Showing papers on "Substituent published in 2012"

Electronic Effects of Linker Substitution on Lewis Acid Catalysis with Metal–Organic Frameworks

Abstract: Functionalized linkers can greatly increase the activity of metal-organic framework (MOF) catalysts with coordinatively unsaturated sites. A clear linear free-energy relationship (LFER) was found between Hammett σ(m) values of the linker substituents X and the rate k(X) of a carbonyl-ene reaction. This is the first LFER ever observed for MOF catalysts. A 56-fold increase in rate was found when the substituent is a nitro group.

Carborane Photochemistry Triggered by Aryl Substitution: Carborane‐Based Dyads with Phenyl Carbazoles

Abstract: probably because the parent carboranes and their derivativeswithout p substituent show little or no absorption at>250 nm, nor any emission. Even for their p-substitutedderivatives,furthermore,nodetailedinvestigationshavebeenperformed, particularly into how the carborane cages affecttheir excited-state behavior.

Direct Amidation of Carboxylic Acids Catalyzed by ortho-Iodo Arylboronic Acids: Catalyst Optimization, Scope, and Preliminary Mechanistic Study Supporting a Peculiar Halogen Acceleration Effect

Abstract: The importance of amides as a component of biomolecules and synthetic products motivates the development of catalytic, direct amidation methods employing free carboxylic acids and amines that circumvent the need for stoichiometric activation or coupling reagents. ortho-Iodophenylboronic acid 4a has recently been shown to catalyze direct amidation reactions at room temperature in the presence of 4A molecular sieves as dehydrating agent. Herein, the arene core of ortho-iodoarylboronic acid catalysts has been optimized with regards to the electronic effects of ring substitution. Contrary to the expectation, it was found that electron-donating substituents are preferable, in particular, an alkoxy substituent positioned para to the iodide. The optimal new catalyst, 5-methoxy-2-iodophenylboronic acid (MIBA, 4f), was demonstrated to be kinetically more active than the parent des-methoxy catalyst 4a, providing higher yields of amide products in shorter reaction times under mild conditions at ambient temperature. ...

Glucose-Sensitivity of Boronic Acid Block Copolymers at Physiological pH

Abstract: Well-defined boronic acid block copolymers were demonstrated to exhibit glucose-responsive disassembly at physiological pH. A boronic acid-containing acrylamide monomer with an electron-withdrawing substituent on the pendant phenylboronic acid moiety was polymerized by reversible addition–fragmentation chain transfer (RAFT) polymerization to yield a polymer with a boronic acid pKa = 8.2. Below this value, a block copolymer of this monomer with poly(N,N-dimethylacrylamide) self-assembled into aggregates. Addition of base to yield a pH > pKa or addition of glucose at pH = 7.4 resulted in aggregate dissociation that may prove promising for controlled delivery applications under physiological relevant conditions.

Hydrogen bonding in 1-butyl- and 1-ethyl-3-methylimidazolium chloride ionic liquids.

Abstract: A detailed investigation of hydrogen bonding in the pure ionic liquids [C4C1im]Cl and [C2C1im]Cl has been carried out using primarily molecular dynamics techniques. Analyses of the individual atom–atom pair radial distribution functions, and in particular those for C···Cl–, have revealed that hydrogen bonding to the first methylene or methyl units of the substituent groups is important. Multiple geometric criteria for defining a hydrogen bond have been applied, and in particular the choice of the cutoff angle has been carefully examined. The interpretation of hydrogen bonding within these ionic liquids is highly angle dependent, and justification is provided for why it may be appropriate to employ a wider angle criteria than the 30° used for water or alcohol systems. The different types of hydrogen bond formed are characterized, and “top” conformations where the Cl anion resides above (or below) the imidazolium ring are investigated. The number of hydrogen bonds undertaken by each hydrogen atom (and the c...

Synthesis of Sequence-Specific Vinyl Copolymers by Regioselective ROMP of Multiply Substituted Cyclooctenes

Abstract: A variety of multisubstituted cyclooctenes were prepared and employed as monomers for ring-opening metathesis polymerization using the Grubbs second or third generation catalysts. The resulting polymers were characterized by NMR spectroscopy, size exclusion chromatography, and differential scanning calorimetry. Monomers possessing a substituent at the 3-position afforded highly regio- and stereoregular polyalkenamers, from which the corresponding sequence-specific vinyl quaterpolymers were obtained upon hydrogenation. Simultaneous control of tacticity was also demonstrated by employing monomers with defined stereochemistry.

Substituent effects on CL···N, S···N, and P···N noncovalent bonds.

Abstract: Cl, S, and P atoms have previously been shown as capable of engaging in a noncovalent bond with the N atom on another molecule. The effects of substituents B on the former atoms on the strength of this bond are examined, and it is found that the binding energy climbs in the order B = CH3 < NH2 < CF3 < OH < Cl < NO2 < F. However, there is some variability in this pattern, particularly for the NO2 group. The A···N bonds (A = Cl, S, P) can be quite strong, amounting to as much as 10 kcal/mol. The binding energy arises from approximately equal contributions from its induction and electrostatic components, although the former becomes more dominant for the stronger bonds. The induction energy is due in large measure to the transfer of charge from the N lone pair to a B–A σ* antibonding orbital of the electron-acceptor molecule containing Cl, S, or P. These A···N bonds typically represent the lowest-energy structure on each potential energy surface, stronger than H-bonds such as NH···F, CH···N, or SH···N.

Ring Structure and Aromatic Substituent Effects on the pK a of the Benzoxaborole Pharmacophore.

Abstract: In this work, we present an investigation into the physical properties of a unique class of aromatic boronic acids, the benzoxaboroles. Using spectrophotometric methods, the ionization constants of a family of substituted benzoxaboroles are determined. Heterocyclic ring modifications are examined to determine their effects on the ionization of the boronic acid moiety. It is also shown that the substituent effects about the aromatic ring follow a Hammett relationship with the compounds' measured pKa values. Finally, these substituent effects are also shown to extend to the sugar binding properties of these compounds under physiologically relevant conditions. Combined, these data will inform medicinal chemists wishing to tailor the ionization and/or ability of this class of compound to bind diol-containing biomolecules.

Spectral properties of substituted coumarins in solution and polymer matrices.

Abstract: The absorption and fluorescence spectra of substituted coumarins (2-oxo-2H-chromenes) were investigated in solvents and in polymer matrices. The substitutions involved were: (1) by groups with varying electron donating ability such as CH3, OCH3 and N(CH3)2, mainly, but not exclusively, in positions 7 and (2), by either CHO or 4-PhNHCONHN=CH- in position 3. While the spectra of non-substituted coumarin-3-carbaldehyde has absorptions at approximately 305 and 350 nm, substitution at position 7 leads to remarkable changes in the shape of the absorption spectrum and shifts the absorption to a longer wavelength. Similarly, the replacement of the formyl group with a semicarbazide group substantially influences the shape of the absorption spectrum, and coumarins which have only N(CH3)2 in position 7 experience small changes. These changes are associated with the increasing intramolecular charge transfer (ICT) character and increasing conjugation length of the chromophoric system, respectively, in the studied molecules. The fluorescence is almost negligible for derivatives which have H in this position. With increasing electron donating ability, and the possibility of a positive mesomeric (+M) effect of the substituent in position 7 of the coumarin moiety, the fluorescence increases, and this increase is most intense when N(CH3)2 substitutes in this position, for both 3-substituted derivatives. Spectral measurements of the studied coumarins in polymer matrices revealed that the absorption and fluorescence maxima lay within the maxima for solvents, and that coumarins yield more intense fluorescence in polymer matrices than when they are in solution. The quantum yield of derivatives which have a dimethylamino group in position 7 in polymer matrices approaches 1, and the fluorescence lifetime is within the range of 0.5–4 ns. The high quantum yield of 7-dimethylamino derivatives qualifies them as laser dyes which have kF higher than knr in the given medium. This is caused by stiffening of the coumarin structure in polar polymer matrices, such as PMMA and PVC, due to higher micro-viscosity than in solution and intermolecular dipole-dipole interaction between chromophore (dopant) and matrix.

Probing Structure–Nanoaggregation Relations of Polyaromatic Surfactants: A Molecular Dynamics Simulation and Dynamic Light Scattering Study

Abstract: Four synthetic perylene bisimide-based polyaromatic (PA) surfactants with a structural or functional group difference in their attached hydrophilic/hydrophobic substituent side chains were used to probe structure–nanoaggregation relations in organic media by molecular dynamics simulations and dynamic light scattering. The results from the simulated radial distribution functions and light scattering experiments indicate that variation in the structure of side chains and polarity of functional groups leads to significant variations in molecular association, dynamics of molecular nanoaggregation and structure of nanoaggregates. The aggregates of PA surfactant molecules grow to much larger sizes in heptane than in toluene. The aromatic solvent is shown to hinder molecular association by weakening π–π stacking, demonstrating the control of molecular aggregation by tuning solvent properties. In aliphatic solvent, the aggregates formed from PA surfactants of aliphatic alkyl groups and phenylalanine derivatives a...

Ligand Steric and Fluoroalkyl Substituent Effects on Enchainment Cooperativity and Stability in Bimetallic Nickel(II) Polymerization Catalysts

Abstract: The synthesis and characterization of two neutrally charged bimetallic NiII ethylene polymerization catalysts, {2,7-di-[2,6-(3,5-di-methylphenylimino)methyl]1,8-naphthalenediolato}-bis-NiII(methyl)(trimethylphosphine) [(CH3)FI2-Ni2] and {2,7-di-[2,6-(3,5-di-trifluoromethyl-phenylimino)methyl]-1,8-naphthalenediolato}-bis-NiII(methyl)(trimethyl-phosphine) [(CF3)FI2-Ni2)], are reported. The diffraction-derived molecular structure of (CF3)FI2-Ni2 reveals a Ni⋅⋅⋅Ni distance of 5.8024(5) A. In the presence of ethylene and Ni(COD)2 or B(C6F5)3 co-catalysts, these complexes along with their monometallic analogues [2-tert-butyl-6-((2,6-(3,5-dimethylphenyl)phenylimino)methyl)-phenolate]-NiII-methyl(trimethylphosphine) [(CH3)FI-Ni] and [2-tert-butyl-6-((2,6-(3,5-ditrifluoromethyl-phenyl)phenylimino)methyl)phenolato]-NiII-methyl-(trimethylphosphine) [(CF3)FI-Ni], produce polyethylenes ranging from highly branched Mw=1400 oligomers (91 methyl branches per 1000 C) to low branch density Mw=92 000 polyethylenes (7 methyl branches per 1000 C). In the bimetallic catalysts, Ni⋅⋅⋅Ni cooperative effects are evidenced by increased product polyethylene branching in ethylene homopolymerizations (∼3× for (CF3)FI2-Ni2 vs. monometallic (CF3)FI-Ni), as well as by enhanced norbornene co-monomer incorporation selectivity, with bimetallic (CH3)FI2-Ni2 and (CF3)FI2-Ni2 enchaining approximately three- and six-times more norbornene, respectively, than monometallic (CH3)FI-Ni and (CF3)FI-Ni. Additionally, (CH3)FI2-Ni2 and (CF3)FI2-Ni2 exhibit significantly enhanced thermal stability versus the less sterically encumbered dinickel catalyst {2,7-di-[(2,6-diisopropylphenyl)imino]-1,8-naphthalenediolato}-bis-NiII(methyl)(trimethylphosphine). The pathway for bimetallic catalyst thermal deactivation is shown to involve an unexpected polymerization-active intermediate, {2,7-di-[2,6-(3,5-di-trifluoromethyl-phenylimino)methyl]-1-hydroxy,8-naphthalenediolato-NiII(methyl)-(trimethylphosphine).

Influence of the difluoromethylene group (CF 2 ) on the conformation and properties of selected organic compounds

Abstract: The CF 2 group has found applications as a substituent in all classes of organic chemical products from medicinal products to materials, although it is less frequently encountered than the C-F and CF 3 substituents. In this review, the geometric influence of placing two fluorine atoms on carbon is considered and in particular, deviations from tetra- hedral geometry are noted. The incorporation of the CF 2 group into CF 2 phosphonates as phosphate mimics is reviewed and the geometric and steric influences of the CF 2 group are considered when the substituent is placed within aliphatic chains and aliphatic ring (cyclo - dodecane) systems.

Platinum-catalyzed hydrosilylations of internal alkynes: harnessing substituent effects to achieve high regioselectivity.

Abstract: The regioselectivity of the presented effective hydrosilylations of internal alkynes is predominantly governed by electronic effects.

Synthesis and characterization of new fluorescent styrene-containing carborane derivatives: the singular quenching role of a phenyl substituent.

Abstract: A set of neutral and anionic carborane derivatives in which the styrenyl fragment is introduced as a fluorophore group has been successfully synthesized and characterized. The reaction of the monolithium salts of 1-Ph-1,2-C2B10H11, 1-Me-1,2-C2B10H11 and 1,2-C2B10H12 with one equivalent of 4-vinylbenzyl chloride leads to the formation of compounds 1–3, whereas the reaction of the dilithium salt of 1,2-C2B10H12 with two equivalents of 4-vinylbenzyl chloride gives disubstituted compound 4. The closo clusters were degraded using the classical method, KOH in EtOH, to afford the corresponding nido species, which were isolated as tetramethylammonium salts. The crystal structure of the four closo compounds 1–4 were analyzed by X-ray diffraction. All compounds, except 1, display emission properties, with quantum yields dependent on the nature of the cluster (closo or nido) and the substituent on the second Ccluster atom. In general, closo compounds 2–4 exhibit high fluorescence emission, whereas the presence of a nido cluster produces a decrease of the emission intensity. The presence of a phenyl group bonded to the Ccluster results in an excellent electron-acceptor unit that produces a quenching of the fluorescence. DFT calculations have confirmed the charge-separation state in 1 to explain the quenching of the fluorescence and the key role of the carboranyl fragment in this luminescent process.

Enhanced Stability of Polymeric Micelles Based on Postfunctionalized Poly(ethylene glycol)-b-poly(γ-propargyl l-glutamate): The Substituent Effect

Abstract: One of the major obstacles that delay the clinical translation of polymeric micelle drug delivery systems is whether these self-assembled micelles can retain their integrity in blood following intravenous (IV) injection. The objective of this study was to evaluate the impact of core functionalization on the thermodynamic and kinetic stability of polymeric micelles. The combination of ring-opening polymerization of N-carboxyanhydride (NCA) with highly efficient “click” coupling has enabled easy and quick access to a family of poly(ethylene glycol)-block-poly(γ-R-glutamate)s with exactly the same block lengths, for which the substituent “R” is tuned. The structures of these copolymers were carefully characterized by 1H NMR, FT-IR, and GPC. When pyrene is used as the fluorescence probe, the critical micelle concentrations (CMCs) of these polymers were found to be in the range of 10–7–10–6 M, which indicates good thermodynamic stability for the self-assembled micelles. The incorporation of polar side groups i...

Amino-grafted graphene as a stable and metal-free solid basic catalyst

Abstract: Metal-free catalysts are essential for various chemical reactions in industrial chemistry. Amino-functionalized carbon is a cheap and environmentally friendly solid basic catalyst. Here, NEt3-graphene solid basic catalyst has been synthesized through a consecutive deprotonation/carbometalation and electrophilic substituent. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), elemental analyses, thermal analyses, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) studies confirmed the formation of the target structure. Catalytic hydrolysis of ethyl acetate reveals that the amino-grafted graphene has a high catalytic activity.

Oxidative Coupling of 1,7,8-Unsubstituted BODIPYs: Synthesis and Electrochemical and Spectroscopic Properties

Abstract: We report the synthesis of BODIPYs with unsubstituted 1,7,8-positions and their dimerization by oxidative coupling with phenyliodine(III)-bis(trifluoroacetate) (PIFA). This dimerization was achieved for BODIPYs substituted in the 3,5-positions with either methyl or thienyl groups. The position and the type of the linkage in the resulting dimers depended on the nature of the substituent. The 3,5-dimethyl-BODIPY dyes were linked either via direct 1,1′-pyrrole–pyrrole coupling or via a 1,3′-methylene bridge. The 3,5-dithienyl-BODIPY dyes provided, in excellent yields, unique compounds linked exclusively via the α-thienyl positions. All dyes were unreactive in the 8-position. Electrochemical and spectroscopic measurements on the monomers and dimers provided evidence of interactions between the two halves of the dimers. Thus, oxidation and reduction potentials were split by up to 210 mV, and modest excitonic coupling and an internal charge transfer were observed in some cases.

Palladium-catalyzed C-H bond direct alkynylation of 5-membered heteroarenes: a well-defined synthetic route to azole derivatives containing two different alkynyl groups.

Abstract: A widely applicable oxidative coupling of 5-membered heteroarenes and terminal alkynes that uses a combination of palladium and silver salts was developed. Under suitable conditions, imidazole and benzimidazole, which are sluggish under similar previously reported oxidative coupling conditions, as well as imidazo[1,5-a]pyridines, oxazole, benzoxazole, thiazole, and benzothiazole could be alkynylated. In addition, the bromine atom on the substrates was intact under the reaction conditions, and conventional Sonogashira coupling did not occur at all. With these reactivities in hand, a well-defined synthetic route to imidazo[1,5-a]pyridines and thiazole containing two different alkynyl groups was achieved in a simple manner. In addition, linear correlations were observed between the fluorescence wavelength and the Hammett substituent constants of aryl groups, not only on the C1- but also on the C3-alkynyl group of the obtained 1,3-bis(arylethynyl)imidazo [1,5-a]pyridines.

Structure–Activity Relationship of Palladium Phosphanesulfonates: Toward Highly Active Palladium‐Based Polymerization Catalysts

Abstract: Palladium phosphanesulfonate [R(2)P(C(6)H(4)-o-SO(3))PdMeL] catalysts permit the copolymerization of an exceptional large number of functional olefins with ethylene. However, these catalysts usually have reduced activity. We here have conducted a systematic study on the influence of the phosphane substituent, R, on activity and molecular weight. Phosphanes with strong σ-donating character are shown to lead to the most active catalysts. Thus, the catalyst based on phosphane bis-tert-butyl-phosphanyl-benzenesulfonic acid (R=tBu) exhibits unprecedented high activity, rapidly polymerizing ethylene at room temperature to yield a linear polymer of high molecular weight (M(w)=116,000 g mol(-1)). The influence of the R group on the catalyst ability to incorporate methyl acrylate is also investigated.

Copper(II) Triflate Catalyzed Amination and Aziridination of 2-Alkyl Substituted 1,3-Dicarbonyl Compounds

Abstract: A method to prepare α-acyl-β-amino acid and 2,2-diacyl aziridine derivatives efficiently from Cu(OTf)(2) + 1,10-phenanthroline (1,10-phen)-catalyzed amination and aziridination of 2-alkyl substituted 1,3-dicarbonyl compounds with PhI═NTs is described. By taking advantage of the orthogonal modes of reactivity of the substrate through slight modification of the reaction conditions, a divergence in product selectivity was observed. In the presence of 1.2 equiv of the iminoiodane, amination of the allylic C-H bond of the enolic form of the substrate, formed in situ through coordination to the Lewis acidic metal catalyst, was found to selectively occur and give the β-aminated adduct. On the other hand, increasing the amount of the nitrogen source from 1.2 to 2-3 equiv was discovered to result in preferential formal aziridination of the C-C bond of the 2-alkyl substituent of the starting material and formation of the aziridine product.