Iridium(I)-Catalyzed α-C(sp3)-H Alkylation of Saturated Azacycles

Ligand-Controlled Regiodivergent Nickel-Catalyzed Hydroaminoalkylation of Unactivated Alkenes

Ligand modulation of transition-metal catalysts to achieve optimal reactivity and selectivity in alkene hydrofunctionalization is a fundamental challenge in synthetic organic chemistry. Hydroaminoalkylation, an atom-economical approach for alkylating amines using alkenes, is particularly significant for amine synthesis in the pharmaceutical, agrochemical, and fine chemical industries. However, the existing methods usually require specific substrate combinations to achieve precise regio- and stereoselectivity, which limits their practical utility. Protocols allowing for regiodivergent hydroaminoalkylation from the same starting materials, controlling both regiochemical and stereochemical outcomes, are currently absent. Herein, we report a ligand-controlled, regiodivergent nickel-catalyzed hydroaminoalkylation of unactivated alkenes with N-sulfonyl amines. The reaction initiates with amine dehydrogenation and involves aza-nickelacycle intermediates. Tritert-butylphosphine promotes branched regioselectivity and syn diastereoselectivity, whereas ethyldiphenylphosphine enables linear selectivity, yielding regioisomers with inverse orientation. Systematic evaluation of diverse monodentate phosphine ligands reveals distinct regioselectivity cliffs, and % Vbur (min), a ligand steric descriptor, was established as a predictive parameter correlating ligand structure to regioselectivity. Computational investigations supported experimental findings, offering mechanistic insights into the origins of regioselectivity. Our method provides an efficient and predictable route for amine synthesis, demonstrating broad substrate scope, excellent tolerance toward various functional groups, and practical advantages. These include the use of readily available starting materials and cost-effective nickel(II) salts as precatalysts.

Electrochemical Vicinal C-H Difunctionalization of Saturated Azaheterocycles

A method to functionalize two vicinal C-H bonds of saturated azaheterocycles is described. The procedure involves subjecting the substrate to a mixture of hydrochloric acid, acetic acid, and acetic anhydride in an undivided electrochemical cell at a constant current, resulting in stereoselective conversion to the corresponding α-acetoxy-β-chloro derivative. The α-position can be readily substituted with a range of other groups, including alkyl, aryl, allyl, alkynyl, alkoxy, or azido functionalities. Furthermore, we demonstrate that the β-chloro position can be engaged in Suzuki cross-coupling. This protocol thus enables the rapid diversification of simple five-, six-, and seven-membered saturated azaheterocycles at two adjacent positions.

The Recent Advances in Iron-Catalyzed C(sp3 )-H Functionalization

The use of iron as a core metal in catalysis has become a research topic of interest over the last few decades. The reasons are clear. Iron is the most abundant transition metal on Earth's crust and it is widely distributed across the world. It has been extracted and processed since the dawn of civilization. All these features render iron a noncontaminant, biocompatible, nontoxic, and inexpensive metal and therefore it constitutes the perfect candidate to replace noble metals (rhodium, palladium, platinum, iridium, etc.). Moreover, direct C-H functionalization is one of the most efficient strategies by which to introduce new functional groups into small organic molecules. The majority of organic compounds contain C(sp3 )-H bonds. Given the enormous importance of organic molecules in so many aspects of existence, the utilization and bioactivity of C(sp3 )-H bonds are of the utmost importance. This review sheds light on the substrate scope, selectivity, benefits, and limitations of iron catalysts for direct C(sp3 )-H bond activations. An overview of the use of iron catalysis in C(sp3 )-H activation protocols is summarized herein up to 2022.

Iridium-Catalyzed Enantioselective Alkene Hydroalkylation via a Heteroaryl-Directed Enolization-Decarboxylation Sequence

Upon exposure to a cationic Ir(I)-complex modified with the chiral diphosphine DuanPhos, hydroalkylations of styrenes and α-olefins with diverse heteroaryl tert-butyl acetates occur with complete branched selectivity and very high enantioselectivity. The initial adducts undergo acid promoted decarboxylation in situ to provide alkylated heteroarenes possessing defined β-stereocenters. The processes are postulated to proceed via a stereodefined chiral Ir-enolate, which arises upon heteroarene directed enolization of the heteroaryl acetate precursor. The method can be classified as an enantioselective decarboxylative C(sp3)-C(sp3) cross-coupling.

Synthesis of Polycyclic Imidazoles via α-C-H/N-H Annulation of Alicyclic Amines

Secondary alicyclic amines are converted to their corresponding ring-fused imidazoles in a simple procedure consisting of oxidative imine formation followed by a van Leusen reaction. Amines with an existing α-substituent undergo regioselective ring-fusion at the α'-position. This method was utilized in a synthesis of fadrozole.

Divergent Access to Chiral C2- and C3-Alkylated Pyrrolidines by Catalyst-Tuned Regio- and Enantioselective C(sp3)-C(sp3) Coupling

Novel-substituted pyrrolidine derivatives are widely used in drugs and bioactive molecules. The efficient synthesis of these valuable skeletons, especially enantiopure derivatives, is still recognized as a key bottleneck to overcome in chemical synthesis. Herein, we report a highly efficient catalyst-tuned regio- and enantioselective hydroalkylation reaction for the divergent synthesis of chiral C2- and C3-alkylated pyrrolidines through desymmetrization of the readily available 3-pyrrolines. The catalytic system consists of CoBr₂ with a modified bisoxazoline (BOX) ligand, which can achieve the asymmetric C(sp³)-C(sp³) coupling via the distal stereocontrol, providing a series of C3-alkylated pyrrolidines in high efficiency. Moreover, the nickel catalytic system allows the enantioselective hydroalkylation to synthesize the C2-alkylated pyrrolidines through the tandem alkene isomerization/hydroalkylation reaction. This divergent method uses readily available catalysts, chiral BOX ligands, and reagents, delivering enantioenriched 2-/3-alkyl substituted pyrrolidines with excellent regio- and enantioselectivity (up to 97% ee). We also demonstrate the compatibility of this transformation with complex substrates derived from a series of drugs and bioactive molecules in good efficiency, which offers a distinct entry to more functionalized chiral N-heterocycles.

Electrochemically Driven Selective Removal of the S═N Bond-Directing Group Using Cyclohexanone Oxime as the Mediator

An inexpensive electrochemical induction system was used for the efficient reductive defunctionalization of sulfoximines through a radical pathway. This practical and robust strategy could be used for the removal of the S═N bond-directing group from various sulfoximines. The practicability of this method was demonstrated by its mild conditions, simple operation, one-pot procedure, gram-scale synthesis, and the undivided cell. Furthermore, preliminary mechanistic studies suggested that the reaction might proceed via a homocoupling reaction and a denitrification procedure.

Regiospecific α-methylene functionalisation of tertiary amines with alkynes via Au-catalysed concerted one-proton/two-electron transfer to O2

Regioselective transformations of tertiary amines, which are ubiquitously present in natural products and drugs, are important for the development of novel medicines. In particular, the oxidative α-C-H functionalisation of tertiary amines with nucleophiles via iminium cations is a promising approach because, theoretically, there is almost no limit to the type of amine and functionalisation. However, most of the reports on oxidative α-C-H functionalisations are limited to α-methyl-selective or non-selective reactions, despite the frequent appearance of α-methylene-substituted amines in pharmaceutical fields. Herein, we develop an unusual oxidative regiospecific α-methylene functionalisation of structurally diverse tertiary amines with alkynes to synthesise various propargylic amines using a catalyst comprising Zn salts and hydroxyapatite-supported Au nanoparticles. Thorough experimental investigations suggest that the unusual α-methylene regiospecificity is probably due to a concerted one-proton/two-electron transfer from amines to O₂ on the Au nanoparticle catalyst, which paves the way to other α-methylene-specific functionalisations.

Asymmetric addition of an N-methyl C(sp3)-H bond to cyclic alkenes enabled by an iridium/phosphine-olefin catalyst

Iridium-catalyzed asymmetric C-H addition of an N-methyl group on 3-trifluoromethyl-2-(N-methylamino)pyridine to cyclic alkenes was realized by using a phosphine-olefin ligand. Indene and its 7-substituted derivatives, acenaphthylene, and some bicyclic alkenes underwent the addition to give the corresponding products in good yields with high enantioselectivity.

Remote and Proximal Hydroaminoalkylation of Alkenes Enabled by Photoredox/Nickel Dual Catalysis

A mild and site-selective hydroaminoalkylation of activated and unactivated alkenes via dual photoredox/Ni catalysis is developed. This dual catalytic strategy enables exclusive access to α-selective products, which is complementary to previously reported photocatalytic hydroaminoalkylation of activated alkenes that provides the β-selective products. The chain-walking of a Ni-H intermediate toward a carbonyl allows for the hydroaminoalkylation of unactivated alkenes at remote sp³ C-H sites. This method tolerates a broad substrate scope of both amines and alkenes as well as providing a streamlined synthesis of value-added β-amino acid derivatives from readily available starting materials.

Regioselective α-Cyanation of Unprotected Alicyclic Amines

Secondary alicyclic amines are converted to α-aminonitriles via addition of TMSCN to their corresponding imines, intermediates that are produced in situ via the oxidation of amine-derived lithium amides with simple ketone oxidants. Amines with an existing α-substituent undergo regioselective α'-cyanation even if the C-H bonds at that site are less activated. Amine α-arylation can be combined with α'-cyanation to generate difunctionalized products in a single operation.

Palladium-Catalyzed Synthesis of Fused Carbo- and Heterocycles: Recent Advances

The use of palladium catalysts in fused ring synthesis has been increasingly noteworthy in recent years in organic synthesis. It has a lot of potential compared to other transition metal catalysts, because of its one-of-a-kind feature that makes them more widely applicable in a variety of disciplines application. Palladium is important in a variety of Heck processes, including intramolecular, intermolecular, and reductive Heck reactions, which produce diverse carbocycles and heterocycles of biological importance. Under optimal reaction conditions, carbocyclization or heterocyclization occurs, resulting in the production of numerous structural building blocks of naturally occurring compounds. Beside intramolecular Heck-type reactions, cycloaddition, cycloalkylation, oxidative coupling, C-H functionalization, cross-coupling reactions, and carboamidation reactions have also been employed extensively to access fused carbo- and heterocycles of immense biological importance. This review article provides a well-summarized discussion (since 2001) on fused carbo- and heterocycle ring synthesis using palladium catalysts, overviewing their applications, and mechanistic insights.

Transition-metal-catalyzed C-H bond alkylation using olefins: recent advances and mechanistic aspects

Transition metal catalysis has contributed immensely to C-C bond formation reactions over the last few decades, and alkylation is no exception. The superiority of such methodologies over traditional alkylation is evident from minimal reaction steps, shorter reaction times, and atom economy while also allowing control over regio- and stereo-selectivity. In particular, hydrocarbonation of alkenes has grabbed increased attention due its fundamental ability to effectively and selectively synthesise a wide range of industrially and pharmaceutically relevant moieties. This review attempts to provide a scientific viewpoint and a systematic analysis of the recent developments in transition-metal-catalyzed alkylation of various C-H bonds using simple and activated olefins. The key features and mechanistic studies involved in these transformations are described briefly.

Hydroaminoalkylation for the Catalytic Addition of Amines to Alkenes or Alkynes: Diverse Mechanisms Enable Diverse Substrate Scope

Hydroaminoalkylation is a powerful, atom-economic catalytic reaction for the reaction of amines with alkenes and alkynes. This C-H functionalization reaction allows for the atom-economic alkylation of amines using simple alkenes or alkynes as the alkylating agents. This transformation has significant potential for transformative approaches in the pharmaceutical, agrochemical, and fine chemical industries in the preparation of selectively substituted amines and N-heterocycles and shows promise in materials science for the synthesis of functional and responsive aminated materials. Different early transition-metal, late transition-metal, and photoredox catalysts mediate hydroaminoalkylation by distinct mechanistic pathways. These mechanistic insights have resulted in the development of new catalysts and reaction conditions to realize hydroaminoalkylation with a broad range of substrates: activated and unactivated, terminal and internal, C-C double and triple bonds with aryl or alkyl primary, secondary, or tertiary amines, including N-heterocyclic amines. By deploying select catalysts with specific substrate combinations, control over regioselectivity, diastereoselectivity, and enantioselectivity has been realized. Key barriers to widespread adoption of this reaction include air and moisture sensitivity for early transition-metal catalysts as well as a heavy dependence on amine protecting or directing groups for late transition-metal or photocatalytic routes. Advances in improved catalyst robustness, substrate scope, and regio-/stereoselective reactions with early- and late transition-metal catalysts, as well as photoredox catalysis, are highlighted, and opportunities for further catalyst and reaction development are included. This perspective shows that hydroaminoalkylation has the potential to be a disruptive and transformative strategy for the synthesis of selectively substituted amines and N-heterocycles from simple amines and alkenes.

Photoinduced α-C-H Amination of Cyclic Amine Scaffolds Enabled by Polar-Radical Relay

Herein, we report a polar-radical relay strategy for α-C-H amination of cyclic amines with N-chloro-N-sodio-carbamates. The relay is initiated by in situ generation of cyclic iminium intermediate using N-iodosuccinimide (NIS) oxidant as an initiator, which then operates through a series of polar (addition and elimination) and radical (homolysis, hydrogen- and halogen atom transfer) reactions to enable the challenging C-N bond formation in a controlled manner. A broad range of α-amino cyclic amines were readily accessed with excellent regioselectivity, and the superb applicability was further demonstrated by functionalization of biologically relevant compounds.

Regio-, Diastereo-, and Enantioselective Decarboxylative Hydroaminoalkylation of Dienol Ethers Enabled by Dual Palladium/Photoredox Catalysis

Intermolecular photocatalytic hydroaminoalkylation (HAA) of alkenes have emerged as a powerful method for the construction of alkyl amines. Although there are some studies aiming at stereoselective photocatalytic HAA reactions, the alkenes are limited to electrophilic alkenes. Herein, we report a highly regio‐, diastereo‐, and enantioselective HAA of electron‐rich dienol ethers and α‐amino radicals derived from α‐amino acids using a unified photoredox and palladium catalytic system. This decarboxylative 1,2‐Markovnikov addition enables the construction of vicinal amino tertiary ethers with high levels of regio‐ (up to >19 : 1 rr), diastereo‐ (up to >19 : 1 dr), and enantioselectivity control (up to >99 % ee). Mechanistic studies support a reversible hydropalladation as a key step.

Synthesis of Polycyclic Isoindolines via α-C-H/N-H Annulation of Alicyclic Amines

Relatively unstable cyclic imines, generated in situ from their corresponding alicyclic amines via oxidation of their lithium amides with simple ketone oxidants, engage aryllithium compounds containing a leaving group on an ortho-methylene functionality to provide polycyclic isoindolines in a single operation. The scope of this transformation includes pyrrolidine, piperidine, azepane, azocane, and piperazines.

Transition-Metal-Catalyzed, Coordination-Assisted Functionalization of Nonactivated C(sp3)-H Bonds

Transition-metal-catalyzed, coordination-assisted C(sp³)-H functionalization has revolutionized synthetic planning over the past few decades as the use of these directing groups has allowed for increased access to many strategic positions in organic molecules. Nonetheless, several challenges remain preeminent, such as the requirement for high temperatures, the difficulty in removing or converting directing groups, and, although many metals provide some reactivity, the difficulty in employing metals outside of palladium. This review aims to give a comprehensive overview of coordination-assisted, transition-metal-catalyzed, direct functionalization of nonactivated C(sp³)-H bonds by covering the literature since 2004 in order to demonstrate the current state-of-the-art methods as well as the current limitations. For clarity, this review has been divided into nine sections by the transition metal catalyst with subdivisions by the type of bond formation. Synthetic applications and reaction mechanism are discussed where appropriate.

Regio- and Stereoselective Alkylboration of Endocyclic Olefins Enabled by Nickel Catalysis

Whereas there is a significant interest in the rapid construction of diversely substituted saturated heterocycles, direct and modular access is currently limited to the mono-, 2,3-, or 3,4-substitution pattern. This Communication describes the straightforward and modular construction of 2,4-substituted saturated heterocycles from readily available materials in a highly stereo- and regioselective manner, which sets the stage for numerous readily accessible drug motifs. The strategy relies on chain walking catalysis.

Iridium-catalyzed enantioselective addition of an N-methyl C-H bond to α-trifluoromethylstyrenes via C-H activation

The Ir-catalyzed enantioselective addition of an N-methyl C-H bond of 2-(methylamino)pyridine derivatives to α-trifluoromethylstyrenes proceeded via C-H activation to give chiral γ-branched amine derivatives having a trifluoromethyl-substituted stereocenter. It was found that a bulky and electron-withdrawing group at the 3-position of 2-(methylamino)pyridines was necessary for the present C-H addition reaction catalyzed by a cationic iridium/chiral bisphosphine complex.

Photocatalytic Hydroaminoalkylation of Styrenes with Unprotected Primary Alkylamines

Catalytic, intermolecular hydroaminoalkylation (HAA) of styrenes provides a powerful disconnection for pharmacologically relevant γ-arylamines, but current methods cannot utilize unprotected primary alkylamines as feedstocks. Metal-catalyzed HAA protocols are also highly sensitive to α-substitution on the amine partner, and no catalytic solutions exist for α-tertiary γ-arylamine synthesis via this approach. We report a solution to these problems using organophotoredox catalysis, enabling a direct, modular, and sustainable preparation of α-(di)substituted γ-arylamines, including challenging electron-neutral and moderately electron-rich aryl groups. A broad range of functionalities are tolerated, and the reactions can be run on multigram scale in continuous flow. The method is applied to a concise, protecting-group-free synthesis of the blockbuster drug Fingolimod, as well as a phosphonate mimic of its in vivo active form (by iterative α-C-H functionalization of ethanolamine). The reaction can also be sequenced with an intramolecular N-arylation to provide a general and modular access to valuable (spirocyclic) 1,2,3,4-tetrahydroquinolines and 1,2,3,4-tetrahydronaphthyridines. Mechanistic and kinetic studies support an irreversible hydrogen atom transfer activation of the alkylamine by the azidyl radical and some contribution from a radical chain. The reaction is photon-limited and exhibits a zero-order dependence on amine, azide, and photocatalyst, with a first-order dependence on styrene.

Sequential Norrish-Yang Cyclization and C-C Cleavage/Cross-Coupling of a [4.1.0] Fused Saturated Azacycle

Methods that functionalize the periphery of azacylic scaffolds have garnered increasing interest in recent years. Herein, we investigate the selectivity of a solid-state Norrish-Yang cyclization (NYC) and subsequent C-C cleavage/cross-coupling reaction of a strained cyclopropane-fused azacyclic system. Surprisingly, the NYC primarily furnished a single lactam constitutional and diastereo-isomer. The regioselectivity of the C-C cleavage of the α-hydroxy-β-lactam moiety could be varied by altering the ligand set used in the coupling chemistry. Experimental and computational observations are discussed.

Decoding Directing Groups and Their Pivotal Role in C-H Activation

Synthetic organic chemistry has witnessed a plethora of functionalization and defunctionalization strategies. In this regard, C-H functionalization has been at the forefront due to the multifarious applications in the development of simple to complex molecular architectures and holds a brilliant prospect in drug development and discovery. Despite been explored tremendously by chemists, this functionalization strategy still enjoys the employment of novel metal catalysts as well metal-free organic ligands. Moreover, the switch to photo- and electrochemistry has widened our understanding of the alternative pathways via which a reaction can proceed and these strategies have garnered prominence when applied to C-H activation. Synthetic chemists have been foraging for new directing groups and templates for the selective activation of C-H bonds from a myriad of carbon-hydrogen bonds in aromatic as well as aliphatic systems. As a matter of fact, by varying the templates and directing groups, scientists found the answer to the challenge of distal C-H bond activation which remained an obstacle for a very long time. These templates have been frequently harnessed for selectively activating C-H bonds of natural products, drugs, and macromolecules decorated with multiple C-H bonds. This itself was a challenge before the commencement of this field as functionalization of a site other than the targeted site could modify and hamper the biological activity of the pharmacophore. Total synthesis and pharmacophore development often faces the difficulty of superfluous reaction steps towards selective functionalization. This obstacle has been solved by late-stage functionalization simply by harnessing C-H bond activation. Moreover, green chemistry and metal-free reaction conditions have seen light in the past few decades due to the rising concern about environmental issues. Therefore, metal-free catalysts or the usage of non-toxic metals have been recently showcased in a number of elegant works. Also, research groups across the world are developing rational strategies for directing group free or non-directed protocols that are just guided by ligands. This review encapsulates the research works pertinent to C-H bond activation and discusses the science devoted to it at the fundamental level. This review gives the readers a broad understanding of how these strategies work, the execution of various metal catalysts, and directing groups. This not only helps a budding scientist towards the commencement of his/her research but also helps a matured mind searching out for selective functionalization. A detailed picture of this field and its progress with time has been portrayed in lucid scientific language with a motive to inculcate and educate scientific minds about this beautiful strategy with an overview of the most relevant and significant works of this era. The unique trait of this review is the detailed description and classification of various directing groups and their utility over a wide substrate scope. This allows an experimental chemist to understand the applicability of this domain and employ it over any targeted substrate.

α,α'-C-H Bond Difunctionalization of Unprotected Alicyclic Amines

A simple one-pot procedure enables the sequential, regioselective, and diastereoselective introduction of the same or two different substituents to the α- and α'-positions of unprotected azacycles. Aryl, alkyl, and alkenyl substituents are introduced via their corresponding organolithium compounds. The scope of this transformation includes pyrrolidines, piperidines, azepanes, and piperazines.

C-H Bond Functionalization of Amines: A Graphical Overview of Diverse Methods

This Graphical Review provides a concise overview of the manifold and mechanistically diverse methods that enable the functionalization of sp3 C-H bonds in amines and their derivatives.

Direct, Catalytic α-Alkylation of N-Heterocycles by Hydroaminoalkylation: Substrate Effects for Regiodivergent Product Formation

Saturated N-heterocycles are prevalent in pharmaceutical and agrochemical industries, yet remain challenging to catalytically alkylate. Most strategies for C-H activation of these challenging substrates use protected amines or high loadings of precious metal catalysts. We report an early transition-metal system for the broad, robust, and direct alkylation of unprotected amine heterocycles with simple alkenes. Short reaction times are achieved using an in situ generated tantalum catalyst that avoids the use of bases, excess substrate, or additives. In most cases, this catalyst system is selective for the branched reaction product, including examples of products that are generated with excellent diastereoselectivity. Alkene electronic properties can be exploited for substrate-modified regioselectivity to access the alternative linear amine alkylation product with a group 5 catalyst. This method allows for the facile isolation of unprotected N-heterocyclic products, as useful substrates for further reactivity.

α-C-H/N-H Annulation of Alicyclic Amines via Transient Imines: Preparation of Polycyclic Lactams

Polycyclic lactams are prepared in a single operation from o-toluamides and cyclic amines in a process that involves transient cyclic imines, species that are conveniently obtained in situ from the corresponding lithium amides and simple ketone oxidants. Imines thus generated, such as 1-pyrroline and 1-piperideine, engage lithiated o-toluamides in a facile annulation process. Undesired side reactions such as imine deprotonation and o-toluamide dimerization are suppressed through the judicious choice of reaction conditions.

Acid- and Base-Switched Palladium-Catalyzed γ-C(sp3)-H Alkylation and Alkenylation of Neopentylamine

The functionalization of remote unactivated C(sp³)-H and the reaction selectivity are among the core pursuits for transition-metal catalytic system development. Herein, we report Pd-catalyzed γ-C(sp³)-H-selective alkylation and alkenylation with removable 7-azaindole as a directing group. Acid and base were found to be the decisive regulators for the selective alkylation and alkenylation, respectively, on the same single substrate under otherwise the same reaction conditions. Various acrylates were compatible for the formation of C(sp³)-C(sp³) and C(sp³)-C(sp²) bonds. The alkenylation protocol could be further extended to acrylates with natural product units and α,β-unsaturated ketones. The preliminary synthetic manipulation of the alkylation and alkenylation products demonstrates the potential of this strategy for structurally diverse aliphatic chain extension and functionalization. Mechanistic experimental studies showed that the acidic and basic catalytic transformations shared the same six-membered dimer palladacycle.

Ru(ii)-Catalyzed cascade decarbonylative annulation and dehydrogenative alkenylation reactions: synthesis of phthalides

A novel Ru(ii)-catalyzed decarbonylative annulation and direct C-H functionalization reaction of phthalic anhydrides with acrylates is reported. This reaction provides an efficient route for the synthesis of 3,7-disubstituted phthalides.

Emergent synthetic methods for the modular advancement of sp3-rich fragments

Fragment-based drug discovery is an important and increasingly reliable technology for the delivery of clinical candidates. Notably, however, sp3-rich fragments are a largely untapped resource in molecular discovery, in part due to the lack of general and suitably robust chemical methods available to aid their development into higher affinity lead and drug compounds. This Perspective describes the challenges associated with developing sp3-rich fragments, and succinctly highlights recent advances in C(sp3)-H functionalisations of high potential value towards advancing fragment hits by 'growing' functionalised rings and chains from unconventional, carbon-centred vectors.

α-C-H Bond Functionalization of Unprotected Alicyclic Amines: Lewis-Acid-Promoted Addition of Enolates to Transient Imines

Enolizable cyclic imines, obtained in situ from their corresponding lithium amides by oxidation with simple ketone oxidants, are readily alkylated with a range of enolates to provide mono- and polycyclic β-aminoketones in a single operation, including the natural product (±)-myrtine. Nitrile anions also serve as competent nucleophiles in these transformations, which are promoted by BF3 etherate. β-Aminoesters derived from ester enolates can be converted to the corresponding β-lactams.

Diversification of Unprotected Alicyclic Amines by C-H Bond Functionalization: Decarboxylative Alkylation of Transient Imines

Despite extensive efforts by many practitioners in the field, methods for the direct α-C-H bond functionalization of unprotected alicyclic amines remain rare. A new advance in this area utilizes N-lithiated alicyclic amines. These readily accessible intermediates are converted to transient imines through the action of a simple ketone oxidant, followed by alkylation with a β-ketoacid under mild conditions to provide valuable β-amino ketones with unprecedented ease. Regioselective α'-alkylation is achieved for substrates with existing α-substituents. The method is further applicable to the convenient one-pot synthesis of polycyclic dihydroquinolones through the incorporation of a SN Ar step.

Asymmetric "Clip-Cycle" Synthesis of Pyrrolidines and Spiropyrrolidines

The development of an asymmetric "clip-cycle" synthesis of 2,2- and 3,3-disubstituted pyrrolidines and spiropyrrolidines, which are increasingly important scaffolds in drug discovery programs, is reported. Cbz-protected bis-homoallylic amines were activated by "clipping" them to thioacrylate via an alkene metathesis reaction. Enantioselective intramolecular aza-Michael cyclization onto the activated alkene, catalyzed by a chiral phosphoric acid, formed a pyrrolidine. The reaction accommodated a range of substitutions to form 2,2- and 3,3-disubstituted pyrrolidines and spiropyrrolidines with high enantioselectivities. The importance of the thioester activating group was demonstrated by comparison to ketone and oxoester-containing substrates. DFT studies supported the aza-Michael cyclization as the rate- and stereochemistry-determining step and correctly predicted the formation of the major enantiomer. The catalytic asymmetric syntheses of N-methylpyrrolidine alkaloids (R)-irnidine and (R)-bgugaine, which possess DNA binding and antibacterial properties, were achieved using the "clip-cycle" methodology.

Catalytic addition of C-H bonds across C-C unsaturated systems promoted by iridium(i) and its group IX congeners

Transition metal-catalyzed hydrocarbonations of unsaturated substrates have emerged as powerful synthetic tools for increasing molecular complexity in an atom-economical manner. Although this field was traditionally dominated by low valent rhodium and ruthenium catalysts, in recent years, there have been many reports based on the use of iridium complexes. In many cases, these reactions have a different course from those of their rhodium homologs, and even allow performing otherwise inviable transformations. In this review we aim to provide an informative journey, from the early pioneering examples in the field, most of them based on other metals than iridium, to the most recent transformations catalyzed by designed Ir(i) complexes. The review is organized by the type of C-H bond that is activated (with C sp², sp or sp³), as well as by the C-C unsaturated partner that is used as a hydrocarbonation partner (alkyne, allene or alkene). Importantly, we discuss the mechanistic foundations of the methods highlighting the differences from those previously proposed for processes catalyzed by related metals, particularly those of the same group (Co and Rh).

Catalytic α-Hydroarylation of Acrylates and Acrylamides via an Interrupted Hydrodehalogenation Reaction

The palladium-catalyzed, α-selective hydroarylation of acrylates and acrylamides is reported. Under optimized conditions, this method is highly tolerant of a wide range of substrates including those with base sensitive functional groups and/or multiple enolizable carbonyl groups. A detailed mechanistic study was undertaken, and the high selectivity of this transformation was shown to be enabled by the formation of a [PdII(Ar)(H)] intermediate, which performs selective hydride insertion into the β-position of α,β-unsaturated carbonyl compounds.