Nickel(ii)-catalyzed reductive silylation of alkenyl methyl ethers for the synthesis of alkyl silanes

A new one pot protocol has been developed for the reductive silylation of alkenyl methyl ethers using Et₃Si–BPin and HSiEt₃ with nickel(ii) catalyst. Styrene type methyl ethers, multi-substituted vinyl methyl ethers, heterocycles and unconjugated vinyl ethers are all tolerated to form alkyl silanes. Mechanistic study reveals that it is a cascade of a C–O bond silylation and vinyl double bond hydrogenation process. Internal nucleophilic substitution or oxidative addition pathways were both acceptable for C–O bond cleavage. The acquired intermediate alkenyl silanes then proceeded through an unconventional reduction process thus providing alkyl silanes.

A Ni(ii)-catalyzed tandem reaction including vinyl C–O bond silylation and olefin hydrogenation has been developed providing structurally diversified alkyl silanes.     

Palladium(ii)-catalyzed synthesis of indenones through the cyclization of benzenecarbaldehydes with internal alkynes

The palladium(ii)-catalyzed carbocyclization of benzenecarbaldehydes with internal alkynes to afford 2,3-disubstituted indenones was reported. The annulation reaction proceeded through the transmetalation of Pd(ii) with an aromatic aldehyde and the insertion of internal alkynes, followed by cyclization via the intramolecular nucleophilic addition of intermediate organopalladium(ii) species to the aldehyde group. This reaction proceeded in moderate to good yields with high regioselectivity.

The palladium(ii)-catalyzed carbocyclization of benzenecarbaldehydes with internal alkynes to afford 2,3-disubstituted indenones was reported.     

Copper(ii)-catalyzed synthesis of multisubstituted indoles through sequential Chan–Lam and cross-dehydrogenative coupling reactions

Starting from arylboronic acids and ester (Z)-3-aminoacrylates, one-pot syntheses of diverse indole-3-carboxylic esters have been described through copper(ii)-catalyzed sequential Chan–Lam N-arylation and cross-dehydrogenative coupling (CDC) reactions. The initial Chan–Lam arylation can proceed in DMF at 100 °C for 24 h to give ester (Z)-3-(arylamino)acrylate intermediates in the presence of Cu(OAc)₂/tri-tert-butylphosphine tetrafluoroborate, a catalytic amount of myristic acid as the additive, KMnO₄ and KHCO₃. Sequentially, these in situ arylated intermediates can undergo an intramolecular oxidative cross-dehydrogenative coupling process in mixed solvents (DMF/DMSO = 2 : 1) at 130 °C to give C3-functionalized multi-substituted indole derivatives.

One-pot syntheses of diverse indole-3-carboxylic esters have been described through copper(ii)-catalyzed sequential oxidative Chan–Lam N-arylation and cross-dehydrogenative coupling (CDC) reaction.     

Rh(iii)-catalyzed spiroannulation of 3-arylquinoxalin-2(1H)-ones with alkynes: practical access to spiroquinoxalinones

The Rh(iii)-catalyzed synthesis of spiroquinoxalinone derivatives from 3-arylquinoxalin-2(1H)-ones and alkynes via a C–H functionalization/[3 + 2] annulation sequence has been developed. This method, featuring low catalyst loading, was amenable to Gram scale synthesis and tolerated a variety of functional groups and substitution patterns on the aryl rings, providing the target products in good to excellent yields.

The use of imines as a H acceptor for Rh(iii)-catalyzed spirocyclization of 3-arylquinoxalinones and alkynes via a C–H functionalization/[3 + 2] annulation sequence has been achieved.     

The Heck reaction of allylic alcohols catalysed by an N-heterocyclic carbene-Pd(ii) complex and toxicity of the ligand precursor for the marine benthic copepod Amphiascoides atopus

The palladium-catalysed reaction of aryl halides and allylic alcohols is an attractive method for obtaining α,β-unsaturated aldehydes and ketones, which represent key intermediates in organic synthesis. In this context, a 1,2,3-triazol-5-ylidene (aNHC)-based palladium(ii) complex formed in situ has been found to be a selective catalyst for the syntheses of building blocks from the corresponding aryl halides and allylic alcohols, with yields ranging from 50% to 90%. The lack of toxic effects of the ligand precursor (1,2,3-triazolium salt) of the palladium(ii) complex for the harpacticoid copepod Amphiascoides atopus allowed us to contrast the efficiency of the catalytic system with the potential impact of the principal waste chemical in global aquatic ecosystems, which has not been previously addressed.

A 1,2,3-triazol-5-ylidene (aNHC)-based palladium(ii) complex formed in situ has been found to be an efficient catalyst in the formation of C–C bonds.     

A convenient approach to difluoromethylated all-carbon quaternary centers via Ni(ii)-catalyzed enantioselective Michael addition

A Ni(ii)-catalyzed enantioselective Michael addition of 2-acetyl azarenes with β-difluoromethyl substituted nitroalkenes was successfully realized, which afforded chiral CF₂H-containing compounds in good enantioselectivities (up to 93% ee). This protocol provides a new convenient approach to all-carbon quaternary stereogenic centers featuring a CF₂H group.

Ni(ii)-catalyzed enantioselective Michael addition afforded compounds with all-carbon quaternary stereogenic centers featuring a CF₂H group in good enantioselectivities.     

Late-stage ortho-C–H alkenylation of 2-arylindazoles in aqueous medium by Manganese(i)-catalysis

Earth-abundant and water-tolerant manganese(i) catalyzed alkenylation of 2-arylindazole with alkyl and aryl alkynes through C–H bond activation is described with a unique level of E-selectivity. The reaction proceeds through the control of C3 nucleophilicity of 2-aryl indazoles. This method is applied to the late-stage functionalization of complex molecules including ethinylestradiol, norethisterone, and N-protected amino acid derivatives. The kinetic isotope studies suggest that the C–H bond activation step may not be the rate-determining step.

Earth-abundant and water-tolerant manganese(i) catalyzed alkenylation of 2-arylindazole with alkyl and aryl alkynes through C–H bond activation is described with a unique level of E-selectivity.     

Computational understanding of catalyst-controlled borylation of fluoroarenes: directed vs. undirected pathway

In this work, density functional theory (DFT) calculations are performed to understand the origin of the regioselective C–H borylation of aromatics catalyzed by Co(i)/^iPrPNP and Ir(iii)/dtbpy (4,4-di-tert-butyl bipyridine). The calculation results indicate that for the Co(i)/^iPrPNP catalytic system, the undirected pathway is 2.9 kcal mol⁻¹ more favoured over the directed pathway leading to ortho-to-fluorine selectivity. In contrast, for the Ir(iii)/dtbpy catalytic system, the directed pathway is 1.2 kcal mol⁻¹ more favoured over the undirected pathway bringing about ortho-to-silyl selectivity. For Co(i)/^iPrPNP catalyzed borylation, the undirected pathway which involves steps of ortho-to-fluorine C–H oxidative addition, C–B reductive elimination, B–B oxidative addition, and B–H reductive elimination is favorable due to the electron deficient character of the ortho-to-fluorine C–H bond. For Ir(iii)/dtbpy catalyzed borylation, the directed pathway consisting of Si–H oxidative addition, B–H reductive elimination, C–H oxidative addition, B–B oxidative addition, C–B reductive elimination, Si–H reductive elimination is favored over the undirected pathway attributed to the directing effect of the hydrosilyl group. The favourable undirected pathway (ortho-to-fluorine selectivity) for Co(i)/^iPrPNP catalyzed borylation and the favourable directed pathway (ortho-to-silyl selectivity) for Ir(iii)/dtbpy catalyzed borylation could explain well the experimentally observed ortho-to-fluorine borylation of hydrosilyl substituted fluoroarenes with cobalt catalyst (J. V. Obligacion, M. J. Bezdek and P. J. Chirik, J. Am. Chem. Soc., 2017, 139, 2825–2832) and ortho-to-silyl selectivity with iridium catalyst (T. A. Boebel and J. F. Hartwig, J. Am. Chem. Soc., 2008, 130, 7534–7535).

In this work, density functional theory (DFT) calculations are performed to understand the origin of the regioselective C–H borylation of aromatics catalyzed by Co(i)/^iPrPNP and Ir(iii)/dtbpy (4,4-di-tert-butyl bipyridine).     

Rhodium(iii)-catalyzed C–H annulation of 2-acetyl-1-arylhydrazines with sulfoxonium ylides: synthesis of 2-arylindoles

An efficient Rh(iii)-catalyzed synthesis of 2-arylindole derivatives via intermolecular C–H annulation of arylhydrazines with sulfoxonium ylides was accomplished. A variety of 2-acetyl-1-arylhydrazines with sulfoxonium ylides were converted into 2-arylindoles in satisfactory yields. Excellent selectivity and good functional group tolerance of this transformation were also observed.

Rh(iii)-catalyzed intermolecular C–H annulation of arylhydrazines with sulfoxonium ylides for synthesis of 2-arylindole derivatives was well established.     

Recovery of cobalt from dilute aqueous solutions using activated carbon–alginate composite spheres impregnated with Cyanex 272

A novel adsorbent was designed for selective recovery of cobalt(ii) from synthetic binary cobalt(ii)–nickel(ii) and cobalt(ii)–manganese(ii) solutions, a synthetic multi-element solution and a real aqueous waste stream from the petrochemical sector. The adsorbent consisted of shaped activated carbon–alginate spheres impregnated with Cyanex 272. The synthesis was followed by characterisation using SEM, infrared spectroscopy, BET analysis and elemental analysis. Good selectivity for cobalt(ii) over nickel(ii) could be achieved during adsorption, while this was not the case for cobalt(ii) over manganese(ii). Cobalt(ii) and manganese(ii) were therefore fully adsorbed and stripped using a dilute sulphuric acid solution. The adsorbent was shown to be reusable in a column setup. Finally, the adsorbent material was used for the purification of a real aqueous waste stream from the petrochemical sector.

Waste water was purified from cobalt(ii) and manganese(ii) by adsorption and desorption on shaped and impregnated activated carbon spheres.     

Nickel(ii)-catalyzed tandem C(sp2)–H bond activation and annulation of arenes with gem-dibromoalkenes

A nickel(ii)/silver(i)-catalyzed tandem C(sp²)–H activation and intramolecular annulation of arenes with dibromoalkenes has been successfully achieved, which offers an efficient approach to the 3-methyleneisoindolin-1-one scaffold. Attractive features of this system include its low cost, ease of operation, and its ability to access a wide range of isoindolinones.

A nickel(ii)/silver(i)-catalyzed tandem C(sp²)–H activation and intramolecular annulation of arenes with dibromoalkenes has been successfully achieved, which offers an efficient approach to the 3-methyleneisoindolin-1-one scaffold.

Over the past years, the transition-metal-catalyzed oxidative C–H/C–H cross-coupling reaction has emerged as a useful, atom- and step-economic synthetic protocol to construct a series of important N-heterocycles.¹ In this context, the synthesis of isoindolinones has attracted considerable attention owing to their interesting biological and pharmaceutical properties,² as well as their usefulness as precursors for the synthesis of structurally diverse and complex molecules (Scheme 1).^2c,3 Several methods have successfully been developed toward isoindolinone synthesis based on Pd,⁴ Cu,⁵ Ru,⁶ and Rh⁷ salts. Among these reactions, the oxidative coupling reactions of benzamides with alkenes^4b,6,7a,f,g or alkynes^5a,5d exhibit high atom economy and the application of this strategy to simple arenes is still largely underdeveloped.⁸ For instance, in 2015, Zhang''s group⁹ revealed cobalt-catalyzed oxidative alkynylation and cyclization of simple arenes and terminal alkynes with silver-cocatalyst via 2-fold C–H bond and N–H bond cleavage and C–C bond and C–N bond formation. In 2016, Song''s group¹⁰ developed a method of a cobalt(ii)-catalyzed decarboxylative C–H activation/annulation of benzamides and alkynyl carboxylic acids and nickel(ii)-catalyzed C(sp²)–H alkynylation/annulation cascade with terminal alkynes to synthesize 3-methyleneiso-indolin-1-ones. Zhang also reported a nickel-catalyzed oxidative alkynylation with amides and terminal acetylenes.^10c In addition, from an environmentally point of view, in 2015, wei''s group¹¹ described an operationally simple, Pd-catalyzed C–H functionalization for the synthesis of important and useful isoindolinones from readily available carboxamides and carboxylic acids or anhydrides. The protocol avoided the use of excess oxidants including benzoquinone, Cu(OAc)₂, or Ag₂CO₃ of previous all the reactions, thus generating stoichiometric amounts of undesired wastes.Open in a separate windowScheme 1Representative isoindolinones with biological and pharmaceutical.To our knowledge, the synthesis of alkynes is among the most fundamental and important synthetic transformations due to the unique reactivity of alkynes including addition, oxidation, reduction, and in particular cyclization.¹² However, the lack of reactivity of alkynes, more electron-deficient than the corresponding alkenes, makes it harder to couple them with heteroarenes. As a consequence, terminal alkyne precursors have been developed to facilitate acetylene exchange.¹³ Halogenoalkynes,¹⁴ hypervalent alkynyliodoniums,¹⁵acetylenic sulfones,¹⁶ copper acetylides¹⁷ and α,β-ynoic acids¹⁸ allowed the generation of more activated alkyne moieties thus broadening the applications of direct alkynylation reactions to heterocycles. Among these alternatives, gem-dihaloalkenes emerged as more efficient coupling partners than the corresponding monohalogenated alkynes along with being inexpensive and readily-available.¹⁹ Indeed, the two geminal halogen atoms on the alkenyl carbon enhance the reactivity of metal complexes thus facilitating cross coupling reactions.²⁰ Stable and readily-available 1,1-dibromo-1-alkenes and our interests in the C–H activation²¹ led us to consider using these reagents in the C–H functionalization to construct the valuable isoindolinones. We can envision that the abundance and structural diversity of the aldehydes (used the preparation of gem-dibromoethylenes via wittig reaction) as well as the merits of C–H functionalization would make the synthetic methods desirable and attractive. Herein, we wish to disclose the nickel(ii)/silver(i)-mediated tandem transformation involving sequential C(sp²)–H/C(sp²)–H alkynylation and intramolecular annulation of unactivated arenes with dibromoethylenes with the assistance of 8-aminoquinoline (Scheme 2). These features of this approach operational simplicity, a wide-ranging substrate scope, and tolerance of various synthetically useful functional groups.Open in a separate windowScheme 2Nickel(ii)/silver(i)-catalyzed alkynylation/annulation of arenes with dibromoalkenes.     

Single molecule magnets of cobalt and zinc homo- and heterometallic coordination polymers prepared by a one-step synthetic procedure

The synthesis of 1D cobalt and zinc monometallic and heterometallic coordination polymers (CPs) was carried out applying one-pot synthetic methods by using either supercritical carbon dioxide or ethanol as the solvent. A collection of four 1D CPs were thus obtained by the combination of a metal (or a mixture of metals) with the linker 1,4-bis(4-pyridylmethyl)benzene. The used metallic complexes were zinc and cobalt hexafluoroacetylacetonate, which can easily incorporate pyridine ligands in the coordination sphere of the metal centre. Independently of the used solvent, the precipitated phases involving Zn(ii), i.e., homometallic CP of Zn(ii) and bimetallic CP of Zn(ii)/Co(ii), were isostructural. Contrarily, homometallic CPs of Co(ii) were precipitated as an isostructural phase of Zn(ii) or with a different structure, depending on the used solvent. All the structures were resolved by XRD using synchrotron radiation. In addition, the magnetic properties of the new CPs involving Co(ii) were studied. Remarkably, at low temperatures with the application of an external field, they acted as field-induced single molecule magnets.

One-pot synthesis of heterometallic (Zn(ii)/Co(ii)) nodes directing CP magnetic behaviour to single molecule magnets.     

Small organic molecules with tailored structures: initiators in the transition-metal-free C–H arylation of unactivated arenes

Simple, small organic molecules containing nitrogen and oxygen atoms in their structures have been disclosed to catalyze transition-metal-free C–H arylation of unactivated arenes with aryl iodides in the presence of ^tBuOK. In this article, an optimized catalytically active molecule, (2-(methylamino)phenyl)methanol, was designed. A broad range of aryl iodides could be converted into the corresponding arylated products at 100 °C over 24 h with good to excellent yields. Mechanistic experiments verified that radicals participated in this catalytic transformation and that the cleavage of the aromatic C–H bond was not the rate determining step. A K⁺ capture experiment by 18-crown-6 emphasized the significance of the cation species of the strong base. Fourier transform infrared spectroscopy proved that the catalytic system was activated by the hydrogen bonds between small organic molecules and ^tBuOK. Also, a clear mechanism was proposed. This transition-metal-free method affords a promising system for efficient and inexpensive synthesis of biaryls via a user-friendly approach, as confirmed by scale-up experiments.

A small organic molecule was tailored for the efficient synthesis of biphenyl and its derivatives from aryl iodides.     

A green route for the cross-coupling of azide anions with aryl halides under both base and ligand-free conditions: exceptional performance of a Cu2O–CuO–Cu–C nanocomposite

A convenient, inexpensive and effective route for the preparation of a Cu₂O–CuO–Cu–C nanocomposite is described here by applying Cu(ii) as a source of copper. Characterization of the nanocomposite was performed with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDX). Analysis of the data showed that the particles of the nanocomposite are uniformly distributed and show high catalytic activity in the cross-coupling of sodium azide with various aryl iodides and bromides. This nanocomposite has a high level of performance, and even led to the synthesis of the products at room temperature. In addition, this is the first report of the synthesis of aryl azides under both base- and ligand-free conditions. For the first time, both ligand- and base-free conditions were applied for the synthesis of aryl azides, which implies exceptional performance of the Cu₂O–CuO–Cu–C nanocomposite. Simultaneous removal of the base and ligand in a green solvent is the main advantage of this reaction. Unfortunately, aryl bromides and aryl iodides with electron-withdrawing functional groups in their scaffold did not give the desired aryl azides.

Ligand-free and base-free conditions were used for the preparation of aryl azides using the cross-coupling of sodium azide and aryl halides catalysed by Cu₂O–CuO–Cu–C.     

Theoretical study of ternary silver fluorides AgMF4 (M = Cu,Ni, Co) formation at pressures up to 20 GPa

Only several compounds bearing the Ag(ii) cation and other paramagnetic transition metal cations are known experimentally. Herein, we predict in silico stability and crystal structures of hypothetical ternary silver(ii) fluorides with copper, nickel and cobalt in 1 : 1 stoichiometry at a pressure range from 0 GPa up to 20 GPa employing the evolutionary algorithm in combination with DFT calculations. The calculations show that AgCoF₄ could be synthesized already at ambient conditions but this compound would host diamagnetic Ag(i) and high-spin Co(iii). Although none of the compounds bearing Ag(ii) could be preferred over binary substrates at ambient conditions, at increased pressure ternary fluorides of Ag(ii) featuring Cu(ii) and Ni(ii) could be synthesized, in the pressure windows of 7–14 and 8–15 GPa, respectively. All title compounds would be semiconducting and demonstrate magnetic ordering. Compounds featuring Ni(ii) and particularly Co(ii) should exhibit fundamental band gaps much reduced with respect to pristine AgF₂. The presence of Cu(ii) and Ni(ii) does not lead to electronic doping to AgF₂ layers, while Co(ii) tends to reduce Ag(ii) entirely to Ag(i).

Only several compounds bearing the Ag(ii) cation and other paramagnetic transition metal cations are known experimentally. Here, we predict as yet unknown AgMF₄ phases and their stability in function of pressure.     

Cu(ii)-alginate-based superporous hydrogel catalyst for click chemistry azide–alkyne cycloaddition type reactions in water

A novel sustainable hydrogel catalyst based on the reaction of sodium alginate naturally extracted from brown algae Laminaria digitata residue with copper(ii) was prepared as spherical beads, namely Cu(ii)-alginate hydrogel (Cu(ii)-AHG). The morphology and structural characteristics of these beads were elucidated by different techniques such as SEM, EDX, BET, FTIR and TGA analysis. Cu(ii)-AHG and its dried form, namely Cu(ii)-alginate (Cu(ii)-AD), are relatively uniform with an average pore ranging from 200 nm to more than 20 μm. These superporous structure beads were employed for the copper catalyzed [3 + 2] cycloaddition reaction of aryl azides and terminal aryl alkynes (CuAAC) via click chemistry at low catalyst loading, using water as a solvent at room temperature and pressure. The catalytic active copper(i) species was generated by the reduction of copper(ii) by terminal alkyne via the oxidative alkyne homocoupling reaction. The prepared catalysts were found to be efficient (85–92%) and regioselective by affording only 1,4-disubstituted-1,2,3-triazoles. They were also recoverable and reused in their dried form for at least four consecutive times without a clear loss of efficiency. A mechanistic study was performed through density functional theory (DFT) calculations in order to explain the regioselectivity outcome of Cu(ii)-alginate in CuAAC reactions. The analysis of the local electrophilicity (ω_k) at the electrophilic reagent and the local nucleophilicity (N_k) at the nucleophilic confirms the polar character of CuAAC. This catalyst has the main advantage of being sustainably ligand-free and recyclable.

The Cu(ii)-alginate-based superporous hydrogel was prepared and used as a heterogenous catalyst in the regioselective click of 1,4-disubstituted-1,2,3-triazoles by CuAAC reactions.     

Pd-catalyzed intramolecular addition of active methylene compounds to alkynes with subsequent cross-coupling with (hetero)aryl halides

We report an efficient protocol for tandem Pd-catalyzed intramolecular addition of active methylene compounds to alkynes, followed by subsequent cross-coupling with (hetero)aryl bromides and chlorides. The reaction proceeds under mild conditions, providing excellent functional group tolerance, including unprotected OH, NH₂ groups, enolizable ketones, or a variety of heterocycles. Mechanistic studies point towards a catalytic cycle involving oxidative addition, intramolecular nucleophilic addition to the Pd(ii)-activated alkyne, and reductive elimination, with 5-exo-dig cyclization being the rate limiting step.

A tandem cyclization/coupling of acetylenic active methylene compounds with aryl halides features broad scope and excellent functional group compatibility. Mechanistic studies identified 5-exo-dig cyclization as the rate limiting step.     

Synthesis of indazoles from 2-formylphenylboronic acids

A method for the synthesis of indazoles was developed which involves a copper(ii) acetate catalysed reaction of 2-formylboronic acids with diazadicaboxylates followed by acid or base induced ring closure. Hydrazine dicarboxylates were also shown as competent reaction partners for the synthesis of indazoles, however, they required a stoichiometric amount of copper(ii) acetate for the C–N bond formation step. The transformation can be efficiently performed as a two step-one pot procedure to give a range of 1N-alkoxycarbonyl indazoles.

A method for the synthesis of indazoles was developed which involves a copper(ii) acetate catalysed reaction of 2-formylboronic acids with diazadicaboxylates followed by acid or base induced ring closure.     

Regio-selective and stereo-selective hydrosilylation of internal alkynes catalyzed by ruthenium complexes

In this study, ruthenium(ii)-catalyzed direct hydrosilylation of internal alkynes with high regio-selectivity and stereo-selectivity is reported. This title transformation led to various vinylsilanes in good to excellent yields. This approach features mild reaction conditions, low catalyst loading, air-stability, and good functional group tolerance. Furthermore, gram-scale preparation and some transformations of vinylsilanes were carried out, which further underscored its synthetic utility and applicability.

In this study, ruthenium(ii)-catalyzed direct hydrosilylation of internal alkynes with high regio-selectivity and stereo-selectivity is reported.     

Rhodium(iii)-catalyzed annulation of enamides with sulfoxonium ylides toward isoquinolines

An efficient rhodium(iii)-catalyzed C–H activation followed by intermolecular annulation between enamides and sulfoxonium ylides has been developed. The transformation proceeds smoothly with a broad range of substrates, affording a series of isoquinoline derivatives in moderate to good yields under additive-free conditions.

An efficient rhodium(iii)-catalyzed C–H activation followed by intermolecular annulation between enamides and sulfoxonium ylides has been developed.