N -Iodosuccinimide (NIS) in Direct Aromatic Iodination

Polar Interactions between Substrate and Flavin Control Iodotyrosine Deiodinase Function

Flavin cofactors offer a wide range of chemical mechanisms to support a great diversity in catalytic function. As a corollary, such diversity necessitates careful control within each flavoprotein to limit its function to an appropriate subset of possible reactions and substrates. This task falls to the protein environment surrounding the flavin in most enzymes. For iodotyrosine deiodinase that catalyzes a reductive dehalogenation of halotyrosines, substrates can dictate the chemistry available to the flavin. Their ability to stabilize the necessary one-electron reduced semiquinone form of flavin strictly depends on a direct coordination between the flavin and α-ammonium and carboxylate groups of its substrates. While perturbations to the carboxylate group do not significantly affect binding to the resting oxidized form of the deiodinase, dehalogenation (kcat/Km) is suppressed by over 2000-fold. Lack of the α-ammonium group abolishes detectable binding and dehalogenation. Substitution of the ammonium group with a hydroxyl group does not restore measurable binding but does support dehalogenation with an efficiency greater than those of the carboxylate derivatives. Consistent with these observations, the flavin semiquinone does not accumulate during redox titration in the presence of inert substrate analogues lacking either the α-ammonium or carboxylate groups. As a complement, a nitroreductase activity based on hydride transfer is revealed for the appropriate substrates with perturbations to their zwitterion.

Room-Temperature Photochemical Copper-Mediated Fluorination of Aryl Iodides

This report describes a method for the photochemical Cu-mediated fluorination of aryl iodides with AgF via putative aryl radical (Ar•) intermediates. It involves irradiating an aryl iodide with UVB light (λmax = 313 nm) in the presence of a mixture of CuI and CuII salts and AgF. Under these conditions, fluorination proceeds at room temperature for substrates containing diverse substituents, including alkoxy and alkyl groups, ketones, esters, sulfonate esters, sulfonamides, and protected amines. This method has been translated to radiofluorination using a combination of K18F, K3PO4, and AgOTf.

C9-Functionalized Doxycycline Analogs as Drug Candidates to Prevent Pathological α-Synuclein Aggregation and Neuroinflammation in Parkinson's Disease Degeneration

Doxycycline, a semi-synthetic tetracycline, is a widely used antibiotic for treating mild-to-moderate infections, including skin problems. However, its anti-inflammatory and antioxidant properties, combined with its ability to interfere with α-synuclein aggregation, make it an attractive candidate for repositioning in Parkinson's disease. Nevertheless, the antibiotic activity of doxycycline restricts its potential use for long-term treatment of Parkinsonian patients. In the search for non-antibiotic tetracyclines that could operate against Parkinson's disease pathomechanisms, eighteen novel doxycycline derivatives were designed. Specifically, the dimethyl-amino group at C4 was reduced, resulting in limited antimicrobial activity, and several coupling reactions were performed at position C9 of the aromatic D ring, this position being one of the most reactive for introducing substituents. Using the Thioflavin-T assay, we found seven compounds were more effective than doxycycline in inhibiting α-synuclein aggregation. Furthermore, two of these derivatives exhibited better anti-inflammatory effects than doxycycline in a culture system of microglial cells used to model Parkinson's disease neuroinflammatory processes. Overall, through structure-activity relationship studies, we identified two newly designed tetracyclines as promising drug candidates for Parkinson's disease treatment.

Tunable Regioselective Allylic Alkylation/Iodination of Imidazoheterocycles in Water

The switchable roles of allylic alcohol and molecular iodine as reagents and catalysts have been demonstrated in the regioselective allylic alkylation and iodination of imidazoheterocycles employing the mixture of allylic alcohol-I2. First, we have explored the catalytic activity of iodine for the allylation of imidazoheterocycles using allylic alcohol in an aqueous medium. The allylation of a library of imidazoheterocycles and other electron-rich heterocycles like indole, pyrazole, 4-hydroxy coumarin, and 6-amino uracil has been achieved by employing this methodology. The efficiency of the I2 catalyst for N-allylation of azoles has also been demonstrated. Next, we have shown that this mixture of allylic alcohol and I2 could be beneficial for the iodination of imidazoheterocycles under room temperature. Mechanistic studies indicate that the activation of allylic alcohol by molecular iodine took place probably through halogen bonding, and NMR studies show that the reaction did not proceed through allylic ether formation.

Does the Intersystem Crossing Rate of β-Iodinated Phosphorus Corrole Depend on Iodine Numbers and/or Positions?

The heavy-atom effect is known to enhance the intersystem crossing (ISC) in organic molecular systems. Effects of iodine numbers and positions on the ISC rate of a few meso-difluorophenyl substituted β-iodinated phosphorus corroles (PCs) with axially ligated fluorine atoms (mI-FPC; m = 1-4) are studied using a time-dependent optimally tuned range-separated hybrid. Solvent effects are accounted for through a polarizable continuum model with a toluene dielectric. Calculations suggest similar thermodynamic stability for all mI-FPCs and also reproduce the experimentally measured 0-0 energies for some of the freebase phosphorus corrole (FPC) systems studied here. Importantly, our results reveal that all mI-FPCs display 10 times larger ISC rate (∼109 s-1) than the fluorescence rate (∼108 s-1), and the higher ISC rate stems from the improved spin-orbit coupling (SOC) introduced by lighter heteroatoms like central P and biaxial F rather than the I heavy-atom effect. However, an enhanced SOC is found with increasing I content for El-Sayed forbidden ISC channels. Research findings reported in this study unveil the impact of light heteroatoms and heavy atoms in promoting ISC in several iodinated PCs, which help in designing visible-light-driven efficient triplet photosensitizers.

Vapor-Phase Halogenation of Hydrogen-Terminated Silicon(100) Using N-Halogen-succinimides

The focus of this study was to demonstrate the vapor-phase halogenation of Si(100) and subsequently evaluate the inhibiting ability of the halogenated surfaces toward atomic layer deposition (ALD) of aluminum oxide (Al2O3). Hydrogen-terminated silicon ⟨100⟩ (H-Si(100)) was halogenated using N-chlorosuccinimide (NCS), N-bromosuccinimide (NBS), and N-iodosuccinimide (NIS) in a vacuum-based chemical process. The composition and physical properties of the prepared monolayers were analyzed by using X-ray photoelectron spectroscopy (XPS) and contact angle (CA) goniometry. These measurements confirmed that all three reagents were more effective in halogenating H-Si(100) over OH-Si(100) in the vapor phase. The stability of the modified surfaces in air was also tested, with the chlorinated surface showing the greatest resistance to monolayer degradation and silicon oxide (SiO2) generation within the first 24 h of exposure to air. XPS and atomic force microscopy (AFM) measurements showed that the succinimide-derived Hal-Si(100) surfaces exhibited blocking ability superior to that of H-Si(100), a commonly used ALD resist. This halogenation method provides a dry chemistry alternative for creating halogen-based ALD resists on Si(100) in near-ambient environments.

Copper-catalyzed nitration of electron-deficient BN-naphthalene

Under Cu-catalysis, a regioselective nitration of 1,8-dihalogenated BN-naphthalene (ABN) compounds (4a-4c) has been established with the use of tert-butyl nitrite as the nitrating reagent. The syntheses of dihalo-ABN nitro products (6a-6c; halo = Cl, Br and I) were case-studied in conjunction with the first synthesis and characterization of diiodo-ABN compound 4c. The molecular structures of these compounds have been spectroscopically characterized and further confirmed by X-ray single crystal diffraction experiments. This method allows direct regioselective nitration of electron-deficient ABN systems, providing a step-economical entry to novel nitro-ABN structural motifs with potential applications in agrochemicals, materials sciences, and the medicinal and pharmaceutical industries.

Electrochemical Iodination through the In Situ Generation of Iodinating Agents: A Promising Green Approach

The synthesis of iodinated compounds using cheap, simple, and green strategies is of fundamental importance. Iodination reactions are mainly used to synthesize useful intermediates, especially in the pharmaceutical field, where they are employed for the production of contrast media or of iodinated active pharmaceutical ingredients. Traditional synthetic methods suffer from the use of erosive, toxic, or hazardous reactants. Approaches which involve the use of molecular iodine as an iodinating agent require the addition of an oxidizing agent, which is often difficult to handle. Electrochemistry can offer a valid and green alternative by avoiding the addition of such oxidizing agents, transforming the iodine source in the active species through the use of electrons as the main reactants. Herein, we report the electrochemical iodination with the generation of iodinating species in situ in water by using iodides as the source of iodine atoms. First of all, the electrochemical behavior of iodide and iodine in water on carbonaceous anodes was studied and, after selecting the suitable potential, in situ electrochemical iodination was successfully applied to 5-hydroxyisophthalic acid and 5-sulfosalicylic acid, comparing the iodinating power of I₂ and iodonium species.

Substrate Electronics Dominate the Rate of Reductive Dehalogenation Promoted by the Flavin-Dependent Iodotyrosine Deiodinase

Iodotyrosine deiodinase (IYD) is unusual in its reliance on flavin to promote reductive dehalogenation of halotyrosines under aerobic conditions. Applications of this activity can be envisioned for bioremediation, but expansion of its specificity requires an understanding of the mechanistic steps that limit the rate of turnover. Key processes capable of controlling steady-state turnover have now been evaluated and described in this study. While proton transfer is necessary for converting the electron-rich substrate into an electrophilic intermediate suitable for reduction, kinetic solvent deuterium isotope effects suggest that this process does not contribute to the overall efficiency of catalysis under neutral conditions. Similarly, reconstituting IYD with flavin analogues demonstrates that a change in reduction potential by as much as 132 mV affects kcat by less than 3-fold. Furthermore, kcat/Km does not correlate with reduction potential and indicates that electron transfer is also not rate determining. Catalytic efficiency is most sensitive to the electronic nature of its substrates. Electron-donating substituents on the ortho position of iodotyrosine stimulate catalysis and conversely electron-withdrawing substituents suppress catalysis. Effects on kcat and kcat/Km range from 22- to 100-fold and fit a linear free-energy correlation with a ρ ranging from -2.1 to -2.8 for human and bacterial IYD. These values are consistent with a rate-determining process of stabilizing the electrophilic and nonaromatic intermediate poised for reduction. Future engineering can now focus on efforts to stabilize this electrophilic intermediate over a broad series of phenolic substrates that are targeted for removal from our environment.

Visible light-enabled regioselective chlorination of coumarins using CuCl2via LMCT excitation

An efficient, regioselective chlorination of coumarins using Earth-abundant and cost-effective CuCl₂ under visible light irradiation is reported. A key feature of this protocol is the photocatalytic dissociation of the copper(II) complex in acetonitrile through ligand-to-metal charge transfer (LMCT) to give the chlorine atom which then selectively chlorinates the coumarin. This method can chlorinate a broad scope of coumarins with either electron-withdrawing or electron-donating substituents to regioselectively afford 3-chlorocoumarins in good to excellent yields and can be further extended to other electron-deficient heterocycles and olefins such as flavones, 8-methoxypsoralen and naphthoquinones.

Semi-Synthetic Analogues of Cryptolepine as a Potential Source of Sustainable Drugs for the Treatment of Malaria, Human African Trypanosomiasis, and Cancer

The prospect of eradicating malaria continues to be challenging in the face of increasing parasite resistance to antimalarial drugs so that novel antimalarials active against asexual, sexual, and liver-stage malaria parasites are urgently needed. In addition, new antimalarials need to be affordable and available to those most in need and, bearing in mind climate change, should ideally be sustainable. The West African climbing shrub Cryptolepis sanguinolenta is used traditionally for the treatment of malaria; its principal alkaloid, cryptolepine (1), has been shown to have antimalarial properties, and the synthetic analogue 2,7-dibromocryptolepine (2) is of interest as a lead toward new antimalarial agents. Cryptolepine (1) was isolated using a two-step Soxhlet extraction of C. sanguinolenta roots, followed by crystallization (yield 0.8% calculated as a base with respect to the dried roots). Semi-synthetic 7-bromo- (3), 7, 9-dibromo- (4), 7-iodo- (5), and 7, 9-dibromocryptolepine (6) were obtained in excellent yields by reaction of 1 with N-bromo- or N-iodosuccinimide in trifluoroacetic acid as a solvent. All compounds were active against Plasmodia in vitro, but 6 showed the most selective profile with respect to Hep G2 cells: P. falciparum (chloroquine-resistant strain K1), IC₅₀ = 0.25 µM, SI = 113; late stage, gametocytes, IC₅₀ = 2.2 µM, SI = 13; liver stage, P. berghei sporozoites IC₅₀ = 6.13 µM, SI = 4.6. Compounds 3-6 were also active against the emerging zoonotic species P. knowlesi with 5 being the most potent (IC₅₀ = 0.11 µM). In addition, 3-6 potently inhibited T. brucei in vitro at nM concentrations and good selectivity with 6 again being the most selective (IC₅₀ = 59 nM, SI = 478). These compounds were also cytotoxic to wild-type ovarian cancer cells as well as adriamycin-resistant and, except for 5, cisplatin-resistant ovarian cancer cells. In an acute oral toxicity test in mice, 3-6 did not exhibit toxic effects at doses of up to 100 mg/kg/dose × 3 consecutive days. This study demonstrates that C. sanguinolenta may be utilized as a sustainable source of novel compounds that may lead to the development of novel agents for the treatment of malaria, African trypanosomiasis, and cancer.

DABCO as a practical catalyst for aromatic halogenation with N-halosuccinimides

A simple and practical synthetic approach for synthesis of aromatic halides is developed. Simple Lewis base, DABCO, is used as the catalyst. This arene halogenation process proceedes conveniently and efficiently at ambient conditions, providing the desired products in good to excellent yields and selectivity.

Triple Emission of 5'-(para-R-Phenylene)vinylene-2-(2'-hydroxyphenyl)benzoxazole (PVHBO). Part I: Dual Emission from the Neutral Species

The effects of 5'-(para-R-phenylene)vinylene (PV) substituents on the emission properties of 2-(2'-hydroxyphenyl)benzoxazole (HBO) are analyzed using steady-state and time-resolved absorption and emission spectroscopies in addition to quantum chemical calculations. All members in the series of PVHBOs are capable of excited-state intramolecular proton transfer (ESIPT) with a solvent sensitivity that is typical of a HBO derivative to produce a normal (aka enol) emission and an excited-state tautomer (aka keto) emission. These two emission bands of the neutral dyes are discussed in the current paper. The intermolecular proton transfer, i.e., the deprotonation, of a PVHBO results in the third band of the triple emission, which is described in the succeeding paper. The placement of an electron-withdrawing substituent R on the PVHBO scaffold increases the intensity of the keto emission relative to the enol emission in hydrogen-bonding solvents. The R substituents do not significantly alter the wavelengths of the enol and keto emission bands, which are located in the blue and green regions, respectively, of the visible spectrum. The ultrafast time-resolved spectroscopies and quantum chemical calculations offer explanations on how the R group and the solvent affect the enol and keto emission properties (i.e., wavelength, lifetime, fluorescence quantum yield, and relative ratio of their emissions). The key findings include the following: (1) the emission energies of both enol and keto forms are not sensitively dependent on the R substituent and (2) the solvent-engaged enol excited state is quenched more efficiently as the R substituent becomes more electron-withdrawing. A PVHBO acts as a fusion of HBO and stilbenoid that intersect at the hydroxyphenyl moiety. Depending on the solvent and other environmental conditions, PVHBOs may exhibit the ESIPT property of HBO or the substituent-dependent emission of stilbenoid. This paper and the succeeding article provide a photophysical model of PVHBOs to explain the wavelengths and relative abundances of the three emission bands (enol, keto, and anion) that these compounds are able to produce. Judicial selection of the environmental factors may drive the emission of a PVHBO into the spectral regions of blue, green, and, in a couple of cases, orange or red.

Selective C-H Iodination of (Hetero)arenes

Iodoarenes are versatile intermediates and common synthetic targets in organic synthesis. Here, we present a strategy for selective C-H iodination of (hetero)arenes with a broad functional group tolerance. We demonstrate the utility and differentiation to other iodination methods of supposed sulfonyl hypoiodites for a set of carboarenes and heteroarenes.

Direct Iodination of Electron-Deficient Benzothiazoles: Rapid Access to Two-Photon Absorbing Fluorophores with Quadrupolar D-π-A-π-D Architecture and Tunable Heteroaromatic Core

Direct iodination of benzothiazoles under strong oxidative/acidic conditions leads to a mixture of iodinated heteroarenes with 1-2 major components, which are easily separable and which structures depend on the I₂ equivalents used. Among the unexpected but dominant products were identified 4,7-diiodobenzothiazoles with a rare substitution pattern for S_EAr reactions of this scaffold. These were employed in the synthesis of 4,7-bis(triarylamine-ethynyl)benzothiazoles - a new class of highly efficient quasi-quadrupolar fluorophores displaying large two-photon absorption cross sections (540-1374 GM) in the near-infrared region.

One-Pot Synthesis of N-Iodo Sulfoximines from Sulfides

This is the first report on the synthesis and characterization of N-iodo sulfoximines. The synthesis was designed as a room temperature one-pot cascade reaction from readily available sulfides as starting compounds, converted into sulfoximines by reaction with ammonium carbonate and (diacetoxyiodo)benzene, followed by iodination with N-iodosuccinimide or iodine in situ, in up to 90% isolated yields, also at a multigram scale. Iodination of aryls with N-iodo sulfoximines, oxidation, and conversion to N-SCF₃ congeners have been demonstrated.

Protecting Group-Controlled Remote Regioselective Electrophilic Aromatic Halogenation Reactions

Being able to utilize a protecting group to influence remote regiocontrol offers a simple alternative approach to direct late-stage functionalization of complex organic molecules. However, protecting groups that have the ability to influence reaction regioselectivity remote to their local chemical environment are not widely reported in the literature. Herein, we report the development of remote regioselective electrophilic aromatic substitution (S_EAr) reactions that are enabled via the application of the tetrafluoropyridyl (TFP) phenol-protecting group. We demonstrate that through sequential reactions and protection/deprotection of the TFP group, substitution patterns that do not conform to classical S_EAr regioselectivity rules can be readily accessed.

Solvent-Controlled Racemic Resolution of C3-Symmetric Trihydroxytribenzotriquinacenes

A racemic C₃-symmetric trihydroxytribenzotriquinacene was resolved on a large scale by fractional crystallization of the corresponding (1S)-camphanic esters, achieving both enantiopure enantiomers (>99% ee) in 35% and 32% yields. The method relies on a distinct solvent-controlled discrimination process between the diastereomers. The enantiopure trihydroxytribenzotriquinacenes were converted into four other enantiopure building blocks, which are valuable precursors for supramolecular and materials chemistry to illustrate the utility of the synthesized compounds.

Triptycenyl Sulfide: A Practical and Active Catalyst for Electrophilic Aromatic Halogenation Using N-Halosuccinimides

A Lewis base catalyst Trip-SMe (Trip = triptycenyl) for electrophilic aromatic halogenation using N-halosuccinimides (NXS) is introduced. In the presence of an appropriate activator (as a noncoordinating-anion source), a series of unactivated aromatic compounds were halogenated at ambient temperature using NXS. This catalytic system was applicable to transformations that are currently unachievable except for the use of Br₂ or Cl₂: e.g., multihalogenation of naphthalene, regioselective bromination of BINOL, etc. Controlled experiments revealed that the triptycenyl substituent exerts a crucial role for the catalytic activity, and kinetic experiments implied the occurrence of a sulfonium salt [Trip-S(Me)Br][SbF₆] as an active species. Compared to simple dialkyl sulfides, Trip-SMe exhibited a significant charge-separated ion pair character within the halonium complex whose structural information was obtained by the single-crystal X-ray analysis. A preliminary computational study disclosed that the π system of the triptycenyl functionality is a key motif to consolidate the enhancement of electrophilicity.

High-Affinity Alkynyl Bisubstrate Inhibitors of Nicotinamide N-Methyltransferase (NNMT)

Nicotinamide N-methyltransferase (NNMT) is a metabolic enzyme that methylates nicotinamide (NAM) using cofactor S-adenosylmethionine (SAM). NNMT overexpression has been linked to diabetes, obesity, and various cancers. In this work, structure-based rational design led to the development of potent and selective alkynyl bisubstrate inhibitors of NNMT. The reported nicotinamide-SAM conjugate (named NS1) features an alkyne as a key design element that closely mimics the linear, 180° transition state geometry found in the NNMT-catalyzed SAM → NAM methyl transfer reaction. NS1 was synthesized in 14 steps and found to be a high-affinity, subnanomolar NNMT inhibitor. An X-ray cocrystal structure and SAR study revealed the ability of an alkynyl linker to span the methyl transfer tunnel of NNMT with ideal shape complementarity. The compounds reported in this work represent the most potent and selective NNMT inhibitors reported to date. The rational design principle described herein could potentially be extended to other methyltransferase enzymes.

Synthesis of Aryl Iodides from Arylhydrazines and Iodine

A metal- and base-free method is developed for the synthesis of aryl iodides from arylhydrazine hydrochlorides and iodine. A wide variety of aryl iodides can be conveniently synthesized by an equimolar reaction of arylhydrazine hydrochlorides and I₂ in dimethyl sulfoxide at 60 °C for 6 h. In the iodination step, arylhydrazines are oxidized by iodine to form arenediazonium salts, which undergo single-electron transfer from iodide anion to give aryl and iodine radicals; subsequent combination of them affords the corresponding aryl iodides.