Triple-Bond Insertion Triggers Highly Regioselective 1,4-Aminomethylamination of 1,3-Enynes with Aminals Enabled by Pd-Catalyzed C–N Bond Activation

Azido-alkynylation of alkenes through radical-polar crossover

We report an azido-alkynylation of alkenes allowing a straightforward access to homopropargylic azides by combining hypervalent iodine reagents and alkynyl-trifluoroborate salts. The design of a photocatalytic redox-neutral radical polar crossover process was key to develop this transformation. A variety of homopropargylic azides possessing electron-rich and -poor aryls, heterocycles or ether substituents could be accessed in 34-84% yield. The products are synthetically useful building blocks that could be easily transformed into pyrroles or bioactive amines.

Palladium-Catalyzed Ring-Closing Aminoalkylative Amination of Unactivated Aminoenynes

An efficient strategy for preventing the β-hydride elimination of alkylpalladium species by ligation of the palladium with adjacent amino-group was developed, which enabled a novel palladium-catalyzed ring-closing aminoalkylative amination of unactivated aminoenynes. The reaction is amenable to aminals, as well as aliphatic aldehydes with secondary amines, which provides straightforward access to structurally diverse exocyclic allenic amines bearing 5 to 12-membered N-heterocycles. With chiral phosphoramidite-ligated palladium complex as the catalyst, an enantioselective variant was achieved with up to 93 % ee. Simultaneously, synthetic transformations of the chiral products were also conducted to afford structurally unique spirodiamines including one pharmaceutically active molecule via axial-to-central chirality transfer.

Deaminative Arylation and Alkenyaltion of Aliphatic Tertiary Amines with Aryl and Alkenylboronic Acids via Nitrogen Ylides

Transition-metal-catalyzed Suzuki-Miyaura coupling has significantly advanced C-C bond formation and has been well recognized in organic synthesis, pharmaceuticals, materials science and other fields. In this rapid development, cross coupling without transition metal catalyst is a big challenge in this field, and using widely existing tertiary amines as electrophiles to directly couple with boronic acids has great hurdles yet significant application prospects. Herein, we report an efficient and general deaminative arylation and alkenylation of tertiary amines (propargyl amines, allyl amines and 1H-indol-3-yl methane amines) with ary and alkenylboronic acids enabled by difluorocarbene under transition-metal-free conditions. Preliminary mechanism experiments suggest that in situ formed difluoromethyl quaternary amine salt, nitrogen ylide and tetracoordinate boron species are the key intermediates, the subsequent 1,2-metallate shift and protodeboronation complete the new coupling reaction.

Advances in Catalytic C–F Bond Activation and Transformation of Aromatic Fluorides

The activation and transformation of C–F bonds in fluoro-aromatics is a highly desirable process in organic chemistry. It provides synthetic methods/protocols for the generation of organic compounds possessing single or multiple C–F bonds, and effective catalytic systems for further study of the activation mode of inert chemical bonds. Due to the high polarity of the C–F bond and it having the highest bond energy in organics, C–F activation often faces considerable academic challenges. In this mini-review, the important research achievements in the activation and transformation of aromatic C–F bond, catalyzed by transition metal and metal-free systems, are presented.

Cascade Multicomponent Assemblies Involving 1,3-Enynes via Auto-Tandem Palladium Catalysis

Here we report a three-component auto-tandem reaction of 1,3-enyne-tethered carbonyls, organoboronic reagents, and suitable nucleophiles catalyzed by palladium, proceeding through consecutive intramolecular vinylogous addition, Suzuki coupling, and allylic alkylation. This process exhibited high chemo- and regioselectivity with 1,3,4-trifunctionalization of the 1,3-enyne motif, and a wide range of 2H-chromenes, 1,2-dihydroquinolines, benzo[b]oxepines, 1,7-annulated indoles, and other frameworks were efficiently constructed in fair to good yields and E/Z selectivity.

Palladium-Catalyzed Aminomethylative Oppolzer-Type Cyclization of Enynes: Access to Aminomethylated Benzofulvenes

A novel palladium-catalyzed Oppolzer-type cyclization reaction aided by the aminomethyl cyclopalladated complex has been developed, which provides rapid access to functionalized benzofulvenes with excellent stereoselectivity. The corresponding products can undergo Diels-Alder reaction with maleimides, providing a series of complex polycyclic compounds with excellent regio- and stereoselectivities.

Catalytic Reactions Directed by a Structurally Well-Defined Aminomethyl Cyclopalladated Complex

The introduction of N-containing moieties into feedstock molecules to build nitrogenated functional molecules has always been widely studied by the organic chemistry community. Progress in this field paves new roads to the synthesis of N-containing molecules, which are of significant importance in biological activities and play vital roles in pharmaceuticals and functional materials. Remarkable progress has been achieved in the field of transition metal-catalyzed C-N bond-forming reactions, typified by alkene hydroamination and the aza-Wacker reaction. However, the poisoning effect of electron-donating amine substrates on late transition metal catalysts presents a key impediment to these reactions, thus limiting the scope of amine substrates to electron-deficient amide derivatives. To address this problem, our group developed a palladium-aminomethyl complex with a three-membered palladacycle structure that allowed for the incorporation of electron-rich amine building blocks via C-C bond instead of C-N bond construction. This Account details the discovery of the well-defined aminomethyl cyclopalladated complex and recapitulates its applications for the catalysis of a series of aminomethylation reactions. We highlight how the understanding of the fundamental structural properties of the defined complex guided us toward tuning the reactivity of nucleophiles to initiate aminomethylation in different modes. Moreover, principles of designing and establishing further cascade reactions are also described.Aminomethyl cyclopalladated complexes can be prepared via the oxidative addition of aminals or N,O-acetals to Pd⁰ species. Thorough structural investigations by single-crystal X-ray diffraction analysis of the cyclopalladated complex suggest the presence of both aminomethylene-Pd^II (3-membered-ring) and Pd⁰-iminium (π-ligated) resonance forms, which indicates that both the palladium center and the methylene site are electrophilic. This is further verified by analysis of charge distribution. Two general types of reactions can be established, differing by the selective affinity of the nucleophiles to the two electrophilic positions, which is relevant to the "hardness suitability" of the nucleophiles with each electrophilic site. Softer nucleophiles such as alkenes prefer to attack the palladium center to initiate the reaction, mainly via migratory insertion into the Pd-C bond on the 3-membered ring with high strain. Through tandem β-hydride or reductive elimination, the Heck-type aminomethylation of styrenes, the aminomethylalkoxylation of electron-rich olefins, and even the aminomethylamination of allenes, dienes, enynes, and carbenoids with full atom-economy have been realized in line with this reaction mode. In contrast, harder nucleophiles tend to attack the harder electrophilic methylene site, leading to the aminomethylation of electron-deficient dienes. For secondary amines, a "C-N bond metathesis" process would be furnished through a reductive elimination, 1,3-proton transfer, and oxidative addition sequence. More intriguingly, when using appropriate "dinucleophile" substrates such as electron-rich amine-tethered dienes, sequential C-N bond metathesis and intramolecular insertion would occur to furnish Pd-catalyzed annulation reactions, which exhibits both the hard and soft nucleophile reactivities mentioned above. These transformations provide convenient methods for the preparation of N-containing molecules, such as amines, diamines, amino acetals, and multiple types of N-heterocycles.

Decarboxylative 1,4-carbocyanation of 1,3-enynes to access tetra-substituted allenes via copper/photoredox dual catalysis

We disclose herein the first example of merging photoredox catalysis and copper catalysis for radical 1,4-carbocyanations of 1,3-enynes. Alkyl N-hydroxyphthalimide esters are utilized as radical precursors, and the reported mild and redox-neutral protocol has broad substrate scope and remarkable functional group tolerance. This strategy allows for the synthesis of diverse multi-substituted allenes with high chemo- and regio-selectivities, also permitting late stage allenylation of natural products and drug molecules.

Palladium-Catalyzed Aminomethylation and Cyclization of Enynol to O-Heterocycle Confined 1,3-Dienes

The rational tuning of the electrophilicity of the allylpalladium intermediates enables the regioselectively intramolecular 1,2-addition of enynol in the presence of aminal. This aminomethylation and cyclization reaction via C-N bond activation and intramolecular nucleophilic addition provides a rare example for the synthesis of O-containing heterocycle-confined 1,3-dienes, which is of synthetic potential for further derivatization. The method possesses broad substrate generality as well as functional group compatibility and efficiently affords a wide range of desired products with 5-, 6-, and 8-membered O-containing heterocycles with various functional groups.

Visible-Light-Induced 1,4-Hydroxysulfonylation of Vinyl Enynes with Sulfonyl Chlorides: The Bridge of Chloride Linking Water and Enynes

A novel visible-light-induced 1,4-hydroxysulfonylation of vinyl enynes with sulfonyl chlorides has been established, providing a highly efficient protocol to access multisubstituted sulfonyl allenic alcohols. Control experiments and mechanistic studies disclose that the target products result from sequential reactions of hydroxyl and tosyl radicals, among which chloride anion plays a key role to generate the requisite ^•OH, thus bridging water and enynes. Moreover, the vinyl pendant is believed to decisively affect the site-selectivity of hydroxyl radical.

Gold-Catalyzed Cascade Cyclization and 1,3-Difunctionalization To Access Polysubstituted Furans

Gold-catalyzed cascade cyclization and 1,3-difunctionalization of 2-(1-alkyny)-2-alken-1-ones with N,O-acetals are described, leading to the discovery of novel 1,3-oxyaminomethylation and 1,3-aminomethylamination. Typically, by varying the reaction conditions especially the gold catalysts, these two distinct reaction pathways can be controlled in a selective manner. Moreover, tandem cyclization and 1,4-oxyaminomethylation have also been achieved.

Selective cleavage and reconstruction of C–N/C–C bonds in saturated cyclic amines: tunable synthesis of lactams and functionalized acyclic amines

Selective cleavage of C–N/C–C bonds in saturated cyclic amines for the tunable synthesis of lactams and functionalized acyclic amines under the promotion of oxoammonium salt and TBHP in the presence of different additives have been developed.

Cu(I)-Catalyzed addition-cycloisomerization difunctionalization reaction of 1,3-enyne-alkylidenecyclopropanes (ACPs)

A copper(i)-catalyzed three-component addition-cycloisomerization difunctionalization reaction of 1,3-enyne-ACPs with Togni I reagent and TMSCN under mild reaction conditions has been developed, affording 3-trifluoroethylcyclopenta[b]naphthalene-4-carbonitrile derivatives. The reaction proceeded through a copper(i)-catalyzed 1,4-addition of conjugated 1,3-enynes via a radical relay process and aromatic cycloisomerization of allene-ACP intermediates.

Conversion of anilines to chiral benzylic amines via formal one-carbon insertion into aromatic C-N bonds

Insertion of atoms into aromatic carbon-nitrogen bonds is an appealing method for the synthesis of nitrogen-containing molecules and it has the advantage of the availability and abundance of anilines. However, the direct cleavage of aromatic carbon-nitrogen bonds is challenging due to the particularly inert and stable nature of these bonds. Here we report a formal, enantioselective one-carbon insertion into an aromatic carbon-nitrogen bond via an aromaticity dissembly-reconstruction process to directly convert anilines to chiral α-branched benzylic amines. The process involves oxidative dearomatization of para-substituted anilines, chiral sulfur ylide-mediated asymmetric aziridination, and subsequent rearrangement. Chiral sulfur ylides serve as one-carbon insertion units.

Atom insertion into aromatic carbon-nitrogen (C-N) bonds is useful for the synthesis of nitrogen-containing molecules, but challenging due to the inert nature of these bonds. Here, the authors report one-carbon insertion into aromatic C-N bonds to directly convert anilines to chiral benzylic amines.

Generation, optimization and characterization of novel anti-prion compounds

Prions are misfolded proteins involved in neurodegenerative diseases of high interest in veterinary and public health. In this work, we report the chemical space exploration around the anti-prion compound BB 0300674 in order to gain an understanding of its Structure Activity Relationships (SARs). A series of 43 novel analogues, based on four different chemical clusters, were synthetized and tested against PrP^Sc and mutant PrP toxicity assays. From this biological screening, two compounds (59 and 65) emerged with a 10-fold improvement in anti-prion activity compared with the initial lead compound, presenting at the same time interesting cell viability.

Palladium-Catalyzed Fluoroarylation of gem-Difluoroenynes to Access Trisubstituted Trifluoromethyl Allenes

Various new transformations of gem-difluoroalkenes leading to trifluoromethyl substituted compounds have been well established in the past years. However, the development of new transformations of gem-difluoroenynes lags much behind. Herein is reported the fluoroarylation of 1,1-difluoro-1,3-enynes with aryl halides in the presence of silver fluoride affording trisubstituted trifluoromethyl allenes under the catalysis of palladium. The reaction features mild conditions, high functional-group tolerance, and high regioselectivity.

Pyridine-Enabled C-N Bond Activation for the Rapid Construction of Amides and 4-Pyridylglyoxamides by Cooperative Palladium/Copper Catalysis

A pyridine-enabled C-N bond activation of peptidomimetics employing cooperative palladium/copper catalysis in water is developed. Diverse amides and 4-pyridylglyoxamides are simultaneously synthesized through two steps from commercially available materials in a rapid, environmentally friendly, and high atom-economical manner.

Palladium-Catalyzed Multistep Tandem Carbonylation/N-Dealkylation/Carbonylation Reaction: Access to Isatoic Anhydrides

A novel and efficient synthesis of isatoic anhydride derivatives was developed via palladium-catalyzed multistep tandem carbonylation/N-dealkylation/carbonylation reaction with alkyl as the leaving group and tertiary anilines as nitrogen nucleophiles. This approach features good functional group compatibility and readily available starting materials. Furthermore, it provided a convenient approach for the synthesis of biologically and medicinally useful evodiamine.

Direct access to pentenedinitriles via Ni-catalyzed dihydrocyanation of 1,3-enynes

A highly regio- and stereoselective dihydrocyanation of 1,3-enynes was implemented by nickel/diphosphine catalysts.

Double C-N bond cleavages of N-alkyl 4-oxopiperidinium salts: access to unsymmetrical tertiary sulfonamides

In this paper, regiospecific, double intraannular C-N bond cleavages of N-alkyl 4-oxopiperidinium salts are presented. The reaction sequence involves a charge-transfer complex, in situ formed between sulfonyl chloride and N-methylmorpholine, which induces S-Cl bond homolysis of sulfonyl chloride, yielding a reactive sulfonyl radical that further induces the double C-N bond cleavages of N-alkyl 4-oxopiperidinium salt. The secondary amine thus produced was trapped by sulfonyl chloride to yield the desired sulfonamide product. The key feature of this protocol is that two intraannular C-N bonds of the 4-oxopiperidine ring are cleaved in one step under metal- and oxidant-free conditions.

Copper Hydride Catalyzed Enantioselective Synthesis of Axially Chiral 1,3-Disubstituted Allenes

The general enantioselective synthesis of axially chiral disubstituted allenes from prochiral starting materials remains a long-standing challenge in organic synthesis. Here, we report an efficient enantio- and chemoselective copper hydride catalyzed semireduction of conjugated enynes to furnish 1,3-disubstituted allenes using water as the proton source. This protocol is sufficiently mild to accommodate an assortment of functional groups including keto, ester, amino, halo, and hydroxyl groups. Additionally, applications of this method for the selective synthesis of monodeuterated allenes and chiral 2,5-dihydropyrroles are described.

ortho-Amide-directed 2,4-dibromohydration of conjugated enynes

A NBS-mediated and ortho-amide-directed 2,4-dibromohydration of enynes is described in this paper.