Engineering Molecular Iodine Catalysis for Alkyl–Nitrogen Bond Formation

Iodine-Mediated δ-Amination of sp3 C-H Bonds

We present a direct δ-amination reaction of sp3 C-H bonds, employing molecular iodine (I2) as the sole oxidant under transition-metal-free conditions. This remote C-H functionalization approach is operationally simple and provides facile, efficient access to pyrrolidines and related heterocyclic derivatives from readily accessible substrates.

Iodine Molecules within Triethylenediamine-Based Metal-Organic Frameworks for Hydrolysis/Alkylation Tandem Reactions

The integration of radionuclide iodine molecules in metal-organic frameworks (MOFs) for organic synthesis is attracting considerable research attention due to their specific catalytic performance. However, understanding the comprehensive catalytic behaviors of different types of molecular iodine encapsulated in MOFs for a sequential organic transformation is a great challenge. To address this issue, we have designed two triethylenediamine-functionalized MOFs assembled from 1,3,5-tricarboxyphenyl-2-(triethylenediaminemethyl)benzene-linker and {Cd(COO)3N} or {Cu4(u3-OH)2(COO)6N} clusters. Both MOFs show good stability and adsorption of I2 in the solution and vapor phases. Catalysts obtained after treatment with ethyl acetate present efficient catalytic activity in hydrolysis/alkylation tandem reactions in water. The mechanistic investigations disclose a sequential catalytic process comprising a "hidden" Brønsted acid catalytic hydrolysis of acetals to aldehydes followed by the I2-bonding Lewis acid catalytic alkylation of aldehydes to 3,3'-disubstituted 1H-indoles.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Applications of the DIB-BBr3 Protocol in Bromination Reactions

Non-symmetrical bromoiodanes are useful for bromination reactions, and some protocols were found to be suitable for specific substrates. Herein, we report the use of a DIB/BBr₃ protocol for various bromination reactions, including electrophilic bromination of arenes, carbonyl C-H monobromination, bromolactonization, bromocarbocyclization, intermolecular bromoetherification of olefin, and light-triggered C(sp³)-H bromination.

Remote C(sp3 )-H Acylation of Amides and Cascade Cyclization via N-Heterocyclic Carbene Organocatalysis

The direct functionalization of inert C(sp³ )-H bonds under environmentally benign catalytic conditions remains a challenging task in synthetic chemistry. Here, we report an organocatalytic remote C(sp³ )-H acylation of amides and cascade cyclization through a radical-mediated 1,5-hydrogen atom transfer mechanism using N-heterocyclic carbene as the catalyst. Notably, a diversity of nitrogen-containing substrates, including simple linear aliphatic carbamates and ortho-alkyl benzamides, can be successfully applied to this organocatalytic system. With the established protocol, over 120 examples of functionalized δ-amino ketones and isoquinolinones with diverse substituents were easily synthesized in up to 99 % yield under mild conditions. The robustness and generality of the organocatalytic strategy were further highlighted by the successful acylation of unactivated C(sp³ )-H bonds and late-stage modification of pharmaceutical molecules. Then, the asymmetric control of the radical reaction was attempted and proven feasible by using a newly designed chiral thiazolium catalyst, and moderate enantioselectivity was obtained at the current stage. Preliminary mechanistic investigations including several control reactions, KIE experiments, and computational studies shed light on the organocatalytic radical reaction mechanism.

Copper-catalyzed intramolecular iminolactonization cyclization reactions of remote C(sp3)–H bonds in carboxamides

A novel and efficient synthetic method for iminolactones by copper-catalyzed intramolecular C(sp3)–H bond functionalization of carboxamides via a cascade process is reported for the first time.

An environmentally benign regioselective synthesis of 2-benzyl-4-arylquinoline derivatives using aryl amines, styrene oxides and aryl acetylenes

A novel and an expedient metal- and solvent-free synthesis of a wide variety of 2-benzyl-4-arylquinoline derivatives is described from readily available aryl amines, styrene oxides and aryl acetylenes in the presence of 10 mol% molecular iodine. This domino reaction occurs under metal- and solvent-free conditions at 120 °C, which avoids the usage of metal catalyst and as a consequence generation of metal waste. The salient features of this methodology are the use of simple starting materials, ease of handling, high regioselectivity, shorter reaction time, atom-economical, step-economical, the formation of one C-N and two C-C bonds and a wide range of functional groups tolerance.

Toolbox for Distal C-H Bond Functionalizations in Organic Molecules

Transition metal catalyzed C-H activation has developed a contemporary approach to the omnipresent area of retrosynthetic disconnection. Scientific researchers have been tempted to take the help of this methodology to plan their synthetic discourses. This paradigm shift has helped in the development of industrial units as well, making the synthesis of natural products and pharmaceutical drugs step-economical. In the vast zone of C-H bond activation, the functionalization of proximal C-H bonds has gained utmost popularity. Unlike the activation of proximal C-H bonds, the distal C-H functionalization is more strenuous and requires distinctly specialized techniques. In this review, we have compiled various methods adopted to functionalize distal C-H bonds, mechanistic insights within each of these procedures, and the scope of the methodology. With this review, we give a complete overview of the expeditious progress the distal C-H activation has made in the field of synthetic organic chemistry while also highlighting its pitfalls, thus leaving the field open for further synthetic modifications.

Formal Fluorinative Ring Opening of 2-Benzoylpyrrolidines Utilizing [1,2]-Phospha-Brook Rearrangement for Synthesis of 2-Aryl-3-fluoropiperidines

A ring expansion of 2-benzoylpyrrolidines, which involves the formal fluorinative ring opening utilizing the [1,2]-phospha-Brook rearrangement under Brønsted base catalysis and a subsequent intramolecular reductive amination, was developed. The operationally simple three-step protocol provides an efficient access to 2-aryl-3-fluoropiperidines. The methodology was further applied to the syntheses of azepanes and tetrahydroquinolines.

Synthesis of Six-Membered N-Heterocycle Frameworks Based on Intramolecular Metal-Free N-Centered Radical Chemistry

The formation of intramolecular C-N bond represents a powerful strategy in organic transformation as the great importance of N-heterocycles in the fields of natural products and bioactive molecules. This personal account describes the synthesis of cyclic phosphinamidation, benzosultam, benzosulfoximine, phenanthridine and their halogenated compounds through transition-metal-free intramolecular oxidative C-N bond formation. Mechanism study reveals that N-X bond is initially formed under the effect of hypervalent halogen, which is very unstable and easily undergoes thermal or light homolytic cleavage to generate nitrogen radical. Then the nitrogen radical is trapped by the arene to give aryl radical. Rearomatization of aryl radical under the oxidant to provide corresponding N-heterocycle. Under suitable conditions, the N-heterocycles can be further transformed to halogenated N-heterocycles.

Intramolecular C-H Amination of N-Alkylsulfamides by tert-Butyl Hypoiodite or N-Iodosuccinimide

1,3-Diamines are an important class of compounds that are broadly found in natural products and are also widely used as building blocks in organic synthesis. Although the intramolecular C-H amination of N-alkylsulfamide derivatives is a reliable method for the construction of 1,3-diamine structures, the majority of these methods involve the use of a transition-metal catalyst. We herein report on a new transition-metal-free method using tert-butyl hypoiodite (t-BuOI) or N-iodosuccinimide (NIS), enabling secondary non-benzylic and tertiary C-H amination reactions to proceed. The cyclic sulfamide products can be easily transformed into 1,3-diamines. Mechanistic investigations revealed that amination reactions using t-BuOI or NIS each proceed via different pathways.

Asymmetric Synthesis of Enantiopure Pyrrolidines by C(sp3 )-H Amination of Hydrocarbons

The asymmetric synthesis of enantiopure pyrrolidines is reported via a streamlined strategy relying on two sequential C-H functionalizations of simple hydrocarbons. The first step is a regio- and stereoselective catalytic nitrene C-H insertion. Then, a subsequent diastereoselective cyclization involving a 1,5-hydrogen atom transfer (HAT) from a N-centered radical leads to the formation of pyrrolidines that can then be converted to their free NH-derivatives.

One-Pot Synthesis of Primary and Secondary Aliphatic Amines via Mild and Selective sp3 C-H Imination

The direct replacement of sp3 C-H bonds with simple amine units (-NH2) remains synthetically challenging, although primary aliphatic amines are ubiquitous in medicinal chemistry and natural product synthesis. We report a mild and selective protocol for preparing primary and secondary aliphatic amines in a single pot, based on intermolecular sp3 C-H imination. The first C-H imination of diverse alkanes, this method shows useful site-selectivity within substrates bearing multiple sp3 C-H bonds. Furthermore, this reaction tolerates polar functional groups relevant for complex molecule synthesis, highlighted in the synthesis of amine pharmaceuticals and amination of natural products. We characterize a unique C-H imination mechanism based on radical rebound to an iminyl radical, supported by kinetic isotope effects, stereoablation, resubmission, and computational modeling. This work constitutes a selective method for complex amine synthesis and a new mechanistic platform for C-H amination.

Metal-free synthesis of quinoline-2,4-dicarboxylate derivatives using aryl amines and acetylenedicarboxylates through a pseudo three-component reaction

An efficient, useful and one-pot protocol for the synthesis of quinoline-2,4-dicarboxylate scaffolds is accomplished from aryl amines and dimethyl/diethyl acetylenedicarboxylates using 20 mol% molecular iodine as a catalyst in acetonitrile at 80 °C. In addition, the mechanistic explanation for the formation of the desired products is disclosed. The pivotal role of molecular iodine in the formation of the major products, diester quinoline derivatives, and the minor product, triesters, in two cases is described in the mechanism. The notable advantages of this method are non-involvement of a metal catalyst, avoiding of metal contamination in the final product as well as waste generation, use of a low cost and eco-friendly catalyst, ease of handling, high regioselectivity, shorter reaction time, the formation of one C-N and two C-C bonds and a broad substrate scope with good yields.

Visible-Light-Induced Carbonylation of Indoles with Phenols under Metal-Free Conditions: Synthesis of Indole-3-carboxylates

A visible-light-induced carbonylation of indoles with phenols for the synthesis of indole-3-carboxylates has been developed. The reaction proceeded via a radical carbonylation process in which elementary I₂ was used as an effective photosensitive initiator and, thus, avoided the use of transition metal catalysts. A series of different aryl indole-3-carboxylates were prepared in moderate to good yields. The broad applicability of this methodology was further highlighted by the late-stage functionalization of several phenol-containing natural products and pharmaceuticals.

Radical-Promoted Distal C-H Functionalization of C(sp3 ) Centers with Fluorinated Moieties

Due to their unique properties, fluorinated scaffolds are pivotal compounds in pharmaceuticals, agrochemicals, and materials science. Over the last years, the development of versatile strategies for the selective synthesis of fluorinated molecules by direct C-H bond functionalization has attracted a lot of attention. In particular, the design of novel transformations based on a radical process was a bottleneck for distal C-H functionalization reactions, offering synthetic solutions for the selective introduction of fluorinated groups. This Minireview highlights the major contributions in this blossoming field. The development of new methodologies for the remote functionalization of aliphatic derivatives with various fluorinated groups based on a 1,5-hydrogen atom transfer process and a β-fragmentation reaction will be showcased and discussed.

Photoinduced triiodide-mediated [3 + 2] cycloaddition of N-tosyl aziridines and alkenes

A photoinduced triiodide-mediated [3 + 2] cycloaddition of N-Ts aziridines and alkenes is described herein. This operationally simple protocol enables regioselective access to a wide range of substituted pyrrolidines under mild-free conditions.

Copper-catalyzed, N-directed remote C(sp3)–H azidation and thiocyanation

A mild and practical protocol is developed for the synthesis of distal azido and thiocyanato alkylamines via N-directed remote C(sp³)–H functionalization.

5-Endo Trig Oxidative Radical Cyclizations of Ugi-3CR Products toward 1,4-Imidazolidinones

A 5-endo trig oxidative radical cyclization of benzylamine-derived Ugi three-component reaction products rapidly affords imidazolidinones with three diversity elements. This adaptation of our previously described multicomponent reaction-oxidation methodology further showcases manipulation of the diversity elements in multicomponent reaction products via oxidative radical cyclizations, which generates highly decorated privileged heterocycles.

Formal α-Allylation of Primary Amines by a Dearomative, Palladium-Catalyzed Umpolung Allylation of N-(Aryloxy)imines

N-(Aryloxy)imines, readily accessible by condensation/tautomerization of (pseudo)benzylic primary amines and 2,6-di-tert-butyl-1,4-benzoquinone, undergo efficient allylation to afford a wide range of homoallylic primary amines following hydrolytic workup. Deprotonation of N-(aryloxy)imines generates a delocalized 2-azaallyl anion-type nucleophile that engages in dearomative C-C bond-forming reactions with allylpalladium(II) electrophiles generated from allylic tert-butyl carbonates. This reactivity umpolung enables the formal α-allylation of (pseudo)benzylic primary amines. Mechanistic studies reveal that the apparent regioselectivity of the desired bond-forming event is a convergent process that is initiated by unselective allylation of N-(aryloxy)imines to give several regioisomeric species, which subsequently rearrange via stepwise [1,3]- or concerted [3,3]-sigmatropic shifts, ultimately converging to provide the desired regioisomer of the amine products.

Iodine Catalysis for C(sp3 )-H Fluorination with a Nucleophilic Fluorine Source

Iodine catalysis was developed for aliphatic fluorination through light-promoted homolytic C-H bond cleavage. The intermediary formation of amidyl radicals enables selective C-H functionalization via carbon-centered radicals. For the subsequent C-F bond formation, previous methods have typically been limited by a requirement for electrophilic fluorine reagents. We here demonstrate that the intermediary instalment of a carbon-iodine bond sets the stage for an umpolung, thereby establishing an unprecedented nucleophilic fluorination pathway.

Iodine-Catalyzed Aerobic Oxidation of Spirovinylcyclopropyl Oxindoles to Form Spiro-1,2-dioxolanes Diastereoselectively

A novel method of iodine-catalyzed aerobic oxidation with spirovinylcyclopropyl oxindoles under mild conditions has been described. A series of spiro-1,2-dioxolanes were prepared in good to excellent yields and considerable diastereoselectivities. The new approach is operationally simple, scalable, and tolerant of various functional groups.

Metal-free oxidative C(sp3)–H functionalization: a facile route to quinoline formaldehydes from methyl-azaheteroarenes

A facile protocol for the synthesis of quinoline formaldehydes via direct oxidative C–H bonds functionalization of methyl-azaheteroarenes in the presence of I2–DMSO has been described. This method is metal-free and easy to operate. This reaction provided a convenient route for the preparation of a range of important quinoline formaldehydes.

Copper-Catalyzed Remote C(sp3)-H Amination of Carboxamides

Here we report a method for the site-selective intermolecular C(sp³)-H amination of carboxamides by merging transition-metal catalysis and the hydrogen atom transfer strategy. The reaction proceeds through a sequence of favorable single-electron transfer, 1,5-hydrogen atom transfer, and C-N cross-coupling steps, thus allowing access to a series of desired products. This reaction could accommodate a wide diversity of nitrogen nucleophiles as well as demonstrate excellent chemoselectivity and functional group compatibility.

Generation and Reactivity of Amidyl Radicals: Manganese-Mediated Atom-Transfer Reaction

A simple and efficient protocol to generate amidyl radicals from amine functionalities through a manganese-mediated atom-transfer reaction has been developed. This approach employs an earth-abundant and inexpensive manganese complex, Mn₂ (CO)₁₀ , as the catalyst and visible light as the energy input. Using this strategy, site-selective chlorination of unactivated C(sp³ )-H bonds of aliphatic amines and intramolecular/intermolecular chloroaminations of unactivated alkenes were readily realized under mild reaction conditions, thus providing efficient access to a range of synthetically valuable alkyl chlorides, chlorinated pyrrolidines, and vicinal chloroamine derivatives. These practical reactions exhibit a broad substrate scope and tolerate a wide array of functional groups, and complex molecules including various marketed drug derivatives.

Crystal engineering of coordination-polymer-based iodine adsorbents using a π-electron-rich polycarboxylate aryl ether ligand

This work revealed that the synergy of microporous channels and convergent arrangements of halogen bonding and charge-transfer interaction sites within coordination polymers facilitated the iodine adsorption process.

A general method for site-selective Csp3-S bond formation via cooperative catalysis

Herein, we report a copper-catalysed site-selective thiolation of Csp³-H bonds of aliphatic amines. The method features a broad substrate scope and good functional group compatibility. Primary, secondary, and tertiary C-H bonds can be converted into C-S bonds with a high efficiency. The late-stage modification of biologically active compounds by this method was also demonstrated. Furthermore, the one-pot preparation of pyrrolidine or piperidine compounds via a domino process was achieved.

Iodine-Catalyzed Functionalization of Primary Aliphatic Amines to Oxazoles, 1,4-Oxazines, and Oxazinones

Unprecedented I₂-catalyzed α,α-C(sp³)-H, decarboxylative α-C(sp³)-H, lactonized α-C(sp³)-H, and α,β-C(sp³)-H functionalized 5- and 6-annulation as well as α-C(sp³)-H activated 6-lactonization of primary aliphatic amines are devised under aerobic conditions. The metal-free sustainable strategy was employed for the diverse construction of valuable five-and six-membered polycyclic N,O-heteroaromatics such as oxazoles, 1,4-oxazines, and oxazin-2-one with a rapid reaction rate and high yield. The viability of this mild nonmetallic catalysis is successfully verified through syntheses of labile chiral heterocyclic analogues. In contrast to the common practice, this method is not limited to use of prefunctionalized amines, directing groups (DGs) and/or transient DGs, metal catalysts, and traditional oxidants. The possible mechanistic pathway of the annulation reaction is investigated by control experiments and ESI-MS data collected for a reaction mixture of the ongoing reaction. The synthesized new compounds are potent organic nanobuilding blocks to achieve valuable organic nanomaterials of different sizes, shapes, and dimensions, which are under investigation for the discovery of high-tech devices of innovative organic nanoelectronics and photophysical properties.

Copper-Catalyzed, N-Directed Csp3-H Trifluoromethylthiolation (-SCF3) and Trifluoromethylselenation (-SeCF3)

A direct and versatile copper-catalyzed trifluoromethylthiolation and trifluoromethylselenation of primary, secondary, and tertiary aliphatic C-H bonds was developed. The reaction provides direct access to molecules containing these emerging moieties in the presence of a wide range of common functional groups and in complex molecular environments.

Alkyliodines in High Oxidation State: Enhanced Synthetic Possibilities and Accelerated Catalyst Turn-Over

In contrast to aryliodine(III) compounds, which have matured into a particularly attractive class of oxidants in modern synthesis, the synthetic potential of related alkyliodine(III) derivatives has remained widely underestimated. This is surprising since several unique synthetic possibilities arise directly from the low stability of their central carbon-iodine bond. In this respect, these high-oxidation-state iodine compounds resemble environmentally benign variants of the prominent metal counterparts such as those derived from palladium, nickel and copper. This Concept article summarizes the general reactivity trends in alkyliodine(III) chemistry and discusses selected examples of their strategic use as highly reactive, transient species in organic synthesis and homogeneous catalysis.