Showing papers by "Virgil Percec published in 2011"

Nickel-Catalyzed Cross-Couplings Involving Carbon-Oxygen Bonds

Abstract: Nickel-Catalyzed Cross-Couplings Involving Carbon-Oxygen Bonds Brad M. Rosen, Kyle W. Quasdorf, Daniella A. Wilson, Na Zhang, Ana-Maria Resmerita, Neil K. Garg,* and Virgil Percec* Roy & Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States

Predicting the Size and Properties of Dendrimersomes from the Lamellar Structure of Their Amphiphilic Janus Dendrimers

Abstract: Dendrimersomes are stable, monodisperse unilamellar vesicles self-assembled in water from amphiphilic Janus dendrimers. Their size, stability, and membrane structure are determined by the chemical structure of Janus dendrimer and the method of self-assembly. Comparative analysis of the periodic arrays in bulk and dendrimersomes assembled by ethanol injection in water of 11 libraries containing 108 Janus dendrimers is reported. Analysis in bulk and in water was performed by differential scanning calorimetry, X-ray diffraction, dynamic light scattering, and cryo-TEM. An inverse proportionality between size, stability, mechanical properties of dendrimersomes, and thickness of their membrane was discovered. This dependence was explained by the tendency of alkyl chains forming the hydrophobic part of the dendrimersome to produce the same local packing density regardless of the branching pattern from the hydrophobic part of the dendrimer. For the same hydrophobic alkyl chain length, the largest, toughest, and m...

Self-Assembly of Dendronized Perylene Bisimides into Complex Helical Columns

Abstract: The synthesis of perylene 3,4:9,10-tetracarboxylic acid bisimides (PBIs) dendronized with first-generation dendrons containing 0 to 4 methylenic units (m) between the imide group and the dendron, (3,4,5)12G1-m-PBI, is reported. Structural analysis of their self-organized arrays by DSC, X-ray diffraction, molecular modeling, and solid-state 1H NMR was carried out on oriented samples with heating and cooling rates of 20 to 0.2 °C/min. At high temperature, (3,4,5)12G1-m-PBI self-assemble into 2D-hexagonal columnar phases with intracolumnar order. At low temperature, they form orthorhombic (m = 0, 2, 3, 4) and monoclinic (m = 1) columnar arrays with 3D periodicity. The orthorhombic phase has symmetry close to hexagonal. For m = 0, 2, 3, 4 ,they consist of tetramers as basic units. The tetramers contain a pair of two molecules arranged side by side and another pair in the next stratum of the column, turned upside-down and rotated around the column axis at different angles for different m. In contrast, for m = ...

Transfer, Amplification, and Inversion of Helical Chirality Mediated by Concerted Interactions of C3-Supramolecular Dendrimers

Abstract: The synthesis, structural, and retrostructural analysis of two libraries containing 16 first and second generation C3-symmetric self-assembling dendrimers based on dendrons connected at their apex via trisesters and trisamides of 1,3,5-benzenetricarboxylic acid is reported. A combination of X-ray diffraction and CD/UV analysis methods demonstrated that their C3-symmetry modulates different degrees of packing on the periphery of supramolecular structures that are responsible for the formation of chiral helical supramolecular columns and spheres self-organizable in a diversity of three-dimensional (3D) columnar, tetragonal, and cubic lattices. Two of these periodic arrays, a 3D columnar hexagonal superlattice and a 3D columnar simple orthorhombic chiral lattice with P2221 symmetry, are unprecedented for supramolecular dendrimers. A thermal-reversible inversion of chirality was discovered in helical supramolecular columns. This inversion is induced, on heating, by the change in symmetry from a 3D columnar si...

SET‐LRP of methyl acrylate catalyzed with activated Cu(0) wire in methanol in the presence of air

Abstract: Single electron transfer-living radical polymerization (SET-LRP) of methyl acrylate (MA) in methanol, catalyzed with nonactivated and activated Cu(0) wires, was performed in the presence of nondeoxygenated reagents and was investigated under a simple blanket of nitrogen. The addition of a small amount of hydrazine hydrate mediates the deoxygenation of the reaction mixture by the consumption of oxygen through its use to oxidize Cu(0) to CuO, followed by the reduction of CuO with hydrazine back to the active Cu(0) catalyst. SET-LRP of MA in methanol in the presence of air requires a smaller dimension of Cu(0) wire, compared to the nonactivated Cu(0) wire counterpart. Activation of Cu(0) wire allowed the polymerization in air to proceed with no induction period, linear first-order kinetics, linear correlation between the molecular weight evolution with conversion, and narrow molecular weight distribution. The retention of chain-end functionality of α,γ-di(bromo) poly(methyl acrylate) (PMA) prepared by SET-LRP was demonstrated by a combination of experiments including H NMR spectroscopy and matrix-assisted laser desorption ionization-time of flight mass spectrometry after thioetherification of α,γ-di(bromo) PMA with thiophenol. In SET-LRP of MA in the presence of limited air, bimolecular termination is observed only above 85% conversion. However, for bifunctional initiators, the small amount of bimolecular termination observed at high conversion maintains a perfectly bifunctional polymer.

Ni(COD)2/PCy3 catalyzed cross-coupling of aryl and heteroaryl neopentylglycolboronates with aryl and heteroaryl mesylates and sulfamates in THF at room temperature.

Abstract: Reaction conditions for the Ni(COD)2/PCy3 catalyzed cross-coupling of aryl neopentylglycolboronates with aryl mesylates were developed. By using optimized reaction conditions, Ni(COD)2/PCy3 was shown to be a versatile catalyst for the cross-coupling of a diversity of aryl neopentylglycolboronates with aryl and heteroaryl mesylates and sulfamates containing both electron-donating and electron-withdrawing substituents in their para, ortho, and meta positions in THF at room temperature. This Ni-catalyzed cross-coupling of aryl neopentylglycolboronates is also effective for the synthesis of heterobiaryls and biaryls containing electrophilic functionalities sensitive to organolithium and organomagnesium derivatives. In combination with the recently developed Ni-catalyzed neopentylglycolborylation, all Ni-catalyzed routes to functional biaryls and heterobiaryls are now easily accessible.

Programming the supramolecular helical polymerization of dendritic dipeptides via the stereochemical information of the dipeptide.

Abstract: Many natural biomacromolecules are homochiral and are built from constituents possessing identical handedness. The construction of synthetic molecules, macromolecules, and supramolecular structures with tailored stereochemical sequences can detail the relationship between chirality and function and provide insight into the process that leads to the selection of handedness and amplification of chirality. Dendritic dipeptides, previously reported from our laboratory, self-assemble into helical porous columns and serve as fundamental mimics of natural porous helix-forming proteins and supramolecular polymers. Herein, the synthesis of all stereochemical permutations of a self-assembling dendritic dipeptide including homochiral, heterochiral, and differentially racemized variants is reported. A combination of CD/UV−vis spectroscopy in solution and in film, X-ray diffraction, and differential scanning calorimetry studies in solid state established the role of the stereochemistry of the dipeptide on the thermody...

Self-Repairing Complex Helical Columns Generated via Kinetically Controlled Self-Assembly of Dendronized Perylene Bisimides

Abstract: The dendronized perylene 3,4:9,10-tetracarboxylic acid bisimide (PBI), (3,4,5)12G1-3-PBI, was recently reported to self-assemble in complex helical columns containing tetramers of PBI as basic repeat unit. These tetramers contain a pair of two molecules arranged side-by-side and another pair in the next stratum of the column turned upside-down and rotated around the column axis. Intra- and intertetramer rotation angles and stacking distances are different. At high temperature, (3,4,5)12G1-3-PBI self-assembles via a thermodynamically controlled process in a 2D hexagonal columnar phase while at low temperature in a 3D orthorhombic columnar array via a kinetically controlled process. Here, we report the synthesis and structural analysis, by a combination of differential scanning calorimetry, X-ray and electron diffraction, and solid-state NMR performed at different temperatures, on the supramolecular structures generated by a library of (3,4,5)nG1-3-PBI with n = 14–4. For n = 11–8, the kinetically controlled...

Disproportionating versus nondisproportionating solvent effect in the SET‐LRP of methyl acrylate during catalysis with nonactivated and activated cu(0) wire

Abstract: The disproportionating solvent effect on the kinetics of single electron transfer living radical polymerization (SET-LRP) during catalysis with nonactivated Cu(0) wire coated with Cu2O and activated Cu(0) wire free of Cu2O was studied. In solvents such as dimethyl sulfoxide, MeOH and ethylene carbonate that in conjunction with Me6-TREN promote extensitve disproportionation of Cu(I)X, faster polymerizations were achieved upon switching from nonactivated Cu(0) wire to activated Cu(0) wire. The results showed that the substantial rate enhancement was accompanied with excellent control of molecular weight evolution and distribution, and high fidelity of chain-end functionality. This can be attributed to a more effective equilibrium between activation and deactivation in the presence of Cu(0) free of Cu2O. In nondisproportionating solvents, the kinetics of SET-LRP of methyl acrylate catalyzed by activated Cu(0) wire resembled that of the polymerizations catalyzed by nonactivated wire. This is the result of a competing effect between rapid activation and insufficient disproportionation. The absence of disproportionation effectively leads to the lack of first order kinetics, broad molecular weight distribution, significant loss of bromide chain-end functionality, and therefore, the absence of a living polymerization. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Acid dissolution of copper oxides as a method for the activation of Cu(0) wire catalyst for SET‐LRP

Abstract: Here we reported the acid dissolution of copper oxides as a methodology for the activation of Cu(0) wire used as catalyst in single-electron transfer living radical polymerization (SET-LRP) In this method, the oxide layer on the surface of commercial Cu(0) wire was removed by dissolution in a concentrated acid such as nitric acid, glacial acetic acid and hydrochloric acid SET-LRP of methyl acrylate catalyzed with Cu(0) wire activated with acids showed comparable k value to that of the nonactivated Cu(0) wire-catalyzed counterpart However, the polymerizations catalyzed with activated Cu(0) wire proceeded with no initial induction period, predictable molecular weight evolution with conversion, and narrow molecular weight distribution Regardless of the activation method, the chain end functionality of α,δ-di(bromo) poly(methyl acrylate) (PMA) prepared from SET-LRP initiated with a bifunctional initiator is extremely high, maintaining a 100% chain end functionality at ∼90% monomer conversion The degree of bimolecular termination increased as the polymerization exceeds 92% conversion However, for binfunctional initiators this small amount of bimolecular termination at high conversion maintains a perfectly bifunctional polymer Structural analysis by MALDI-TOF upon thioetherification of α,δ- di(bromo) PMA with thiophenol and 4-fluorothiophenol confirmed the high fidelity of bromide chain ends

Improving the initiation efficiency in the single electron transfer living radical polymerization of methyl acrylate with electronic chain‐end mimics

Abstract: Computational studies on the heterolytic bond dissociation energies and electron affinities of methyl 2-bromopropionate (MBP) and ethyl 2-bromoisobutyrate (EBiB) in the dissociative electron transfer (DET) step of single electron transfer living radical polymerization (SET-LRP) of methyl acrylate (MA) combined with kinetic experiments were performed in an effort to design the most efficient initiation system. This study suggests that EBiB is more effective than MBP in the SET-LRP of acrylates catalyzed by Cu(0) wire, thus being a true electronic mimic of the dormant PMA species. EBiB allows for a more predictable dependence of the molecular weight evolution and distribution. This is exemplified by the absence of a deviation in the PMA molecular weight from theoretical values at low conversions, as a result of a faster SET activation with EBiB than with MBP. The enhanced control over molecular weight evolution was also observed in the SET-LRP of MA initiated with bifunctional initiators similar in structure to MBP and EBiB, suggesting a higher reactivity than MBP in the SET activation, which matches closely that of the polymer dormant chains. The use of bifunctional initiators in conjunction with activated Cu(0) wire in SET-LRP allows for dramatically accelerated polymerizations, although still providing for exceptional control of the molecular weight evolution and distribution.

Why Are Biological Systems Homochiral

Abstract: Self-assembling and self-organizable dendrons, dendrimers, and dendronized polymers produce chiral supramolecular architectures that have been developed as biological mimics. Here we review our work on the self-assembly of homochiral, heterochiral, and racemic dendritic dipeptides, and address one of the most fundamental questions of biological systems: Why are biological systems homochiral and not heterochiral or racemic and, if they were heterochiral or racemic, how would they look and function by comparison with contemporary homochiral biological systems?

Set-LRP polymerization of acrylates in the presence of acids

Abstract: SET-LRP polymerization of acrylic monomers under acidic conditions is described. The source of the acidity may be the solvent (e.g., an acetic acid-containing solvent) or in the monomer content (e.g., acrylic acid or methacrylic acid, optionally in combination with other monomers such as methyl methacrylate).

Zero-Valent Metals Accelerate the Neopentylglycolborylation of Aryl Halides Catalyzed by NiCl2-Based Mixed-Ligand Systems.

Abstract: In the presence of Zn powder and NiCl2(dppp)/dppf, a wide range of aryl halides including o-substituted ones undergoes rapid and efficient neopentylglycolborylation.