron rich arenes has been developed, providing straightforward access to enantioenriched alkyl(hetero)arenes with high enantioselectivity. Mechanistic studies suggest that the reaction underwent a CoH-mediated hydrogen atom transfer (HAT) with alkenes, followed by a pivotal catalyst-controlled S_N 2-like pathway between in situ generated organocobalt(IV) species and aromatic nucleophiles. This is the first CoH-catalyzed asymmetric hydrofunctionalization using carbon nucleophiles, providing a new strategy for asymmetric Friedel-Crafts type alkylation.

Remote Site-Selective Asymmetric Protoboration of Unactivated Alkenes Enabled by Bimetallic Relay Catalysis

A remote C(sp³ )-H bond asymmetric borylation of unactivated alkenes was achieved by bimetallic relay catalysis. The reaction proceeded through reversible and consecutive β-H elimination/olefin insertion promoted by CoH species generated in situ, followed by copper-catalyzed asymmetric protoboration. The use of this synergistic Co/Cu catalysis protocol allowed the enantioselective protoboration of various unactivated terminal alkenes and internal alkenes, as well as an unrefined mixture of olefin isomers, at the distal less-reactive β-position to a functional group, leading to chiral organoboronates.

Development and Mechanistic Studies of the Iridium-Catalyzed C-H Alkenylation of Enamides with Vinyl Acetates: A Versatile Approach for Ketone Functionalization

Ketone functionalization is a cornerstone of organic synthesis. Herein, we describe the development of an intermolecular C-H alkenylation of enamides with the feedstock chemical vinyl acetate to access diverse functionalized ketones. Enamides derived from various cyclic and acyclic ketones reacted efficiently, and a number of sensitive functional groups were tolerated. In this iridium-catalyzed transformation, two structurally and electronically similar alkenes-enamide and vinyl acetate-underwent selective cross-coupling through C-H activation. No reaction partner was used in large excess. The reaction is also pH- and redox-neutral with HOAc as the only stoichiometric by-product. Detailed experimental and computational studies revealed a reaction mechanism involving 1,2-Ir-C migratory insertion followed by syn-β-acetoxy elimination, which is different from that of previous vinyl acetate mediated C-H activation reactions. Finally, the alkenylation product can serve as a versatile intermediate to deliver a variety of structurally modified ketones.

Highly Enantioselective Iridium(I)-Catalyzed Hydrocarbonation of Alkenes: A Versatile Approach to Heterocyclic Systems Bearing Quaternary Stereocenters

We report a versatile, highly enantioselective intramolecular hydrocarbonation reaction that provides a direct access to heteropolycyclic systems bearing chiral quaternary carbon stereocenters. The method, which relies on an iridium(I)/bisphosphine chiral catalyst, is particularly efficient for the synthesis of five-, six- and seven-membered fused indole and pyrrole products, bearing one and two stereocenters, with enantiomeric excesses of up to >99 %. DFT computational studies allowed to obtain a detailed mechanistic profile and identify a cluster of weak non-covalent interactions as key factors to control the enantioselectivity.

Iridium-Catalyzed Hydroarylation via C-H Bond Activation

Hydroarylation reactions via C-H activation, which compensate for shortcomings of classical methods based on the Friedel-Crafts reaction, is one of the most attractive methods to synthesize substituted arenes. This Personal Account reviews our recent studies on iridium-catalyzed intermolecular hydroarylation of vinyl ethers, alkynes, bicycloalkenes, and 1,3-dienes, and intramolecular hydroarylation of m-allyloxyphenyl ketones, where asymmetric addition reactions are included. A cationic iridium catalyst, which is generated from chloroiridium [IrCl] and NaBAr^F ₄ [Ar^F =3,5-(CF₃ )₂ C₆ H₃ ], or a hydroxoiridium [Ir(OH)] complex is effective in catalyzing the hydroarylation depending on the substrates. 1,5-Cyclooctadiene (cod), chiral dienes, and conventional bisphosphines function as ligands controlling the high reactivity and selectivity of the catalysts in the hydroarylation. H/D exchange reaction of alkenes by use of a key intermediate of the hydroarylation reaction is also described.

Nickel-Catalyzed Migratory Hydrocyanation of Internal Alkenes: Unexpected Diastereomeric-Ligand-Controlled Regiodivergence

A regiodivergent nickel-catalyzed hydrocyanation of a broad range of internal alkenes involving a chain-walking process is reported. When appropriate diastereomeric biaryl diphosphite ligands are applied, the same starting materials can be converted to either linear or branched nitriles with good yields and high regioselectivities. DFT calculations suggested that the catalyst architecture determines the regioselectivity by modulating electronic and steric interactions. In addition, moderate enantioselectivities were observed when branched nitriles were produced.

Emerging Organometallic Methods for the Synthesis of C-Branched (Hetero)aryl, Alkenyl, and Alkyl Glycosides: C-H Functionalization and Dual Photoredox Approaches

Transition-metal-catalyzed C-H functionalization and photoredox nickel dual catalysis have emerged as innovative and powerful avenues for the synthesis of C-branched glycosides. These two concepts have been recently established and provide efficient and mild methods for accessing a series of valuable complex C-branched glycosides of great interest. Herein, recent developments in the synthesis of C-branched aryl/alkenyl/alkyl glycosides through these two approaches are highlighted.

Enantioselective Nickel-Catalyzed Migratory Hydrocyanation of Nonconjugated Dienes

Metal-catalyzed chain-walking reactions have recently emerged as a powerful strategy to functionalize remote positions in organic molecules. However, a chain-walking protocol for nonconjugated dienes remains scarcely reported, and developments are currently ongoing. In this Communication, a nickel-catalyzed asymmetric hydrocyanation of nonconjugated dienes involving a chain-walking process is demonstrated. The reaction exhibits excellent regio- and chemoselectivity, and a wide range of substrates were tolerated, delivering the products in high yields and enantioselectivities. Deuterium-labeling experiments support the chain-walking process, which involves an iterative β-H elimination and reinsertion processes. Gram-scale synthesis, regioconvergent experiments, and downstream transformations gave further insights into the high potential of this transformation.

Branched-Selective Direct α-Alkylation of Cyclic Ketones with Simple Alkenes

Herein, we describe an intermolecular direct branched-selective α-alkylation of cyclic ketones with simple alkenes as the alkylation agents. Through an enamine-transition metal cooperative catalysis mode, the α-alkylation is realized in an atom- and step-economic manner with excellent branched selectivity for preparing β-branched ketones. Employment of a pair of bulky Brønsted acid and base as additives is responsible for enhanced efficiency. Promising enantioselectivity (74 % ee) has been obtained. Experimental and computational mechanistic studies suggest that a pathway through alkene migratory insertion into the Ir-C bond followed by C-H reductive elimination is involved for the high branched selectivity.

Remote Nucleophilic Allylation by Allylrhodium Chain Walking

Metal migration through a carbon chain is a versatile method for achieving remote functionalization. However, almost all known examples involve the overall net migration of alkylmetal species. Here, we report that allylrhodium species obtained from hydrorhodation of 1,3-dienes undergo chain walking toward esters, amides, or (hetero)arenes over distances of up to eight methylene units. The final, more highly conjugated allylrhodium species undergo nucleophilic allylation with aldehydes and with an imine to give Z-homoallylic alcohols and amines, respectively.