Metal-Catalyzed Oxidations of C–H to C–N Bonds

Investigation of transition metal-catalyzed nitrene transfer reactions in water

Transition metal-catalyzed nitrene transfer is a powerful method for incorporating new CN bonds into relatively unfunctionalized scaffolds. In this communication, we report the first examples of site- and chemoselective CH bond amination reactions in aqueous media. The unexpected ability to employ water as the solvent in these reactions is advantageous in that it eliminates toxic solvent use and enables reactions to be run at increased concentrations with lower oxidant loadings. Using water as the reaction medium has potential to expand the scope of nitrene transfer to encompass a variety of biomolecules and highly polar substrates, as well as enable pH control over the site-selectivity of CH bond amination.

Metal-free, base promoted sp2 C-H functionalization in the sulfonamidation of 1,4-naphthoquinones

A novel, metal-free, base promoted sp2 C-H functionalization in the sulfonamidation of 1,4-naphthoquinones via a [3 + 2] cycloaddition reaction using sulfonyl azides under mild reaction conditions is reported. In this straightforward atom- and step-economical protocol, the active alkene moiety of quinone undergoes a thermal azide-alkene [3 + 2] cycloaddition followed by proton abstraction, ring opening and elimination of a nitrogen molecule to form sulfonamidation products in good yield and all the synthesized sulfonamidation derivatives exhibit good absorption and emission characteristics. In addition, the electrochemical properties of both 1,4-naphthoquinone and menadione sulfonamidation derivatives are studied and significant redox potentials are observed. Other important features of this methodology are readily accessible and easy to handle starting materials, milder conditions, reaction with a wide range of substrates and shorter reaction times with good yields.

Iodine promoted dual oxidative C(sp3)-H amination of 2-methyl-3-arylquinazolin-4(3H)-ones: a facile route to 1,4-diarylimidazo[1,5-a]quinazolin-5(4H)-ones

An iodine promoted tandem oxidative condensation of benzylamines and 2-methylquinazolin-4-(3H)-ones was developed to yield imidazo[1,5-a]quinazolin-5(4H)-ones via dual C(sp³)-H amination under metal free conditions in a greener way using molecular oxygen as a terminal oxidant. This tandem transformation provides an efficient approach to construct various functionalized imidazo[1,5-a]quinazolin-5(4H)-ones in a straightforward manner via a sequential amination-oxidation-annulation-aromatisation.

Natural products as inspiration for the development of new synthetic methods

Natural products have played an important role in shaping modern synthetic organic chemistry. In particular, their complex molecular skeletons have stimulated the development of many new synthetic methods. We highlight in this article some recent examples of synthetic design inspired by the biosynthesis of natural products.

Inverting Steric Effects: Using "Attractive" Noncovalent Interactions To Direct Silver-Catalyzed Nitrene Transfer

Nitrene transfer (NT) reactions represent powerful and direct methods to convert C-H bonds into amine groups that are prevalent in many commodity chemicals and pharmaceuticals. The importance of the C-N bond has stimulated the development of numerous transition-metal complexes to effect chemo-, regio-, and diastereoselective NT. An ongoing challenge is to understand how subtle interactions between catalyst and substrate influence the site-selectivity of the C-H amination event. In this work, we explore the underlying reasons why Ag(tpa)OTf (tpa = tris(pyridylmethyl)amine) prefers to activate α-conjugated C-H bonds over 3° alkyl C(sp3)-H bonds and apply these insights to reaction optimization and catalyst design. Experimental results suggest possible roles of noncovalent interactions (NCIs) in directing the NT; computational studies support the involvement of π···π and Ag···π interactions between catalyst and substrate, primarily by lowering the energy of the directed transition state and reaction conformers. A simple Hess's law relationship can be employed to predict selectivities for new substrates containing competing NCIs. The insights presented herein are poised to inspire the design of other catalyst-controlled C-H functionalization reactions.

Catalytic Asymmetric Intermolecular Allylic Functionalization of Unactivated Internal Alkenes

The asymmetric allylic functionalization of unactivated internal alkenes is an emerging strategy for the conversion of simple unsaturated starting materials into a diverse range of enantioenriched products. This Perspective summarizes the development of reactions wherein a chiral catalyst facilitates the intermolecular stereoselective reaction between an achiral unactivated internal alkene and a reagent.

Regio- and Stereoselective Iron-Catalyzed Oxyazidation of Enamides Using a Hypervalent Iodine Reagent

A novel regio- and diastereoselective iron-catalyzed intermolecular oxyazidation of enamides using various azidobenziodoxolone (ABX) derivatives is presented. A variety of α-N₃ amino derivatives and of α-N₃ piperidines were synthesized in good yields and under mild reaction conditions. The reaction involves a radical process using cheap FeCl₂ as the initiator.

Tunable, Chemo- and Site-Selective Nitrene Transfer Reactions through the Rational Design of Silver(I) Catalysts

Carbon-nitrogen (C-N) bonds are ubiquitous in pharmaceuticals, agrochemicals, diverse bioactive natural products, and ligands for transition metal catalysts. An effective strategy for introducing a new C-N bond into a molecule is through transition metal-catalyzed nitrene transfer chemistry. In these reactions, a metal-supported nitrene can either add across a C═C bond to form an aziridine or insert into a C-H bond to furnish the corresponding amine. Typical catalysts for nitrene transfer include Rh₂L_n and Ru₂L_n complexes supported by bridging carboxylate and related ligands, as well as complexes based on Cu, Co, Ir, Fe, and Mn supported by porphyrins and related ligands. A limitation of metal-catalyzed nitrene transfer is the ability to predictably select which specific site will undergo amination in the presence of multiple reactive groups; thus, many reactions rely primarily on substrate control. Achieving true catalyst-control over nitrene transfer would open up exciting possibilities for flexible installation of new C-N bonds into hydrocarbons, natural product-inspired scaffolds, existing pharmaceuticals or biorenewable building blocks. Silver-catalyzed nitrene transfer enables flexible control over the position at which a new C-N bond is introduced. Ag(I) supported by simple N-donor ligands accommodates a diverse range of coordination geometries, from linear to tetrahedral to seesaw, enabling the electronic and steric parameters of the catalyst to be tuned independently. In addition, the ligand, Ag salt counteranion, Ag/ligand ratio and the solvent all influence the fluxional and dynamic behavior of Ag(I) complexes in solution. Understanding the interplay of these parameters to manipulate the behavior of Ag-nitrenes in a predictable manner is a key design feature of our work. In this Account, we describe successful applications of a variety of design principles to tunable, Ag-catalyzed aminations, including (1) changing Ag/ligand ratios to influence chemoselectivity, (2) manipulating the steric environment of the catalyst to achieve site-selective C-H bond amination, (3) promoting noncovalent interactions between Ag/substrate or substrate/ligand to direct C-H functionalization, and (4) dictating the substrate's trajectory of approach to the Ag-nitrene. Our catalysts distinguish between the aminations of various types of C-H bonds, including tertiary C(sp³)-H, benzylic, allylic, and propargylic C-H bonds. Efforts in asymmetric nitrene transfer reactions catalyzed by Ag(I) complexes are also described.

Chemo- and Enantioselective Intramolecular Silver-Catalyzed Aziridinations

Asymmetric nitrene-transfer reactions are a powerful tool for the preparation of enantioenriched amine building blocks. Reported herein are chemo- and enantioselective silver-catalyzed aminations which transform di- and trisubstituted homoallylic carbamates into [4.1.0]-carbamate-tethered aziridines in good yields and with ee values of up to 92 %. The effects of the substrate, silver counteranion, ligand, solvent, and temperature on both the chemoselectivity and ee value were explored. Stereochemical models were proposed to rationalize the observed absolute stereochemistry of the aziridines, which undergo nucleophilic ring opening to yield enantioenriched amines with no erosion in stereochemical integrity.

Catalytic allylic oxidation of internal alkenes to a multifunctional chiral building block

The stereoselective oxidation of hydrocarbons is one of the most notable advances in synthetic chemistry over the past fifty years. Inspired by nature, enantioselective dihydroxylations, epoxidations and other oxidations of unsaturated hydrocarbons have been developed. More recently, the catalytic enantioselective allylic carbon-hydrogen oxidation of alkenes has streamlined the production of pharmaceuticals, natural products, fine chemicals and other functional materials. Allylic functionalization provides a direct path to chiral building blocks with a newly formed stereocentre from petrochemical feedstocks while preserving the olefin functionality as a handle for further chemical elaboration. Various metal-based catalysts have been discovered for the enantioselective allylic carbon-hydrogen oxidation of simple alkenes with cyclic or terminal double bonds. However, a general and selective allylic oxidation using the more common internal alkenes remains elusive. Here we report the enantioselective, regioselective and E/Z-selective allylic oxidation of unactivated internal alkenes via a catalytic hetero-ene reaction with a chalcogen-based oxidant. Our method enables non-symmetric internal alkenes to be selectively converted into allylic functionalized products with high stereoselectivity and regioselectivity. Stereospecific transformations of the resulting multifunctional chiral building blocks highlight the potential for rapidly converting internal alkenes into a broad range of enantioenriched structures that can be used in the synthesis of complex target molecules.

Synthesis, Characterization, and Variable-Temperature NMR Studies of Silver(I) Complexes for Selective Nitrene Transfer

An array of silver complexes supported by nitrogen-donor ligands catalyze the transformation of C═C and C-H bonds to valuable C-N bonds via nitrene transfer. The ability to achieve high chemoselectivity and site selectivity in an amination event requires an understanding of both the solid- and solution-state behavior of these catalysts. X-ray structural characterizations were helpful in determining ligand features that promote the formation of monomeric versus dimeric complexes. Variable-temperature ¹H and DOSY NMR experiments were especially useful for understanding how the ligand identity influences the nuclearity, coordination number, and fluxional behavior of silver(I) complexes in solution. These insights are valuable for developing improved ligand designs.

Enantioselective, intermolecular benzylic C-H amination catalysed by an engineered iron-haem enzyme

C-H bonds are ubiquitous structural units of organic molecules. Although these bonds are generally considered to be chemically inert, the recent emergence of methods for C-H functionalization promises to transform the way synthetic chemistry is performed. The intermolecular amination of C-H bonds represents a particularly desirable and challenging transformation for which no efficient, highly selective, and renewable catalysts exist. Here we report the directed evolution of an iron-containing enzymatic catalyst-based on a cytochrome P450 monooxygenase-for the highly enantioselective intermolecular amination of benzylic C-H bonds. The biocatalyst is capable of up to 1,300 turnovers, exhibits excellent enantioselectivities, and provides access to valuable benzylic amines. Iron complexes are generally poor catalysts for C-H amination: in this catalyst, the enzyme's protein framework confers activity on an otherwise unreactive iron-haem cofactor.

Expeditious diastereoselective synthesis of elaborated ketones via remote Csp3-H functionalization

The quest for selective C–H functionalization reactions, able to provide new strategic opportunities for the rapid assembly of molecular complexity, represents a major focus of the chemical community. Examples of non-directed, remote Csp³–H activation to forge complex carbon frameworks remain scarce due to the kinetic stability and thus intrinsic challenge associated to the chemo-, regio- and stereoselective functionalization of aliphatic C–H bonds. Here we describe a radical-mediated, directing-group-free regioselective 1,5-hydrogen transfer of unactivated Csp³–H bonds followed by a second Csp²–H functionalization to produce, with exquisite stereoselectivity, a variety of elaborated fused ketones. This study demonstrates that aliphatic acids can be strategically harnessed as 1,2-diradical synthons and that secondary aliphatic C–H bonds can be engaged in stereoselective C–C bond-forming reactions, highlighting the potential of this protocol for target-oriented natural product and pharmaceutical synthesis.

C-H activation is a powerful method to form functionalised molecules, but is particularly challenging for unactivated sp³ sites. Here the authors report a directing-group-free radical cascade process for converting vinyl azides and carboxylic acids to tetralone derivatives in high diastereoselectivity.

Transition Metal-Catalyzed C-H Amination: Scope, Mechanism, and Applications

Catalytic transformation of ubiquitous C-H bonds into valuable C-N bonds offers an efficient synthetic approach to construct N-functionalized molecules. Over the last few decades, transition metal catalysis has been repeatedly proven to be a powerful tool for the direct conversion of cheap hydrocarbons to synthetically versatile amino-containing compounds. This Review comprehensively highlights recent advances in intra- and intermolecular C-H amination reactions utilizing late transition metal-based catalysts. Initial discovery, mechanistic study, and additional applications were categorized on the basis of the mechanistic scaffolds and types of reactions. Reactivity and selectivity of novel systems are discussed in three sections, with each being defined by a proposed working mode.

A biocatalytic cascade for the amination of unfunctionalised cycloalkanes

Here we describe a one-pot, three-enzyme, cascade involving a cytochrome P450 monooxygenase, an alcohol dehydrogenase and a reductive aminase for the synthesis of secondary amines from cycloalkanes.

Tunable differentiation of tertiary C–H bonds in intramolecular transition metal-catalyzed nitrene transfer reactions

Metal-catalyzed nitrene transfer reactions are an appealing and efficient strategy for accessing tetrasubstituted amines through the direct amination of tertiary C–H bonds.

Transition metal-catalyzed iodine(iii)-mediated nitrene transfer reactions: efficient tools for challenging syntheses

The main synthetic applications of catalytic C(sp³)–H amination and alkene aziridination reactions are discussed in the context of natural product synthesis. The examples highlight that these synthetic methods now firmly belong in the organic chemist's toolbox.

Catalyst-Controlled and Tunable, Chemoselective Silver-Catalyzed Intermolecular Nitrene Transfer: Experimental and Computational Studies

The development of new catalysts for selective nitrene transfer is a continuing area of interest. In particular, the ability to control the chemoselectivity of intermolecular reactions in the presence of multiple reactive sites has been a long-standing challenge in the field. In this paper, we demonstrate examples of silver-catalyzed, nondirected, intermolecular nitrene transfer reactions that are both chemoselective and flexible for aziridination or C-H insertion, depending on the choice of ligand. Experimental probes present a puzzling picture of the mechanistic details of the pathways mediated by [(tBu3tpy)AgOTf]2 and (tpa)AgOTf. Computational studies elucidate these subtleties and provide guidance for the future development of new catalysts exhibiting improved tunability in group transfer reactions.

Ligand-Promoted meta-C-H Amination and Alkynylation

Using a modified norbornene (methyl bicyclo[2.2.1]hept-2-ene-2-carboxylate) as a transient mediator, meta-C-H amination and meta-C-H alkynylation of aniline and phenol substrates have been developed for the first time. Both the identification of a monoprotected 3-amino-2-hydroxypyridine/pyridone-type ligand and the use of a modified norbornene as a mediator are crucial for the realization of these two unprecedented meta-C-H transformations. A variety of substrates are compatible with both meta-C-H amination and meta-C-H alkynylation. Amination and alkynylation of heterocyclic substrates including indole, indoline, and indazole afford the desired products in moderate to high yields.

DFT Study of Pd(0)-Promoted Intermolecular C-H Amination with O-Benzoyl Hydroxylamines

Computational studies were carried out to explore the mechanism of Pd-catalyzed intermolecular C-H amination with O-benzoyl hydroxylamines in which both Pd(0) and Pd(II) catalysts are effective. For the Pd(0)-catalyzed reaction, the generally assumed Pd(0)/Pd(II) catalytic cycle might not be feasible. Instead, Pd(0), being essentially a catalyst precursor, could be oxidized to Pd(II), and the C-H amination proceeds through the Pd(II)/Pd(IV) catalytic cycle.

Intramolecular 1,5-C(sp3)-H Radical Amination via Co(II)-Based Metalloradical Catalysis for Five-Membered Cyclic Sulfamides

Co(II)-based metalloradical catalysis (MRC) proves effective for intramolecular 1,5-C-H amination of sulfamoyl azides under neutral and nonoxidative conditions, providing a straightforward approach to access strained 5-membered cyclic sulfamides with nitrogen gas as the only byproduct. The metalloradical amination system is applicable to different types of C(sp3)-H bonds and has a high degree of functional group tolerance. Additional features of the Co(II)-catalyzed 1,5-C-H amination include excellent chemoselectivity toward allylic and propargylic C-H bonds. The unique reactivity and selectivity profile of the Co(II)-catalyzed 1,5-C-H amination is attributed to the underlying radical mechanism of MRC.