Photoredox cooperative N-heterocyclic carbene/palladium-catalysed alkylacylation of alkenes

Efficient synergistic photocatalysis of supported Ir/Pd dual metal catalysts based on flexible polymeric brushes

Dual metal complexes, Pd(PPh3)4 and Ir(ppy)2(dtbbpy)PF6, are simultaneously anchored onto SiO2-grafted poly(4-vinyl benzyl chloride) (PVBC) brushes via Friedel-Crafts alkylation in one pot, affording supported dual Pd/Ir complexes which gave good yields in photocatalytic α-allylation.

Enantioselective Three-Component α-Allylic Alkylation of α-Amino Esters by Synergistic Photoinduced Pd/Carbonyl Catalysis

Photoinduced excited-state Pd catalysis has emerged as an intriguing strategy for unlocking new reactivity potential of simple substrates. However, the related transformations are still limited and the enantiocontrol remains challenging. Organocatalysis displays unique capability in substrate activation and stereocontrol. Combination of organocatalysis and photoinduced excited-state Pd catalysis may provide opportunities to develop new enantioselective reactions from simple substrates. By applying cooperative triple catalysis including excited-state Pd catalysis, ground-state Pd catalysis, and carbonyl catalysis, we have successfully realized enantioselective α-allylic alkylation of α-amino esters with simple styrene and alkyl halide starting materials. The reaction allows rapid modular assembly of the three reaction partners into a variety of chiral quaternary α-amino esters in good yields with 90-99 % ee, without protecting group manipulations at the active NH2 group. The cooperation of the chiral pyridoxal catalyst and the chiral phosphine ligand accounts for the excellent chirality induction.

Iron-Mediated Dialkylation of Alkenylarenes with Benzyl Bromides

We disclose a method for the dibenzylation of alkenylarenes with benzyl bromides using iron powder. This reaction generates branched alkyl scaffolds adorned with functionalized aryl rings through the formation of two new C(sp3)-C(sp3) bonds at the vicinal carbons of alkenes. This protocol tolerates electron-rich, electron-neutral, and electron-poor benzyl bromides and alkenylarenes. Mechanistic studies suggest the formation of benzylic radical intermediates as a result of single-electron transfer from the iron, which is intercepted by alkenylarenes.

Visible light-mediated organocatalyzed 1,3-aminoacylation of cyclopropane employing N-benzoyl saccharin as bifunctional reagent

The carboamination of unsaturated molecules using bifunctional reagents is considered an attractive approach for the synthesis of nitrogen-containing compounds. However, bifunctional C-N reagents have never been employed in the carboamination of cyclopropane. In this study, we use an N-heterocyclic carbene (NHC), N-benzoyl saccharin, as a bifunctional reagent and a photoredox catalyst for a dual-catalyzed 1,3-aminoacylation of cyclopropane. NHCs play multiple roles, functioning as Lewis base catalysts to activate C-N bonds, promoting the oxidative quenching process of PC*, and acting as efficient acyl radical transfer catalysts for the formation of C-C bonds. The oxidative quenching process between the excited-state PC* and acyl NHC adduct is the key to the photooxidation generality of aryl cyclopropanes.

Carbene and photocatalyst-catalyzed 3-acylation of indoles for facile access to indole-3-yl aryl ketones

3-Acyl indoles play important roles in both organic synthesis and diverse types of functional molecules. Herein, a facile nitrogen heterocyclic carbene (NHC) and photocatalyst cooperatively-catalyzed 3-acylation of indoles was disclosed. The reaction proceeded via radical cross-coupling of indole-based aryl radical cations with NHC-bound ketyl radical species, which are less explored in radical NHC catalysis. The reaction exhibits mild reaction conditions, broad substrate scope, and good functional group tolerance. Mechanistic studies support our proposed reaction pathway. The synthesis of structurally diverse analogs of an aldose reductase inhibitor and antibacterial activity investigation further demonstrated the utility of the current acylation reaction.

N-Heterocyclic Carbene/Transition Metal Dual Catalysis

N-heterocyclic carbene catalysis has been developed as a versatile method for the enantioselective synthesis of complex organic molecules in organic chemistry. Merging of N-heterocyclic carbene catalysis with transition metal catalysis holds the potential to achieve unprecedented transformations with broad substrate scope and excellent stereoselectivity, which are unfeasible with individual catalyst. Thus, this dual catalysis has attracted increasing attention, and numerous elegant dual catalytic systems have been established. In this review, we summarize the recent achievements of dual NHC/transition metal catalysis, including the reaction design, mechanistic studies and practical applications.

Photoinduced Palladium-Catalyzed Radical Germylative Arylation of Alkenes with Chlorogermanes

We describe a visible light-induced palladium-catalyzed radical germylative arylation of alkenes with easily accessible chlorogermanes. This protocol provides expedient access to germanium-substituted indolin-2-ones in good to excellent yields under mild reaction conditions. The key step for this strategy lies in the reductive activation of germanium-chloride bonds with an excited palladium complex under visible light irradiation. The involvement of germanium radicals was evidenced by electron paramagnetic resonance spectroscopy experiments.

Catalytic Asymmetric Cascade Dearomatization of Indoles via a Photoinduced Pd-Catalyzed 1,2-Bisfunctionalization of Butadienes

Photoinduced Pd-catalyzed bisfunctionalization of butadienes with a readily available organic halide and a nucleophile represents an emerging and attractive method to assemble versatile alkenes bearing various functional groups at the allylic position. However, enantiocontrol and/or diastereocontrol in the C-C or C-X bond-formation step have not been solved due to the open-shell process. Herein, we present a cascade asymmetric dearomatization reaction of indoles via photoexcited Pd-catalyzed 1,2-biscarbonfunctionalization of 1,3-butadienes, wherein asymmetric control on both the nucleophile and electrophile part is achieved for the first time in photoinduced bisfunctionalization of butadienes. This method delivers structurally novel chiral spiroindolenines bearing two contiguous stereogenic centers with high diastereomeric ratios (up to >20 : 1 dr) and good to excellent enantiomeric ratios (up to 97 : 3 er). Experimental and computational studies of the mechanism have confirmed a radical pathway involving excited-state palladium catalysis. The alignment and non-covalent interactions between the substrate and the catalyst were found to be essential for stereocontrol.

N-Heterocyclic carbene/palladium synergistic catalysis in organic synthesis

The cooperation of two distinct catalytic cycles to activate different reactive centers leading to a chemical transformation has been classified as synergistic catalysis. The synergistic combination of NHC with palladium catalysis has emerged as a powerful strategy in the last few years. Merging the ability of NHCs to inverse the polarity of a functional group with the unique reactivity of palladium enables transformations that cannot be accomplished by either of these catalysts alone. Despite the associated challenges, such as quenching of catalysts, reactivity mismatch etc., significant development has been achieved in the field of NHC/Pd synergistic catalysis. The recent incorporation of photoredox catalysis with NHC/Pd synergistic catalysis has further advanced this area. This review highlights the developments made in the area of NHC/Pd synergistic catalysis.

Stereoselective Fluoroalkylacylation of Alkynes via Cooperative N-Heterocyclic Carbene/Palladium Catalysis

Herein, a cooperative N-heterocyclic carbene- and palladium-catalyzed three-component reaction of alkynes with aldehydes and fluoroalkyl iodides is developed. A series of biologically valuable CF2R-incorporated α-substituted enones was obtained in moderate to good yields. This mild catalytic method exhibits exclusive regio- and stereoselectivity, excellent functional group tolerance, and a broad substrate scope including terminal and internal alkynes. Mechanistic investigations disclose that this alkyne fluoroalkylacylation proceeds via a radical relay process in which vinyl iodides serve as putative reaction intermediates.

Phosphonylacylation of Alkenes Enabled by Visible-Light-Induced N-Heterocyclic Carbene Catalysis

Herein, we report the phosphonylacylation of alkenes via visible-light-induced N-heterocyclic carbene (NHC) catalysis to afford a series of γ-ketophosphonates in moderate to good yields. This protocol features mild conditions, free of photocatalyst, and good compatibility of functional groups. The excited Breslow enolate intermediate was proposed to undergo single-electron transfer with oxime phosphonate to generate the corresponding ketyl radical and phosphonyl radical.

Light-Driven Enantioselective Carbene-Catalyzed Radical-Radical Coupling

An enantioselective carbene-catalyzed radical-radical coupling of acyl imidazoles and racemic Hantzsch esters is disclosed. This method involves the coupling of an N-heterocyclic carbene-derived ketyl radical and a secondary sp3 -carbon radical and allows access to chiral α-aryl aliphatic ketones in moderate-to-good yields and enantioselectivities without any competitive epimerization. The utility of this protocol is highlighted by the late-stage functionalization of various pharmaceutical compounds and is further demonstrated by the transformation of the enantioenriched products to biologically relevant molecules. Computational investigations reveal the N-heterocyclic carbene controls the double-facial selectivity of the ketyl radical and the alkyl radicals, respectively.

Synthesis of α-Branched Enones via Chloroacylation of Terminal Alkenes

Here, we show the conversion of unactivated alkenes into α-branched enones via regioselective chloroacylation with acyl chlorides. The method relies upon the initial in situ generation of chlorine radicals directly from the acyl chloride precursor under cooperative nickel/photoredox catalysis. Subsequent HCl elimination provides enones and α,β-unsaturated esters that are not accessible via the conventional acylation approaches that provide the other, linear constitutional isomer.

Visible-Light-Mediated Three-Component Decarboxylative Coupling Reactions to Synthesize 1,4-Diol Monoethers

An efficient approach for 1,2-difunctionalization of aromatic olefins and the synthesis of functionalized 1,4-diols monoethers has been established via a photoinduced three-component reaction of an α-alkoxycarboxylic acid, an aromatic olefin, and an aldehyde. The reaction proceeds by photoinduced oxidative decarboxylation of the carboxylic acid followed by the addition of the α-alkoxyalkyl radical to the olefin, one-electron reduction of the addition radical, and the nucleophilic attack of the resulting carbanion to the aldehyde. Besides the convenient one-pot protocol of the three-component reaction, this method offers several other advantages, including good functional group tolerance for the three substrates, gentle reaction conditions, and ease of scaling up. The reaction mechanism has been investigated through free radical trapping experiment and isotope labeling experiments.

NHC-Catalyzed Regioselective Intramolecular Radical Cyclization Reaction for the Synthesis of Benzazepine Derivatives

A novel and efficient strategy for the synthesis of a series of structurally interesting benzazepine derivatives via an N-heterocyclic carbene-catalyzed regioselective intramolecular radical cyclization has been developed. This protocol features good regioselectivity, good functional-group compatibility, and wide substrate scope, providing a transition-metal- and oxidant-free pathway to access the seven-membered rings under mild reaction conditions. Additionally, further transformation of benzazepines and a large-scale experiment were also conducted.

[IPr#-PEPPSI]: A Well-Defined, Highly Hindered and Broadly Applicable Pd(II)-NHC (NHC = N-Heterocyclic Carbene) Precatalyst for Cross-Coupling Reactions

In this Special Issue, "Featured Papers in Organometallic Chemistry", we report on the synthesis and characterization of [IPr#-PEPPSI], a new, well-defined, highly hindered Pd(II)-NHC precatalyst for cross-coupling reactions. This catalyst was commercialized in collaboration with MilliporeSigma, Burlington, ON, Canada (no. 925489) to provide academic and industrial researchers with broad access to reaction screening and optimization. The broad activity of [IPr#-PEPPSI] in cross-coupling reactions in a range of bond activations with C-N, C-O, C-Cl, C-Br, C-S and C-H cleavage is presented. A comprehensive evaluation of the steric and electronic properties is provided. Easy access to the [IPr#-PEPPSI] class of precatalysts based on modular pyridine ligands, together with the steric impact of the IPr# peralkylation framework, will facilitate the implementation of well-defined, air- and moisture-stable Pd(II)-NHC precatalysts in chemistry research.

Ketones from aldehydes via alkyl C(sp3)-H functionalization under photoredox cooperative NHC/palladium catalysis

Direct synthesis of ketones from aldehydes features high atom- and step-economy. Yet, the coupling of aldehydes with unactivated alkyl C(sp³)-H remains challenging. Herein, we develop the synthesis of ketones from aldehydes via alkyl C(sp³)-H functionalization under photoredox cooperative NHC/Pd catalysis. The two-component reaction of iodomethylsilyl alkyl ether with aldehydes gave a variety of β-, γ- and δ-silyloxylketones via 1,n-HAT (n = 5, 6, 7) of silylmethyl radicals to generate secondary or tertiary alkyl radicals and following coupling with ketyl radicals from aldehydes under photoredox NHC catalysis. The three-component reaction with the addition of styrenes gave the corresponding ε-hydroxylketones via the generation of benzylic radicals by the addition of alkyl radicals to styrenes and following coupling with ketyl radicals. This work demonstrates the generation of ketyl radical and alkyl radical under the photoredox cooperative NHC/Pd catalysis, and provides two and three component reactions for the synthesis of ketones from aldehydes with alkyl C(sp³)-H functionalization. The synthetic potential of this protocol was also further illustrated by the late-stage functionalization of natural products.

Silver/chiral pyrrolidinopyridine relay catalytic cycloisomerization/(2 + 3) cycloadditions of enynamides to asymmetrically synthesize bispirocyclopentenes as PDE1B inhibitors

Significant progress has been made in asymmetric synthesis through the use of transition metal catalysts combined with Lewis bases. However, the use of a dual catalytic system involving 4-aminopyridine and transition metal has received little attention. Here we show a metal/Lewis base relay catalytic system featuring silver acetate and a modified chiral pyrrolidinopyridine (PPY). It was successfully applied in the cycloisomerization/(2 + 3) cycloaddition reaction of enynamides. Bispirocyclopentene pyrazolone products could be efficiently synthesized in a stereoselective and economical manner (up to >19:1 dr, 99.5:0.5 er). Transformations of the product could access stereodivergent diastereoisomers and densely functionalized polycyclic derivatives. Mechanistic studies illustrated the relay catalytic model and the origin of the uncommon chemoselectivity. In subsequent bioassays, the products containing a privileged drug-like scaffold exhibited isoform-selective phosphodiesterase 1 (PDE1) inhibitory activity in vitro. The optimal lead compound displayed a good therapeutic effect for ameliorating pulmonary fibrosis via inhibiting PDE1 in vivo.

On the Redox Properties of the Dimers of Thiazol-2-ylidenes That Are Relevant for Radical Catalysis

We report the isolation and study of dimers stemming from popular thiazol-2-ylidene organocatalysts. The model featuring 2,6-di(isopropyl)phenyl (Dipp) N-substituents was found to be a stronger reducing agent (E_ox = -0.8 V vs SCE) than bis(thiazol-2-ylidenes) previously studied in the literature. In addition, a remarkable potential gap between the first and second oxidation of the dimer also allows for the isolation of the corresponding air-persistent radical cation. The latter is an unexpected efficient promoter of the radical transformation of α-bromoamides into oxindoles.

Direct allylic acylation via cross-coupling involving cooperative N‑heterocyclic carbene, hydrogen atom transfer, and photoredox catalysis

Herein, we report a mild, operationally simple, multicatalytic method for the synthesis of β,γ-unsaturated ketones via allylic acylation of alkenes. Specifically, the method combines N‑heterocyclic carbene catalysis, hydrogen atom transfer catalysis, and photoredox catalysis for cross-coupling reactions between a wide range of feedstock carboxylic acids and readily available olefins to afford structurally diverse β,γ-unsaturated ketones without olefin transposition. The method could be used to install acyl groups on highly functionalized natural-product-derived compounds with no need for substrate pre-activation, and C-H functionalization proceed with excellent site selectivity. To demonstrate the potential applications of the method, we convert a representative coupling product into various useful olefin synthons.

Synthesis of β-Ketonitriles via N-Heterocyclic-Carbene-Catalyzed Radical Coupling of Aldehydes and Azobis(isobutyronitrile)

Herein, an NHC (N-heterocyclic carbene)-catalyzed radical coupling reaction between aldehydes and azobis(isobutyronitrile) (AIBN) has been developed. This method provides an efficient and convenient approach for the synthesis of β-ketonitriles containing a quaternary carbon center (31 examples, up to >99% yield) utilizing commercially available substrates. This protocol features broad substrate scope, good functional group tolerance, and high efficiency under metal-free and mild reaction conditions.

Construction of C(CO)-C(CO) Bond via NHC-Catalyzed Radical Cross-Coupling Reaction

A C(sp²)-C(sp²) bond can be constructed via a photoredox/N-heterocyclic carbene (NHC)-cocatalyzed radical cross-coupling reaction, which provides a complementary strategy to classic electron pair processes. The present protocol represents the first example of an NHC-catalyzed two-component radical cross-coupling reaction involving C(sp²)-centered radical species. The decarboxylative acylation of oxamic acid with acyl fluoride was conducted under mild conditions and allowed the preparation of a variety of useful α-keto amides, including sterically congested ones.

Iminoacylation of Alkenes via Photoredox N-Heterocyclic Carbene Catalysis

The iminoacylation of alkenes via photoredox N-heterocyclic carbene catalysis is developed with the employment of alkene-tethered α-imino-oxy acids and acyl imidazoles. The corresponding substituted 3,4-dihydro-2H-pyrroles were afforded in moderate to good yields with good to high diastereoselectivities in most cases. The reaction involves the 5-exo-trig radical cyclization of an alkene-tethered iminyl radical and the following coupling with a ketyl radical from acyl imidazole under NHC catalysis.