Site-Selective C-H Alkenylation of N-Heteroarenes by Ligand-Directed Co/Al and Co/Mg Cooperative Catalysis

We report herein the design and development of Co/Al and Co/Mg bimetallic catalysts, supported by a phosphine/secondary phosphine oxide (PSPO) bifunctional ligand, for the site-selective C-H alkenylation of nitrogen-containing heteroarenes with alkynes. These catalysts enable the alkenylation of pyridine, pyridone, and imidazo[1,2-a]pyridine derivatives at the C-H site proximal to the Lewis basic nitrogen or oxygen atom, which represents a selectivity profile distinct from that of the previously developed cobalt-diphosphine/aluminum catalyst. The alkenylated products were obtained in moderate to good yields using various heterocycles and differently substituted internal alkynes. Kinetic isotope effect experiments suggest the irreversibility of the C-H activation step, the relevance of which to the rate-limiting step depends on the reaction conditions. Density functional theory calculations indicate that ligand-to-ligand hydrogen transfer is the common mechanism of C-H activation.

Ruthenium(II)/Imidazolidine Carboxylic Acid-Catalyzed C-H Alkylation for Central and Axial Double Enantio-Induction

Enantioselective C-H activation has surfaced as a transformative toolbox for the efficient assembly of chiral molecules. However, despite of major advances in rhodium and palladium catalysis, ruthenium(II)-catalyzed enantioselective C-H activation has thus far largely proven elusive. In contrast, we herein report on a ruthenium(II)-catalyzed highly regio-, diastereo- and enantioselective C-H alkylation. The key to success was represented by the identification of novel C2-symmetric chiral imidazolidine carboxylic acids (CICAs), which are easily accessible in a one-pot fashion, as highly effective chiral ligands. This ruthenium/CICA system enabled the efficient installation of central and axial chirality, and featured excellent branched to linear ratios with generally >20 : 1 dr and up to 98 : 2 er. Mechanistic studies by experiment and computation were carried out to understand the catalyst mode of action.

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.

Nickel-Catalyzed Hydroarylation Reaction: A Useful Tool in Organic Synthesis

This article describes the recent advances in the field of nickel-catalyzed hydroarylation reaction of alkenes, alkynes, and arenes. All reactions proceeded either through internal hydride transfer or in presence of...

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.

Accessing C2-Functionalized 1,3-(Benz)azoles through Transition Metal-Catalyzed C-H Activation

The skeletal presence of 1,3-azoles in a variety of bioactive natural products, pharmacophores, and organic materials demands the derivatization of such heteroarenes regioselectively. Plenty of cross-coupling as well as cyclocondensation reactions have been performed to build up these skeletons but remained commercially unrealizable. A couple of severe drawbacks are faced by these traditional protocols that require a more straightforward strategy to obviate them. Transition metal-catalyzed C-H functionalization has emerged as a superior alternative in that context. 1,3-Azoles and their benzo counterparts have been extensively functionalized exploiting both noble and earth-abundant transition metals. Lately, C-2 functionalization have gained much traction due to the ease of attaining high regioselectivity and installation of synthetically manipulative functionalities. This critical review presents a bird's eye view of all major C-2 functionalization of (benz)azoles catalyzed by a diverse set of metals performed over the past 15 years.

Enantioselective Three-Component Coupling of Heteroarenes, Cycloalkenes and Propargylic Acetates

Asymmetric coupling proceeds efficiently between propargylic acetates, cycloalkenes and electron-rich heteroarenes including indoles, pyrroles, activated furans and thiophenes. 2,3-Disubstituted tetrahydrofurans and pyrrolidines are produced in trans configuration and excellent enantiomeric ratios. The reaction proceeds via Wacker-type attack of nucleophilic heteroarenes on alkenes activated by allenyl Pd^II species.

Enantioselective Pallada-Electrocatalyzed C-H Activation by Transient Directing Groups: Expedient Access to Helicenes

Asymmetric pallada-electrocatalyzed C-H olefinations were achieved through the synergistic cooperation with transient directing groups. The electrochemical, atroposelective C-H activations were realized with high position-, diastereo-, and enantio-control under mild reaction conditions to obtain highly enantiomerically-enriched biaryls and fluorinated N-C axially chiral scaffolds. Our strategy provided expedient access to, among others, novel chiral BINOLs, dicarboxylic acids and helicenes of value to asymmetric catalysis. Mechanistic studies by experiments and computation provided key insights into the catalyst's mode of action.

Mechanism and Origin of MAD-Induced Ni/N-Heterocyclic Carbene-Catalyzed Regio- and Enantioselective C-H Cyclization of Pyridines with Alkenes

This work presents a DFT-based computational study on the regio- and enantioselective C-H functionalization of pyridines with alkenes at the relatively unreactive C4-position, which was successfully achieved by Shi et al. [J. Am. Chem. Soc. 2019, 141, 5628-5634] using Ni⁰ /N-heterocyclic carbene (NHC) catalysis under the assistance of an aluminum-based Lewis acid additive (2,6-tBu₂ -4-Me-C₆ H₂ O)₂ AlMe (MAD). The calculations indicate that the selective functionalization involves a three-step mechanism in which a unique H-migration assisted oxidation metalation (HMAOM) step is identified as the rate- and enantioselectivity-determining step. The newly proposed mechanism can well rationalize the experimental observation that the preferred product is the endo-type (vs. exo-type), R-configuration (vs. S-configuration) product at the C4 (vs. C2) position, and also unveil the reasons that the NHC ligand and the MAD additive can facilitate the reaction.

Enantioselective Twofold C-H Annulation of Formamides and Alkynes without Built-in Chelating Groups

Twofold C-H annulation of readily available formamides and alkynes without built-in chelating groups was achieved. Ni-Al bimetallic catalysis enabled by a bulky BINOL-derived chiral secondary phosphine oxide (SPO) ligand proved to be critical for high reactivity and high selectivity. This reaction uses readily available formamides as starting materials and provides a concise synthetic pathway to a broad range of chiral ferrocenes in 40-98 % yield and 93-99 % ee.

Nickel/NHC-Catalyzed Asymmetric C-H Alkylation of Fluoroarenes with Alkenes: Synthesis of Enantioenriched Fluorotetralins

Chiral polyfluoroarene derivatives are an important scaffold in chemistry. An unprecedented enantioselective C-H alkylation of polyfluoroarenes with alkenes is described. The reaction employs bulky chiral N-heterocyclic carbene (NHC) ligands for nickel catalysts to enable exclusive activation of C-H bonds over C-F bonds and complete endo-selective C-H annulation and excellent enantioselectivity. A wide variety of chiral fluorotetralins, compounds otherwise difficultly accessed but serve as important bioisosteric analogs of both tetralin and heterocycle units for drug design, are expediently synthesized from easily available substrates. To our knowledge, this is the first example of catalytic enantioselective C-H functionalization of polyfluoroarenes.