Palladium-catalyzed carbonylative annulation of internal alkynes: synthesis of 3,4-disubstituted coumarins

The palladium-catalyzed annulation of internal alkynes by o-iodophenols in the presence of CO results in exclusive formation of coumarins. No isomeric chromones have been observed. The best reaction conditions utilize the 2-iodophenol, 5 equiv of alkyne, 1 atm of CO, 5 mol % Pd(OAc)2, 2 equiv of pyridine, and 1 equiv of n-Bu4NCl in DMF at 120 degrees C. The use of a sterically unhindered pyridine base is essential to achieve high yields. A wide variety of 3,4-disubstituted coumarins containing alkyl, aryl, silyl, alkoxy, acyl, and ester groups have been prepared in moderate to good yields. Mixtures of regioisomers have been obtained when unsymmetrical alkynes are employed. 2-iodophenols with electron-withdrawing and electron-donating substituents and 3-iodo-2-pyridone are effective in this annulation process. The reaction is believed to proceed via (1) oxidative addition of the 2-iodophenol to Pd(0), (2) insertion of the alkyne triple bond into the aryl-palladium bond, (3) CO insertion into the resulting vinylic carbon-palladium bond, and (4) nucleophilic attack of the phenolic oxygen on the carbonyl carbon of the acylpalladium complex with simultaneous regeneration of the Pd(0) catalyst. This annulation process is the first example of intermolecular insertion of an alkyne occurring in preference to CO insertion.     

Structural differentiation of uronosyl substitution patterns in acidic heteroxylans by electrospray tandem mass spectrometry

The structures of two oligomers of acidic xylo-oligosaccharides (XOS) of the same molecular weight (634 Da), Xyl(2)MeGlcAHex and Xyl(2)GlcA(2) were differentiated by electrospray tandem mass spectrometry (ESI-MS/MS). These oligomers were present in a mixture of XOS obtained by acid hydrolysis of heteroxylans extracted from Eucalyptus globulus wood (Xyl(2)MeGlcAHex) and Olea europaea olive fruit (Xyl(2)GlcA(2)). In the ESI-MS spectra of the XOS, ions at m/z 657 and 652 were observed and assigned to [M + Na](+) and [M + NH(4)](+), respectively. The ESI-MS/MS spectrum of [M + Na](+) ion of Xyl(2)MeGlcAHex showed the loss of Hex residue from the reducing end followed by the loss of MeGlcA moiety. Simultaneously, the loss of a Xyl residue from either the reducing or the non-reducing ends was detected. On the other hand, the fragmentation of Xyl(2)GlcA(2) occurs mainly by the loss of one and two GlcA residues or by the loss of the GlcAXyl moiety, due to the glycosidic bond cleavage between the two Xyl residues. Loss of one and two CO(2) molecules was only observed for this oligomer, where the GlcA are in vicinal Xyl residues. The ESI-MS/MS spectra of [M + NH(4)](+) of both oligomers showed the loss of NH(3), resulting in the protonated molecule, where the presence of ions assigned as protonated molecules of aldobiuronic acid residues, [MeGlcA - Xyl + H](+) and [GlcA - Xyl + H](+), are diagnostic ions of the presence of MeGlcA and GlcA moieties in XOS. Since these structures occur in small amounts in complex acidic XOS mixtures and are very difficult, if possible, to isolate, tandem mass spectrometry revealed to be a powerful tool for the characterization of these types of substitution patterns present in heteroxylans.     

Quantum variational transition state theory revisited

A quantum version of classical variational transition state theory suggested by McLafferty and Pechukas is refined. In this new quantum version, the variational property of the theory leads to the identification of an optimal smeared dividing surface. This optimal function is shown to be the eigenfunction associated with the lowest eigenvalue of a positive quantum transition state theory operator. The lowest eigenvalue is the optimal bound on the quantum rate. Application of the theory to the parabolic barrier provides better bounds but does not give an essential improvement when compared to previous quantum transition state theories.     

Combined use of calorimetric and adsorption kinetic methods for the study of the mechanisms of catalytic processes

Consideration is given to the main results obtained in studies based on calorimetric methods at high temperatures (above room temperature) of the following aspects of the mechanisms of catalytic and sorption processes on powder catalysts: 1) relation between chemisorption and dissolution of gases in the subsurface layers of solids; 2) influence of surface-adsorbed substance on the adsorption of another substance from the gas phase; 3) nature of intermediate species formed during catalytic processes on the catalyst surface. Results are presented of the application of calorimetric methods to measuring the enthalpy change of polymerization in a dry system: gaseous monomer — solid catalyst — solid polymer, and to investigating the mechanisms of such processes. Anomalous solubility of gases in the subsurface layers of solids has been shown to be significant for the mechanism of nucleation during phase transitions in solids.

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Zusamenfassung Die wichtigsten Ergebnisse der auf kalorimetrischen Methoden bei hohen Temperaturen (über Raumtemperatur) beruhenden Untersuchungen der Mechanismen von katalytischen und Sorptionsprozessen an pulverförmigen Katalysatoren werden behandelt: 1) Zusammenhang zwischen Chemisorption und Lösung von Gasen in den Schichten von Festkörpern unterhalb der Oberfläche; 2) Einfluß der an der Oberfläche adsorbierten Substanz auf die Adsorption einer anderen Substanz aus der Gasphase; 3) Beschaffenheit der während des katalytischen Vorgangs an der Katalysator-Oberfläche entstandenen Intermediärteilchen.Ergebnisse der Anwendung der kalorimetrischen Methode zur Messung der Enthalpie-Änderungen der Polymerisation im trockenen System: gasförmiges Monomer — fester Katalysator — festes Polymer und zur Untersuchung der Mechanismen solcher Vorgänge werden mitgeteilt. Es wurde gezeigt, daß die anomale Löslichkeit von Gasen in den Schichten unterhalb der Oberfläche von Festkörpern für den Mechanismus der Keimbildung während des Phasenüberganges in Festkörpern von Bedeutung ist.

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Résumé On examine les principaux résultats obtenus lors des études effectuées à l'aide de méthodes calorimétriques à hautes températures (au-dessus de la température ambiante), sous les aspects suivants du mécanisme des réactions catalytiques et de Sorption sur des catalyseurs en poudre: 1) relation entre la chimisorption et la dissolution des gaz dans les sous-couches superficielles des solides, 2) influence d'une substance adsorbée en surface sur l'adsorption d'une autre substance de la phase gazeuse, 3) nature des particules intermédiaires formées lors de la réaction catalytique à la surface du catalyseur. Les résultats de l'application de la méthode calorimétrique à la mesure des variations d'enthalpie au cours de la polymérisation en système «sec» sont présentés: monomère gazeux — catalyseur solide — polymère solide, ainsi que ceux relatifs à l'étude du mécanisme de ces réactions. On montre que la solubilité anormale des gaz dans les sous-couches superficielles des solides joue un rôle important sur le mécanisme de la nucléation lors des transitions de phases dans les solides.

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NMR Studies of Mixed-Ligand Lanthanide (III) Complexes. Peculiarities of Molecular Structure, Dynamics and Paramagnetic Properties for Cerium Subgroup Chelates with Crown Ethers

The molecular structure, dynamics and paramagnetic properties of the complex cations [Ln(ptfa)₂ (18-crown-6)]⁺ in deuterated toluene were studied for Ln = La, Ce, Pr and Nd. The activation enthalpy values of 68 ± 5, 55 ± 15 and 60 ± 13 kJ mol^-1 for the 18-crown-6 conformationalinversion processes for the complexcations of Ce, Pr and Nd, respectively,were obtained. Quantitativeinvestigation of the lanthanide-induced chemical shifts shows a monotonic change of a spatial structure and magnetic susceptibility in comparison with the Bleaney predicted dependence. The free energy of molecular inversion activation for 18-crown-6 molecules in the complex cation [Ln(fod)₂(18-crown-6)]⁺ is 74 ± 9 kJ mol^-1 at 363 K, which is a little more than the value of the free energy of activation 64 ± 9 kJ mol^-1 at 363 K in the complex cation [Ln(ptfa)₂(18-crown-6)]⁺.     

Parallel liquid-phase synthesis of N-substituted 6-aminosulfonyl-2-oxo-1,2-dihydroquinoline-4-carboxamide and 6-aminosulfonylquinoline-4-carboxamide derivatives

Two efficient strategies for solution-phase parallel synthesis of libraries of quinoline derivatives are described. The first synthetic pathway features the Pfitzinger reaction of isatin with diethyl malonate and sulfochlorination of the resulting 2-oxo-1,2-dihydroquinoline-4-carboxylate followed by generation of sulfonamide library. The second strategy employs the unusual behavior of 5-sulfamoylisatins in Pfitzinger reactions, which results in formation of 6-sulfamoyl-4-carboxyquinolines instead of the anticipated 2-oxo-1,2-dihydroquinoline structures. The obtained carboxylates appeared to be convenient synthetic intermediates for the generation of the corresponding carboxamide libraries. Using these reagents, the parallel solution-phase synthesis of more than 500 substituted quinoline and 2-oxo-1,2-dihydroquinoline derivatives has been accomplished on the 50-100-mg scale. Simple manual techniques for parallel reactions using special CombiSyn synthesizers were coupled with easy purification procedures to give high-purity final products. The scope and limitations of the developed approaches are discussed.     

Copper(II) hypophosphite: the α‐ and β‐forms at 270 and 100 K,and the γ‐form at 270 K

Copper(II) hypophosphite has been shown to exist as several polymorphs. The crystal structures of monoclinic α‐, ortho­rhombic β‐ and ortho­rhombic γ‐Cu(H₂PO₂)₂ have been determined at different temperatures. The geometry of the hypophosphite anion in all three polymorphs is very close to the idealized one, with point symmetry mm2. Despite having different space groups, the structures of the α‐ and β‐polymorphs are very similar. The polymeric layers formed by the Cu atoms and the hypophosphite ions, which are identical in the α‐ and β‐polymorphs, stack in the third dimension in different ways. Each hypophosphite anion is coordinated to three Cu atoms. On cooling, a minimum amount of contraction was observed in the direction normal to the layers. The structure of the polymeric layers in the γ‐­polymorph is quite different. There are two symmetry‐independent hypophosphite anions; the first is coordinated to two Cu atoms, while the second is coordinated to four Cu atoms. In all three polymorphs, the Cu atoms are coordinated by six O atoms of six hypophosphite anions, forming tetragonal bipyramids; in the α‐ and β‐polymorphs, there are four short and two long Cu—O distances, while in the γ‐polymorph, there are four long and two short Cu—O distances.     

Syntheses and rearrangements of tris(hydroxymethyl)phosphine and tetrakis(hydroxymethyl)phosphonium salts

Abstract

Tetrakis(hydroxymethyl)phosphonium (HOCH₂)₄P⁺ salts, particularly the chloride (THPC) and sulfate (THPS), are among the most accessible organophosphorus compounds that can be made quantitatively from PH₃ and formaldehyde in aqueous media under ambient conditions. These phosphonium salts are air-stable, and are widely used as reactants in organic syntheses, as well as in textile or oil industries. Synthesis of tris(hydroxymethyl)phosphine (THP), an active component of the salts, is more complicated because direct synthesis from PH₃ and CH₂O requires either pressure and high temperature, at which THP rearranges into bis(hydroxymethyl)methylphosphine oxide, CH₃P(O)(CH₂OH)₂, or a metal catalyst that is usually difficult or impossible to recover. Syntheses of THP based on neutralization of the salts are more convenient but require pH control and additional steps to separate THP from CH₂O. This review summarizes literature data on syntheses and purification of the salts and THP, and side reactions such as thermal rearrangement and oxidation.     

Dicationic triple-decker complexes with a bridging boratabenzene ligand

New dicationic triple-decker complexes with a bridging boratabenzene ligand [Cp*Fe(μ-η:η-C₅H₅BMe)ML]X₂ (ML=CoCp*, 6(CF₃SO₃)₂; RhCp, 7(BF₄)₂; IrCp, 8(CF₃SO₃)₂; Ru(η-C₆H₆), 9(CF₃SO₃)₂; Ru(η-C₆H₃Me₃-1,3,5), 10(CF₃SO₃)₂; Ru(η-C₆Me₆), 11(CF₃SO₃)₂) were synthesized by stacking reactions of Cp*Fe(η-C₅H₅BMe) (2) with the corresponding half-sandwich fragments [ML]²⁺. The structure of 10(CF₃SO₃)₂ was determined by X-ray diffraction study.     

Studies relevant to catalytic reduction of dinitrogen to ammonia by molybdenum triamidoamine complexes

In this paper we explore several issues surrounding the catalytic reduction of dinitrogen by molybdenum compounds that contain the [(HIPTNCH2CH2)3N]3- ligand (where HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3). Four additional plausible intermediates in the catalytic dinitrogen reduction have now been crystallographically characterized; they are MoN= NH (Mo = [(HIPTNCH2CH2)3N]Mo), [Mo=NNH2][BAr'4] (Ar' = 3,5-(CF3)2C6H3), [Mo=NH][BAr'4], and Mo(NH3). We also have crystallographically characterized a 2,6-lutidine complex, Mo(2,6-Lut)+, which is formed upon treatment of MoH with [2,6-LutH][B(C6F5)4]. We focus on the synthesis of compounds that have not yet been isolated, which include Mo=NNH2, Mo=NH, and Mo(NH2). Mo=NNH2, formed by reduction of [Mo=NNH2]+, has not been observed. It decomposes to give mixtures that contain two or more of the following: MoN=NH, Mo triple bond N, Mo(NH3)+, Mo(NH3), and ammonia. Mo=NH, which can be prepared by reduction of [Mo=NH]+, is stable for long periods in the presence of a small amount of CrCp*2, but in the absence of CrCp*2, and in the presence of Mo=NH+ as a catalyst, Mo=NH is slowly converted into a mixture of Mo triple bond N and Mo(NH2). Mo(NH2) can be produced independently by deprotonation of Mo(NH3)+ with LiN(SiMe3)2 in THF, but it decomposes to Mo triple bond N upon attempted isolation. Although catalytic reduction of dinitrogen could involve up to 14 intermediates in a "linear" sequence that involves addition of "external" protons and/or electrons, it seems likely now that several of these intermediates, along with ammonia and/or dihydrogen, can be produced in several reactions between intermediates that themselves behave as proton and/or electron sources.