Intermolecular C-h amination of complex molecules: insights into the factors governing the selectivity

Transition-metal-catalyzed C-H amination via nitrene insertion allows the direct transformation of a C-H into a C-N bond. Given the ubiquity of C-H bonds in organic compounds, such a process raises the problem of regio- and chemoselectivity, a challenging goal even more difficult to tackle as the complexity of the substrate increases. Whereas excellent regiocontrol can be achieved by the use of an appropriate tether securing intramolecular addition of the nitrene, the intermolecular C-H amination remains much less predictable. This study aims at addressing this issue by capitalizing on an efficient stereoselective nitrene transfer involving the combination of a chiral aminating agent 1 with a chiral rhodium catalyst 2. Allylic C-H amination of terpenes and enol ethers occurs with excellent yields as well as with high regio-, chemo-, and diastereoselectivity as a result of the combination of steric and electronic factors. Conjugation of allylic C-H bonds with the π-bond would explain the chemoselectivity observed for cyclic substrates. Alkanes used in stoichiometric amounts are also efficiently functionalized with a net preference for tertiary equatorial C-H bonds. The selectivity, in this case, can be rationalized by steric and hyperconjugative effects. This study, therefore, provides useful information to better predict the site of C-H amination of complex molecules.     

Heterotricyclodecane XXI 2-Aza-7-thia-twistane (bzw. 2-Thia-7-aza-twistane)

2-Aza-7-thia-twistanes (or 2-thia-7-aza-twistanes, respectively) The suitably substituted 9-thiabicylo[3.3.1]nonanes 7–9 and 17 were synthesized and subsequently cyclized to the 2-aza-7-thia-twistanes 21–23 and 19 , respectivelly, from which several other twistane derivatives ( 20 and 24–29 ) were prepared.     

Isotope effects in liquid water by infrared spectroscopy. IV. No free OH groups in liquid water

The presence of free OH (OH not H-bonded) in bulk water is a key element for the determination of its molecular structure. The OH covalent bond infrared (IR) absorption is highly sensitive to the molecular environment. For this reason, IR spectroscopy is used for the determination of free OH. A workable definition of this is obtained with methanol (MeOH) in hexane where minute quantities of free OH are present. These absorb at 3654?cm(-1) (a 27?cm(-1) redshift from the gas position) with a full width at half height of 35?cm(-1). The IR spectrum of water between room temperature and 95?°C does not display such a band near 3650?cm(-1). This indicates that we do not see, in the IR spectra, the "free" OH group. From this we conclude that it is not present in liquid water at least down to the 1000 ppm level which is the limit of detectivity of our spectrometer. Other spectroscopic considerations of methanol and water in acetonitrile solutions indicate that weak H-bonds are also not present in liquid water.     

Structural characterization of poly(amino)ester dendrimers and related impurities by electrospray tandem mass spectrometry

An acid‐terminated poly(amino)ester dendrimer was studied by electrospray ionization tandem mass spectrometry to establish its fragmentation pathways, with the aim of using them to investigate the structure of any defective molecules generated during the dendrimer synthesis. This poly(amino)ester dendrimer could be ionized in both polarities but the most structurally relevant dissociation pathways were found from the deprotonated molecule in negative ion mode. The dissociation pattern of this dendrimer is fully described and supported by accurate mass measurements. The main dissociation reactions of the negatively charged polyacidic dendrimer were shown to consist of (i) the release of carbon dioxide and ethene within a branch, which proceeds as many times as intact neutral branches are available; and (ii) the elimination of an entire dendrimer arm. Monitoring the occurrence of these reactions together with any deviation from these two main routes allowed six major dendritic impurities to be structurally characterized. Copyright © 2010 John Wiley & Sons, Ltd.     

MALDI-MS/MS with Traveling Wave Ion Mobility for the Structural Analysis of N-Linked Glycans

The preference for singly charged ion formation by MALDI makes it a better choice than electrospray ionization for profiling mixtures of N-glycans. For structural analysis, fragmentation of negative ions often yields more informative spectra than fragmentation of positive ones but such ions are more difficult to produce from neutral glycans under MALDI conditions. This work investigates conditions for the formation of both positive and negative ions by MALDI from N-linked glycans released from glycoproteins and their subsequent MS/MS and ion mobility behaviour. 2,4,6-Trihydroxyacetophenone (THAP) doped with ammonium nitrate was found to give optimal ion yields in negative ion mode. Ammonium chloride or phosphate also yielded prominent adducts but anionic carbohydrates such as sulfated N-glycans tended to ionize preferentially. Carbohydrates adducted with all three adducts (phosphate, chloride, and nitrate) produced good negative ion CID spectra but those adducted with iodide and sulfate did not yield fragment ions although they gave stronger signals. Fragmentation paralleled that seen following electrospray ionization providing superior spectra than could be obtained by PSD on MALDI-TOF instruments or with ion traps. In addition, ion mobility drift times of the adducted glycans and the ability of this technique to separate isomers also mirrored those obtained following ESI sample introduction. Ion mobility also allowed profiles to be obtained from samples whose MALDI spectra showed no evidence of such ions allowing the technique to be used in conditions where sample amounts were limiting. The method was applied to N-glycans released from the recombinant human immunodeficiency virus glycoprotein, gp120.     

Trivalent Cation Substitution Effect into Layered Double Hydroxides Co2Fey Al1−y(OH)6Cl·nH2O: Study of the Local Order: Ionic Conductivity and Magnetic Properties

A series of layered double hydroxide materials of composition Co₂Fe_yAl_1−y(OH)₆Cl·nH₂O (0≤y≤1) was prepared via chimie douce. The crystalline parameter related to the cation to cation distance obeys the expected variation, showing that the substitution is effective over the entire range. Local order around Co and Fe cations is studied by X-ray absorption spectroscopy. Moduli of the Fourier transform at the Fe K-edge are superimposable, in agreement with an ordered model, although present in small domains since no superlattice is depicted. The ionic resistivity of the samples is highly dependent on the water molecule content. The conductivity is found to be thermally assisted, and the variation of the slope in the Arrhenius diagram is explained by a Vogel-Tamman-Fulcher-type behavior. Magnetic susceptibility measurements support the proposed cation composition and indicate the onset of local magnetic order at low temperature (below 10 K). The inter-sheet distance influences the magnetic response at low temperature, showing the presence of weak interactions between lamellae.     

Uncatalyzed Hydroamination of Electrophilic Organometallic Alkynes: Fundamental,Theoretical, and Applied Aspects

Simple reactions of the most used functional groups allowing two molecular fragments to link under mild, sustainable conditions are among the crucial tools of molecular chemistry with multiple applications in materials science, nanomedicine, and organic synthesis as already exemplified by peptide synthesis and “click” chemistry. We are concerned with redox organometallic compounds that can potentially be used as biosensors and redox catalysts and report an uncatalyzed reaction between primary and secondary amines with organometallic electrophilic alkynes that is free of side products and fully “green”. A strategy is first proposed to synthesize alkynyl organometallic precursors upon addition of electrophilic aromatic ligands of cationic complexes followed by endo hydride abstraction. Electrophilic alkynylated cyclopentadienyl or arene ligands of Fe, Ru, and Co complexes subsequently react with amines to yield trans‐enamines that are conjugated with the organometallic group. The difference in reactivities of the various complexes is rationalized from the two‐step reaction mechanism that was elucidated through DFT calculations. Applications are illustrated by the facile reaction of ethynylcobalticenium hexafluorophosphate with aminated silica nanoparticles. Spectroscopic, nonlinear‐optical and electrochemical data, as well as DFT and TDDFT calculations, indicate a strong push–pull conjugation in these cobalticenium– and Fe– and Ru–arene–enamine complexes due to planarity or near‐planarity between the organometallic and trans‐enamine groups involving fulvalene iminium and cyclohexadienylidene iminium mesomeric forms.     

Synthesis and characterization of iPP‐sPP stereoblock produced by a binary metallocene system

Stereoblock polypropylenes comprising of iPP and sPP segments are synthesized by polymerization of the following binary system of metallocenes: the C_s‐symmetric [2,7‐t‐Bu₂(Flu)₂Ph₂C(Cp)ZrCl₂] and the C₂‐symmetric rac‐Me₂Si(2‐Me‐4‐Ph‐Ind)₂ZrCl₂. Blends of samples made either by each catalyst individually (solution blend) with materials obtained with the mixed catalyst system (reactor blend) are compared. The simultaneous presence of MAO and DEZ, enhancing fast and reversible transfer of the growing chains between the two active centers, leads to the formation of a stereoblock microstructure. In this case, low molecular weight polymers are obtained. The junction between the blocks is qualitatively observed in ¹³C NMR. When made in toluene, the stereoblock material consists of a majority of syndiotactic sequences, whereas the ratio is more equilibrated when the polymerization was conducted in the more polar chlorobenzene. This is confirmed by the results obtained with ¹³C NMR, CRYSTAF, HT HPLC, DSC, SSA, WAXD, and optical microscopy. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1422–1434     

Paper strengthening by polyaminoalkylalkoxysilane copolymer networks applied by spray or immersion: a model study

Two di-alkoxysilanes, with (AMDES, aminopropylmethyldiethoxysilane) or without (DMDES, dimethyldiethoxysilane) an amine function, and a tri-alkoxy aminosilane (APTES, aminopropyltriethoxysilane) as well as their mixtures were introduced in paper as fiber strengthening agents. The polymerization and copolymerization of these polysiloxanes in the paper were investigated. In all the cases where APTES was present, the formation of networks was established by measuring the soluble fraction amount extracted from the treated papers. A slight decrease of the opacity of the paper sheets when AMDES was part of the treatment was noted. The presence of APTES reduced this opacity loss. The study of the physicochemical properties of the treated paper (mechanical strength and alkalinity) demonstrated that, besides the required deacidification feature, the different treatments allowed an efficient strengthening of the cellulose fibers to various extents. Contact angle measurements indicated a decrease of the hydrophilic character of papers treated with the mixture APTES/AMDES and the occurrence of a hydrophobic character of the papers treated with APTES alone. These results were consistently obtained for both spray and immersion treatment processes.