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Showing papers in "Helvetica Chimica Acta in 2017"


Journal ArticleDOI
TL;DR: In this article, the fluorescence properties of the BODIPY dye and its two meso-substituted derivatives, tert-butyl- and phenyl-bodipy, are rationalized.
Abstract: The fluorescence properties of the BODIPY dye and its two meso-substituted derivatives, tert-butyl- and phenyl-BODIPY, are rationalized. The non emissive behavior of the latter two are attributed to the energetically accessible low-lying conical intersection between the ground state and the lowest excited singlet state. Both intramolecular non-covalent interactions and excited state charge transfer character are identified as being crucial for “stabilizing” the intersection and prompting the nonradiative decay. Similar crossing was located in the bare BODIPY dye, however, being energetically less accessible, which correlates well with the high fluorescence quantum yields of the parent dye.

43 citations


Journal ArticleDOI
TL;DR: Structural modification of the sulfenylating agents had a significant and salutary effect on the enantioselectivity of sulfenofunctionalization which was demonstrated to be general for trans disubstituted alkenes.
Abstract: The full details of mechanistic investigation on enantioselective sulfenofunctionalization of alkenes under Lewis base catalysis are described. Solution spectroscopic identification of the catalytically active sulfenylating agent has been accomplished along with the spectroscopic identification of putative thiiranium ion intermediates generated in the enantiodetermining step. The structural insights gleaned from these studies informed the design of new catalyst architectures to improve enantioselectivity. In addition, structural modification of the sulfenylating agents had a significant and salutary effect on the enantioselectivity of sulfenofunctionalization which was demonstrated to be general for trans disubstituted alkenes. Whereas electronic modulation had little effect on the rate and selectivity, steric bulk on arylsulfenylphthalimides was very beneficial. This article is protected by copyright. All rights reserved.

32 citations


Journal ArticleDOI
TL;DR: In this article, the question of whether scientists in the perovskite and nanomaterials science communities refer CH3NH3PbY3, as well as other perovsite derivatives falling into the same category, as organometallic was discussed.
Abstract: Methylammonium lead trihalide perovskite solar cells (CH3NH3PbY3, where Y = I(3–x)Brx=1–3, I(3 – x)Clx=1–3, Br(3-x)Cl x=1–3, and IBrCl) are photonic semiconducting materials. Researches on various fundamental and technological aspects of these materials are extensively on-going to make them stable environmentally and for commercialization. Research studies addressing these materials as organometallic are massively and repeatedly appearing in reputable and high-profile peer-reviewed journal publications (viz. Energy and Environmental Science, Nature Chemistry, Nature Communication, Advanced Materials, Science, ACS Nano, ACS Energy Letters, and in many other chemistry and materials based international journals). Herein, I candidly addresses the question: whether should scientists in the perovskite and nanomaterials science communities refer CH3NH3PbY3, as well as other perovskite derivatives falling into the same category, as organometallic? This article is protected by copyright. All rights reserved.

27 citations


Journal ArticleDOI
TL;DR: In this paper, the authors describe polarizing materials incorporating nitroxide radicals within the walls of the solids to minimize interactions between the radicals and the solute, which can hyperpolarize pure pyruvic acid, a particularly important substrate of clinical interest.
Abstract: Hyperpolarization by dynamic nuclear polarization relies on the microwave irradiation of paramagnetic radicals dispersed in molecular glasses to enhance the nuclear magnetic resonance (NMR) signals of target molecules. However, magnetic or chemical interactions between the radicals and the target molecules can lead to attenuation of the NMR signal through paramagnetic quenching and/or radical decomposition. Here we describe polarizing materials incorporating nitroxide radicals within the walls of the solids to minimize interactions between the radicals and the solute. These materials can hyperpolarize pure pyruvic acid, a particularly important substrate of clinical interest, while nitroxide radicals cannot be used, even when incorporated in the pores of silica, because of reactions between pyruvic acid and the radicals. The properties of these materials can be engineered by tuning the composition of the wall by introducing organic functionalities.

23 citations


Journal ArticleDOI
TL;DR: In this article, a triarylamine−Ru(bpy)32+−anthraquinone triad was excited with two temporally delayed laser pulses of different color and monitored the resulting photoproducts.
Abstract: Controlling light‐induced accumulation of electrons or holes is desirable in view of multi‐electron redox chemistry, for example for the formation of solar fuels or for photoredox catalysis in general. Excitation with multiple photons is usually required for electron or hole accumulation, and consequently pump‐pump‐probe spectroscopy becomes a valuable spectroscopic tool. In this work, we excited a triarylamine‐Ru(bpy)32+‐anthraquinone triad (bpy = 2,2′‐bipyridine) with two temporally delayed laser pulses of different color and monitored the resulting photoproducts. Absorption of the first photon by the Ru(bpy)32+ photosensitizer generated a triarylamine radical cation and an anthraquinone radical anion by intramolecular electron transfer. Subsequent selective excitation of either one of these two radical ion species then induced rapid reverse electron transfer to yield the triad in its initial (ground) state. This shows in direct manner that after absorption of a first photon and formation of the primary photoproducts, the absorption of a second photon can lead to unproductive electron transfer events that counteract further charge accumulation. In principle, this problem is avoidable by careful excitation wavelength selection in combination with good molecular design.

21 citations


Journal ArticleDOI
TL;DR: In this paper, it has been suggested that the origin of regio-and stereoselectivity in Michael addi- tions of pyrrolidine enamines is achieved by thermodynamic rather than kinetic control through distinct conformational preferences of the enamines.
Abstract: It has been suggested that the origin of regio- and stereoselectivity in Michael addi- tions of pyrrolidine enamines is achieved by thermodynamic rather than kinetic control through distinct conformational preferences of the enamines. We assess this proposal by elaboration of a computational protocol that warrants sufficient accuracy. The small energy differences between the conformers necessitate a high accuracy of the electronic structure method which, in addition, must allow for computationally feasible calcula- tions of a large number of conformers. Our protocol is based on density functional theory which we validated against explicitly correlated coupled cluster theory. The results are in agreement with the available experimental data, but illustrate that conformational preferences determined for one enamine are not readily transferable to other types of enamines. We found that an appropriate conformational sampling is inevitable to arrive at meaningful conclusions. Most prominently, s-cis and s-trans conformers are similarly stable for aldehyde- and ketone-derived enamines. The regio- and stereoselectivity in Michael additions of pyrrolidine-derived enamines can, however, not be explained by pronounced stability differences of the enamine isomers and conformers in general, dis- proving the thermodynamic-control hypothesis. The elucidation of the origin of regio- and stereoselectivity requires further theoretical investigations of the elementary steps of Michael additions. This article is protected by copyright. All rights reserved.

20 citations


Journal ArticleDOI
TL;DR: In this article, a series of twisted push-pull probes with turn-on sulfide donors was designed and evaluated in lipid bilayer membranes of different thickness and fluidity from liquid-disordered to liquid-ordered and solid-ordered phases.
Abstract: Planarizable and polarizable dithieno[3,2-b;2′,3′-d]thiophene (DTT) dimers have been introduced recently as fluorescent probes that report on membrane fluidity with red shifts in excitation, i.e. planarization in the ground state. In this study, we elaborate on the hypothesis that twisted push-pull probes could perform best in the presence of one unorthodox substituent that acts as a weak acceptor with electron-rich and as a strong donor with electron-poor aromatics. According to Hammett constants, we thought that sulfides could provide access to such a conceptually innovative donor-acceptor switch. To elaborate on this hypothesis, we here describe the design, synthesis and evaluation of a comprehensive series of twisted push-pull probes with turn-on sulfide donors. Their planarization is explored in lipid bilayer membranes of different thickness and fluidity from liquid-disordered to liquid-ordered and solid-ordered phases. Results from membranes are compared to the planarization of turn-on mechanophores in crystals, proteins, and cyclodextrin macrocycles of varied diameter.

17 citations


Journal ArticleDOI
TL;DR: The potassium acyltrifluoroborate amide-forming ligation is orthogonal to both thiol-Michael and strain promoted azide alkyne cycloadditions and enables stepwise covalent protein immobilization of multiple BSA-derivatives onto the hydrogel scaffold regardless of the order of addition.
Abstract: The covalent immobilization of peptides, proteins, and other biomolecules to hydrogels provides a biologically mimicking environment for cell and tissue growth. Bioorthogonal chemical reactions can serve as a tool for this, but the paucity of such reactions and mutual incompatibilities limits the number of distinct molecules that can be introduced. We now report that the potassium acyltrifluoroborate (KAT) amide-forming ligation is orthogonal to both thiol-Michael and strain promoted azide alkyne cycloadditions (SPAAC) and the requisite functional groups – KATs and hydroxylamines – are stable and compatible to hydrogel formation, protein modification, and post-assembly immobilization of biomolecules onto hydrogels. In combination these ligations enables stepwise covalent protein immobilization of multiple BSA-derivatives onto the hydrogel scaffold regardless of the order of addition.

17 citations


Journal ArticleDOI
TL;DR: In this article, a polynuclear zinc complexes containing tridentate Schiff base ligands were successfully synthesized and fully characterized using single crystal X-ray diffraction, which showed good catalytic activity towards the ring-opening polymerization (ROP) of rac-lactide (rac-LA) and ǫcaprolactone (ǫ-CL) under solvent free conditions.
Abstract: New polynuclear zinc complexes containing tridentate Schiff base ligands were successfully synthesized and fully characterized. The solid-state structure of the complexes was determined using single crystal X-ray diffraction. The complexes display a tetranuclear cubane-like core structure [Zn4O4] and sowed good catalytic activity towards the ring-opening polymerization (ROP) of rac-lactide (rac-LA) and ɛ-caprolactone (ɛ-CL) under solvent-free conditions. The polylactic acid (PLA) obtained from rac-LA showed isotactic enrichment, as proved by homonuclear decoupled 1H NMR analysis. These complexes also showed good activity and superior control towards the ROP of rac-LA and ɛ-CL in the presence of benzyl alcohol as a co-initiator. Furthermore, Kinetic studies demonstrated that the ROP of rac-LA and ɛ-CL has a first order dependence on both monomer (rac-LA and ɛ-CL) and catalyst concentration. This article is protected by copyright. All rights reserved.

17 citations


Journal ArticleDOI
TL;DR: In this paper, a rhodium(II)-catalyzed reaction of N-sulfonyl-1,2,3-triazoles with 2-alkenols is reported.
Abstract: A study on a rhodium(II)-catalyzed reaction of N-sulfonyl-1,2,3-triazoles with 2-alkenols is reported. The reaction is initiated by insertion of an α-imino carbene into the O–H linkage of alcohol, forming a 2-alkenoxy enamide intermediate. A thermal [3,3]-sigmatropic rearrangement follows to yield 2-substituted 2-amino ketone in a stereoselective manner. The successful application of this methodology to a formal synthesis of (–)-α-conhydrine is also described.

16 citations


Journal ArticleDOI
TL;DR: In this article, the authors reported the serendipitous discovery and first use in the C-H functionalization of arenes of IndoleBX and PyrroleBX, new thermally highly stable benziodoxol(on)e hypervalent iodine reagents.
Abstract: The development of reagents allowing the reversal of the standard reactivity (Umpolung) of small building blocks is an important field of research in chemistry, as it allows increasing the flexibility of organic synthesis. Indoles and pyrroles are ubiquitous heterocycles in natural products and drugs. They are usually functionalized making use of their high nucleophilicity. In contrast, only few methods are based on the use of electrophilic indole and pyrrole synthons, as the needed reagents are highly unstable or can be used only with a very narrow scope. Herein, we report the serendipitous discovery and first use in the C-H functionalization of arenes of IndoleBX and PyrroleBX, new thermally highly stable benziodoxol(on)e hypervalent iodine reagents. IndoleBX and PyrroleBX could be obtained in one step from the corresponding heterocycles and acetoxy benziodoxolone using a Lewis acid catalyst. The mild reactions conditions allowed the introduction of a broad range of functional groups, including ethers, halogens and boronic esters. The new reagents could then be used in the rhodium- and ruthenium- catalyzed C-H heteroarylation of arene rings bearing heterocyclic or benzamide directing groups. Such transformations could not be realized using previously reported C-H functionalization procedures. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
TL;DR: In this article, a general method to covalently modify the sidechains of poly(disulfide)s was proposed, which results in a 20-fold increase in activity with preserved sensitivity toward inhibition with the Ellman's reagent.
Abstract: Cell-penetrating poly(disulfide)s (CPDs) have been introduced recently to explore new ways to enter into cells. In this report, we disclose a general method to covalently modify the sidechains of CPDs. Compatibility of copper-catalyzed alkyne-azide cycloaddition (CuAAC) with the addition of either strained cyclic disulfides of varied ring tension or increasing numbers of guanidinium and phosphonium cations is demonstrated. Reloading CPDs with disulfide ring tension results in an at least 20-fold increase in activity with preserved sensitivity toward inhibition with the Ellman's reagent. The cumulation of permanent positive charges by sidechain engineering affords Ellman-insensitive CPDs with similarly increased activity. Co-localization experiments indicate the CPDs reach endosomes, cytosol and nucleus, depending on their nature and their concentration. Supported by pertinent controls, these trends confirm that CPDs operate with combination of counterion- and thiol-mediated uptake, and that the balance between the two can be rationally controlled. For the most active CPDs, uptake can be observed at substrate (fluorophore) concentrations as low as 5 nM. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
TL;DR: In this article, a new hypervalent iodine reagent based on 1,2-dihydro-3,3-dimethyl-1, 2-benziodoxole containing a 4-methyl-7-tetrafluoroethoxycoumarin unit as a specific UV absorber was described.
Abstract: A new hypervalent iodine reagent (5) based on 1,2-dihydro-3,3-dimethyl-1,2-benziodoxole containing a 4-methyl-7-tetrafluoroethoxycoumarin unit as a specific UV absorber was prepared and fully characterized, including X-ray crystal structural analysis. The high reactivity of this compound towards thiols has been exploited for the selective tagging of several targets with the UV chromophore coumarin, including e.g. glutathione.

Journal ArticleDOI
TL;DR: In this paper, arylation and alkenylation of 1,4-diaza-bicyclo[2.2] octane was proposed to obviate the inefficiencies associated with the preparation of these target molecules.
Abstract: Unsymmetrical piperazines are key constituents of many pharmaceuticals. Given that the selective introduction of an aryl and alkyl motif onto the piperazine is not always straightforward, direct arylation and alkenylation of 1,4-diaza-bicyclo[2.2.2]octane would obviate the inefficiencies associated with the preparation of these target molecules. We have utilized alkyl halides, aryl or alkenyl triflates, and 1,4-diaza-bicyclo[2.2.2]octane for the synthesis of N-alkyl-N′-aryl or alkenylpiperazines. The optimum conditions are developed using CuCl, t-BuOLi in NMP. Alkenyl triflates requires N, N′-dimethyl ethylenediamine and higher temperature to afford the desired cross-coupled product. Substrates bearing electron-deficient and electron-rich groups were successfully coupled under the optimum reaction conditions. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
TL;DR: In this paper, it was shown that deprotonated [K(crypt-222)]+ is highly unstable even at as low as 195 K. The previously considered 1:1 CHF3 clathrate of deproptonated [k(crypt)-222]+ (crystallographically indistinguishable from 1) is ruled out on the basis of all available data.
Abstract: [K(crypt-222)]+ CF3- (1) and [K(crypt-222)]+ BF4- (3) are isostructural, displaying nearly identical unit cell parameters. The two structures are similar to the extent that the previously reported [K(crypt-222)]+ CF3- model can be refined against the new data for [K(crypt-222)]+ BF4-, with extra electron density being observed from the fourth fluorine atom of the BF4-. In agreement with experimental observations, theoretical calculations suggest that deprotonated [K(crypt-222)]+ is highly unstable even at as low as 195 K. The previously considered 1:1 CHF3 clathrate of deprotonated [K(crypt-222)]+ (crystallographically indistinguishable from 1) is ruled out on the basis of all available data. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
Lan Hu1, Yan Zhao1
TL;DR: By molecularly imprinting template molecules within doubly cross-linked micelles, protein-sized nanoparticles with catalytically functionalized binding sites are created with accelerated hydrolysis of activated esters thousands of times over the background reaction, whereas the analogous catalytic group was completely inactive in bulk solution under the same conditions.
Abstract: Enzymes have substrate-tailored active sites with optimized molecular recognition and catalytic features. Although many different platforms have been used by chemists to construct enzyme mimics, it is challenging to tune the structure of their active sites systematically. By molecularly imprinting template molecules within doubly cross-linked micelles, we created protein-sized nanoparticles with catalytically functionalized binding sites. These enzyme mimics accelerated the hydrolysis of activated esters thousands of times over the background reaction, whereas the analogous catalytic group (a nucleophilic pyridyl derivative) was completely inactive in bulk solution under the same conditions. The template molecules directly controlled the size and shape of the active site and modulated the resulting catalyst's performance at different pHs. The synthetic catalysts displayed Michaelis–Menten enzymatic behavior and, interestingly, reversed the intrinsic reactivity of the activated esters during the hydrolysis. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
TL;DR: In this paper, the effect of a decrease in the solvent polarity on rotameric equilibrium is probed using molecular dynamics simulation. But, in an aqueous environment, solvent-exposed H-bonds are likely to represent only a marginal (possibly adverse) conformational driving as well as steering force.
Abstract: Intramolecular hydrogen-bonding (H-bonding) is commonly regarded as a major determinant of the conformation of (bio)molecules. However, in an aqueous environment, solvent-exposed H-bonds are likely to represent only a marginal (possibly adverse) conformational driving as well as steering force. For example, the hydroxymethyl rotamers of glucose and galactose permitting the formation of an intramolecular H-bond with the adjacent hydroxyl group are not favored in water but, in the opposite, least populated. This is because the solvent-exposed H-bond is dielectrically screened as well as subject to intense H-bonding competition by the water molecules. In the present study, the effect of a decrease in the solvent polarity on this rotameric equilibrium is probed using molecular dynamics simulation. This is done by considering six physical solvents (H2O, DMSO, MeOH, CHCl3, CCl4, and vacuum), along with 19 artificial water-like solvent models for which the dielectric permittivity and H-bonding capacity can be modulated independently via a scaling of the O–H distance and of the atomic partial charges. In the high polarity solvents, the intramolecular H-bond is observed, but arises as an opportunistic consequence of the proximity of the H-bonding partners in a given rotameric state. Only when the polarity of the solvent is decreased does the intramolecular H-bond start to induce a conformational pressure on the rotameric equilibrium. The artificial solvent series also reveals that the effects of the solvent permittivity and of its H-bonding capacity mutually enhance each other, with a slightly larger influence of the permittivity. The hydroxymethyl conformation in hexopyranoses appears to be particularly sensitive to solvent-polarity effects because the H-bond involving the hydroxymethyl group is only one out of up to five H-bonds capable of forming a network around the ring.

Journal ArticleDOI
TL;DR: In this paper, a new procedure for the synthesis of highly substituted 1,3-diaminopyrazoles is described, where 1-alkynyltriazenes and imines are used as substrates.
Abstract: A new procedure for the synthesis of highly substituted 1,3-diaminopyrazoles is described. As substrates, we have employed 1-alkynyltriazenes and imines. The formation of pyrazoles was achieved by two-fold C-N coupling reactions in the presence of (JohnPhos)AuCl and AgNTf2 as catalyst precursors. The regioselectivity of the reaction was inferred from a crystallographic analysis of one reaction product. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
TL;DR: The reaction of indoles and stabilized cyclopropyl alkynes under gold and/or gold & Bronsted acid-catalysis provided access to highly substituted tetrahydrocarbazoles.
Abstract: The reaction of indoles and stabilized cyclopropyl alkynes under gold- and/or gold & Bronsted acid-catalysis provided access to highly substituted tetrahydrocarbazoles. A mechanistic study revealed the complex mechanism underlying these processes and the opportunistic cooperation of Lewis and Bronsted acid-catalysts towards the formation of complex molecular scaffolds.


Journal ArticleDOI
TL;DR: In this article, the authors reported the direct chemical synthesis of coenzyme B12 (AdoCbl) from Coβ-5'-deoxyadenosylcobyric acid (AdOCby) and the preparation of the latter from crystalline CN,H2O-cobyrics acid (CN,H 2OCby) in one-step in over 85% yield, by covalent attachment in aqueous solution of the integral B12-nucleotide moiety.
Abstract: Here we report the direct chemical synthesis of coenzyme B12 (AdoCbl) from Coβ-5’-deoxyadenosylcobyric acid (AdoCby) and the preparation of the latter from crystalline CN,H2O-cobyric acid (CN,H2OCby). AdoCby is a suggested common key intermediate in the biosynthesis of AdoCbl and of other cobamides in microorganisms. AdoCby was thoroughly characterized by spectroscopic means, including homo-nuclear and hetero-nuclear NMR, as such data are not available in published work. AdoCbl was prepared from AdoCby in one-step in over 85% yield, by covalent attachment in aqueous solution of the integral B12-nucleotide moiety using 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC*HCl) and N-hydroxybenzotriazole (HOBt) as coupling reagents. By the same procedure crystalline vitamin B12 (CNCbl) was also prepared in 92% yield from CN,H2OCby. Coordination of the B12-nucleotide base at the Coα-face of AdoCby or of CN,H2OCby was indicated to assist in the efficient covalent coupling at the activated f-side chain function to furnish the complete corrinoids AdoCbl and CNCbl. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
TL;DR: In this paper, the SnAP hydrazine reagents undergo a molecular oxygen-assisted oxidative cyclization, which accommodates a broad range of groups including electron-rich, electron-poor aromatic, heteroaromatic, and aliphatic aldehydes and is amenable to gram scale synthesis.
Abstract: SnAP (Sn (tin) amine protocol) hydrazine reagents and aldehydes undergo oxidative, copper mediated coupling to form substituted 1,4,5-oxadiazepanes and 1,2,5-triazepanes. Unlike all prior reactions involving SnAP reagents, the SnAP hydrazine reagents undergo a molecular oxygen-assisted oxidative cyclization. The air- and moisture tolerant transformation accommodates a broad range of groups including electron-rich, electron-poor aromatic, heteroaromatic, and aliphatic aldehydes and is amenable to gram scale synthesis. These unusual, chiral heterocycles have unexpectedly large optical rotations, which may find use in optical materials. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
TL;DR: In this article, the preparation of the first soluble quaterrylene derivative featuring peripheral tert-butyl substituents and sterically hindering, core-anchored triflate groups has been achieved.
Abstract: The preparation of the first soluble quaterrylene derivative featuring peripheral tert-butyl substituents and sterically hindering, core-anchored triflate groups has been achieved. This involves a facile synthetic route based on an oxidative coupling of perylene precursors in the presence of H2O2 as oxidant. The steric hindrance between the OTf substituents at the central bay position of the quaterrylene board triggers a strong deformation of the central perylene planarity, which forces the quaterrylene platform to adopt a twisted geometry as shown by X-Ray analysis. Exceptionally, photophysical investigations show that the core-twisted quaterrylene phosphoresces in the NIR spectral region at 1716 nm. Moreover, third-order nonlinear optical (NLO) measurments on solutions and thin film containing the relevant molecule showed very large second hyperpolarizability values, as predicted by theoretical calculations at the CAM-B3LYP/6-31G** level of theory, making this material very appealing for photonic applications. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
TL;DR: In this paper, the metalation of tetradentate pyrphyrin by copper substrate atoms on a Cu(111) surface was studied using thermal desorption spectroscopy (TDS) and N 1s X-ray photoelectron spectrograph (XPS).
Abstract: The metalation of the tetradentate molecule pyrphyrin by copper substrate atoms on a Cu(111) surface is studied. Pyrphyrin, in contrast to porphyrin, consists of four fused pyridine groups instead of pyrrol groups. Using thermal desorption spectroscopy (TDS) and N 1s X-ray photoelectron spectroscopy (XPS), we show that metalation of the monolayer of pyrphyrin with Cu atoms from the Cu(111) surface occurs at 377 K. The formation of an extended two-dimensional (2D) network is observed with scanning tunneling microscopy (STM). A honeycomb-like lattice of metalated pyrphyrin molecules is formed by intermolecular connection via the two cyano groups at the periphery of pyrphyrin as well as Cu adatoms. Dehydrogenation at the periphery of the molecule is observed during annealing at 520 K. The surface-adsorbed metal-pyrphyrin has the potential to serve as a molecular catalyst.

Journal ArticleDOI
TL;DR: A systematic investigation of structural modifications of brassinosteroids is presented in this paper, which includes significant improvements of previously reported protocols as well as access to new analogs with functional modifications of the steroid skeleton and of the C(17)-attached side chain.
Abstract: A systematic investigation of structural modifications of brassinosteroids is presented. We describe in detail their synthetic preparation, which includes significant improvements of previously reported protocols as well as access to new analogs with functional modifications of the steroid skeleton and of the C(17)-attached side chain. We report the biological potency of the prepared brassinosteroid analogs as plant hormones, which were carefully established in the French bean second internode elongation assay and discuss our observations in light of the recently reported structural data detailing the molecular interactions between brassinolide in the trimeric complex with the protein receptors kinases BRASSINOSTEROID INSENSITIVE 1 (BRI1) and SOMATIC EMBRYOGENESIS RECEPTOR KINASE 1 (SERK1). In a further part of this work we describe the preparation of water-soluble pro-forms of 24-epicastasterone and we discuss their physical properties, hydrolytic stabilities and biological activity. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
TL;DR: In this article, the synthesis and characterization of chitosan-silica sulfate nano hybrid (CSSNH) as a novel and efficient heterogeneous nano catalyst involving acid-base bifunctional activity is described.
Abstract: The synthesis and characterization of chitosan-silica sulfate nano hybrid (CSSNH) as a novel and efficient heterogeneous nano catalyst involving acid-base bifunctional activity is described. The catalytic potency and activity of this eco-friendly catalyst was investigated in regioselective ring opening of epoxides with carboxylic acids to access structurally diverse 1,2-diol mono-esters in good to excellent yields. CSSNH catalyst was characterized using different microscopic and spectroscopic techniques encompassing scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), N2 adsorption isotherm, and Fourier transform infrared spectroscopy (FT-IR). The green nature, cheapness, efficiency, ease of preparation, handling and reusability of this new catalyst makes this catalyst to be useful for green industrial processes. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
TL;DR: A collection of excerpts from letters written by R. B. Woodward to his friends, colleagues, young students and others is presented in this article, which illustrate many aspects of his personality and philosophy of life.
Abstract: A collection of excerpts from letters written by R. B. Woodward to his friends, colleagues, young students and others is presented. These excerpts are representative of Woodward's lengthy correspondence and illustrate many aspects of his personality and philosophies of life. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
TL;DR: In this article, the influence of the linker on the efficiency of dynamic nuclear polarization was investigated, and it was shown that the DNP efficiency was similar as in materials with flexible linkers, both for magic angle spinning at 105 K and dissolution DNP at 4 K.
Abstract: Hyperpolarization of NMR-active nuclei is key to gather high quality spectra of rare species and insensitive isotopes. We have recently established that silica-based materials containing regularly distributed nitroxyl radicals connected to the silica matrix by flexible linkers can serve as promising polarization matrices for dynamic nuclear polarization (DNP). Here we investigate the influence of the linker on the efficiency of the polarization. The materials were fully characterized and exhibit high surface areas and narrow pore size distributions with a tunable amount of phenyl azide groups over a broad range of concentrations. The phenyl azide groups can be easily functionalized via a two-step procedure with 4-carboxy-2,2,6,6-tetramethyl-1-oxylpiperidine (TEMPO) to give polarizing matrices with controllable radical content. The DNP efficiency was found to be similar as in materials with flexible linkers, both for magic angle spinning at 105 K and dissolution DNP at 4 K.

Journal ArticleDOI
TL;DR: In this article, a straightforward synthetic route to monothiomalonates (MTMs) that proceeds through nucleophilic ring-opening of Meldrum's acid derivatives followed by O-alkylation of the resulting malonic acid half thioesters with alkyl triflates or acetimidates as electrophiles is presented.
Abstract: Monothiomalonates (MTMs) are surrogates of thioester enolates that allow for stereoselective C–C bond formations under mild conditions and thereby afford access to synthetically versatile thioester derivatives. Here we present a straightforward synthetic route to monothiomalonates (MTMs) that proceeds through nucleophilic ring-opening of Meldrum's acid derivatives followed by O-alkylation of the resulting malonic acid half thioesters (MAHTs) with alkyl triflates or acetimidates as electrophiles. The method affords MTMs in overall yields of 34–92 % and allows for variations of the oxo- and thioester moieties as well as the substituent at the Cα position. This article is protected by copyright. All rights reserved.

Journal ArticleDOI
TL;DR: In this paper, the authors synthesized Mo(NC6F5)(CHCMe2Ph)(TPPO)(PPhMe2)Cl (TPPO = 2,3,5,6-tetraphenylphenoxide), Mo( NC6F 5), CHCMe 2Ph)(TTBTO), PPhMe 2), and CF3Pyr, in order to evaluate them as catalysts for the homocoupling of 3methyl-1-butene.
Abstract: We synthesized Mo(NC6F5)(CHCMe2Ph)(TPPO)(PPhMe2)Cl (TPPO = 2,3,5,6-tetraphenylphenoxide), Mo(NC6F5)(CHCMe2Ph)(TTBTO)(PPhMe2)Cl (TTBTO = 2,6-di(3’,5’-di-tert-butylphenyl)phenoxide), and Mo(NC6F5)(CHCMe2Ph)(TPPO)(PPhMe2)(CF3Pyr) (CF3Pyr = 3,4-bistrifluoromethylpyrrolide), in order to evaluate them as catalysts for the homocoupling of 3-methyl-1-butene. They were compared with Mo(NC6F5)(CHCMe2Ph)(HMTO)(PPhMe2)Cl (HMTO = 2,6-dimesitylphenoxide), Mo(NC6F5)(CHCMe2Ph)(HIPTO)(PPhMe2)Cl (HIPTO = 2,6-di(2’,4’,6’-triisopropylphenyl)phenoxide), and several other Mo and Ru catalysts. In the best cases turnover numbers (TONs) of 400-700 were observed for the homocoupling of 3-methyl-1-butene in a closed vessel (ethylene not removed). This article is protected by copyright. All rights reserved.