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Showing papers on "Substituent published in 2018"


Journal ArticleDOI
TL;DR: Getting the green light!
Abstract: Getting the green light! Substituted arylazopyrazoles (AAPs) have been investigated as supramolecular photoswitches in aqueous solution. Selective photostationary states (PSSs) and improved binding affinities to β-cyclodextrin have been determined. The experimental findings are supported by results from DFT calculations.

78 citations


Journal ArticleDOI
04 Oct 2018-Nature
TL;DR: This approach introduces an aryl substituent to the α-carbon of amino acids without a transition-metal catalyst, and uses the inherent chirality of the amino acid itself as the source of asymmetry.
Abstract: Quaternary amino acids, in which the α-carbon that bears the amino and carboxyl groups also carries two carbon substituents, have an important role as modifiers of peptide conformation and bioactivity and as precursors of medicinally important compounds1,2. In contrast to enantioselective alkylation at this α-carbon, for which there are several methods3-8, general enantioselective introduction of an aryl substituent at the α-carbon is synthetically challenging9. Nonetheless, the resultant α-aryl amino acids and their derivatives are valuable precursors to bioactive molecules10,11. Here we describe the synthesis of quaternary α-aryl amino acids from enantiopure amino acid precursors by α-arylation without loss of stereochemical integrity. Our approach relies on the temporary formation of a second stereogenic centre in an N'-arylurea adduct12 of an imidazolidinone derivative6 of the precursor amino acid, and uses readily available enantiopure amino acids both as a precursor and as a source of asymmetry. It avoids the use of valuable transition metals, and enables arylation with electron-rich, electron-poor and heterocyclic substituents. Either enantiomer of the product can be formed from a single amino acid precursor. The method is practical and scalable, and provides the opportunity to produce α-arylated quaternary amino acids in multi-gram quantities.

77 citations


Journal ArticleDOI
TL;DR: The proposed palladium oxidative addition complexes demonstrate the potential for palladium-mediated methods to serve as a platform for the development of future cross-linking techniques for peptides and proteins with natural amino acid residues.
Abstract: A new method for cysteine–lysine cross-linking in peptides and proteins using palladium oxidative addition complexes is presented. First, a biarylphosphine-supported palladium reagent is used to transfer an aryl group bearing an O-phenyl carbamate substituent to a cysteine residue. Next, this carbamate undergoes chemoselective acyl substitution by a proximal lysine to form a cross-link. The linkage so formed is stable toward acid, base, oxygen, and external thiol nucleophiles. This method was applied to cross-link cysteine with nearby lysines in sortase A*. Furthermore, we used this method for the intermolecular cross-linking between a peptide and a protein based on the p53-MDM2 interaction. These studies demonstrate the potential for palladium-mediated methods to serve as a platform for the development of future cross-linking techniques for peptides and proteins with natural amino acid residues.

75 citations


Journal ArticleDOI
TL;DR: An asymmetric aza-Wacker-type cyclization of N-Ts hydrazine-tethered tetrasubstituted olefins, affording optically active pyrazolines bearing chiral tetrasubsituted carbon stereocenters, is developed, giving the desired chiral products with high enantioselectivities.
Abstract: We have developed an asymmetric aza-Wacker-type cyclization of N-Ts hydrazine-tethered tetrasubstituted olefins, affording optically active pyrazolines bearing chiral tetrasubstituted carbon stereocenters. This reaction is tolerant to a broad range of substrates under mild reaction conditions, giving the desired chiral products with high enantioselectivities. Generation of two vicinal stereocenters on the C═C double bonds was also achieved with high selectivities, a process which has been rarely studied for Wacker-type reactions. A mechanistic study revealed that this aza-Wacker-type cyclization undergoes a syn-aminopalladation process. It was also found that for substrates bearing two linear alkyl substituents on the outer carbon atom of the olefin, both of which are larger than a methyl group, the alkyl substituent that is cis to the intranucleophilic group participates more readily in β-hydride elimination. When one of the two alkyl substituents on the outer carbon atom of the olefin is a methyl group,...

73 citations


Journal ArticleDOI
TL;DR: In this article, a series of highly sterically hindered acenaphthene-based α-diimine nickel complexes with the remote R group in 4-position of diarylmethyl moiety have been synthesized and characterized.

73 citations


Journal ArticleDOI
TL;DR: In this article, the authors deduced that β-Diketiminato (BDI) calcium alkyl derivatives undergo hydrogenolysis with H2 to regenerate [(BDI)-CaH]2, allowing the catalytic hydrogenation of a wide range of 1-alkenes and norbornene under very mild conditions (2 bar H2, 298 K).
Abstract: β-Diketiminato (BDI) calcium alkyl derivatives undergo hydrogenolysis with H2 to regenerate [(BDI)CaH]2 , allowing the catalytic hydrogenation of a wide range of 1-alkenes and norbornene under very mild conditions (2 bar H2 , 298 K). The reactions are deduced to take place with the retention of the dimeric structures of the calcium hydrido-alkyl and alkyl intermediates via a well-defined sequence of Ca-H/C=C insertion and Ca-C hydrogenation events. This latter deduction is strongly supported by DFT calculations (B3PW91) performed on the 1-hexene/H2 system, which also indicates that the hydrogenation transition states display features which discriminate them from a classical σ-bond metathesis mechanism. In particular, NBO analysis identifies a strong second order interaction between the filled α-methylene sp3 orbital of the n-hexyl chain and the σ* orbital of the H2 molecule, signifying that the H-H bond is broken by what is effectively the nucleophilic displacement of hydride by the organic substituent.

72 citations


Journal ArticleDOI
TL;DR: The methodology paves the way for further application of linear free energy relationships and physical organic methodologies to mechanochemical reactions, and the characterization of new force probes should enable additional, quantitative studies of force-coupled molecular behavior in polymeric materials.
Abstract: We report the effect of substituents on the force-induced reactivity of a spiropyran mechanophore. Using single molecule force spectroscopy, force-rate behavior was determined for a series of spiropyran derivatives substituted with H, Br, or NO2 para to the breaking spirocyclic C-O bond. The force required to achieve the rate constants of ∼10 s-1 necessary to observe transitions in the force spectroscopy experiments depends on the substituent, with the more electron withdrawing substituent requiring less force. Rate constants at 375 pN were determined for all three derivatives, and the force-coupled rate dependence on substituent identity is well explained by a Hammett linear free energy relationship with a value of ρ = 2.9, consistent with a highly polar transition state with heterolytic, dissociative character. The methodology paves the way for further application of linear free energy relationships and physical organic methodologies to mechanochemical reactions, and the characterization of new force probes should enable additional, quantitative studies of force-coupled molecular behavior in polymeric materials.

71 citations


Journal ArticleDOI
TL;DR: It is demonstrated for the first time that an XB can be engineered to stabilize and increase the activity of an enzyme, with the increased stabilizing potential of the HeX-B further extending the application of halogenated amino acids in the design of more stable protein therapeutics.
Abstract: The construction of more stable proteins is important in biomolecular engineering, particularly in the design of biologics-based therapeutics. We show here that replacing the tyrosine at position 18 (Y18) of T4 lysozyme with the unnatural amino acid m-chlorotyrosine (mClY) increases both the thermal stability (increasing the melting temperature by ∼1 °C and the melting enthalpy by 3 kcal/mol) and the enzymatic activity at elevated temperatures (15% higher than that of the parent enzyme at 40 °C) of this classic enzyme. The chlorine of mClY forms a halogen bond (XB) to the carbonyl oxygen of the peptide bond at glycine 28 (G28) in a tight loop near the active site. In this case, the XB potential of the typically weak XB donor Cl is shown from quantum chemical calculations to be significantly enhanced by polarization via an intramolecular hydrogen bond (HB) from the adjacent hydroxyl substituent of the tyrosyl side chain, resulting in a distinctive synergistic HB-enhanced XB (or HeX-B for short) interaction...

63 citations


Journal ArticleDOI
TL;DR: The results indicate that the combination of good planarity of the extended D-π-A framework with multiple alkoxy/alkyl chains may compose an effective optimizing strategy for designing and synthesizing excellent cosensitizers for porphyrins.
Abstract: Relatively high efficiencies have been achieved for porphyrin-based dye-sensitized solar cells. For the purpose of designing efficient cosensitizers, we herein report systematically optimized dyes XC1–XC5 employing a triphenylamine donor, a benzothiadiazole moiety as the auxiliary acceptor, and a benzoic acid acceptor. One hexyl and four hexyloxy groups were introduced, and an ethynylene moiety was introduced between the donor and the auxiliary acceptor to afford XC1. To further extend the absorption wavelength, a second ethynylene moiety was introduced between the acceptor and the auxiliary acceptor to afford XC2. XC3 and XC4 were designed by introducing one and two methyl substituents, respectively, into the meta-positions of the anchoring carboxyl group. XC5 was further synthesized by inserting a cyano substituent into one of the ortho-positions of the carboxyl group with the purpose to strengthen the intramolecular charge-transfer effect and thus broaden the absorption wavelength. As expected, compare...

63 citations


Journal ArticleDOI
TL;DR: In this paper, the solvatochromic behavior of seven new synthetic pyrazoline derivatives, (4-[3-(4-hydroxyphenyl)-5-aryl-4,5-dihydro-pyrazol-1-yl]benzenesulfonamides, compound 1-7), were determined in different solvents.
Abstract: Pyrazoline derivatives are among solvatochromic dyes and they are the most widely used in many fields such as sensors, labeling agent and optoelectronic devices. Therefore, synthesis of new pyrazoline derivatives and determination of their optical behavior in different solvents are a very important research area. In this study, the solvatochromic behaviors of seven new synthetic pyrazoline derivatives, (4-[3-(4-Hydroxyphenyl)-5-aryl-4,5-dihydro-pyrazol-1-yl]benzenesulfonamides, compound 1-7), were determined in different solvents. As a result of the measurements, a large red shifts in the fluorescence spectra of the compounds studied were observed for each compound, when the polarity of the solvent increased. Kamlet-Taft and Catalan parameters were used to describe the solvent-soluble interactions. The obtained results showed that the solvatochromic behavior of the compounds is dependent on their solvent polarity as well as the effect of the hydrogen-bonding properties of the solvents. It was also found that the increases in the polarity of the solvent was facilitated the non-radiative transition. Furthermore, the changes in the dipole moments of compounds 1-7 in different solvents at room temperature were calculated by using the Lippert–Mataga equation. It was very impressive that calculated fluorescence quantum yield values of the compounds studied here by using different solvents were remarkably high than the ones reported in the literatures for pyrazolines. In addition, the effect of substituent on the photo-physical properties of the compounds was also investigated. It was observed that the fluorescence intensity of the substituted compounds 2-6 increased comparing to the compound 1, which is a non-substituted derivative. These changes were not dependent on electronic nature of the substituent on phenyl ring (i.e. electron donating and electron withdrawing substituent). According to data obtained it can be stated that these novel solvatochromic dyes studied here can be find application in pharmaceutical industry, as labeling agent, sensor applications as biosensors or analytical sensors and/or optoelectronic devices.

57 citations


Journal ArticleDOI
TL;DR: In this paper, four N-[2-(dimethylamino)ethyl]-1,8-naphthalimide derivatives (ANNs) with different substituents (bromo group, primary amine, secondary amine and tertiary amine) were synthesized and incorporated into multi-component photoinitiating systems for free radical or cationic photopolymerization under the irradiation of various LEDs.

Journal ArticleDOI
TL;DR: Mechanistic studies suggest that a 2 : 2 mixed anhydride is expected to be the only active species, and the ortho-substituent of boronic acid plays a key role in preventing the coordination of amines to the boron atom of the Active species, thus accelerating the amidation.

Journal ArticleDOI
TL;DR: In this paper, four new Schiff base ester liquid crystal compounds, 4-((4ʹ-substituted phenylimino) methyl)phenyl-4″-alkoxybenzoates, Ina-d, were prepared and investigated for their mesophase formation and stability.
Abstract: Four new groups of Schiff base ester liquid crystal compounds, 4-((4ʹ-substituted phenylimino) methyl)phenyl–4″-alkoxybenzoates, Ina–d, were prepared and investigated for their mesophase formation and stability. Each group constitutes four homologous series that differ from each other by the polar substituent X (CH3O, CH3, H, and Cl). Within each homologous series, the number (n) of carbons in the alkoxy chain varies between 6, 8, 10, and 12. Molecular structures of the prepared compounds were confirmed via FT-IR, 1H NMR, mass spectroscopy, and elemental analysis. The mesomorphic properties were investigated by differential scanning calorimetry and polarised light microscopy. Comparative studies between the present series and previously investigated 4-(4-substituted phenylazo)phenyl 4″-alkoxybenzoates revealed that the phenylimino mesogenic core increases the mesophase stability rather than the azo analogues.

Journal ArticleDOI
TL;DR: It was found that a hydroxy group at the 2-position of the benzene ring exerted a positive effect on the affinities, while a 4-hydroxy substituent had a negative influence.

Journal ArticleDOI
TL;DR: In this article, the adsorption and corrosion inhibition properties of three triazepines carboxylate substituted compounds, namely, ethyl 4,9-bis(4-chlorophenyl)-8-cyano-3-hydroxy-2,7-dioxo, 1,2,5,7tetrahydropyrido[1,2-b]triazepine-10-carboxylates (2Cl-Et), methyl 4, 9-bis (4- chlorophenyl), 8-cyclo-3
Abstract: The adsorption and corrosion inhibition properties of three triazepines carboxylate substituted compounds, namely, ethyl 4,9-bis(4-chlorophenyl)-8-cyano-3-hydroxy-2,7-dioxo-1,2,5,7-tetrahydropyrido[1,2-b]triazepine-10-carboxylate (2Cl–Et), methyl 4,9-bis(4-chlorophenyl)-8-cyano-3-hydroxy-2,7-dioxo-1,2,5,7-tetrahydropyrido[1,2-b]triazepine-10-carboxylate (2Cl–Me), and ethyl 9-(4-chlorophenyl)-8-cyano-3-hydroxy-2,7-dioxo-4-(p-tolyl)-1,2,5,7-tetrahydropyrido[1,2-b]triazepine-10-carboxylate (Cl–Me–Et) on mild steel corrosion in 1.0 M of hydrochloride acid were investigated by using weight loss, electrochemical, and theoretical investigations. It is found that these compounds act as excellent inhibitors and their inhibition efficiencies follow the order: 2Cl–Me > 2Cl–Et > Cl–Me–Et, and they depend on the type of the substituent. In addition, the possible relations between the experimental and theoretical parameters were discussed. It is found that the theoretical parameters confirmed those obtained by experimental studies. Surface roughness morphology for mild steel before and after hydrochloric acid attack is investigated and discussed using 3D-profilometry images. The scanning electron microscope (SEM) images of the corrosion product confirmed the protection offered by triazepines carboxylate derivatives on the surface of the metal immersed in both media.

Journal ArticleDOI
TL;DR: An experimentally verified theoretical approach based on multi-conformer transition state theory is used to calculate rate constants for a systematic set of H-shifts and shows that reactions leading to secondary carbon radicals (alkyl substituent) are 100 times faster than those lead to primary carbon radicals, and those leading to tertiary carbon radicals a factor of 30 faster thanThose leading tosecondary carbon radicals.
Abstract: Peroxy radical hydrogen shift (H-shift) reactions are key to the formation of highly oxidized organic molecules and particle growth in the atmosphere. In an H-shift reaction, a hydrogen atom is transferred to the peroxy radical from within the same molecule to form a hydroperoxy alkyl radical, which can undergo O2 uptake and further H-shift reactions. Here we use an experimentally verified theoretical approach based on multi-conformer transition state theory to calculate rate constants for a systematic set of H-shifts. Our results show that substitution at the carbon, from which the hydrogen is abstracted, with OH, OOH, and OCH3 substituents lead to increases in the rate constant by factors of 50 or more. Reactions with C═O and C═C substituents lead to resonance stabilized carbon radicals and have rate constants that increase by more than a factor of 400. In addition, our results show that reactions leading to secondary carbon radicals (alkyl substituent) are 100 times faster than those leading to primary...

Journal ArticleDOI
TL;DR: For the first time, the copolymerization of E and polar styrenes (SR) is reported by using an oxophilic scandium catalyst that exhibits higher turnover frequencies than both E and SR homopolymerizations when R is an electron-withdrawing group.
Abstract: Copolymerization of ethylene (E) and polar vinyl monomers remains a problem because E propagation is hindered. Herein, for the first time, we report the copolymerization of E and polar styrenes (SR ) by using an oxophilic scandium catalyst that exhibits higher turnover frequencies than both E and SR homopolymerizations when R is an electron-withdrawing group. This positive comonomer effect was elucidated through computing reaction profiles of E/SF copolymerization at the DFT (B3PW91) level of theory. It reveals that the secondary interaction between Sc3+ and phenyl of the last and penultimate inserted SF units leads to a decrease of the E insertion barrier, because the electron-withdrawing substituent enhances the electrophilicity of Sc3+ by an inductive effect mediated by the secondary interaction. After three consecutive insertions of the E units, the secondary interaction is lost and the SF insertion is kinetically preferred over the E insertion. This process is in line with the NMR spectrum analyses which show that the resultant copolymers mainly contain SR (E)x SR sequences where x≤3.

Journal ArticleDOI
TL;DR: Based on the design of electronic-spin structure of polycyclic carbon-centered π-radicals, the authors realized a peculiarly stable neutral π radical without bulky substituent groups, 4,8,12-trioxotriangulene (TOT).
Abstract: To stabilize organic neutral radicals, which are usually very unstable chemical species in air atmosphere, “steric protection” is the most general and indispensable method. Based on the design of electronic-spin structure of polycyclic carbon-centered π-radicals, we have for the first time realized a peculiarly stable neutral π-radical without bulky substituent groups, 4,8,12-trioxotriangulene (TOT), whose decomposition point is higher than 240 °C in the solid state under air. This remarkably high air-stability as a neutral radical is achieved by spin-delocalization originating from the symmetric and expanded π-electronic structure of TOT. The oxo-functionalities also highly contribute to the high stability through electronic-spin modulation, where the largest electronic spin density located at the central carbon atom further decreases the spin densities of the peripheral carbon atoms. In the solution state, TOT is in the equilibrium between the monomer and highly symmetric π-dimer, as stabilized by the f...

Journal ArticleDOI
TL;DR: Nickel complexes bearing bidentate alkylphophine-phenolate ligands were synthesized and applied to the polymerization of propylene and the copolymerization of polypropylenes.
Abstract: Nickel complexes bearing bidentate alkylphophine–phenolate ligands were synthesized and applied to the polymerization of propylene and the copolymerization of propylene and polar monomers. Therein, the use of bulky alkyl substituents on the phosphorus atom was crucial for the formation of highly regioregular polypropylenes. Theoretical calculations suggested that the 1,2-insertion of propylene is favored over the 2,1-insertion in these nickel-catalyzed (co)polymerization reactions. The substituent at the ortho position relative to the oxido group greatly influences the polymerization activity, the molecular weight, and stereoregularity of the polypropylenes. This method delivers moderately isotactic ([mm] ≤ 68%) crystalline polypropylenes. The present system represents the first example for a nickel-catalyzed regiocontrolled copolymerization of propylene and polar monomers such as but-3-en-1-ol, but-3-en-1-yl acetate, and tert-butyl allylcarbamate.

Journal ArticleDOI
TL;DR: Surprisingly, it is found that polymers with moderate steric hindrance exhibit the highest Tg values, and this interesting structure-property relationship is attributed to a complex interplay between methyl-induced steric strain and the concerted rotations along the polymer backbone.
Abstract: Converting biomass-based feedstocks into polymers not only reduces our reliance on fossil fuels, but also furnishes multiple opportunities to design biorenewable polymers with targeted properties and functionalities. Here we report a series of high glass transition temperature (Tg up to 184 °C) polyesters derived from sugar-based furan derivatives as well as a joint experimental and theoretical study of substituent effects on their thermal properties. Surprisingly, we find that polymers with moderate steric hindrance exhibit the highest Tg values. Through a detailed Ramachandran-type analysis of the rotational flexibility of the polymer backbone, we find that additional steric hindrance does not necessarily increase chain stiffness in these polyesters. We attribute this interesting structure-property relationship to a complex interplay between methyl-induced steric strain and the concerted rotations along the polymer backbone. We believe that our findings provide key insight into the relationship between structure and thermal properties across a range of synthetic polymers.

Journal ArticleDOI
TL;DR: A complex interplay of steric, electronic effects and hydrogen bonding as factors in dictating the stability of Z-isomers in arylazo-1 H-3,5-dimethylpyrazoles has been demonstrated.
Abstract: The electronic and steric effects of aryl substituents and the influence of hydrogen bonding in Z-isomer stability of phenylazopyrazole derivatives have been investigated. In this regard, 38 substituted phenylazopyrazole derivatives and 6 N-methyl phenylazopyrazoles (with meta substitutions) have been synthesized. Their photoswitching behavior, photostationary states (PSS), and kinetics of thermal reverse isomerization were evaluated experimentally using UV–vis and NMR spectroscopic techniques. Furthermore, density functional theory (DFT) computations have been performed for more detailed insights. Despite the presence of substantial substituent effects inferred through Taft and Hammett relationships, the concentration dependency in controlling the isomerization rates has also been observed. Kinetics studies at different concentrations, solvent effects, and computations have confirmed the decisive role of hydrogen bonding and solvent-assisted tautomerism in this regard. Through this study, a complex inter...

Journal ArticleDOI
TL;DR: Analysis of substituent effects on the physicochemical properties, toxicity, chemical reactivity and its relation with the bacteriostatic activity of selected sulfonamides by means of DFT-M06-2X calculations in aqueous solution using quantum chemical and docking descriptors finds a correlation between the theoretical acidity and the pKa experimental values.
Abstract: Relationships among physicochemical properties, the chemical structure and antibacterial activity of sulfonamides have not been completely explicated yet. Nevertheless, from a therapeutics and prodrugs design point of view, a substituent group can modify the electronic structure, the physicochemical features and chemical reactivity which are critical for the biological activity. In this work, we analyze the substituent effects on the physicochemical properties, toxicity, chemical reactivity and its relation with the bacteriostatic activity of selected sulfonamides by means of DFT-M06-2X calculations in aqueous solution, using quantum chemical and docking descriptors. A correlation between the theoretical acidity and the pKa experimental values has been found. The more active sulfonamides have a larger acidity. The acidity increases with electron-withdrawing substituents. The main reactivity takes place on N4 atoms linked to aromatic ring, and in the SO2NH moiety, which are influenced by substituents. Docking descriptors showed binding affinities between sulfonamides and target receptor, the dihydropteroate synthase (DHPS).

Journal ArticleDOI
TL;DR: The first copper-mediated diastereoselective C–H thiolation of ferrocenes has been developed and tolerated various functional groups on arylthiols, implying its potential application in N,S-bidentate planar chiral ligands.

Journal ArticleDOI
TL;DR: An efficient 1H NMR spectroscopic approach for determining the relative configurations of lignans with a 7,9':7',9-diepoxy moiety has been established and the rule should be applied carefully when the C-2 or C-6 substituent of the aromatic rings may alter the dominant conformers of the furofuran moiety.
Abstract: An efficient 1H NMR spectroscopic approach for determining the relative configurations of lignans with a 7,9′:7′,9-diepoxy moiety has been established. Using the chemical shift differences of H2-9 and H2-9′ (ΔδH-9 and ΔδH-9′), the configurations of 8-H and 8-OH furofuran lignans can be rapidly and conveniently determined. The rule is applicable for data acquired in DMSO-d6, methanol-d4, or CDCl3. Notably, the rule should be applied carefully when the C-2 or C-6 substituent of the aromatic rings may alter the dominant conformers of the furofuran moiety.

Journal ArticleDOI
TL;DR: Alcohol solvents are significantly more polar than expected based on the measured H-bonding properties of monomeric alcohols in dilute solution.
Abstract: Alcohol solvents are significantly more polar than expected based on the measured H-bonding properties of monomeric alcohols in dilute solution. Self-association of alcohols leads to formation of cyclic aggregates and linear polymeric chains that have a different polarity from the alcohol monomer. Cyclic aggregates are less polar than the monomer, and the chain ends of linear polymers are more polar. The solvation properties of alcohols therefore depend on the interplay of these self-association equilibria and the equilibria involving interactions with solutes. Twenty-one different molecular recognition probes of varying polarity were used to probe the solvation properties of alkane–alcohol mixtures across a wide range of different solvent compositions. The results allow dissection of the complex equilibria present in these systems. Formation of a H-bond between two alcohol molecules leads to polarisation of the hydroxyl groups, resulting in an increase in binding affinity for subsequent interactions with the unbound donor and acceptor sites. The H-bond donor parameter (α) for these sites increases from 2.7 to 3.5, and the H-bond acceptor parameter (β) increases from 5.3 to 6.9. Polarisation is a short range effect limited to the first H-bond in a chain, and formation of subsequent H-bonds in longer chains does not further increase the polarity of chain ends. H-bond donor sites involved in a H-bond are unavailable for further interactions, because the formation of a bifurcated three-centre H-bond is three orders of magnitude less favourable than formation of a conventional two-centre H-bond. These findings are reproduced by quantum chemical calculations of the molecular electrostatic potential surfaces of alcohol aggregates. Thus, the overall solvation properties of alcohols depend on the speciation of different aggregates, the polarities of these species and the polarities of the solutes. At low alcohol concentrations, polar solutes are solvated by alcohol monomers, and at higher alcohol concentrations, solutes are solvated by the more polar chain ends of linear polymers. The less polar cyclic aggregates are less important for interactions with solutes. Similar behavior was found for ten different alcohol solvents. Tertiary alcohols are marginally less polar solvents than primary alcohols, due to steric interactions that destabilises the formation of polymeric aggregates leading to lower concentrations of polar chain ends. One alcohol with an electron-withdrawing substituent was studied, and this solvent showed slightly different behavior, because the H-bond donor and acceptor properties are different.

Journal ArticleDOI
TL;DR: Group 15 biradicals with two different radical sites display regioselectivity upon addition of small molecules and can be utilized to activate small molecules bearing single, double and triple bonds as well as to trap labile in situ generated fragments.
Abstract: Readily accessible group 15 biradicals of the type [E(μ-NR)]2 (E = P–Bi) form planar 6π-electronic-4-membered heterocycles featuring open-shell singlet biradical character. They can be utilized to activate small molecules bearing single, double and triple bonds as well as to trap labile in situ generated fragments. In the reaction with CO and R–NC (R = small alkyl or aryl substituent) pnictogen analogues of cyclopentane-1,3-diyl are formed which represent robust molecular switches. Group 15 biradicals with two different radical sites display regioselectivity upon addition of small molecules.

Journal ArticleDOI
TL;DR: In this paper, two novel imidazoliums with N3-adamantyl substituent along with four other cationic ions with N 3-methyl and butyl-substituents were synthesized and systematically assessed the impact of N 3 and C 2 substitutions on their chemical stabilities.

Journal ArticleDOI
TL;DR: In this paper, a synthetic strategy to convert biomass-derived unsubstituted furan to aromatics at high selectivity, especially to ethylbenzene via alkylation/Diels-Alder cycloaddition using ethanol, while greatly reducing the formation of the main side product, benzofuran, over zeolite catalysts.
Abstract: Herein, we report a synthetic strategy to convert biomass-derived unsubstituted furan to aromatics at high selectivity, especially to ethylbenzene via alkylation/Diels–Alder cycloaddition using ethanol, while greatly reducing the formation of the main side product, benzofuran, over zeolite catalysts. Using synchrotron X-ray powder diffraction and first-principles calculations, it is shown that the above methodology favors the formation of aromatic products due to ready alkylation of furan by the first ethanol molecule, followed by Diels–Alder cycloaddition with ethylene derived from the second ethanol molecule on a Bronsted acid site in a one-pot synthesis. This gives a double-promoting effect: an alkyl substituent(s) on furan creates steric hindrance to inhibit self-coupling to benzofuran while an alkylated furan (diene) undergoes a Diels–Alder reaction more favorably due to higher HOMO energy.

Journal ArticleDOI
TL;DR: A highly enantioselective transfer hydrogenation of non-ortho-substituted aryl N-heteroaryl ketones, using readily available chiral diamine-derived iridium complex (S,S)-1f as a catalyst and sodium formate as a hydrogen source in a mixture of H2O/i-PrOH (v/v = 1:1) under ambient conditions, is described.

Journal ArticleDOI
TL;DR: In this article, single-crystal analysis results revealed the influence of steric constraints around the catalytically active center and established a relationship between the product selectivity and the P-Cr-P bite angle.
Abstract: The N-substituents on the backbone of Ph2PN(R)Si(CH3)2CH2PPh2- and [Ph2PCH2Si(CH3)2]2N(R)-type silylated-diphosphinoamine (Si-PNP) systems have been observed to have a significant impact on their catalytic performance in ethylene oligomerization reactions. Cr precatalyst 3, bearing an isopropyl (iPr) substituent, affords the most efficient catalytic system and exhibited the highest selectivity (83%) toward 1-octene (1-C8) and showed a catalytic activity of more than 76,700 g(product)·g(Cr)−1·h–1 under experimental conditions. Single-crystal analysis results revealed the influence of steric constraints around the catalytically active center and established a relationship between the product selectivity and the P–Cr–P bite angle. Furthermore, DFT calculations indicate that the catalytic system based on precatalyst 3 faces a low energy barrier in the formation of 1-C8 and therefore shows high selectivity toward the 1-C8 fraction. Modification in the backbone length may alter the binding mode of the ligands f...