Aryl Units Research Articles

ChemInform Abstract: Anionic Chiral Tridentate N‐Donor Pincer Ligands in Asymmetric Catalysis

AbstractReview: 95 refs.

A Practical Aryl Unit for Azlactone Dynamic Kinetic Resolution: Orthogonally Protected Products and A Ligation‐Inspired Coupling Process

AbstractThe first strategy for bringing about enantioselective azlactone dynamic kinetic resolution to generate orthogonally protected amino acids has been developed. In the presence of a C2‐symmetric squaramide‐based catalyst, benzyl alcohol reacts with novel yet readily prepared tetrachloroisopropoxycarbonyl‐substituted azlactones to generate trapped phthalimide products of significant synthetic interest with excellent enantiocontrol. These materials are masked amino acids which are demonstrably orthogonally protected: cleavage of the phthalimide can be achieved in the presence of the ester and vice versa. This process could be utilized to bring about a highly stereoselective ligation‐type coupling of protected serines (at stoichiometric loadings) with racemic azlactones derived from both natural and abiotic amino acids. After deprotection, a subsequent base‐mediated O→N acyl transfer occurs to form a dipeptide.

A practical aryl unit for azlactone dynamic kinetic resolution: orthogonally protected products and a ligation-inspired coupling process.

The first strategy for bringing about enantioselective azlactone dynamic kinetic resolution to generate orthogonally protected amino acids has been developed. In the presence of a C2-symmetric squaramide-based catalyst, benzyl alcohol reacts with novel yet readily prepared tetrachloroisopropoxycarbonyl-substituted azlactones to generate trapped phthalimide products of significant synthetic interest with excellent enantiocontrol. These materials are masked amino acids which are demonstrably orthogonally protected: cleavage of the phthalimide can be achieved in the presence of the ester and vice versa. This process could be utilized to bring about a highly stereoselective ligation-type coupling of protected serines (at stoichiometric loadings) with racemic azlactones derived from both natural and abiotic amino acids. After deprotection, a subsequent base-mediated O→N acyl transfer occurs to form a dipeptide.

Synthesis and biological evaluation of novel biaryl type α-noscapine congeners

Natural α-noscapine, a known antitussive drug, is also now known to possess weak anticancer efficacy with relatively safe toxicity profile. In this study, we report synthesis and evaluation of novel biaryl type α-noscapine congeners designed by adding aryl unit to the tetrahydroisoquinoline part of natural α-noscapine core. Palladium catalyzed Suzuki cross coupling of 9-bromo α-noscapine with aryl boronic acids was employed using mild and inexpensive reagents to attain desired noscapinoids 5a–g in excellent yields. Screening anti-proliferative activity for new noscapinoids 5b–g, on human cancer cell lines resulted three compounds 5b, 5d and 5f as potent analogues, active against human breast epithelial (MCF-7), human cervix cancer (HeLa) and human lung adenocarcinoma epithelial (A549) cell lines.

Cyclo-meta-phenylene revisited: nickel-mediated synthesis, molecular structures, and device applications.

From a one-pot nickel-mediated Yamamoto-type coupling reaction of m-dibromobenzene, five congeners of [n]cyclo-meta-phenylenes were synthesized and fully characterized. The [n]cyclo-meta-phenylenes possessed a commonly shared arylene unit and intermolecular contacts but varied in packing structures in the crystalline solid state. Columnar assembly of larger congeners yielded nanoporous crystals with carbonaceous walls to capture minor protic or aliphatic solvent molecules. The concise and scalable synthesis allowed exploration of the macrocyclic hydrocarbons as bipolar charge carrier transport materials in organic light-emitting diode devices.

Anionic chiral tridentate N-donor pincer ligands in asymmetric catalysis.

Tridentate monoanionic ligands known as "pincers" have gained a prominent place as ligands for transition metals and, more recently, for main-group metals and lanthanides. They have been widely employed as ancillary ligands for metal complexes studied inter alia in bond activation steps relevant to catalytic processes. The central formally anionic aryl or heteroaryl unit acts as an "anchor" in the coordination to the metal, which kinetically stabilizes the resulting complexes. Their stability, activity, and reactivity can be tuned by subtle modifications of substitution patterns on the pincer ligand or by modifying the donor atoms. The challenges in pincer ligand design for enantioselective catalysis have been met by their assembly from rigid heterocycles and chiral ligating units in the "wingtip" positions, which generally contain the stereochemical information. The resulting well-defined geometry and shape of the reactive sector of the molecular catalyst favor orientational control of the substrates. On the other hand, the kinetic stability allows reduced catalyst loadings. Recently, a new generation of tridentate anionic N(∧)N(∧)N pincer ligands has been developed which give rise to highly enantioselective transformations. Their applications in asymmetric catalysis have focused primarily on the asymmetric Nozaki-Hiyama-Kishi coupling of aldehydes with halogenated hydrocarbons as well as Lewis acid catalysis involving enantioselective electrophilic attack onto metal-activated β-keto esters, oxindoles, and related substrates. These include highly selective protocols for Friedel-Crafts alkylations with Michael acceptors, electrophilic fluorinations, trifluoromethylations, azidations, and alkylations and subsequent transformations. Increasingly, these stereodirecting ligands are being employed in other types of transformations, including hydrosilylations, cyclopropanations, and epoxidations. The stability and well-defined nature of the molecular catalysts have made them attractive targets for mechanistic studies into a wide range of these transformations, thus providing the type of insight required for a more rational approach to catalyst development. This Account reviews work performed by us and other groups in the field and places it into perspective in relation to general research efforts in enantioselective catalysis.

ChemInform Abstract: Tricks of Light on Helices: Transformation of Helical Polymers by Photoirradiation

AbstractReview: [39 refs.

Follow the Yellow Brick Road: Structural Optimization of Vibrant Yellow-to-Transmissive Electrochromic Conjugated Polymers

A series of conjugated polymers were designed and synthesized to extract structure–property relationships with the goal of yielding yellow-to-transmissive switching electrochromes. The polymers are based on repeat units of propylenedioxythiophene (ProDOT) in alternation with a variety of arylenes including 1,4-phenylene (ProDOT-Ph), 2,7-fluorene (ProDOT-Fl), 2,7-carbazole (ProDOT-Cbz), 2,5-dimethoxy-1,4-phenylene (ProDOT-Ph(MeO)2), and 2,7-pyrene (ProDOT-Py). Additionally, a random copolymer containing ProDOT and two different arylene units was produced: ProDOT-phenylene-ProDOT-dimethoxyphenylene (R-ProDOT-Ph/Ph(MeO)2) and two polymers with a ProDOT dimer in alternation with pyrene and phenylene composed ProDOT2-pyrene (ProDOT2-Py) and ProDOT2-phenylene (ProDOT2-Ph), respectively. The polymers were synthesized using Suzuki polycondensation. Examinations of the optoelectronic properties via UV–vis–NIR spectroscopy, differential pulse voltammetry, and spectroelectrochemistry show that varying the electron richness of the polymer by utilizing more electron rich arylenes, dimers of ProDOT, or less electron rich arylenes, the oxidation potential could be decreased or increased, respectively, ranging from 270 to 650 mV. Through subtle C–H ortho interactions from the arylene unit, yellow neutral state colors were maintained with transmissive or near-transmissive oxidized states. Colorimetry utilizing L*a*b*, where a*b* values correlate to the chroma or saturation of a color (note: −a* and +a* correspond to green and red and −b* and +b* correspond to blue and yellow, respectively) and L* represents the lightness, was used to show the maintenance of yellow colors in the neutral states. Herein, the yellow polymers had L* values above 84.0, a* values ranging from −11.6 to 24.8, and b* values greater than 47.6. In the oxidized states, the most transmissive forms had L* values above 70.0, a* values ranging from −2.1 to 2.0, and b* values ranging from −6.8 to −0.1. These structure–property relationships grant access to conjugated polymers with high energy absorbance in the visible, while allowing variability in redox potentials, providing a deeper understanding in yielding yellow-to-transmissive electrochromic polymers.

Substituted N-(biphenyl-4'-yl)methyl (R)-2-acetamido-3-methoxypropionamides: potent anticonvulsants that affect frequency (use) dependence and slow inactivation of sodium channels.

We prepared 13 derivatives of N-(biphenyl-4′-yl)methyl (R)-2-acetamido-3-methoxypropionamide that differed in type and placement of a R-substituent in the terminal aryl unit. We demonstrated that the R-substituent impacted the compound’s whole animal and cellular pharmacological activities. In rodents, select compounds exhibited excellent anticonvulsant activities and protective indices (PI = TD50/ED50) that compared favorably with clinical antiseizure drugs. Compounds with a polar, aprotic R-substituent potently promoted Na+ channel slow inactivation and displayed frequency (use) inhibition of Na+ currents at low micromolar concentrations. The possible advantage of affecting these two pathways to decrease neurological hyperexcitability is discussed.

Hindered aryl bromides for regioselective palladium-catalysed direct arylation at less favourable C5-carbon of 3-substituted thiophenes.

The use of the congested aryl bromide 2-bromo-1,3-dichlorobenzene as coupling partner allows to modify the regioselectivity of the arylation of 3-substituted thiophene derivatives in favour of carbon C5. The coupling of this aryl bromide with a variety of 3-substituted thiophenes gave in all cases the desired 5-arylation products in moderate to good yields using only 0.5 mol % of a phosphine-free and air-stable palladium catalyst. Then, from these 5-arylthiophenes, a second palladium-catalysed C–H bond functionalization at C2 of the thiophene ring allows the synthesis of 2,5-diarylthiophenes with two different aryl units.

EDOT-Based Copolymers with Pendant Anthraquinone Units: Analysis of Their Optoelectronic Properties within the Double-Cable Context

Here we present the synthesis and optical and structural characterization of a new series of alternating donor–acceptor copolymers based on 3,4-ethylenedioxythiophene (EDOT). The donor consists of phenylene, alkylated fluorene, or diethynyl-1,4-phenylene as the aryl units copolymerized with the EDOT group with and without a 9,10-anthraquinone (AQ) side chain as the acceptor group, namely, as PEX-AQ and PEX, respectively. The changes in the electronic absorption/emission spectra and redox properties are analyzed in detail as a function of (i) the composition of the copolymers and (ii) the inclusion of the pendant 9,10-anthraquinone. Density functional theory calculations are used to interpret the experimental results. Special emphasis is placed on the flexibility of the phenylene-based copolymer spine which favors the tendency toward aggregation and confers interesting solvathocromic, thermochromic, and sonochromic properties when applying external stimuli. This is explained by means of an order–disorder c...

Tricks of light on helices: transformation of helical polymers by photoirradiation.

The following polymer structural transitions were achieved using light: preferred-handed helix formation for poly(9,9-di-n-octylfluoren-2,7-diyl), helix racemization (helix-helix transition) for poly(2,7-bis(4-t-butylphenyl)fluoren-9-yl acrylate) and poly(2,5-bis[4-((S)-2-methylbutyloxy)phenyl]styrene), and helix decomposition for poly(2,7-bis(4-t-butylphenyl)-9-methylfluoren-9-yl acrylate) and poly(2,7-bis(4-t-butylphenyl)fluoren-9-ylmethyl methacrylate). Although these types of transitions and chemical transformations have been studied mainly using heat or chemicals as stimuli, light can also cause these structural alterations. In the helix construction and the helix-helix transition, a key transition is a twist-coplanar conformational change of a biphenyl or an aryl-aryl unit in the side chain or the main chain of the polymer. Furthermore, the helix-helix transition was caused only by light and not by heat. The examples discussed in this review are expected to trigger off a new direction in synthesis and reaction of chiral polymers.

2,6-Bis(arylsulfonyl)anilines as Fluorescent Scaffolds through Intramolecular Hydrogen Bonds: Solid-State Fluorescence Materials and Turn-On-Type Probes Based on Aggregation-Induced Emission.

A series of 2,6-bis[aryl(alkyl)sulfonyl]anilines were synthesized by nucleophilic aromatic substitution of 2,6-dichloronitrobenzene with various aryl or alkyl thiolates (benzyl-, phenyl-, 2-naphthyl-, and 2-aminophenyl thiolate), followed by hydrogenation and subsequent oxidation. All prepared 2,6-bis[aryl-(alkyl)sulfonyl]anilines showed high fluorescence emissions in the solid state; X-ray structures revealed well-defined intramolecular hydrogen bonds, which served to immobilize the rotatable amino group and generate a fluorescence enhancement in addition to improved photostability. Moreover, absorption and fluorescence spectra showed redshifts in the order of benzyl<phenyl<2-naphthyl in solution and the solid state, and DFT calculations confirmed charge transfer from the central aniline unit to the side aryl groups through the sulfonyl bridges, which together indicated the push-pull effect with an extended π-conjugated system comprising the 2,6-bis(arylsulfonyl)aniline unit. Furthermore, 2,6-bis(2-aminophenylsulfonyl)aniline, with rotatable amino groups at the flanking aryl units, was affected by fluorescence quenching in solution. This nonemissive state showed an enhanced environmental sensitivity to solvent polarity and viscosity, along with a "turn-on" fluorescence response in frozen solvent and in the aggregated form. 2,6-Bis(2-aminophenylsulfonyl)aniline behaved as a turn-on fluorescence probe for selective detection of DNA based on its aggregation-induced emission.

Impact of linking arylene units off-axis on the photophysical properties of polyfluorenes

The design of new organic molecules with the first triplet (T1) excitation energy high enough for use as host materials for blue phosphorescence is challenging because the energy transfer from host to guest requires the host to have a T1 energy greater than the already high T1 energy of the guest phosphor. By using density functional theory methods on polyfluorenes as an example, we show that moving the inter-ring linkage on arylene units away from the para position can raise the T1 energy by as much as 0.36eV While the ortho linkage raises the T1 energy by a greater extent than the meta, it is the latter that opens the transport gap wider. With either linkage position, electron density at the transport levels remain fully delocalized over the joined units, a necessary condition for efficient charge transport.

High temperature ethylene polymerization catalyzed by titanium(IV) complexes with tetradentate aminophenolate ligands in cis-O, N, N chelating mode.

A series of titanium trichloride complexes , ligated with claw-type tetradentate aminophenolate ligands were synthesized from the direct reaction of TiCl4(THF)2 with 1 equiv. of the corresponding aminophenol in the presence of triethylamine. For comparison purposes, titanium isopropoxide complexes were also synthesized via the reaction of Ti(O(i)Pr)4 and 1 equiv. of the proligand. Similar reactions of ZrCl4(THF)2 with the corresponding aminophenol ligands in the presence of triethylamine only allowed the isolation of zirconium complex . The X-ray diffraction studies reveal that titanium trichloride complexes , and titanium triisopropoxide complex all possess a distorted octahedral geometry with the tetradentate aminophenolate ligand in cis-O, N, N chelating mode, where the methoxy group of the aryl unit does not coordinate with the metal center in the solid state. Upon activation with MMAO, these titanium and zirconium(iv) complexes exhibited moderate to high catalytic activities for ethylene polymerization at 30-120 °C, producing high-molecular-weight polyethylenes with broad distributions (Mw/Mn = 10.2-34.8). The activities of titanium trichloride complexes are significantly higher than those of titanium isopropoxide and zirconium trichloride complexes at high temperatures. The highest activity of 15 456 kg (mol-Ti h)(-1) could be achieved by titanium trichloride complex with bromo groups on both ortho- and para-positions of the phenolate ring of the ligand at 120 °C.

A Photochromic Diarylethene Bearing a Six-Membered Aryl Unit

A new unsymmetrical photochromic diarylethene compound1awas synthesized and its photochromism were investigated, such as photochromism and kinetics in solution. The compound exhibited good photochromism in solution with alternating irradiation by UV/Vis light, and the maxima absorption of its closed-ring isomer1bare 515nm. The open-ring isomer of the diarylethene1aexhibited relatively strong fluorescence at 424 nm when excited at 280 nm. Its fluorescence intensity decreased along with the photochromism from open-ring isomers to closed-ring isomers upon irradiation with 297 nm UV light.

Ultrafast Excited-State Dynamics of Donor–Acceptor Biaryls: Comparison between Pyridinium and Pyrylium Phenolates

The excited-state dynamics of two donor-acceptor biaryls that differ by the strength of the acceptor, a pyridinium or a pyrylium moiety, have been investigated using a combination of steady-state solvatochromic absorption, ultrafast fluorescence, as well as visible and infrared transient absorption spectroscopies. The negative solvatochromic behavior of pyridinium phenolate indicates that the permanent electric dipole moment experiences a decrease upon S1 ← S0 excitation, implying that the ground state possesses more zwitterionic character than the excited state. In contrast, pyrylium phenolate exhibits a weakly positive solvatochromic behavior corresponding to a small increase in the dipole moment upon excitation, implying more zwitterionic character in the excited than the ground state. Both compounds are therefore situated at different sides of the cyanine-limit structure, which has equally polar ground and excited states. Despite these differences, both molecules exhibit qualitatively similar excited-state properties. They are characterized by a very short fluorescence lifetime, increasing from about 1 to 20 ps, when varying solvent viscosity from 0.4 to 11 cP. There are, however, characteristic differences between the two compounds: The excited-state lifetimes of the pyrylium dye are shorter and also depend somewhat on polarity. The ensemble of spectroscopic data can be explained with a model where the emitting Franck-Condon excited state relaxes upon twisting around the single bond between the aryl units to a point where the excited- and ground-state surfaces are very close or intersect. After internal conversion to the ground state, the distorted molecule relaxes back to its equilibrium planar configuration, again largely dependent upon solvent viscosity. However, in this case, the kinetics for the pyrylium dye are slower than for the pyridinium dye and the polar solvent-induced acceleration is significantly stronger than in the excited state. This difference of kinetic behavior between the two compounds is a direct consequence of the change of the electronic structure from a normal to an overcritical merocyanine evidenced by steady-state spectroscopy.

Hierarchical Preferences of Hydroxylated Oxanorbornane-Based Achiral Amphiphiles

Achiral amphiphiles with hydroxylated oxanorbornane headgroups showed specific morphological characteristics and hierarchical preferences depending upon the nature of lipophilic units. Detailed scanning electron microscopic (SEM) studies showed that twisted ribbonlike aggregates are characteristic of monoalkoxyaryl lipids with hydrocarbon chain length in the range C10-C13; these systems also had a preference toward lamellar arrangement. Asymmetric packing of these lipids is a unique occurrence and shows that the presence of molecular chirality is not an absolute requirement for curvature effects in their supramolecular assemblies. Aryl units in these systems were found to be important for the observed morphological preferences, which became evident from comparative studies involving simple long chain esters without this moiety. Single-crystal X-ray diffraction analysis of one of the lipids from the latter group gave finer details of strong and weak secondary interactions, which operate during their assembly process. Introduction of more than one alkyl chain on the aromatic ring caused a notable shift in the packing propensity toward columnar arrangement. Most of these cone-shaped molecules were found to give doughnut-shaped aggregates from acetone solution through the intermediary of fibrous structures, which was confirmed through SEM, transmission electron microscopic, and atomic force microscopic studies.

Solid-phase Pd-catalysed cross-coupling methods for the construction of π-conjugated peptide nanomaterials

We describe the development of two procedures for the synthesis of peptides that are embedded with a variety of π-conjugated semi-conducting oligomers. These procedures utilise solid-phase variants of classical palladium-catalysed cross-couplings commonly used to prepare π-conjugated oligomers. The resulting peptide–π–electron hybrids are soluble in aqueous media and self-assemble to produce 1D nanostructures, simultaneously forming networks of π-stacked conduits. The procedures have allowed for the inclusion of complex chromophores including mixed aryl units, ethynylene linkers and sexithiophenes where the latter peptide's nanostructures demonstrated substantial conductivity when employed as an active layer in a field-effect transistor.

Synthesis of Novel Optically Active Poly(phenyleneethynylene–aryleneethynylene)s Bearing Hydroxy Groups. Examination of the Chiroptical Properties and Conjugation Length

Novel optically active poly(phenyleneethynylene-aryleneethynylene)s bearing hydroxy groups with various arylene units [poly(1–2), poly(1–3a), poly(1–3b), poly(1–4)] were synthesized by the Sonogashira–Hagihara coupling polymerization of (S)-3,5-diiodo-4-hydroxy-C6H4CONHCH(CH3)COOC12H25 (1) with HC≡C–Ar–C≡CH [2 (Ar = 1,4-phenylene), 3a (Ar = 2,7-naphthylene), 3b (Ar = 1,4-naphthylene) and 4 (Ar = 1,6-pyrenylene), and the optical properties were compared. Polymers with number-average molecular weights (Mn) of 5,300–11,300 were obtained in 88–94% yields. CD and UV–vis spectroscopic analysis revealed that all the polymers formed predominantly one-handed helical structures in THF. The order of absorption maxima (λmax) of the polymers was poly(1–3a) < poly(1–2) < poly(1–3b) < poly(1–4). Poly(1–2), poly(1–3a), poly(1–3b), and poly(1–4) emitted blue, purplish blue, green and yellow fluorescence, respectively.