Shared posts

28 Oct 09:47

Topology‐Reset Execution: Repeatable Postcyclization Recyclization of Cyclic Polymers

by Satoshi Honda, Minami Oka, Hideaki Takagi, Taro Toyota
Angewandte Chemie International Edition Topology‐Reset Execution: Repeatable Postcyclization Recyclization of Cyclic Polymers

Mighty T‐rex: Phototriggered topology‐reset execution (T‐rex) on cyclic poly(dimethylsiloxane)s (PDMSs) functionalized in‐chain with hexaarylbiimidazole moieties enabled the formation of cyclic PDMSs with a desired mixing state of ring sizes, depending on the UV‐irradiation conditions (see picture). T‐rex enabled repeatable and dramatic changes in the rheological properties of PDMS while maintaining its liquid state.


Abstract

Repeatable topological transformation of polymers for the modulation of material functions is a challenge. We have developed a method for repeatedly resetting a cyclic macromolecular architecture to a linear architecture by photostimulation, namely, topology‐reset execution (T‐rex) based on the photochemistry of hexaarylbiimidazoles (HABIs). We synthesized cyclic poly(dimethylsiloxane)s (PDMSs) of various ring sizes with HABIs linked in the chains. UV irradiation of the cyclic PDMSs produced telechelic linear PDMSs with triphenylimidazolyl radical (TPIR) end groups. After termination of UV irradiation, end‐to‐end recyclization occurred by the recoupling of TPIRs. The cyclic PDMSs also responded to ultrasound, which decreased their molecular weight (MW) by site‐specific cleavage of in‐chain HABI moieties, and we are able to reset the MWs by subsequent phototriggered T‐rex. Furthermore, T‐rex enabled solvent‐free switching of the rheological properties of the materials while retaining the liquid character of PDMS.

28 Oct 09:46

Amino Acid Coordination Driven Self‐Assembly for Enhancing both the Biological Stability and Tumor Accumulation of Curcumin

by Xuehai Yan, Yongxin Li, Qianli Zou, Chengqian Yuan, Shukun Li, Ruirui Xing
Angewandte Chemie International Edition Amino Acid Coordination Driven Self‐Assembly for Enhancing both the Biological Stability and Tumor Accumulation of Curcumin

Special agents on the job: Supramolecular curcumin nanoagents were fabricated by amino acid coordination driven self‐assembly (see picture). The biological stability of curcumin was significantly improved through both coordination and molecular stacking in the nanoagents, the size of which could be readily manipulated. These favorable therapeutic features substantially enhanced the antitumor activity of curcumin without discernible side effects.


Abstract

Clinical translation of curcumin has been highly obstructed by the rapid degradation and poor tissue absorption of this agent. Herein, we report on the generation of supramolecular curcumin nanoagents through amino acid coordination driven self‐assembly to simultaneously increase the biological stability and tumor accumulation of curcumin. The biological stability of curcumin was significantly improved both through coordination and through molecular stacking. The sizes of these nanoagents can be readily manipulated to facilitate tumor accumulation. These favorable therapeutic features, together with high drug‐loading capacities and responses to pH and redox stimuli, substantially enhanced the antitumor activity of curcumin without discernible side effects. Hence, supramolecular curcumin nanoagents may hold promise in moving forward the clinical application of curcumin as an effective anticancer drug.

28 Oct 09:46

Bond Energies of Adsorbed Intermediates to Metal Surfaces: Correlation with Hydrogen–Ligand and Hydrogen–Surface Bond Energies and Electronegativities

by Charles T. Campbell, Spencer J. Carey, Wei Zhao
Angewandte Chemie International Edition Bond Energies of Adsorbed Intermediates to Metal Surfaces: Correlation with Hydrogen–Ligand and Hydrogen–Surface Bond Energies and Electronegativities

Putting a number on it: An equation is derived based on Pauling's equation and is shown to accurately estimate the bond enthalpies of small molecular fragments to transition‐metal surfaces.


Abstract

Understanding what controls the strength of bonding of adsorbed intermediates to transition‐metal surfaces is of central importance in many technologies, especially catalysis and electrocatalysis. Our recently measured bond enthalpies of −OH, −OCH3, −O(O)CH and −CH3 to Pt(111) and Ni(111) surfaces are fit well (standard deviation of 7.2 kJ mol−1) by a predictive equation involving only known parameters (gas‐phase ligand–hydrogen bond enthalpies, bond enthalpies of adsorbed H atoms to that surface, electronegativities of the elements, and group electronegativities of the ligands). This equation is based upon Pauling's equation, with improvements introduced by Matcha, derived here following manipulations of Matcha's equation similar to (but going beyond) those introduced by Schock and Marks to explain ligand–metal bond enthalpy trends in organometallic complexes.

26 Jan 12:51

Cooperative Reductive Elimination: The Missing Piece in the Oxidative-Coupling Mechanistic Puzzle

by Ignacio Funes-Ardoiz, Feliu Maseras

Abstract

The reaction between benzoic acid and methylphenylacetylene to form an isocoumarin is catalyzed by Cp*Rh(OAc)2 in the presence of Cu(OAc)2(H2O) as an oxidant and a leading example of oxidative-coupling reactions. Its mechanism was elucidated by DFT calculations with the B97D functional. The conventional mechanism, with separate reductive-elimination and reoxidation steps, was found to yield a naphthalene derivative as the major product by CO2 extrusion, contradicting experimental observations. The experimental result was reproduced by an alternative mechanism with a lower barrier: In this case, the copper acetate oxidant plays a key role in the reductive-elimination step, which takes place through a transition state containing both rhodium and copper centers. This cooperative reductive-elimination step would not be accessible with a generic oxidant, which, again, is in agreement with available experimental data.

Thumbnail image of graphical abstract

Joint effort: The reaction between benzoic acid and methylphenylacetylene to form an isocoumarin is catalyzed by Cp*Rh(OAc)2 in the presence of Cu(OAc)2(H2O) as the oxidant. Its mechanism was elucidated by DFT calculations with the B97D functional, which showed that the overall transformation proceeds by cooperative reductive elimination with a transition state containing both rhodium and copper centers.

06 Apr 22:53

Stereocontrolled Synthesis of syn-β-Hydroxy-α-Amino Acids by Direct Aldolization of Pseudoephenamine Glycinamide

by Ian B. Seiple, Jaron A. M. Mercer, Robin J. Sussman, Ziyang Zhang, Andrew G. Myers

Abstract

β-Hydroxy-α-amino acids figure prominently as chiral building blocks in chemical synthesis and serve as precursors to numerous important medicines. Reported herein is a method for the synthesis of β-hydroxy-α-amino acid derivatives by aldolization of pseudoephenamine glycinamide, which can be prepared from pseudoephenamine in a one-flask protocol. Enolization of (R,R)- or (S,S)-pseudoephenamine glycinamide with lithium hexamethyldisilazide in the presence of LiCl followed by addition of an aldehyde or ketone substrate affords aldol addition products that are stereochemically homologous with L- or D-threonine, respectively. These products, which are typically solids, can be obtained in stereoisomerically pure form in yields of 55–98 %, and are readily transformed into β-hydroxy-α-amino acids by mild hydrolysis or into 2-amino-1,3-diols by reduction with sodium borohydride. This new chemistry greatly facilitates the construction of novel antibiotics of several different classes.

Thumbnail image of graphical abstract

On aldol: Enolization of (R,R)- or (S,S)-pseudoephenamine glycinamide with lithium hexamethyldisilazide (LiHMDS) in the presence of LiCl followed by addition of either an aldehyde or ketone substrate affords aldol addition products which are stereochemically homologous with L- or D-threonine, respectively. These products can be obtained in stereoisomerically pure form in yields of 55–98 %, and are readily transformed into β-hydroxy-α-amino acids by mild hydrolysis or into 2-amino-1,3-diols by reduction.

29 Sep 22:06

Iron-Catalyzed Oxidative CH/CH Cross-Coupling: An Efficient Route to α-Quaternary α-Amino Acid Derivatives

by Kaizhi Li, Guangying Tan, Jingsheng Huang, Feijie Song, Jingsong You
Thumbnail image of graphical abstract

Fully loaded: A coordinating activation strategy has been developed to furnish α-quaternary α-amino acids through the iron(III)-catalyzed oxidative functionalization of α-C(sp3)[BOND]H bonds of α-tertiary α-amino acid esters. The reaction exhibits a broad substrate scope for both α-amino acids and nucleophiles (Nu) as well as good functional-group tolerance (see scheme, DTBP=di-tert-butyl peroxide, DCE=1,2-dichloroethane).