Catalytic Enantioselective Methylene C(sp 3 )−H Amidation of 8‐Alkylquinolines Using a Cp*Rh III /Chiral Carboxylic Acid System

Intermolecular Enantioselective Amination Reactions Mediated by Visible Light and a Chiral Iron Porphyrin Complex

In the presence of 1 mol % of a chiral iron porphyrin catalyst, various 3-arylmethyl-substituted 2-quinolones and 2-pyridones underwent an enantioselective amination reaction (20 examples; 93-99 % ee). The substrates were used as the limiting reagents, and fluorinated aryl azides (1.5 equivalents) served as nitrene precursors. The reaction is triggered by visible light which allows a facile dediazotation at ambient temperature. The selectivity of the reaction is governed by a two-point hydrogen bond interaction between the ligand of the iron catalyst and the substrate. Hydrogen bonding directs the amination to a specific hydrogen atom within the substrate that is displaced by the nitrogen substituent either in a concerted fashion or by a rebound mechanism.

(SCp)Rhodium-Catalyzed Asymmetric Satoh-Miura Reaction for Building-up Axial Chirality: Counteranion-Directed Switching of Reaction Pathways

Satoh-Miura reaction is an important method for extending π-systems by forging multi-substituted benzene rings via double aryl C-H activation and annulation with alkynes. However, the development of highly enantioselective Satoh-Miura reaction remains rather challenging. Herein, we report an asymmetric Satoh-Miura reaction between 1-aryl benzo[h]isoquinolines and internal alkynes enabled by a SCpRh-catalyst. Judiciously choosing the counteranion of the Rh-catalyst is crucial for the desired reactivity over the competitive formation of azoniahelicenes. Detailed mechanistic studies support the proposal of counteranion-directed switching of reaction pathways in Rh-catalyzed asymmetric 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(III)/Chiral Carboxylic Acid-Catalyzed Enantioselective Synthesis of Benzothiadiazine-1-oxides via C-H Activation

Among sulfoximine derivatives containing a chiral sulfur center, benzothiadiazine-1-oxides are important for applications in medicinal chemistry. Here, we report that the combination of an achiral cobalt(III) catalyst and a pseudo-C₂ -symmetric H₈ -binaphthyl chiral carboxylic acid enables the asymmetric synthesis of benzothiadiazine-1-oxides from sulfoximines and dioxazolones via enantioselective C-H bond cleavage. With the optimized protocol, benzothiadiazine-1-oxides with several functional groups can be accessed with high enantioselectivity.

Synthesis of Axially Chiral Aldehydes by N-Heterocyclic-Carbene-Catalyzed Desymmetrization Followed by Kinetic Resolution

Axially chiral aldehydes have received increasing attention in enantioselective catalysis. However, only very few catalytic methods have been developed to construct structurally diverse axially chiral aldehydes. We herein describe an NHC-catalyzed atroposelective esterification of biaryl dialdehydes as a general and practical strategy for the construction of axially chiral aldehydes. Mechanistic studies indicate that coupling proceeds through a novel combination of NHC-catalyzed desymmetrization of the dialdehydes and kinetic resolution. This protocol features excellent enantioselectivity, mild conditions, good functional-group tolerance, and applicability to late-stage functionalization and provides a modular platform for the synthesis of axially chiral aldehydes and their derivatives.

Biocatalytic, Intermolecular C-H Bond Functionalization for the Synthesis of Enantioenriched Amides

Directed evolution of heme proteins has opened access to new-to-nature enzymatic activity that can be harnessed to tackle synthetic challenges. Among these, reactions resulting from active site iron-nitrenoid intermediates present a powerful strategy to forge C-N bonds with high site- and stereoselectivity. Here we report a biocatalytic, intermolecular benzylic C-H amidation reaction operating at mild and scalable conditions. With hydroxamate esters as nitrene precursors, feedstock aromatic compounds can be converted to chiral amides with excellent enantioselectivity (up to >99 % ee) and high yields (up to 87 %). Kinetic and computational analysis of the enzymatic reaction reveals rate-determining nitrenoid formation followed by stepwise hydrogen atom transfer-mediated C-H functionalization.

Synthesis of Chiral Spirolactams via Sequential C-H Olefination/Asymmetric [4+1] Spirocyclization under a Simple CoII /Chiral Spiro Phosphoric Acid Binary System

An unprecedented enantioselective synthesis of spiro-γ-lactams via a sequential C-H olefination/asymmetric [4+1] spirocyclization under a simple Co^II /chiral spiro phosphoric acid (SPA) binary system is reported. A range of biologically important spiro-γ-lactams are obtained with high levels of enantioselectivity (up to 98 % ee). The concise, asymmetric synthesis of an aldose reductase inhibitor was successfully achieved. Notably, contrast to previous reports that relied on the use of cyclopentadienyl or its derivatives (achiral Cp*, Cp^tBu , or chiral Cp^x ) ligated Co^III complexes requiring tedious steps to prepare, cheap and commercially available cobalt(II) acetate tetrahydrate was used as an efficient precatalyst.

A New Dioxazolone for the Synthesis of 1,2-Aminoalcohols via Iridium(III)-Catalyzed C(sp3 )-H Amidation

Vicinal aminoalcohols are widespread structural motifs in bioactive molecules. We report the development of a new dioxazolone reagent containing a p-nitrophenyldifluoromethyl group, which 1. displays a good safety profile; 2. shows a remarkably high reactivity in the oxime-directed iridium(III)-catalyzed amidation of unactivated C(sp3 )-H bonds; 3. leads to amide products which can be hydrolyzed under mild conditions. The amidation reaction is mild, general and compatible with both primary C-H bonds of tertiary and secondary alcohols, as well as secondary C-H bonds of cyclic secondary alcohols. This method provides an easy access to free 1,2-aminoalcohols after efficient and mild cleavage of the oxime directing group and activated amide.

Interweaving Visible-Light and Iron Catalysis for Nitrene Formation and Transformation with Dioxazolones

Herein, visible-light-driven iron-catalyzed nitrene transfer reactions with dioxazolones for intermolecular C(sp³ )-N, N=S, and N=P bond formation are described. These reactions occur with exogenous-ligand-free process and feature satisfactory to excellent yields (up to 99 %), an ample substrate scope (109 examples) under mild reaction conditions. In contrast to intramolecular C-H amidations strategies, an intermolecular regioselective C-H amidation via visible-light-induced nitrene transfer reactions is devised. Mechanistic studies indicate that the reaction proceeds via a radical pathway. Computational studies show that the decarboxylation of dioxazolone depends on the conversion of ground sextet state dioxazolone-bounding iron species to quartet spin state via visible-light irradiation.

Merging kinetic resolution with C-H activation: an efficient approach for enantioselective synthesis

In the last two decades tremendous progress has been made in transition-metal (TM)-catalyzed C-H bond functionalization, paving the way to design complex molecules. Despite significant advances, enantioselective C-H activation is still in the age of infancy. For the enantioselective synthesis, several TM catalyst based approaches are well known, including kinetic resolution (KR) and its advanced versions [dynamic kinetic resolution (DKR) and parallel kinetic resolution (PKR)]. These strategies have recently been successfully applied synergetically with the TM catalyzed C-H activation to achieve enantioselective synthesis in a more economical and sustainable way. This review will summarize the recent advancements made towards merging KR with TM-catalysed C-H activation for enantioselective synthesis.

Chiral Bicyclo[2.2.2]octane-Fused CpRh Complexes: Synthesis and Potential Use in Asymmetric C-H Activation

A new class of chiral cyclopentadienyl rhodium(I) complexes (CpRh^I ) bearing C₂ -symmetric chiral bridged-ring-fused Cp ligands was prepared. The complexes were successfully applied to the asymmetric C-H activation reaction of N-methoxybenzamides with quinones, affording a series of chiral hydrophenanthridinones in up to 82 % yield with up to 99 % ee. Interestingly, structure analysis reveals that the side wall of the optimal chiral CpRh^I catalyst is vertically more extended, horizontally less extended, and closer to the metal center in comparison with the classic binaphthyl and spirobiindanyl CpRh^I complexes, and may thus account for its superior catalytic performance.

Ultrasound-Facilitated Direct meta-C-H Functionalization of Arenes: A Time-Economical Strategy under Ambient Temperature with Improved Yield and Selectivity

The horizon of ultrasound-assistance has been expanded to palladium-catalyzed distal C-H functionalization of arenes. Compared to thermal conditions, operationally simple ultrasound mediated distal C-H functionalization occurred with a shorter reaction time and enhanced reactivity of reactants to give superior yields with improved selectivity both in terms of meta:others and mono:di. A wide variety of meta-functionalizations such as olefination, alkylation, acetoxylation, allylation and cyanation were successfully carried out under ambient temperature.

Enantioselective Cpx RhIII -Catalyzed Carboaminations of Acrylates

Enantioselective carboaminations of olefins constitute an attractive strategy for a rapid increase in molecular complexity from readily available starting materials. Reported here is an intermolecular asymmetric carboamination of acrylates using rhodium(III)-catalyzed alkenyl C-H activations of N-enoxysuccinimides to generate the nitrogen and carbon portion for the transfer. A rhodium complex equipped with a tailored bulky trisubstituted chiral Cp^x ligand ensures carboamination chemoselectivity as well high levels of enantioinduction. The transformation operates under mild reaction conditions at ambient temperatures and provides access to a variety of α-amino esters in good yields and excellent enantiomeric ratios of >99.5:0.5.

Diverse Approaches for Enantioselective C-H Functionalization Reactions Using Group 9 Cpx MIII Catalysts

Transition-metal-catalyzed C-H functionalization reactions with Cp*M^III catalysts (M=Co, Rh, Ir) have found a wide variety of applications in organic synthesis. Albeit the intrinsic difficulties in achieving catalytic stereocontrol using these catalysts due to their lack of additional coordination sites for external chiral ligands and the conformational flexibility of the Cp ligand, catalytic enantioselective C-H functionalization reactions using the Group 9 metal triad with Cp-type ligands have been intensively studied since 2012. In this minireview, the progress in these reactions according to the type of the chiral catalyst used are summarized and discussed. The development of chiral Cp^x ligands the metal complexes thereof, artificial metalloenzymes, chiral carboxylate-assisted enantioselective C-H activations, enantioselective alkylations assisted by chiral carboxylic acids or chiral sulfonates, and chiral transient directing groups are discussed.

Synthesis of Axially Chiral Styrenes through Pd-Catalyzed Asymmetric C-H Olefination Enabled by an Amino Amide Transient Directing Group

The atroposelective synthesis of axially chiral styrenes remains a formidable challenge due to their relatively lower rotational barriers compared to the biaryl atropoisomers. Herein, we describe the construction of axially chiral styrenes through Pd^II -catalyzed atroposelective C-H olefination, using a bulky amino amide as a transient chiral auxiliary. Various axially chiral styrenes were produced with good yields and high enantioselectivity (up to 95 % yield and 99 % ee). Carboxylic acid derivatives of the resulting axially chiral styrenes showed superior enantiocontrol over the biaryl counterparts in Co^III -catalyzed enantioselective C(sp³ )-H amidation of thioamide. Mechanistic studies suggest that C-H cleavage is the enantioselectivity-determining step.