Showing papers in "Journal of Organic Chemistry in 2006"

Energy, charge, and spin transport in molecules and self-assembled nanostructures inspired by photosynthesis.

Abstract: Electron transfer in biological molecules provides both insight and inspiration for developing chemical systems having similar functionality. Photosynthesis is an example of an integrated system in which light harvesting, photoinduced charge separation, and catalysis combine to carry out two thermodynamically demanding processes, the oxidation of water and the reduction of carbon dioxide. The development of artificial photosynthetic systems for solar energy conversion requires a fundamental understanding of electron-transfer reactions between organic molecules. Since these reactions most often involve single-electron transfers, the spin dynamics of photogenerated radical ion pairs provide important information on how the rates and efficiencies of these reactions depend on molecular structure. Given this knowledge, the design and synthesis of large integrated structures to carry out artificial photosynthesis is moving forward. An important approach to achieving this goal is the development of small, functional building blocks, having a minimum number of covalent bonds, which also have the appropriate molecular recognition sites to facilitate self-assembly into a complete, functional artificial photosynthetic system.

Nucleus-independent chemical shifts (NICS): distance dependence and revised criteria for aromaticity and antiaromaticity.

Abstract: Nucleus-independent chemical shifts (NICS) have been used extensively for the identification of aromaticity properties of molecules, ions, intermediates, and transition states since their introduction in 1996 by Schleyer et al. Initially, probes (bq's) were placed at the centers of systems (NICS(0)) and later, 1A above the molecular planes (NICS(1)). However, contradicting assignments of aromaticity by NICS and other methods were found for some systems. In this article, an alternative NICS-based method is introduced. The method is based on scanning NICS values over a distance and separating them into in-plane and out-of plane contributions. The shapes of the plots of the chemical shifts and their components as a function of the distance of the NICS probe (bq) from the molecular plane give a clear indication of diamagnetic and paramagnetic ring currents. This method is applied to several (4n + 2)- and 4n π-electron systems (molecules and ions) in the singlet and triplet electronic states, including some of...

Parallel Synthesis of a Library of Benzoxazoles and Benzothiazoles Using Ligand-Accelerated Copper-Catalyzed Cyclizations of ortho-Halobenzanilides

Abstract: A general method for the formation of benzoxazoles via a copper-catalyzed cyclization of ortho-haloanilides is reported. This approach complements the more commonly used strategies for benzoxazole formation which require 2-aminophenols as substrates. The reaction involves an intramolecular C−O cross-coupling of the ortho-haloanilides and is believed to proceed via an oxidative insertion/reductive elimination pathway through a Cu(I)/Cu(III) manifold. The reaction is also applicable to the formation of benzothiazoles. A variety of ligands including 1,10-phenanthroline and N,N‘-dimethylethylenediamine were shown to provide ligand acceleration/stabilization in the reaction. Optimal conditions for cyclization used a catalyst combination of CuI and 1,10-phenanthroline (10 mol %). The method was amenable to a parallel-synthesis approach, as demonstrated by the synthesis of a library of benzoxazoles and benzothiazoles substituted at various positions in the ring. Most examples utilized the cyclization of ortho-br...

Efficient synthesis of meso-substituted corroles in a H2O-MeOH mixture.

Abstract: New and efficient conditions for the synthesis of meso-substituted corroles were developed. The first step, involving the reaction of aldehydes with pyrrole, was carried out in a water-methanol mixture in the presence of HCl. A relatively narrow distribution of aldehyde-pyrrole oligocondensates was obtained through careful control of their solubility in the reaction medium. After thorough optimization of various reaction parameters (cosolvent, reagent, and acid concentration), high yields of bilanes were obtained. Additionally, the bilane derived from 4-cyanobenzaldehyde was isolated, and the oxidative macrocyclization reaction was performed under various reaction conditions (different solvents, different concentrations, and various oxidants). As a result, triphenylcorrole was obtained in the highest yield (32%) reported to date. The scope and limitations studied showed that this method was particularly efficient for moderately reactive aldehydes and those bearing electron-donating groups (yields 14-27%). Using these conditions, corroles bearing strongly electron-donating groups were obtained for the first time. In addition, it was found that the reaction of unhindered dipyrromethanes with aldehydes under analogous conditions afforded trans-A2B-corroles in very high yields (45-56%; 8-fold higher than previously reported) without scrambling. The fact that scrambling was not observed in this reaction despite a very high HCl concentration (0.3 M) is unprecedented. Detailed studies on the oxidation of bilane, derived from sterically hindered dipyrromethane, allowed us to unequivocally establish that the yield of macrocyclization is insensitive to the concentration. It was found the 1H NMR spectra of corroles in deuterated TFA gave very sharp signals.

New BODIPY derivatives as OFF-ON fluorescent chemosensor and fluorescent chemodosimeter for Cu2+: cooperative selectivity enhancement toward Cu2+.

Abstract: New 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives (1 and 2) were synthesized as an “off−on” fluorescent chemosensor and fluorescent chemodosimeter for Cu2+ and Pb2+. Compound 1 displayed selective and large chelation enhanced fluorescence effects with Pb2+ and Cu2+ among the metal ions examined. On the other hand, compound 2, a fluorescent chemodosimeter, effectively recognized Cu2+ via a selective hydrolysis of the acetyl group.

Detecting Hg2+ ions with an ICT fluorescent sensor molecule: remarkable emission spectra shift and unique selectivity.

Abstract: A fluorescent ratiometric Hg2+ ion sensor RMS, based on a coumarin platform coupled with a tetraamide receptor, is presented. This sensor, employing the ICT mechanism, could be used to specifically detect Hg2+ ions in a neutral buffered water solution with an approximately 100-nm blue shift in emission spectra.

Small molecule natural products in the discovery of therapeutic agents: the synthesis connection.

Abstract: Natural products have been a rich source of agents of value in medicine. They have also inspired, at various levels, the fashioning of nonnatural agents of pharmaceutical import. Hitherto, these nonnatural derivatives have been primarily synthesized by manipulating the natural product. As a consequence of major innovations in the subscience of synthetic methodology, the capacity of synthesis to deal with molecules of considerable complexity has increased dramatically. In this paper, we show by example some total syntheses which draw from strategy-enabling advances in methodology. Moreover, we show how these capabilities can be used to discover and develop new agents of potential pharmaceutical value without recourse to the natural product itself.

A Comprehensive Self-Consistent Spectrophotometric Acidity Scale of Neutral Brønsted Acids in Acetonitrile

Abstract: For the first time, the self-consistent spectrophotometric acidity scale of neutral Bronsted acids in acetonitrile (AN) spanning 24 orders of magnitude of acidities is reported. The scale ranges from pKa 3.7 to 28.1 in AN. The scale includes 93 acids that are interconnected by 203 relative acidity measurements (ΔpKa measurements) and contains compounds with gradually changing acidities, including representatives from all of the conventional families of OH (alcohols, phenols, carboxylic acids, sulfonic acids), NH (anilines, diphenylamines, disulfonimides), and CH acids (fluorenes, diphenylacetonitriles, phenylmalononitriles). The CH acids were particularly useful in constructing the scale because they do not undergo homo- or heteroconjugation processes and their acidities are rather insensitive to traces of water in the medium. The scale has been fully cross-validated: the relative acidity of any two acids on the scale can be found by combining at least two independent sets of ΔpKa measurements. The consi...

Nucleophilic reactivities of indoles.

Abstract: The kinetics of the coupling of indole (1a), N-methylindole (1b), 5-methoxyindole (1c), and 5-cyanoindole (1d) with a set of reference benzhydryl cations have been investigated in acetonitrile and/or dichloromethane. The second-order rate constants for the reactions correlate linearly with the electrophilicity parameter E of the benzhydryl cations. This allows the determination of the reactivity parameters, N and s, characterizing the nucleophilicity of 1a−d according to the linear free enthalpy relationship log k(20 °C) = s(N + E) (Acc. Chem. Res. 2003, 36, 66). The nucleophilicity parameters thus defined describe nicely the reactions of 1a−d with 4,6-dinitrobenzofuroxan (2), a neutral superelectrophilic heteroaromatic whose electrophilicity (E) has been recently determined. On this ground, the kinetics of the coupling of 2 with a large variety of indole structures have been studied in acetonitrile, leading to a ranking of this family of π-excessive carbon nucleophiles over a large domain of the nucleoph...

Chromogenic oxazines for cyanide detection.

Abstract: We have designed two heterocyclic compounds for the colorimetric detection of cyanide. The skeleton of both molecules fuses a benzooxazine ring to an indoline fragment and can be assembled efficiently in three synthetic steps starting from commercial precursors. The two compounds differ in the nature of the substituent on the carbon atom at the junction of the fused heterocycles, which can be either a methyl or a phenyl group. In the presence of cyanide, both molecules are converted quantitatively into cyanoamines with the concomitant appearance of an intense band in the visible region of the absorption spectrum. The developing absorption is a result of the opening of the benzooxazine ring with the formation of a 4-nitrophenylazophenolate chromophore. Nuclear magnetic resonance spectroscopy and X-ray crystallographic analyses demonstrate that the covalent attachment of a cyanide anion to the indoline fragment is responsible for these transformations. The chromogenic process is particularly fast for the me...

ZnO as a new catalyst for N-formylation of amines under solvent-free conditions.

Abstract: The treatment of amines with formic acid in the presence of ZnO under solvent-free conditions brings about highly and efficient N-formylation to give the corresponding formamides in excellent yields. The N-formylation reaction not only involves mild conditions, simple operation, and high yields but also high chemoselectivity.

Aqueous N-Heterocyclization of Primary Amines and Hydrazines with Dihalides: Microwave-Assisted Syntheses of N-Azacycloalkanes, Isoindole, Pyrazole, Pyrazolidine, and Phthalazine Derivatives

Abstract: The synthesis of nitrogen-containing heterocycles from alkyl dihalides (ditosylates) and primary amines and hydrazines via a simple and efficient cyclocondensation in an alkaline aqueous medium that occurs under microwave irradiation is described. This improved greener synthetic methodology provides a simple and straightforward one-pot approach to the synthesis of a variety of heterocycles, such as substituted azetidines, pyrrolidines, piperidines, azepanes, N-substituted 2,3-dihydro-1H-isoindoles, 4,5-dihydropyrazoles, pyrazolidines, and 1,2-dihydrophthalazines.

Facile N-arylation of amines and sulfonamides and o-arylation of phenols and arenecarboxylic acids.

Abstract: An efficient, transition-metal-free procedure for the N-arylation of amines, sulfonamides, and carbamates and O-arylation of phenols and carboxylic acids has been achieved by allowing these substrates to react with a variety of o-silylaryl triflates in the presence of CsF. Good to excellent yields of arylated products are obtained under very mild reaction conditions. This chemistry readily tolerates a variety of functional groups.

Iron-phosphine, -phosphite, -arsine, and -carbene catalysts for the coupling of primary and secondary alkyl halides with aryl Grignard reagents.

Abstract: Simple catalysts formed in situ from iron chloride and a wide range of monodentate and bidentate phosphines and arsines have been screened in the coupling of alkyl halides bearing β-hydrogens with aryl Grignard reagents. The best of these show excellent activity, as do catalysts formed in situ with monodentate trialkyl and triaryl phosphite ligands. N-heterocyclic carbene-based precatalysts, either preformed or made in situ, also show excellent performance.

Adventures in sulfur-nitrogen chemistry.

Abstract: This account reviews our efforts over the past 37 years to understand the chemistry of a select group of sulfur−nitrogen compounds including sulfinimines (N-sulfinyl imines) and N-sulfonyloxaziridines. Our early exploration of the thermal properties of sulfenamides, a class of sulfur−nitrogen compounds about which little was known, resulted in a new procedure, the silver-assisted method, for the construction of sulfenimines (N-sulfenyl imines). Selective oxidations of these compounds resulted in the production of N-sulfinyl imines (sulfinimines) and N-sulfonyloxaziridines. N-Sulfonyloxaziridines turned out to be a new class of aprotic neutral oxidizing reagents. Enantiomerically pure examples afford high ee values in the oxidation of enolates to α-hydroxy carbonyl compounds and in the oxidation of sulfides and selenides to sulfoxides and selenoxides. Additions of organometallic reagents to enantiomerically pure sulfinimines provide the best and most versatile method for the asymmetric construction of the ...

Suzuki−Miyaura, α-Ketone Arylation and Dehalogenation Reactions Catalyzed by a Versatile N-Heterocyclic Carbene−Palladacycle Complex

Abstract: The activity of the complex (IPr)PdCl(η2-N,C-C12H7NMe2), 1 [IPr = (N,N‘-bis(2,6-diisopropylphenyl)imidazol)-2-ylidene], in the Suzuki−Miyaura cross-coupling reaction involving unactivated aryl chlorides and triflates with arylboronic acids at room temperature in technical grade 2-propanol is described. These conditions allow for the synthesis of di- and tri-ortho-substituted biaryls in very short reaction times. This complex also displays very high activity for α-ketone arylation and dehalogenation reactions of activated and unactivated aryl chlorides.

A Study of the Scope and Regioselectivity of the Ruthenium-Catalyzed [3 + 2]-Cycloaddition of Azides with Internal Alkynes

Abstract: [3 + 2]-Cycloadditions of alkyl azides with various unsymmetrical internal alkynes in the presence of Cp*RuCl(PPh3)2 as catalyst in refluxing benzene have been examined, leading to 1,4,5-trisubstituted-1,2,3-triazoles. Whereas alkyl phenyl and dialkyl acetylenes undergo cycloadditions to afford mixtures of regioisomeric 1,2,3-triazoles, acyl-substituted internal alkynes react with complete regioselectivity. In addition, propargyl alcohols and propargyl amines were found to react with azides to afford single regioisomeric products.

Synthesis and optical and redox properties of core-substituted naphthalene diimide dyes.

Abstract: 2,6-Dichloronaphthalene dianhydride has been synthesized by a modified procedure. The imidization of this dichlorinated anhydride with amines and subsequent stepwise nucleophilic exchange of the chlorine atoms by alkyl- or arylamines afforded a series of hitherto unknown monoamino- and diamino-substituted naphthalene diimides. An alternative route for the synthesis of diamino-substituted naphthalene diimides is also reported. Optical and electrochemical properties of the newly synthesized amino-functionalized naphthalene diimides were studied in detail. The absorption maxima (530−620 nm) of these dyes are appreciably bathochromically shifted compared to those of the corresponding core-unsubstituted compounds. At the naphthalene core alkylamino-substituted diimides exhibit fluorescence quantum yields up to 60%.

Perspective on dirhodium carboxamidates as catalysts.

Abstract: Dirhodium compounds are emerging as highly efficient catalysts for diverse reactions, and those with carboxamidate ligands have the broadest applications. The unique features of these compounds are their structural rigidity, ease of ligand exchange, open diaxial sites for coordination with Lewis bases, and their low oxidation potential. As consequences of this, dirhodium carboxamidates are efficient and effective catalysts for metal carbene reactions, Lewis acid-catalyzed processes, and chemical oxidations. With chiral carboxamidate ligands these dirhodium compounds show exceptional enantiocontrol for intramolecular cyclopropanation and carbon-hydrogen insertion reactions of diazoacetates, and they are also highly efficient and selective for hetero-Diels-Alder reactions. Their limitations lie in their moderate reactivities for metal carbene generation and Lewis acid catalysis and in the cost of the precious metal rhodium.

One-pot synthesis of dithiocarbamates accelerated in water.

Abstract: Highly efficient one-pot reactions of amines and carbon disulfide with α,β-unsaturated compounds were carried out in water under a mild and green procedure with high yields.

Modified Palladium-Catalyzed Sonogashira Cross-Coupling Reactions under Copper-, Amine-, and Solvent-Free Conditions

Abstract: PdCl2(PPh3)2 combined with TBAF under solvent-free conditions provided general and fast Sonogashira cross-coupling reactions of aryl halides with terminal alkynes. In particular, this protocol could be applied to the reactions of deactivated aryl chlorides. In the presence of 3 mol % of PdCl2(PPh3)2 and 3 equiv of TBAF, a number of ArX species (X = I, Br, Cl) were coupled with alkynes to afford the corresponding products in moderate to excellent yields under copper-, amine-, and solvent-free conditions.

A general and efficient FeCl3-catalyzed nucleophilic substitution of propargylic alcohols

Abstract: A general and efficient FeCl3-catalyzed substitution reaction of propargylic alcohols with carbon- and heteroatom-centered nucleophiles such as allyl trimethylsilane, alcohols, aromatic compounds, thiols, and amides, leading to the construction of C−C, C−O, C−S and C−N bonds, has been developed.

First Synthesis of Free Cholesterol−BODIPY Conjugates

Abstract: Analogues of cholesterol (compounds 1 and 2) and coprostanol (compound 3) containing the BODIPY fluorophore in the aliphatic tail of the free sterol have been synthesized starting with bisnorcholen...

Microwave Assisted “Click” Chemistry for the Synthesis of Multiple Labeled-Carbohydrate Oligonucleotides on Solid Support†

Abstract: A versatile approach has been developed for the multiple labeling of oligonucleotides. First, three linkers as a H-phosphonate monoester derivative were condensed on a solid-supported T12 to introduce H-phosphonate diester linkages which were oxidized in the presence of propargylamine. Second, three galactosyl azide derivatives were conjugated to the solid-supported three-alkyne-modified T12 by a 1,3-cycloaddition so-called “click chemistry” in the presence of Cu(I) assisted by microwaves.

Asymmetric Friedel−Crafts Alkylations of Indoles with Nitroalkenes Catalyzed by Zn(II)−Bisoxazoline Complexes

Abstract: A novel asymmetric Friedel−Crafts alkylation of indoles with nitroalkenes catalyzed by Zn(II)−bisoxazoline complexes has been developed. The nitroalkylated indoles are synthesized in excellent yields and high enantioselectivities (up to 90% ee). The effects of ligand structure, metal salt, and solvent on the reaction are discussed. The substrates of the reaction can be aromatic, heteroaromatic, and even aliphatic nitroalkenes. The high reactivity and selectivity of the reaction are presumptively attributed to the activation and asymmetric induction of chiral Lewis acids coordinated by nitroalkene substrates through a 1,3-metal bonding model.

Development of isomerization and cycloisomerization with use of a ruthenium hydride with N-heterocyclic carbene and its application to the synthesis of heterocycles.

Abstract: A pure ruthenium hydride complex with N-heterocyclic carbene (NHC) ligand was efficiently generated from the reaction of a second-generation Grubbs ruthenium catalyst with vinyloxytrimethylsilane and unambiguously characterized. This ruthenium hydride complex showed high catalytic activity for the selective isomerization of terminal olefin and for the cycloisomerization of 1,6-dienes. These reactions of N-allyl-o-vinylaniline lead to novel synthetic methods for heterocycles such as indoles and 3-methylene-2,3-dihydroindoles, which are useful synthons for bioactive natural products. These procedures address an important issue in diversity-oriented synthesis.

Diatomite-supported Pd nanoparticles: an efficient catalyst for Heck and Suzuki reactions.

Abstract: The Pd nanoparticles immobilized on natural diatomite were achieved by a simple procedure. The catalysts are highly active for Heck and Suzuki reactions and can be recovered and reused many times. The catalytic process was also investigated.

Revisiting nucleophilic substitution reactions: microwave-assisted synthesis of azides, thiocyanates, and sulfones in an aqueous medium.

Abstract: A practical, rapid, and efficient microwave (MW) promoted synthesis of various azides, thiocyanates, and sulfones is described in an aqueous medium. This general and expeditious MW-enhanced nucleophilic substitution approach uses easily accessible starting materials such as halides or tosylates in reaction with alkali azides, thiocyanates, or sulfinates in the absence of any phase-transfer catalyst, and a variety of reactive functional groups are tolerated.

On the mechanism of the Skraup-Doebner-Von Miller quinoline synthesis.

Abstract: The mechanism of the formation of substituted quinolines from anilines and α,β-unsaturated ketones has been studied by the use of 13C-labeled ketones in cross-over experiments. In the reaction of doubly labeled 13C(2,4) mesityl oxide, a 100% scrambling of the label in the quinoline product was observed, whereas only a small (5−10%) amount of the starting mesityl oxide showed scrambling of the label. Similarly, the reaction of triply labeled pulegone clearly shows that the label in the product is 100% scrambled, whereas the label in the starting pulegone is retained. On the basis of these studies, a mechanistic pathway for the Skraup quinoline synthesis is proposed that involves a fragmentation−recombination mechanism. The aniline component condenses with the α,β-unsaturated ketone initially in a conjugate fashion, followed by a fragmentation to the corresponding imine and the ketone itself. These fragments recombine to form the quinoline product.

Synthesis of aryl ketones or ketimines by palladium-catalyzed arene C-H addition to nitriles.

Abstract: The unprecedented palladium-catalyzed C−H addition of arenes to nitriles provides moderate to excellent yields of aryl ketones or the corresponding hindered imines. The addition of a small amount of DMSO increases the yields dramatically. Both intermolecular and intramolecular reactions are successful, although the intramolecular reactions tend to be more sluggish. This novel chemistry is believed to involve palladium-catalyzed C−H activation of the arene by electrophilic aromatic substitution, followed by the unusual carbopalladation of a nitrile. Similar reactions have been successfully developed employing arylboronic acids and nitriles. A concise route to xanthones starting from cheap starting materials has been developed employing this synthetic protocol.