Palladium‐Catalyzed Insertion of an Allene into an Aminal: Aminomethylamination of Allenes by CN Bond Activation

Cobalt-catalyzed amination of aziridines and azetidines toward 1,2- and 1,3-diamines

Diamines play important roles in synthetic organic chemistry and thus facilitate life and materials sciences. Herein we report a cobalt-catalyzed ring opening, nucleophilic amination of aziridines and azetidines with N-fluorosulfonamides toward a wide range of 1,2- and 1,3-diamine derivatives in moderate to good yields under mild conditions.

Silver-catalyzed chemodivergent assembly of aminomethylated isochromenes and naphthols

A silver-catalyzed chemodivergent cyclization of alkyne-tethered aldehydes with aminals to aminomethylated 1H-isochromenes and naphthols is described by tuning the reaction conditions. The reaction exhibits broad substrate generality and functional group compatibility. Mechanistic studies have disclosed that the aminomethylated naphthols are generated from the resulting N,O-aminal containing isochromenes via a silver-catalyzed unusual rearrangement process.

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.

Metal-free enaminone C-N bond cyanation for the stereoselective synthesis of (E)- and (Z)-β-cyano enones

A highly practical method for C-CN bond formation by C-N bond cleavage on enaminones leading to the efficient synthesis of β-cyano enones is developed. The reactions take place efficiently to provide (E)-β-cyano enones with only a molecular iodine catalyst. In addition, the additional employment of oxalic acid enables the selective synthesis of (Z)-β-cyano enones.

Palladium-Catalyzed Ring-Closing Reaction for the Synthesis of Saturated N-Heterocycles with Aminodienes and N,O-Acetals

An efficient palladium-catalyzed ring-closing reaction of aminodienes with N,O-acetals for the synthesis of saturated N-heterocycles is described. The reaction is consistently operated at room temperature and tolerates a wide range of functional groups with volatile MeOH as the sole byproduct. This method provides rapid and practical access to a broad range of saturated N-heterocycles with diverse structural backbones that are useful building blocks in natural product synthesis and drug discovery.

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.

Bifunctional reagents in organic synthesis

Developments in synthetic chemistry are increasingly driven by improvements in the selectivity and sustainability of transformations. Bifunctional reagents, either as dual coupling partners or as a coupling partner in combination with an activating species, offer an atom-economic approach to chemical complexity, while suppressing the formation of waste. These reagents are employed in organic synthesis thanks to their ability to form complex organic architectures and empower novel reaction pathways. This Review describes several key bifunctional reagents by showcasing selected cornerstone research areas and examples, including radical reactions, C-H functionalization, cross-coupling, organocatalysis and cyclization reactions.

Deconstructive Functionalizations of Unstrained Carbon-Nitrogen Cleavage Enabled by Difluorocarbene

Transition-metal- or oxidant-promoted deconstructive functionalizations of noncyclic carbon-nitrogen bonds are well established, usually only leaving one moiety functionalized toward the final product. In contrast, concomitant C- and N-functionalizations via the unstrained C(sp3)-N bond under metal- and oxidant-free conditions are very rare, which would favorably confer versatility and product diversity. Disclosed herein is the first difluorocarbene-induced deconstructive functionalizations embodying successive C(sp3)-N bond cleavage of cyclic amines and synchronous functionalization of both constituent atoms which would be preserved in the eventual molecular outputs under transition-metal-free and oxidant-free conditions. Correspondent access to deuterated formamides with ample isotopic incorporation was demonstrated by a switch to heavy water which is conceivably useful in pharmaceutical sciences. The current strategy remarkably administers a very convenient, operationally simple and novel method toward molecular diversity from readily available starting materials. Therefore, we project that these findings would be of broad interest to research endeavors encompassing fluorine chemistry, carbene chemistry, C-N bond activation, as well as medicinal chemistry.

A novel one-pot method for the stereoselective synthesis of tetrahydropyrimidinones in a low melting mixture

A direct and metal free one-pot method has been developed for the stereoselective synthesis of tetrahydropyrimidinone derivatives from a vinyl arene and formaldehyde using a tartaric acid-dimethylurea (TA : DMU) melt as a green reaction medium. The substrate scope of this method is very general and the tetrahydropyrimidinone (THPM) derivatives are synthesized in good yields with a high degree of diastereoselectivity. In this reaction, the melt plays a triple role as the solvent, catalyst and reagent.

The DFT Quest for Possible Reaction Pathways, Catalytic Species, and Regioselectivity in the InCl3-Catalyzed Cycloaddition of N-Tosyl Formaldimine with Olefins or Allenes

The present work focuses on a theoretical investigation of the plausible mechanism, determination of catalytically active species, and understanding of the regioselectivity in the InCl₃-catalyzed cycloaddition of N-tosyl formaldimine with alkenes or allenes. InCl₃ and InCl₂⁺ coordinated by dichloroethane (InCl₂⁺-DCE) were investigated as model catalytic systems. DFT data supported that InCl₂⁺-DCE represent the plausible in situ generated catalytic species. The catalytic cycle starts from the coordination of N-tosyl formaldimine to InCl₂⁺-DCE, generating an In-complexed iminium intermediate. This then undergoes intermolecular reaction (aza-Prins) with alkene substrate to form a carbocation intermediate, which is chemoselectively attacked by the second N-tosyl formaldimine molecule to form a formaldiminium intermediate. In a final step, this intermediate undergoes the ring closure, leading to hexahydropyrimidine along with the regeneration of catalyst. In addition, our DFT results indicate that N-tosyl formaldimine not only acts as a reactant but also accelerates the 1,3-H-shift as a proton acceptor, giving an experimentally observed allylamide product. Also, the "iminium/alkene/imine" path was supported by calculation results for diastereoselective [2 + 2 + 2] reaction using an internal alkene. Finally, the regioselectivity of the InCl₃-catalyzed cycloaddition using allenes along with N-tosyl formaldimine was also analyzed.

A General Acid-Mediated Hydroaminomethylation of Unactivated Alkenes and Alkynes

In comparison to the extensively studied metal-catalyzed hydroamination reaction, hydroaminomethylation has received significantly less attention despite its considerable potential to streamline amine synthesis. State-of-the-art protocols for hydroaminomethylation of alkenes rely largely on transition-metal catalysis, enabling this transformation only under highly designed and controlled conditions. Here we report a broadly applicable, acid-mediated approach to the hydroaminomethylation of unactivated alkenes and alkynes. This methodology employs cheap, readily available, and bench-stable reactants and affords the desired amines with excellent functional group tolerance and impeccable regioselectivity. The broad scope of this transformation, as well as mechanistic investigations and in situ domino functionalization reactions are reported.

Copper-catalyzed 1,3-aminoazidation of arylcyclopropanes: a facile access to 1,3-diamine derivatives

Copper-catalyzed 1,3-aminoazidation of arylcyclopropanes with N-fluorobenzenesulfonimide (NFSI) and trimethylsilyl azide (TMSN₃) has been developed, providing various 1,3-diamine derivatives in moderate to good yields under mild conditions.

Palladium-catalyzed formal insertion of carbenoids into N,O-aminals: direct access to α-alkoxy-β-amino acid esters

A new and efficient reaction has been developed to synthesize α-alkoxy-β-amino acid esters via palladium-catalyzed formal insertion of carbenoids into N,O-aminals.

One-step construction of molecular complexity by tert-butyl nitrite (TBN)-initiated cascade α,β-sp3 C-H bond difunctionalization and C-N bond cleavage

TBN-initiated cascade multifunctionalization of inert C-H and C-N bonds in N-arylazacycles has been realized under transition-metal free conditions, in which the synthetically useful oxime, N-NO and aldehyde functional groups were constructed in only one synthetic step.

Formal group insertion into aryl C‒N bonds through an aromaticity destruction-reconstruction process

Given the abundance and the ready availability of anilines, the selective insertion of atoms into the aryl carbon–nitrogen bonds will be an appealing route for the synthesis of nitrogen-containing aromatic molecules. However, because aryl carbon–nitrogen bonds are particularly inert, anilines are normally activated by conversion to more reactive intermediates such as aryldiazonium salts to achieve functionalization of the aryl carbon–nitrogen bonds, but the nitrogen atom is usually not incorporated into products, instead being discarded. The selective insertion of groups into aryl carbon–nitrogen bonds remains an elusive challenge and an unmet need in reaction design. Here we show an aromaticity destruction-reconstruction process that selectively inserts a trimethylenemethane (TMM) group into the aromatic carbon–nitrogen bond of anilines concomitant with a benzylic carbon–hydrogen bond functionalization. This process provides a transformative mode for anilines, and the insertion products are versatile precursor to various nitrogen-containing aromatic molecules through simple conversions.

Activation of C–N bonds of anilines requires transformation of the amino group into a more reactive functionality. Here, the authors report an aromaticity destruction-reconstruction process that converts abundant anilines into valuable amines through group insertion into the C–N bond and benzylic C–H functionalization.

Copper-Catalyzed Difunctionalization of Allenes with Sulfonyl Iodides Leading to ( E)-α-Iodomethyl Vinylsulfones

A highly regioselective iodosulfonylation of allenes in the presence of CuI and 1,10-phenanthroline has been developed for the synthesis of various useful ( E)-α-iodomethyl vinylsulfones in moderate to excellent yields. This practical reaction is fast, operationally simple, and in particular, proceeds under very mild conditions to afford the target products with high regio- and stereoselectivity. The selectivity was illustrated by a conceptual DFT analysis.

Catalyst-Free Aminomethylamination of o-Hydroxystyrenes with Aminals to 1,3-Diamines

A new catalyst-free protocol for aminomethylamination of o-hydroxystyrenes with simple aminals is described. This direct and operationally simple method provides a fundamentally novel and rapid approach for the synthesis of 1,3-diamines. This novel reaction occurs under mild reaction conditions and provides a fundamentally unique and efficient strategy for the synthesis of 1,3-diamines with good to excellent yields.

Nickel-Catalyzed Alkylarylation of Activated Alkenes with Benzyl-amines via C-N Bond Activation

A nickel-catalyzed alkylarylation of active alkenes with tertiary benzylamines was achieved by charge-transfer-complex promoted C-N bond activation. The reaction proceeded through initial Ni-catalyzed C-N bond activation, followed by sequential radical addition, redox and proton abstraction with cleaved amine moiety in the absence of oxidant, and provides an efficient method to prepare various alkyl-substituted oxindoles and dihydroquinolinones in good yields.

Regioselective 1,2-addition of allenamides with N-haloimides: synthesis of 2-halo allylic aminal derivatives

A strategy for the synthesis of 2-halo allylic aminal derivatives through regioselective 1,2-addition of allenamides with N-haloimides is presented. This reaction was conducted under very mild conditions and gave up to 99% yield. Moreover, the obtained halides allow functional group diversification by palladium-catalyzed coupling reactions, which could act as potential intermediates for the synthesis of valuable compounds.

Featuring Xantphos

This review highlights the use of the bisphosphine ligand group in homogeneous catalysis.

Nucleophile-controlled mono- and bis-phosphonation of amino-2-en-1-ones via catalyst-free C(sp3)–N bond cleavage

Selective mono- and bis-phosphonation of amino-2-en-1-ones without a catalyst is developed. The selectivity is controlled by the nucleophilicity of the phosphorus sources.

Palladium-Catalyzed Hydroaminocarbonylation of Alkynes with Tertiary Amines via C-N Bond Cleavage

An efficient strategy for the cleavage of the C-N bond of tertiary amines was developed with DTBP as an oxidant, in which the cleaved H atom and amine moiety were successfully transferred to the desired products. This strategy has enabled an efficient palladium-catalyzed hydroaminocarbonylation of alkynes with tertiary amines. Notably, the catalyst loading could be lowered from 5 to 0.1 mol %, which represents the lowest catalyst loading among the reported work on carbonylation via C-N bond activation.

Catalytic [2 + 2 + 2] cycloaddition with indium(iii)-activated formaldimines: a practical and selective access to hexahydropyrimidines and 1,3-diamines from alkenes

Catalytic [2 + 2 + 2] cycloaddition with three double bond systems has been developed for construction of hexahydropyrimidine and 1,3-diamine derivatives.

Catalytic [2 + 2 + 2] cycloaddition with imines has, for the first time, been developed as a practical and selective approach for direct construction of hexahydropyrimidine derivatives from various alkenes. With formaldimines as reagents and simple InCl₃ as the catalyst, this ionic [2 + 2 + 2] approach is applicable for a wide scope of alkenes and allenes with various electronic and steric properties, as well as substitution patterns. Through facile hydrolysis of the resulting hexahydropyrimidines, this catalytic process also provides a new synthetic strategy for the aminomethylamination of alkenes and allenes to practically access 1,3-diamine derivatives.