Selenium-catalyzed oxidative halogenation

Organoselenides catalyze the oxidation of halides by H₂O₂. Furthermore, these selenides catalyze the transfer of oxidized halogens from N-halosuccinimides to olefins and ketones. Thus, organoselenides catalyze oxidative halogenation reactions including halolactonization, α-halogenation of ketones, and allylic halogenation. The ability of selenium to undergo reversible 2e⁻ oxidation-reduction chemistry facilitates halogenation through selenium-bound halogen intermediates.     

Complementary Site-Selective Halogenation of Nitrogen-Containing (Hetero)Aromatics with Superacids

Site-selective functionalization of arenes that is complementary to classical aromatic substitution reactions remains a long-standing quest in organic synthesis. Exploiting the generation of halenium ion through oxidative process and the protonation of the nitrogen containing function in HF/SbF₅, the chlorination and iodination of classically inert Csp²−H bonds of aromatic amines occurs. Furthermore, the superacid-promoted (poly)protonation of the molecules acts as a protection, favoring the late-stage selective halogenation of natural alkaloids and active pharmaceutical ingredients     

Highly regioselective halogenation of 1-phenyl-3-(3,5-dimethoxyphenyl)-propane-1,3-dione

Halogenation of 1-phenyl-3-(3,5-dimethoxyphenyl)-propane-1,3-dione (1) with N-X reagents take place regioselectively at the α position (except for fluorination), while halogenation of its BF₂ derivative 3 take place regioselectively at position 2 in the activated phenyl ring. When the molar ratio of substrate to reagent is changed from 1:1.1 to 1:2.1 or 1:2.8, halogenation takes place at positions 2 and 6 of the aromatic ring. Crystallization of a BF₂ derivative from protic solvent led to hydrolysis of the BF₂ group.     

Über Eisen(III)-Komplexe mit Phenolen. V. Komplexbildung mit 4,5-Dibrombrenzcatechin-disulfonsäure(3,6)

The chelate formation between Fe³⁺ and 4.5-dibromopyrocatechol-3.6-disulfonic acid in aqueous solution has been investigated spectrophotometrically. The stability constants and the light absorption properties of the chelates are communicated. Compared with the bromine-free tiron, the ? values increase and the λ_max positions are shifted bathochromically on halogenation. However, due to the changed position of the sulfo groups, the bond-weakening effect of the halogen substituents is covered, leading to a distinct increase of pK_S1, but to only slight alterations of all the other constants. Poor solubility and accessability of the chelating reagent hinder its application in analysis.     

Halogenation of 4,5-dicarba-arachno- nonaborane(13),4,5-C2B7H13

The AlX₃-catalyzed (X = Cl, Br, and I) halogenation of arachno-4,5-C₂B₇H₁₃ with anhydrous hydrogen halides produces a series of 6-substituted derivatives, 6-X-4,5-C₂B₇H₁₂. The same compounds along with 6,8-I₂-4,5-C₂B₇H₁₁ are obtained in non-catalyzed reactions with elemental halogens. The electrophile-induced nucleophilic substitution concept (EINS) of the substitution with hydrogen halides is suggested. The constitution of all compounds isolated was unambiguously determined via ¹H, ¹³C, ¹¹B, and two-dimensional (2-D) ¹¹B-¹¹B NMR spectra.     

Non-Heme-Iron-Mediated Selective Halogenation of Unactivated Carbon−Hydrogen Bonds

Oxidation of the iron(II) precursor [(L¹)Fe^IICl₂], where L¹ is a tetradentate bispidine, with soluble iodosylbenzene (^sPhIO) leads to the extremely reactive ferryl oxidant [(L¹)(Cl)Fe^IV=O]⁺ with a cis disposition of the chlorido and oxido coligands, as observed in non-heme halogenase enzymes. Experimental data indicate that, with cyclohexane as substrate, there is selective formation of chlorocyclohexane, the halogenation being initiated by C−H abstraction and the result of a rebound of the ensuing radical to an iron-bound Cl⁻. The time-resolved formation of the halogenation product indicates that this primarily results from ^sPhIO oxidation of an initially formed oxido-bridged diiron(III) resting state. The high yield of up to >70 % (stoichiometric reaction) as well as the differing reactivities of free Fe²⁺ and Fe³⁺ in comparison with [(L¹)Fe^IICl₂] indicate a high complex stability of the bispidine-iron complexes. DFT analysis shows that, due to a large driving force and small triplet-quintet gap, [(L¹)(Cl)Fe^IV=O]⁺ is the most reactive small-molecule halogenase model, that the Fe^III/radical rebound intermediate has a relatively long lifetime (as supported by experimentally observed cage escape), and that this intermediate has, as observed experimentally, a lower energy barrier to the halogenation than the hydroxylation product; this is shown to primarily be due to steric effects.     

Halogenation and amination dual properties of haloamines in Raschig environment: A chlorine transfer reaction between chloramine and 3-azabicyclo[3,3,0]octane

The chlorine transfer reaction between 3-azabicyclo[3,3,0]octane “AZA” and chloramine was studied over pH 8–13 in order to follow both the amination and halogenation properties of NH₂Cl. The results show the existence of two competitive reactions which lead to the simultaneous formation of N-amino- and N-chloro- 3-azabicyclo[3,3,0]octane by bimolecular kinetics. The halogenation reaction is reversible and the chlorine derivative obtained, which is thermolabile and unstable in the pure state, was identified by electrospray mass spectrometry. These phenomena were quantified by a reaction between neutral species according to an apparent S_N2-type mechanism for the amination process and a ionic mechanism involving a reaction between chloramine and protonated amine for the halogenation process. Amination occurs only in strongly basic solutions (pH ≥ 13) while chlorination occurs at lower pH's (pH ≤ 8). At intermediate pH's, a mixture of these two compounds is obtained. The relative proportions of the products are a function of intrinsic rate constants, pH and pK_a of the reactants. The rate constants and thermodynamic activation parameters are the following: k₁ = 45.5 × 10⁻³ M⁻¹ s⁻¹; ΔH₁^0# = 59.8 kJ mol⁻¹; ΔS₁^0# = − 86.5 J mol⁻¹ K⁻¹ for amination; k₂ = 114 × 10⁻³ M⁻¹ s⁻¹; ΔH₂^0# = 63.9 kJ mol⁻¹; and ΔS₂^0# = − 48.3 J mol⁻¹ K⁻¹ for chlorination. The ability of an interaction corresponding to a specific (NH₃Cl⁺/RR′NH) or general (NH₂Cl/RR′NH) acid catalysis has been also discussed. © 1997 John Wiley & Sons, Inc.     

Synthesis and anti-proliferative activity studies of 2-(2-(trifluoromethyl)-6-(substituted)imidazo[1,2-b]pyridazin-3-yl)-N-(substituted)acetamide derivatives

A series of novel imidazo[1,2-b]pyridazin-3-yl acetamide derivatives (9a-9j) were synthesized from a 3,6-dichloropyridazine. We have developed a simple strategy for the synthesis of functionally diverse imidazole, and pyridiazine derivatives were reported via a series of steps. The work involves bicyclic imidazo-pyridazine ring formation, halogenation, cynation, hydrolysis, peptide coupling, and Buchwald reaction. The structure of the synthesized compounds was confirmed by IR, ¹H NMR, ¹³C NMR,¹⁹F NMR, mass spectra, and elemental analysis, and purity is checked by HPLC. All synthesized compounds were screened for anticancer activity against A-549 and Du-145 cancer cell lines by MTT assay. The preliminary bioassay suggests that most of the compounds show anti-proliferation with different degrees; doxorubicin was used as positive control. The synthesized compound shows IC₅₀ values in the range of 1.74μM to 16.17μM in both cell lines. The compounds 9e , 9g , and 9h were active compared with doxorubicin in both the cell lines. The compounds having cyclopentyl ring are active compared with higher and lower carbon analogues.     

Direct and initiated halogenation of carbon nanomaterials at low temperatures

Cryochemical reactions of the direct and initiated (by photolysis and radiolysis) halogenation of carbon nanomaterials (C₆₀ fullerene, nanotubes, and nanofibers) at 77–240 K were investigated by the ESR, IR spectroscopy, and elemental analysis techniques. A high reactivity of C₆₀ in reactions with fluorine and chlorine with the formation of corresponding derivatives was shown. High concentrations of radical intermediates indicating the radical chain halogenation of C₆₀ were detected (the kinetic chain length for the chlorination process reaches 10⁴–10⁶ units). The amount of chlorine attached to fullerene is ～35% and practically does not depend on the initiation mode (UV or γ-irradiation at doses up to 350 kGy). The mechanism of the cryochemical halogenation of C₆₀ is considered within the limits of the model of multicenter synchronous transitions in a molecular complex consisting of several reactant molecules including molecular fluorine or chlorine and ensuring a net exothermic effect. The amount of chlorine added to nanotubes and nanofibers did not exceed 2.5–8%, thereby indicating a low reactivity of these materials under cryogenic conditions.     

Über die Darstellung der Dichlormethylen-halogensulfenium-Salze Cl2CSCl+ AsF6− und Cl2CSBr+ AsF6− [1]

The Synthesis of the Dichloromethylene-halogenosulfenium Salts Cl₂CSCl⁺ AsF₆^? and Cl₂CSBr⁺ AsF₆^? The sulfenium salts Cl₂CSCl⁺ AsF₆^? and Cl₂CSBr⁺ AsF₆^? are synthesized by oxidative halogenation of thiophosgene, Cl₂CS with X₂/AsF₅ (X = Cl, Br) at 195 K and are characterized by vibrational as well as NMR spectroscopy.     

Bis(trifluoromethyl)-containing 1-azatricycloheptanes

Methods for the synthesis ofanti-3-halo-7, 7-bis(trifluoromethyl)-1-azatricyclo[2.2.1.0^2,6]heptanes by conjugated halogenation of 3,3-bis(trifluoromethyl)-2-azabicyclo[2.2.1]hept-5-ene have been developed. Hydrohalogenation of the synthesized 1-azatricyclic compounds gives exclusively 6,7-dihalo-3,3-bis(trifluoromethyl)-2-azabicyclo[2.2.1]heptanes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1649–1653, September, 1997.     

Characteristics of the halogenation of 2-substituted 6-benzhydryl-4(3H)-pyrimidinones

The halogenation of 2-substituted 6-benzhydryl-4(3H)-pyrimidinones has been investigated. Bromination with Py·Br₂ and iodination with I₂ solution in alkali occurs exclusively at the CH hydrogen of the benzhydryl group. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, 1418–1420, September, 2006.     

Heterogeneous Photocatalytic Recycling of FeX2/FeX3 for Efficient Halogenation of C−H Bonds Using NaX

Environmental-friendly halogenation of C−H bonds using abundant, non-toxic halogen salts is in high demand in various chemical industries, yet the efficiency and selectivity of laboratory available protocols are far behind the conventional photolytic halogenation process which uses hazardous halogen sources. Here we report an FeX₂ (X=Br, Cl) coupled semiconductor system for efficient, selective, and continuous photocatalytic halogenation using NaX as halogen source under mild conditions. Herein, FeX₂ catalyzes the reduction of molecular oxygen and the consumption of generated oxygen radicals, thus boosting the generation of halogen radicals and elemental halogen for direct halogenation and indirect halogenation via the formation of FeX₃. Recycling of FeX₂ and FeX₃ during the photocatalytic process enables the halogenation of a wide range of hydrocarbons in a continuous flow, rendering it a promising method for applications.     

Mechanochemical synthesis of platinum(<Emphasis Type="SmallCaps">IV</Emphasis>) complexes with <Emphasis Type="Italic">N</Emphasis>-heterocyclic carbenes

A new method for the synthesis of complexes Pt^IV(NHC)X₄L (NHC is N-heterocyclic carbene of imidazole or benzimidazole series; X = Cl, Br; L is N-coordinated pyridine or NHC) based on mechanochemical oxidation of complexes Pt^II(NHC)X₂L with dichloroiodobenzene (PhICl₂) or pyridinium hydrobromide perbromide (PyHBr₃) was proposed. Mechanochemical activation led to reduction in the synthesis time and increase in the selectivity of halogenation and yields of the target Pt^IV complexes (74–98%) as compared to the reaction in solutions.     

A New Entry to the Thiazolo(3,2-a)Pyridine Ring System: Bromination of 1-Alkenyl-1,2-Dihydropyridine-2-Thiones

Derivatives of the thiazolo(3,2-a)pyridine ring system possess a variety of pharmacological activities like antiinflammatory² hypoglycemic³ and analgesic⁴ activity. Classical methods for the preparation of hydrosoluble thiazolo(3,2-a)pyridinium halides involve reactions of 1H-1,2-dihydropyridine-2-thiones with a-halocarbonyl compounds⁵ I acid-catalyzed dehydration of their 1-phenacyl derivatives6 and halogenation of 2-vinylthiopyridines.⁷     

A One Pot Synthesis of Dehydrohedione (DHH) from a Hedione® Precursor

Dehydrohedione (DHH) 1a was obtained via a one pot, three step, domino procedure in 54% overall yield from 2a , by treatment with CuBr₂, in MeOH at 65 °C. We demonstrated that the direct transformation of malonate derivative 2a into DHH 1a under CuX₂ Kochi's conditions goes preferentially through the pathway involving intermediates 2b / 2c and 7a , rather than 3a / 3b or 8a / 8b , essentially via α‐halogenation/dehydrohalogenation of the ketone moiety, both mediated by CuX₂, while in‐situ decarbomethoxylation is promoted by the resulting CuX in refluxing MeOH.     

Bildung siliciumorganischer Verbindungen. 66. (H2Si?CH2)2 und Si-substituierte Derivate

Formation of Organosilicon Compounds. 66. (H₂Si? CH₂)₂ and Si-substituted Derivatives (H₂Si? CH₂)₂ 1 is formed in the reaction of (Cl₂Si? CH₂)₂ with LiAlH₄. In 1 , the halogenation of the SiH bond is so much preferred compared to the ring cleavage reaction, that 1 reacts with Cl₂ or Br₂ to form successively all compounds form 1-monochlor-1,3-disilacyclobutane to (X₂Si? CH₂)₂ (X = Cl, Br). The stability of the 1,3-disilacyclobutane skeleton towards HBr or Br₂ increases as the electronegativity of the Si-substituents increases. Thus, (Cl₂Si? CH₂)₂ is cleaved neither by HBr nor by Br₂, whereas e. g. [H(C₆H₅)Si? CH₂]₂ reacts to [Br(C₆H₅)Si? CH₂]₂ with Br₂, but yields meH(C₆H₅)Si? CH₂? SiBr(C₆H₅)H (me = CH₃) with HBr. In [me(C₆H₅)Si? CH₂]₂, the four-membered ring is cleaved by Br₂ as well as by HBr. The ¹H-, ²⁹Si- and ¹³C-n.m.r. data are reported.     

Nucleophilic assistance in methane activation by superelectrophiles with halogen-centered cationic sites

Simulation of fragments of potential energy surface for systems CH₄ + CBr ₃ ⁺ , CH₄ + CBr ₃ ⁺ AlBr ₄ ^? , CH₄ + CCl ₃ ⁺ AlCl ₄ ^? , and CH₄ + CCl ₃ ⁺ Al₂Cl ₇ ^? was performed by DFT-B3LYP and DFT-PBE methods. The important role of nucleophilic assistance in methane halogenation by these superelectrophiles was confirmed. These reactions occur with a synchronous hydride transfer from methane to the electrophile within the cyclic transition states in linear C-H-C fragment of the rings and a generation of a C-Hlg bond between the carbon atom of the arising methyl group and the halogen atom of the electrophile. The nucleophilic assistance from the unshared electron pair of this halogen atom provides the lowering of the potential barriers to methane halogenation by complexes CBr ₃ ⁺ AlBr ₄ ^? , CCl ₃ ⁺ AlCl ₄ ^? , and CCl ₃ ⁺ Al₂Cl ₇ ^? to the values of the order of 20 kcal mol^?1. These essential features of the mechanism of methane halogenation are independent of the halogen nature and are retained on going from the model electrophiles to the real ones.     

Silver-halogen cluster compounds Ag n X m (n≥2; 0≤m≤n;X=F,Br)

Nonstoichiometric silver-halogen cluster compounds Ag _n X _m (0≤m≤n;X=F, Br) are generated by cocondensation of Ag atoms and AgX species using a slightly modified gas aggregation technique. The AgX molecules are produced by partial decomposition of SF₆ and Br₂ respectively at the surface of the hot silver containing crucible, followed by the reaction of halogen atoms with silver, giving rise to the formation of AgX molecules. In a heterogeneous nucleation between these molecules and evaporated Ag atoms the afore mentioned cluster compounds are formed. The degree of halogenation can either be controlled by the adjustment of the silver evaporation rate, or even more easily by controlling the partial pressure of the halogenating agent. The mass spectra of singly charged halogenated clusters, which are generated by electron impact ionization, reflect the stability of ions. These mass spectra demonstrate that there is an alternation in the intensity pattern up to a relatively high degree of halogenation (m) for each of the investigated compound series Ag _n X _m ,n≤8. This behavior is similar to the well-known odd-even effect for pure metal clusters, allowing us to postulate the existence of a “metallic” core which governs the stability of the cluster ion (at least for not too high degree of halogenation).     

Photopolymerization of Pyrrole Initiated by the Ferrocene- and Iron-Arene Salts-Chlorinated Solvents Complexes

Abstract

Ferrocene- and iron-arene salts can photoinitiate polymerization of pyrrole in the presence of halogenated solvents, such as CH₂CH₂Cl₂, CHCl₃, and CCl₄, when irradiated with UV light (254 nm). The polypyrroles obtained are black colloidal powders and have low conductivity in the range of 10^?5 S cm^?1 and rather poor electrochemical properties, which can be the result of loss of conjugation by halogenation. Polypyrrole samples contain both covalently bounded CCl₃ groups (from CCl₄) and ionic FeCl^? ₄. A mechanism of photoinitiated polymerization of pyrrole has been proposed.