Highly stereo- and regioselective hydrostannylation of internal alkynes promoted by simple boric Acid in air

Facile hydrostannylation of alkynes in air: The ability to prepare vinylstannanes of high regio- and sterochemical fidelity in a safe manner employing a simple operational set up, especially on a large scale, has remained elusive. This study has shown that all of the autoxidation products of Et(3) B and boronic acids are capable of promoting hydrostannylation. Most importantly, simple boric acid itself can also promote the hydrostannylation of highly functionalized internal alkynes with complete selectivity under very mild conditions (RT to 80?°C) in air.     

Copper-catalyzed hydrative amide synthesis with terminal alkyne, sulfonyl azide, and water

It is shown for the first time that N-sulfonyl amides can be efficiently prepared by an unconventional approach of the hydrative reaction between terminal alkynes, sulfonyl azides, and water in the presence of copper catalyst and amine base under very mild conditions. The present route is quite general, and a wide range of alkynes and sulfonyl azides are readily coupled catalytically with water to furnish amides in high yields. A variety of labile functional groups are tolerated under the conditions, and the reaction is regioselective in that only terminal alkynes react while double or internal triple bonds are intact. The reaction can be readily scaled up and is also adaptable to a solid-phase procedure with high efficiency.     

Synthesis of 3-iodoindoles by electrophilic cyclization of N,N-dialkyl-2-(1-alkynyl)anilines

[reaction: see text] A wide variety of N-alkyl-3-iodoindoles are readily prepared under very mild reaction conditions by the palladium/copper-catalyzed cross-coupling of N,N-dialkyl-o-iodoanilines and terminal alkynes, followed by electrophilic cyclization by I(2)(). Alkyl-, aryl-, and vinylic-substituted alkynes all undergo iodocyclization in excellent yields.     

The mechanism of the phosphine-free palladium-catalyzed hydroarylation of alkynes

The mechanism of the Pd-catalyzed hydroarylation and hydrovinylation reaction of alkynes has been studied by a combination of experimental and theoretical methods (B3LYP), with an emphasis on the phosphine-free version. The regioselectivity of the hydroarylation and hydrovinylation shows unexpected differences, which could be attributed mainly to the higher steric demand of the cyclohexenyl group as compared to the phenyl group. Hydroarylation of alpha,beta-acetylenic carbonyl substrates yields a very unusual anti-Michael selectivity, which is shown to result from reaction of the nonconjugated double bond, leaving the conjugation intact. In all cases were the regioselectivities reproduced by the calculations.     

Terminal Alkynes as One-Carbon Donors in [5+1] Heteroannulation: Synthesis of Pyridines via Ynimine Intermediates and Application in the Total Synthesis of Anibamine B

Transition-metal-catalyzed [4+2] heteroannulation of α,β-unsaturated oximes and their derivatives with alkynes has been developed into a powerful strategy for the synthesis of pyridines. It nevertheless lacks regioselectivity when unsymmetrically substituted alkynes are used. We report herein the unprecedented synthesis of polysubstituted pyridines by a formal [5+1] heteroannulation of two readily accessible building blocks. A copper-catalyzed aza-Sonogashira cross-coupling between β,γ-unsaturated oxime esters and terminal alkynes affords ynimines, which, without isolation, undergo an acid-catalyzed domino reaction involving ketenimine formation, 6π-electrocyclization and aromatization to afford pyridines. Terminal alkynes served as a one-carbon donor to the pyridine core in this transformation. Di- through pentasubstituted pyridines are accessible with complete regioselectivity and excellent functional-group compatibility. The first total synthesis of anibamine B, an indolizinium alkaloid with potent antiplasmodial activity, was accomplished featuring this reaction as a key step.     

Chemoselective and scalable preparation of alkyl tosylates under solvent-free conditions

The improved method for the efficient tosylation of alcohols has been reported using two procedures (A and B). Procedure A: methanol, ethanol, benzyl alcohols, and valuable ethylene glycols can be converted into their corresponding alkyl tosylates in very fast, simple, and efficient grinding method using potassium carbonate as solid base. Other primary and secondary alcohols need to add potassium hydroxide to reaction mixture (procedure B). High selectivity of tosylation was observed for these two procedures. Scale up ability was found in this method even in 100 mmol of substrate. The present method is the example of solid-state tosylation using tosyl chloride, and is a green chemical process due to solvent-free conditions.     

Bis(1-Methyl-ortho-Carboranyl)Borane

The Lewis superacid, bis(1-methyl-ortho-carboranyl)borane, is rapidly accessed in two steps. It is a very effective hydroboration reagent capable of B−H addition to alkenes, alkynes, and cyclopropanes. To date, this is the first identified Lewis superacidic secondary borane and most reactive neutral hydroboration reagent.     

Synthesis of bis(indolyl)alkanes by a site-selective gold-catalyzed addition of indoles to butynol derivatives

A new site-selective hydroarylation reaction of alkynes catalyzed by gold complexes and directed by an internal hydroxyl group has been developed. Thus, the treatment of 3-butyn-1-ol derivatives with indoles and a catalytic amount of an in situ formed cationic gold complex leads to the formation of bis(indolyl)alkane derivatives. Particularly interesting is the reaction with terminal alkynes as the double addition of the indol occurs at the terminal carbon of the triple bond. The reaction conditions are very mild and the final bis(indolyl)alkanes are obtained in high yields.     

Influence of sample pre-treatment on the determination of trace silver and cadmium in geological and environmental samples by quadrupole inductively coupled plasma mass spectrometry

The suitability of three different digestion procedures has been evaluated with regard to the quantitative determination of trace amounts of silver and cadmium in geological and environmental samples. Procedure A: extraction with aqua regia in an open vessel system; Procedure B: extraction with aqua regia in a closed pressurized vessel system; Procedure C: digestion with HF + HNO₃ in a closed pressurized vessel system. It was found that procedure A represented an effective way of extracting analytes from different types of geological samples while the effect of extracting interfering metals is low. Because it is interference-free, poses a low risk of contamination and is time-saving, sample treatment procedure A was preferred. For this digestion procedure, a series of Chinese geological reference materials was determined by ICP-MS, the results of which were found to be in reasonable agreement with the certified values.     

Transition-metal-promoted or -catalyzed exocyclic alkyne insertion via zirconacyclopentene with carborane auxiliary: formation of symmetric or unsymmetric benzocarboranes

Reactions of Cp(2)Zr(μ-Cl)(μ-C(2)B(10)H(10))Li(OEt(2))(2) with alkynes R(1)C≡CR(2) gave as insertion products zirconacyclopentenes incorporating a carboranyl unit, 1,2-[Cp(2)ZrC(R(1))═C(R(2))]-1,2-C(2)B(10)H(10) (1). Treatment of 1 with another type of alkyne R(3)C≡CR(4) in the presence of stoichiometric amounts of NiCl(2) and FeCl(3) or a catalytic amount of NiCl(2) afforded symmetric or unsymmetric benzocarboranes. The regioselectivity was dominated by the polarity of the corresponding alkynes. These reactions could also be carried out in one pot, leading to the equivalent of a three-component [2 + 2 + 2] cycloaddition of carboryne and two different alkynes promoted by transition metals. A reaction mechanism was proposed after the isolation and structural characterization of the key intermediate nickelacycle. These results show that nickel complexes are more reactive than the iron ones toward the insertion of alkynes but that the latter do not initiate the trimerization of alkynes, making the insertion of activated alkynes possible. This work also demonstrates that a catalytic amount of nickel works as well as a stoichiometric amount of nickel in the presence of excess FeCl(3) for the reactions. Such a catalytic reaction may shed some light on the development of zirconocene-based catalytic reactions.     

Facial Synthesis of o‐Carborane‐Substituted Alkenes and Allenes by a Regioselective Ene Reaction of 1,3‐Dehydro‐o‐carborane

1,3‐Dehydro‐o‐carborane is a useful synthon for selective cage boron functionalization of o‐carboranes. It reacts readily with alkenes or alkynes to give a variety of cage B(3)‐alkenyl/allenyl o‐carboranes by ene reactions in very high yields and excellent regioselectivity. This can be ascribed to the highly polarized cage C?B multiple bond, which lowers the activation barriers of the ene reaction.     

Copper catalyzed regioselective coupling of allylic halides and alkynes promoted by weak inorganic bases

Allylic halides and terminal alkynes couple under CuI catalysis in DMSO or DMF solution. In most cases, sodium carbonate or bicarbonate is sufficient to promote the reaction; less reactive alkynes require catalytic amounts of DBU. Bifunctional alkynes and halides can be reacted selectively according to the stoichiometry used. Trimethylsilyl, hydroxyl, ester and halide groups are tolerated in the alkyne. Most halides react without allylic rearrangement. The method is suitable for the synthesis of functionalized enynes.     

Enantioselective synthesis of binaphthyl polymers using chiral asymmetric phenolic coupling catalysts: oxidative coupling and tandem glaser/oxidative coupling

A series of functionalized and optically active polybinaphthyls have been synthesized from achiral substrates by asymmetric oxidative phenolic coupling using a chiral 1,5-diaza-cis-decalin copper catalyst. In most cases, a copper tetrafluoroborate catalyst was found to be superior to the copper iodide catalyst, as ortho-iodination of the substrates could be prevented. Three methods for the formation of chiral polymers are described. In the first method, two 2-naphthols linked together at C-6 are subjected to the optimized asymmetric oxidative phenolic coupling conditions to form chiral polynaphthyls. A combination of NMR and HPLC measurements secured the selectivity of the asymmetric coupling. In the second method, substrates containing only one naphthalene were utilized. By incorporating a 2-naphthol and a terminal alkyne, the chiral copper catalysts effect both Glaser-Hay coupling of the alkyne and oxidative asymmetric coupling of the 2-naphthol with remarkable chemoselectivity. The relative reaction rates of various moieties with the chiral catalysts follows the order: benzyl cyanides > aryl alkynes > electron-rich 2-naphthols > electron-deficient 2-naphthols > alkyl alkynes. Because of high chemoselectivity, this approach is useful for the organized assembly of multifunctional substrates in a single operation. In all cases, no cross-coupling is observed between the alkyne and the 2-naphthol. This approach was thus applied to a set of highly functionalized precursors. In this third case, the biaryl coupling was performed first and a Glaser-Hay coupling was performed in a separate step to generate a highly functionalized polymer. In some cases, the resultant chiral polymers exhibit very large optical rotations.     

Palladium-catalyzed highly chemo- and regioselective formal [2+2+2] sequential cycloaddition of alkynes: a renaissance of the well known trimerization reaction?

A new concept of highly chemo- and regioselective formation of the benzene ring by a palladium-catalyzed formal [2 + 2 + 2] sequential intermolecular trimerization of alkynes is proposed. Homodimerization of terminal alkynes and subsequent [4 + 2] benzannulation with diynes gives tetrasubstituted benzenes in moderate to good yields. The introduction of two different alkynes (terminal and internal) in the first step of the sequence allows for construction of pentasubstituted benzenes from three different acyclic acetylenic units. In all cases the tetra- and pentasubstituted benzenes are formed as a single reaction product without being accompanied by any of regio- or chemoisomers. A significant acceleration of the sequential trimerization reaction in the presence of Lewis acid/phosphine combined system was observed. Mechanistic studies reveal that the Lewis acid assisted isomerization of the E-enyne formed in the first step of the sequence to the more reactive Z-isomer is responsible for the observed acceleration effect. The proposed methodology provides a conceptually new and synthetically useful route to multifunctional aromatic compounds.     

Carbohydrate-derived amino-alcohol ligands for asymmetric alkynylation of aldehydes

[reaction: see text] Conformationally restricted amino alcohols based on carbohydrate scaffolds provide flexible and fine-tuneable libraries that greatly expand the range of ligands available in the Zn(OTf)(2)-mediated addition of alkynes to aldehydes, in some cases with very high stereoselectivities.     

Synthesis of highly substituted aniline and o-phenylenediamine derivatives containing various substitution patterns

The Diels-Alder reactions of a variety of 2H-pyran-2-ones 1 with alkynes 2 yielding highly substituted aniline and o-phenylenediamine derivatives 4 with novel (and in some cases known but very rare and useful) structural patterns are presented. The effect of substituents of both reactants on the reaction rates was investigated. The reactions were carried out under thermal conditions, as well as at high pressures.     

Nickel‐Catalyzed Reductive Coupling of Aldehydes with Alkynes Mediated by Alcohol†

A nickel‐catalyzed reductive coupling of aldehydes with alkynes using 1‐phenylethanol as reducing agent has been developed. The key achievement of this work is that we demonstrate environmentally benign 1‐phenylethanol can serve as a viable alternative reducing agent to Et₃B, ZnEt₂ and R₃SiH for the nickel‐catalyzed reductive coupling reaction of aldehyde and alkynes.     

New intra-intermolecular criss-cross cycloaddition of unsymmetrical allenylazines with alkynes leading to three fused five-membered heterocycles

This report describes a new type of intra-intermolecular criss-cross cycloadditions. Thermal reactions of unsymmetrical allenylazines in the presence of alkynes led to three fused five-membered heterocycles in some cases. In the case of unsymmetrical substituted alkynes, a regioselectivity was observed. The molecular structures of all products are discussed. One X-ray crystal structure is also reported.     

Development,Scope and Mechanisms of Multicomponent Reactions of Asymmetric Electron‐Deficient Alkynes with Amines and Formaldehyde

Based on the reactive behaviour of the substrates, two synthetic routes to polysubstituted pyrimidine derivatives are presented herein: 1) A catalyst‐free multicomponent reaction of electron‐deficient alkynes, aliphatic amines and formaldehyde and 2) Ag^I‐catalyzed synthesis of pyrimidines from electron‐deficient alkynes, anilines and formaldehyde by a domino reaction. Under optimized conditions, the multicomponent reactions were accomplished with high regioselectivity and excellent yields. A computational study was carried out by using the B3LYP density functional theory to elucidate the mechanisms of the catalyst‐free hydroamination reaction. Calculations showed the activation free energies of aliphatic amines were lower than those of anilines, which is consistent with the experimental results.     

Synthesis of 2-substituted pyridines catalyzed by cobalt(I) complexes and promoted by light

A series of pyridines substituted in the 2-position were synthesized by cobalt(I)-catalyzed cotrimerization of alkynes and nitriles. The reactions are usually enhanced by light. The reaction conditions are very mild. Activity and selectivity are satisfactory with respect to formation of pyridines, but depend on the type of nitrile.