Mechanism of Rhodium-Catalyzed C-H Functionalization: Advances in Theoretical Investigation

Isatoic anhydride as a masked directing group and internal oxidant for Rh(III)-catalyzed decarbonylative annulation through C-H activation: insights from DFT calculations

Density functional theory calculations uncovered a new mechanism for the rhodium-catalyzed decarbonylative annulation of isatoic anhydride with alkynes, in which the acyloxy group formed from the N-H deprotonation and C-O bond cleavage of isatoic anhydride acts as the directing group to assist the ortho C-H activation. From the generated five-membered rhodacycle intermediate, the final aminoisocoumarin product could be formed by subsequent steps of alkyne insertion, reductive elimination, decarbonylation, and protonation. The isocyanate moiety contained in the annulation intermediate was uncovered as a novel internal oxidant for the reaction, which oxidizes the Rh(I) to Rh(III) by decarbonylation.

Gold(III) Auracycles Featuring C(sp3)-Au-C(sp2) Bonds: Synthesis and Mechanistic Insights into the Cycloauration Step

The direct auration of arenes is a key step in numerous gold-catalyzed reactions. Although reported more than 100 years ago, understanding of its underlying mechanism has been hampered by the difficulties in the isolation of relevant intermediates given the propensity of gold(III) species to undergo reductive elimination. Here, we report the synthesis and isolation of a new family of intriguing zwitterionic [C(sp3)^C(sp2)]-auracyclopentanes, as well as of their alkyl-gold(III) precursors and demonstrate their value as mechanistic probes to study the C(sp2)-Au bond-forming event. Experimental investigations employing Kinetic Isotope Effects (KIE), Hammett plot, and Eyring analysis provided important insights into the formation of the auracycle. The data suggest a SEAr mechanism wherein the slowest step might be the π-coordination between the arene and the gold(III) center, en route to the Wheland intermediate. We also show that these auracyclopentanes can work as catalysts in several gold-promoted transformations.

Hydrated cation-π interactions of π-electrons with hydrated Mg2+ and Ca2+ cations

Hydrated cation-π interactions at liquid-solid interfaces between hydrated cations and aromatic ring structures of carbon-based materials are pivotal in many material, biological, and chemical processes, and water serves as a crucial mediator in these interactions. However, a full understanding of the hydrated cation-π interactions between hydrated alkaline earth cations and aromatic ring structures, such as graphene remains elusive. Here, we present a molecular picture of hydrated cation-π interactions for Mg2+ and Ca2+ by using the density functional theory methods. Theoretical results show that the graphene sheet can distort the hydration shell of the hydrated Ca2+ to interact with Ca2+ directly, which is water-cation-π interactions. In contrast, the hydration shell of the hydrated Mg2+ is quite stable and the graphene sheet interacts with Mg2+ indirectly, mediated by water molecules, which is the cation-water-π interactions. These results lead to the anomalous order of adsorption energies for these alkaline earth cations, with hydrated Mg2+-π < hydrated Ca2+-π when the number of water molecules is large (n ≥ 6), contrary to the order observed for cation-π interactions in the absence of water molecules (n = 0). The behavior of hydrated alkaline earth cations adsorbed on a graphene surface is mainly attributed to the competition between the cation-π interactions and hydration effects. These findings provide valuable details of the structures and the adsorption energy of hydrated alkaline earth cations adsorbed onto the graphene surface.

N^N^C-Cyclometalated rhodium(III) complexes with isomeric pyrimidine-based ligands: unveiling the impact of isomerism on structural motifs, luminescence and cytotoxicity

The impact of isomerism of pyrimidine-based ligands and their rhodium(III) complexes with regard to their structures and properties was investigated. Two isomeric ligands, 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2,5-diphenylpyrimidine (HL2,5) and 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2,6-diphenylpyrimidine (HL2,6), were synthesized. The ligands differ by the degree of steric bulk: the molecular structure of HL2,5 is more distorted due to presence of pyrazolyl and phenyl groups in the neighbouring positions 4 and 5 of the pyrimidine ring. The complexation of HL2,5 and HL2,6 with RhCl3 leads to the sp2 C-H bond activation, resulting in the isolation of two complexes, [RhL2,5(Solv)Cl2]·nEtOH and [RhL2,6(Solv)Cl2]·nEtOH (Solv = H2O, EtOH), with the deprotonated forms of the pyrazolylpyrimidine molecules which coordinate the Rh3+ ion as N^N^C-tridentate ligands. According to DFT modelling, the mechanism of the deprotonation involves (i) the C-H bond breaking in the 2-phenyl group followed by the coordination of the C atom to the Rh atom, (ii) the protonation of coordinated chlorido ligand, (iii) the ejection of the HCl molecule and (iv) the coordination of the H2O molecule. The ligand isomerism has an impact on emission properties and cytotoxicity of the complexes. Although the excited states of the complexes effectively deactivate through S0/T1 and S0/S1 crossings associated with the cleavage of the weak H2O ligands upon excitation, the [RhL2,5(Solv)Cl2]·nEtOH complex appeared to be emissive in the solid state, while [RhL2,6(Solv)Cl2]·nEtOH is non-emissive at all. The complexes show significant cytotoxic activity against cancerous HepG2 and Hep2 cell lines, with the [RhL2,6(Solv)Cl2]·nEtOH complex being more active than its isomer [RhL2,5(Solv)Cl2]·nEtOH. On the other hand, noticeable cytotoxicity of the latter against HepG2 is supplemented by its non-toxicity against non-cancerous MRC-5 cells.

Recent advances in Rh(I)-catalyzed enantioselective C-H functionalization

Chiral carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds are pervasive and very essential in natural products, bioactive molecules, and functional materials, and their catalytic construction has emerged as one of the hottest research fields in synthetic organic chemistry. The last decade has witnessed vigorous progress in Rh(I)-catalyzed asymmetric C-H functionalization as a complement to Rh(II) and Rh(III) catalysis. This review aims to provide the most comprehensive and up-to-date summary covering the recent advances in Rh(I)-catalyzed C-H activation for asymmetric functionalization. In addition to the development of diverse reactions, chiral ligand design and mechanistic investigation (inner-sphere mechanism, outer-sphere mechanism, and 1,4-Rh migration) will also be highlighted.

Enantioselective Rhodium-Catalyzed C-H Arylation Enables Direct Synthesis of Atropisomeric Phosphines

Atropisomeric phosphines hold considerable significance in asymmetric catalysis, yet their synthesis presents a formidable challenge owing to intricate multistep procedures. In this context, a groundbreaking methodology has been presented for their preparation. This innovative approach entails an atroposelective rhodium-catalyzed C-H activation employing aryl and heteroaryl halides, chelated by a P(III) center. The essence of this strategy lies in its ability to directly construct chiral phosphine ligands in a single step, thereby exhibiting exceptional efficiency in terms of atom and redox economy. Illustrative examples serve to demonstrate the immense potential of in situ-formed ligands in asymmetric catalysis. Mechanistic experiments have further provided invaluable insights into this transformation.

Non-Invasive Delivery of Insulin for Breaching Hindrances against Diabetes

Insulin is recognized as a crucial weapon in managing diabetes. Subcutaneous (s.c.) injections are the traditional approach for insulin administration, which usually have many limitations. Numerous alternative (non-invasive) slants through different routes have been explored by the researchers for making needle-free delivery of insulin for attaining its augmented absorption as well as bioavailability. The current review delineating numerous pros and cons of several novel approaches of non-invasive insulin delivery by overcoming many of their hurdles. Primary information on the topic was gathered by searching scholarly articles from PubMed added with extraction of data from auxiliary manuscripts. Many approaches (discussed in the article) are meant for the delivery of a safe, effective, stable, and patient friendly administration of insulin via buccal, oral, inhalational, transdermal, intranasal, ocular, vaginal and rectal routes. Few of them have proven their clinical efficacy for maintaining the glycemic levels, whereas others are under the investigational pipe line. The developed products are comprising of many advanced micro/nano composite technologies and few of them might be entering into the market in near future, thereby garnishing the hopes of millions of diabetics who are under the network of s.c. insulin injections.

Annulative π-Extension by Cp*Co(III)-Catalyzed Ketone-Directed peri-Annulation: An Approach to Access Fused Arenes

A masked-bay-region selective first-row transition-metal Cp*Co(III)-catalyzed annulative π-extension of arene-derived ketones is achieved to afford K-region-functionalized benzo[e]pyrenes, benzotetraphenes, and pyrenes. Comprehensive density functional theory studies buttress the mechanistic pathway comprising key steps like peri-C-H activation, alkyne 1,2-migratory insertion, and nucleophilic attack toward ketone, this attack being the rate-determining step. In addition, π-conjugated 1,1'-bipyrenes, potential photocatalyst pyrene-quinones, and putative n-type semiconductor cyano group-containing dibenzo[de,qr]tetracenes are also accessed.

Expedient C-H allylation of sulfoxonium ylides: merging C-H and C-C/C-het bond activation

Sulfoxonium ylide chelation-assisted C-H allylation of arenes has been accomplished utilizing strained vinyl carbo/heterocycles as the allyl surrogates via sequential C-H and C-C/het bond activation. Broad substrate scope, Co-catalysis, selectivity, and late-stage drug mutation are the important practical features.

Is Enol Always the Culprit? The Curious Case of High Enantioselectivity in a Chiral Rh(II) Complex Catalyzed Carbene Insertion Reaction

The mechanism of Rh2 (S-NTTL)4 catalyzed carbene insertion into C(3)-H of indole is investigated using DFT methods. Since the commonly accepted enol mechanism cannot account for enantioinduction, a concerted oxocarbenium pathway was proposed in an earlier work using a model catalyst. However, after considering the full catalytic system, this study finds that akin to other reactions, here, too, the enol pathway is of lower energy, which now naturally raises a conundrum regarding the mode of chiral induction. Herein, a new water promoted mechanistic pathway involving a metal-associated enol intermediate hydrogen bonding and stereochemical model are proposed to solve this puzzle. It is shown how the catalyst bowl-shaped structure along with substrate-catalyst binding is crucial for achieving high levels of enantioselectivity. A stereodetermining water-assisted proton transfer is proposed and confirmed through deuterium-labeling experiments. The water molecules are held together by H-bonding interactions with the carboxylate ligands that is reminiscent of enzyme catalysis. Although several previous studies have aimed at understanding the mechanism of metal catalyzed carbene insertion reactions, the origin of high stereoinduction especially with chiral metal complexes remains unclear, and till date there is no transition state model that can explain the high enantioselectivity with such chiral Rh complexes. The metal-associated enol pathway is currently underrepresented in catalytic cycles and may play a crucial role in catalyst design. Since the enol pathway is commonly adopted in other metal-catalyzed X-H insertion reactions involving a diazoester, the presented results are not specific to the current reaction. Therefore, this study could provide the direction for achieving high levels of enantioselectivity which is otherwise difficult to achieve with a single metal catalyst.

2-Hydroxypyridine-based Ligands as Promoter in Ruthenium(II) Catalyzed C-H Bond Activation/Arylation Reactions

A class of 2-hydroxypyridine based ligands are explored to achieve enhanced catalytic activity for ortho-C-H bond activation/arylation reaction over [(η6 -p-cymene)RuCl2 ]2 catalyst in water. Extensive studies using a series of substituted 2-hydroxypyridine based ligands (L1-L6) inferred that 5-trifluoromethyl-2-hydroxypyridine (L6) exhibited favorable effects to enhance the catalytic activity of Ru(II) catalyst for ortho C-H bond arylation of 2-phenylpyridine by 8 folds compared to those performed without ligands. The (η6 -p-cymene)Ru - L6 system also exhibited enhanced catalytic activity for ortho C-H bond arylation of 2-phenylpyridine using a variety of aryl halides. NMR and mass investigations inferred the presence of several ligand coordinated Ru(II) species, suggesting the involvement of these species in C-H bond activation reaction. Further in concurrence with the experimental findings, the density functional theory (DFT) calculations also evidenced the prominent role of 2-hydroxypyridine based ligands in Ru(II) catalyzed C-H bond arylation of 2-phenylpyridine with lower energy barrier for the C-H activation step.

[RhCp*Cl2]2-Catalyzed Indole Functionalization: Synthesis of Bioinspired Indole-Fused Polycycles

Polycyclic fused indoles are ubiquitous in natural products and pharmaceuticals due to their immense structural diversity and biological inference, making them suitable for charting broader chemical space. Indole-based polycycles continue to be fascinating as well as challenging targets for synthetic fabrication because of their characteristic structural frameworks possessing biologically intriguing compounds of both natural and synthetic origin. As a result, an assortment of new chemical processes and catalytic routes has been established to provide unified access to these skeletons in a very efficient and selective manner. Transition-metal-catalyzed processes, in particular from rhodium(III), are widely used in synthetic endeavors to increase molecular complexity efficiently. In recent years, this has resulted in significant progress in reaching molecular scaffolds with enormous biological activity based on core indole skeletons. Additionally, Rh(III)-catalyzed direct C-H functionalization and benzannulation protocols of indole moieties were one of the most alluring synthetic techniques to generate indole-fused polycyclic molecules efficiently. This review sheds light on recent developments toward synthesizing fused indoles by cascade annulation methods using Rh(III)-[RhCp*Cl2]2-catalyzed pathways, which align with the comprehensive and sophisticated developments in the field of Rh(III)-catalyzed indole functionalization. Here, we looked at a few intriguing cascade-based synthetic designs catalyzed by Rh(III) that produced elaborate frameworks inspired by indole bioactivity. The review also strongly emphasizes mechanistic insights for reaching 1-2, 2-3, and 3-4-fused indole systems, focusing on Rh(III)-catalyzed routes. With an emphasis on synthetic efficiency and product diversity, synthetic methods of chosen polycyclic carbocycles and heterocycles with at least three fused, bridged, or spiro cages are reviewed. The newly created synthesis concepts or toolkits for accessing diazepine, indol-ones, carbazoles, and benzo-indoles, as well as illustrative privileged synthetic techniques, are included in the featured collection.

An Aldehyde-Driven, Fe(0)-Mediated, One-Pot Reductive Cyclization: Direct Access to 5,6-Dihydro-quinazolino[4,3-b]quinazolin-8-ones and Photophysical Study

A short, proficient, and regioselective synthesis of biheterocyclic 5,6-dihydro-quinazolino[4,3-b]quinazolin-8-ones has been revealed via an Fe(0)-powder-mediated, one-pot reductive cyclization protocol. Mechanistic investigation proved that water acts as a source of hydrogen for the reduction of the nitro group and the reaction rate was accelerated by an aldehyde. The designed transformation works under aerobic conditions, providing a series of bio-inspired molecular scaffolds. In addition, the photophysical study showed blue fluorescence emission with a good fluorescence quantum yield.

Computational Insights into the Influence of Ligands on Hydrogen Generation with [Cp*Rh] Hydrides

This work reports a computational investigation of the effect of ancillary ligands on the activity of an Rh catalyst for hydrogen evolution based on the [Cp*Rh] motif (Cp* = η⁵-pentamethylcyclopentadienyl). Specifically, we investigate why a bipyridyl (bpy) ligand leads to H₂ generation but diphenylphosphino-based (dpp) ligands do not. We compare the full ligands to simplified models and systematically vary structural features to ascertain their effect on the reaction energy of each catalytic step. The calculations based on density functional theory show that the main effect on reactivity is the choice of linker atom, followed by its coordination. In particular, P stabilizes the intermediate Rh-hydride species by donating electron density to the Rh, thus inhibiting the reaction toward H₂ generation. Conversely, N, a more electron-withdrawing center, favors H₂ generation at the price of destabilizing the hydride intermediate, which cannot be isolated experimentally and makes determining the mechanism of this reaction more difficult. We also find that the steric effects of bulky substituents on the main ligand scaffold can lead to large effects on the reactivity, which may be challenging to fine-tune. On the other hand, structural features like the bite angle of the bidentate ligand have a much smaller impact on reactivity. Therefore, we propose that the choice of linker atom is key for the catalytic activity of this species, which can be further fine-tuned by a proper choice of electron-directing groups on the ligand scaffold.

Tunably strained metallacycles enable modular differentiation of aza-arene C-H bonds

The precise activation of C-H bonds will eventually provide chemists with transformative methods to access complex molecular architectures. Current approaches to selective C-H activation relying on directing groups are effective for the generation of five-membered, six-membered and even larger ring metallacycles but show narrow applicability to generate three- and four-membered rings bearing high ring strain. Furthermore, the identification of distinct small intermediates remains unsolved. Here, we developed a strategy to control the size of strained metallacycles in the rhodium-catalysed C-H activation of aza-arenes and applied this discovery to tunably incorporate the alkynes into their azine and benzene skeletons. By merging the rhodium catalyst with a bipyridine-type ligand, a three-membered metallacycle was obtained in the catalytic cycle, while utilizing an NHC ligand favours the generation of the four-membered metallacycle. The generality of this method was demonstrated with a range of aza-arenes, such as quinoline, benzo[f]quinolone, phenanthridine, 4,7-phenanthroline, 1,7-phenanthroline and acridine. Mechanistic studies revealed the origin of the ligand-controlled regiodivergence in the strained metallacycles.

Cascade C-H Activation/Annulation of Sulfoxonium Ylides with Vinyl Cyclopropanes: Access to Cyclopropane-Fused α-Tetralones

Rh-catalyzed weak and traceless directing-group-assisted cascade C-H activation and annulation of sulfoxonium ylides with vinyl cyclopropanes as a coupling partner have been accomplished to furnish functionalized cyclopropane-fused tetralones at moderate temperature. The C-C bond formation, cyclopropanation, functional group tolerance, late-stage diversifications of drug molecules, and scale-up are the important practical features.

Bridging Chemical Knowledge and Machine Learning for Performance Prediction of Organic Synthesis

Recent years have witnessed a boom of machine learning (ML) applications in chemistry, which reveals the potential of data-driven prediction of synthesis performance. Digitalization and ML modelling are the key strategies to fully exploit the unique potential within the synergistic interplay between experimental data and the robust prediction of performance and selectivity. A series of exciting studies have demonstrated the importance of chemical knowledge implementation in ML, which improves the model's capability for making predictions that are challenging and often go beyond the abilities of human beings. This Minireview summarizes the cutting-edge embedding techniques and model designs in synthetic performance prediction, elaborating how chemical knowledge can be incorporated into machine learning until June 2022. By merging organic synthesis tactics and chemical informatics, we hope this Review can provide a guide map and intrigue chemists to revisit the digitalization and computerization of organic chemistry principles.

Rh-Catalyzed [3+2] Annulation of Cyclic Ketimines and Alkynyl Chloride: A Strategy for Accessing Unsymmetrically Substituted and Highly Functionalizable Indenes

Alkynyl chlorides were found to be extraordinarily novel electrophiles, which could afford a single regioisomer of the [3+2] annulation adducts with cyclic ketimines by rhodium catalysis. The alkenyl chloride moiety in the products provided a valuable functional handle for further diverse transformations. Therefore, this research provided not only a synthetic protocol for accessing unsymmetrically substituted indenyl amines but also a highly divergent solution for decorating the substituting group by postmanipulation of the chloride.

A comprehensive understanding of carbon-carbon bond formation by alkyne migratory insertion into manganacycles

Migratory insertion (MI) is one of the most important processes underpinning the transition metal-catalysed formation of C-C and C-X bonds. In this work, a comprehensive model of MI is presented, based on the direct observation of the states involved in the coupling of alkynes with cyclometallated ligands, augmented with insight from computational chemistry. Time-resolved spectroscopy demonstrates that photolysis of complexes [Mn(C^N)(CO)4] (C^N = cyclometalated ligand) results in ultra-fast dissociation of a CO ligand. Performing the experiment in a toluene solution of an alkyne results in the initial formation of a solvent complex fac-[Mn(C^N)(toluene)(CO)3]. Solvent substitution gives an η2-alkyne complex fac-[Mn(C^N)(η2-R1C2R2)(CO)3] which undergoes MI of the unsaturated ligand into the Mn-C bond. These data allowed for the dependence of second order rate constants for solvent substitution and first order rate constants for C-C bond formation to be determined. A systematic investigation into the influence of the alkyne and C^N ligand on this process is reported. The experimental data enabled the development of a computational model for the MI reaction which demonstrated that a synergic interaction between the metal and the nascent C-C bond controls both the rate and regiochemical outcome of the reaction. The time-resolved spectroscopic method enabled the observation of a multi-step reaction occurring over 8 orders of magnitude in time, including the formation of solvent complexes, ligand substitution and two sequential C-C bond formation steps.

How Strain-Release Determines Chemoselectivity: A Mechanistic Study of Rhodium-Catalyzed Bicyclo[1.1.0]butane Activation

Bicyclo[1.1.0]butane (BCB) derivatives are versatile coupling partners, and various reaction modes for their activation and transformation have been proposed. In this work, three BCB-activation modes in Rh-catalyzed BCB transformations that construct diastereoselective α-quaternary β-lactones were investigated by density functional theory calculations. Our results show that, compared with C1-C3 insertion and C-C3 oxidative addition, C2-C3 oxidative addition is more favorable. The whole catalytic cycle involves five main steps: C-H activation, oxidative addition, β-C elimination/reductive elimination, Rh walking, and aldehyde insertion/protonation. Independent gradient model, intrinsic reaction coordinate, distortion-interaction energy, and Laplacian electron-density analyses were carried out to investigate the mode of BCB activation. Our calculation also showed that aldehyde-insertion is the diastereoselectivity determining step, which is controlled by the steric effect between the ligand, methyl group, and aldehyde.

Insights into the Activation Mode of α-Carbonyl Sulfoxonium Ylides in Rhodium-Catalyzed C-H Activation: A Theoretical Study

A computational study has been performed to investigate the mechanism of RhIII -catalyzed C-H bond activation using sulfoxonium ylides as a carbene precursor. The stepwise and concerted activation modes for sulfoxonium ylides were investigated. Detailed theoretical results showed that the favored stepwise pathway involves C-H bond activation, carbonization, carbene insertion, and protonation. The free energy profiles for dialkylation of 2-phenylpyridine were also calculated to account for the low yield of this reaction. Furthermore, the substituent effect was elucidated by comparing the energy barriers for the protonation of meta- and para-substituted sulfoxonium ylides calculated by density functional theory.

Modern metal-catalyzed and organocatalytic methods for synthesis of coumarin derivatives: a review

Coumarin is an important pharmaceutical structural motif, abundantly found in numerous commonly used drugs. Compounds containing this core show a broad spectrum of medicinal properties and biological activities. The increasing importance and wide usages of coumarin derivatives have drawn attention to its synthetic methods, among which metal-catalyzed and organocatalytic methods have proved the most effective. Several metal-catalyzed and/or organocatalytic synthetic strategies for coumarin have been investigated and reported in recent decades. This review focuses on more recent reports on catalysis methods for synthesizing coumarin and coumarin-like structures (including light-mediated methods and nano-catalysts), exploring the mechanistic aspects, simplicity, efficiency, repeatability, and other advantages and disadvantages of these methods.

A novel quinoline-based NNN-pincer Cu(II) complex as a superior catalyst for oxidative esterification of allylic C(sp3)-H bonds

We report for the first time that the quinoline-based NNN-pincer Cu(II) complex acts as an air stable superior catalyst for the oxidative cross-coupling of the allyl sp³ C-H bond with an acid for the synthesis of allyl esters in a homogeneous system at ambient temperature. The synthesized catalyst, 1, has been well characterized by various analytical techniques (HRMS, single crystal X-ray diffraction, CV, EPR, UV-vis spectroscopy) and showed excellent catalytic activity for the oxidative esterification of allylic C(sp³)-H bonds at 40 °C within a very short period of time (1 h) using only 1 mol% of the catalyst. A wide variety of aromatic allylic esters were synthesized in moderate to good yields, which could be extended to aliphatic allyl esters as well.

Oxidative [4 + 2] annulation of 1-naphthols with alkynes accelerated by an electron-deficient rhodium(III) catalysts

The 1,3-diethoxycarbonyl-2,4,5-trimethylcyclopentadienyl (Cp^E) rhodium(III) complex displayed high efficacy in the catalytic oxidative annulation of 1-naphthols with internal alkynes under mild conditions. DFT calculations revealed that lower activation energies for the concerted metalation-deprotonation and the reductive elimination steps are the key to improved reactivity.

Recent progress in polymeric non-invasive insulin delivery

The design of carriers for insulin delivery has recently attracted major research attentions in the biomedical field. In general, the release of drug from polymers is driven via a variety of polymers. Several mechanisms such as matrix release, leaching of drug, swelling, and diffusion are usually adopted for the release of drug through polymers. Insulin is one of the most predominant therapeutic drugs for the treatment of both diabetes mellitus; type-I (insulin-dependent) and type II (insulin-independent). Currently, insulin is administered subcutaneously, which makes the patient feel discomfort, pain, hyperinsulinemia, allergic responses, lipodystrophy surrounding the injection area, and occurrence of miscarried glycemic control. Therefore, significant research interest has been focused on designing and developing new insulin delivery technologies to control blood glucose levels and time, which can enhance the patient compliance simultaneously through alternative routes as non-invasive insulin delivery. The aim of this review is to emphasize various non-invasive insulin delivery mechanisms including oral, transdermal, rectal, vaginal, ocular, and nasal. In addition, this review highlights different smart stimuli-responsive insulin delivery systems including glucose, pH, enzymes, near-infrared, ultrasound, magnetic and electric fields, and the application of various polymers as insulin carriers. Finally, the advantages, limitations, and the effect of each non-invasive route on insulin delivery are discussed in detail.

How the electron-deficient Cp ligand facilitates Rh-catalyzed annulations with alkynes

The dominant factors for the CpX ligand effects (Cp* versus CpE) on the reactivity for alkyne insertion into cationic and neutral rhodacycles are identified based on energy decomposition analysis.

Transition metal catalysed direct sulfanylation of unreactive C-H bonds: an overview of the last two decades

Transition metal catalysed direct sulfanylations of unreactive C-H bonds have become a unique and straightforward synthetic strategy in late-stage C-S bond formation of relevant complex molecules. Such transformations have represented...

Cationic π-extended heteroaromatics via a catalytic C-H activation annulative alkyne-insertion sequence

Cationic π-conjugated organic molecules have broad applications in materials science as next-generation organic materials. The annulative alkyne-insertion π-extension (AAIPEX) strategy has emerged as a promising synthetic approach for the rapid synthesis of cationic polycyclic heteroaromatic compounds (cPHACs) in a single step. The AAIPEX reaction provides a synthetic shortcut to achieve complex organic molecules from simple (hetero)arene templates and alkynes as π-extending partners, which would otherwise be difficult to achieve using traditional methods. In general, a step-economic AAIPEX protocol proceeds via C-H activation of unfunctionalized heteroarene templates, followed by alkyne insertion-annulation to furnish cPHACs. In this Feature Article, recent progress in the AAIPEX strategy to construct cPHACs is described along with brief illustrations of the resulting cPHACs in luminescence-related applications.

Recent Advances in the High-Valent Cobalt-Catalyzed C-H Functionalization of N-Heterocycles

Direct functionalization of heterocycles using C-H activation widely relies on the precious metal complexes. In past decade, the use of earth abundant and inexpensive transition metal to functionalize heterocycles has become an attractive alternate strategy. This concept is also interesting due to the unique reactivity pattern of these inexpensive metals. In this context we and other research groups have utilized the high-valent cobalt complexes as an inexpensive and readily available catalyst for the functionalization of heterocycles. In this review, we intend to brief recent progress made in the area of high-valent cobalt complexes catalyzed C-H functionalization of N-containing heterocycles.

Recent advances in organic electrosynthesis employing transition metal complexes as electrocatalysts

Organic electrosynthesis has been widely used as an environmentally conscious alternative to conventional methods for redox reactions because it utilizes electric current as a traceless redox agent instead of chemical redox agents. Indirect electrolysis employing a redox catalyst has received tremendous attention, since it provides various advantages compared to direct electrolysis. With indirect electrolysis, overpotential of electron transfer can be avoided, which is inherently milder, thus wide functional group tolerance can be achieved. Additionally, chemoselectivity, regioselectivity, and stereoselectivity can be tuned by the redox catalysts used in indirect electrolysis. Furthermore, electrode passivation can be avoided by preventing the formation of polymer films on the electrode surface. Common redox catalysts include N-oxyl radicals, hypervalent iodine species, halides, amines, benzoquinones (such as DDQ and tetrachlorobenzoquinone), and transition metals. In recent years, great progress has been made in the field of indirect organic electrosynthesis using transition metals as redox catalysts for reaction classes including C-H functionalization, radical cyclization, and cross-coupling of aryl halides-each owing to the diverse reactivity and accessible oxidation states of transition metals. Although various reviews of organic electrosynthesis are available, there is a lack of articles that focus on recent research progress in the area of indirect electrolysis using transition metals, which is the impetus for this review.

Stereoselective rhodium-catalyzed 2-C-H 1,3-dienylation of indoles: dual functions of the directing group

A rhodium-catalyzed intermolecular highly stereoselective 1,3-dienylation at the 2-position of indoles with non-terminal allenyl carbonates has been developed by using 2-pyrimidinyl or pyridinyl as the directing group. The reaction tolerates many functional groups affording the products in decent yields under mild conditions. In addition to C-H bond activation, the directing group also played a vital role in the determination of Z-stereoselectivity for the C-H functionalization reaction with 4-aryl-2,3-allenyl carbonates, which is confirmed by the E-selectivity observed with 4-alkyl-2,3-allenyl carbonates. DFT calculations have been conducted to reveal that π-π stacking involving the directing 2-pyrimidinyl or pyridinyl group is the origin of the observed stereoselectivity. Various synthetic transformations have also been demonstrated.

Computational insights into Ir(iii)-catalyzed allylic C-H amination of terminal alkenes: mechanism, regioselectivity, and catalytic activity

Computational studies on Ir(iii)-catalyzed intermolecular branch-selective allylic C–H amination of terminal olefins with methyl dioxazolone have been carried out to investigate the mechanism, including the origins of regioselectivity and catalytic activity difference. The result suggests that the reaction proceeds through generation of active species, alkene coordination, allylic C–H activation, decarboxylation, migratory insertion, and protodemetalation. The presence of AgNTf₂ could thermodynamically promote the formation of catalytically active species [Cp*Ir(OAc)]⁺. Both the weaker Ir–C(internal) bond and the closer interatomic distance of N⋯C(internal) in the key allyl-Ir(v)-nitrenoid intermediate make the migratory insertion into Ir–C(internal) bond easier than into the Ir–C(terminal) bond, leading to branch-selective allylic C–H amidation. The high energy barrier for allylic C–H activation in the Co system could account for the observed sluggishness, which is mainly ascribed to the weaker coordination capacity of alkenes to the triplet Cp*Co(OAc)⁺ and the deficient metal⋯H interaction to assist hydrogen transfer.

DFT studies on Ir(iii)-catalyzed branch-selective allylic C–H amination of terminal olefins with methyl dioxazolone have been carried out to investigate the mechanism, including the origins of regioselectivity and catalytic activity difference.