Wusheng Guo

Sustainable conversion of carbon dioxide: the advent of organocatalysis

The conversion of carbon dioxide (CO2), an abundant renewable carbon reagent, into chemicals of academic and industrial interest is of imminent importance to create a higher degree of sustainability in chemical processing and production. Recent progress in this field is characterised by a plethora of organic molecules able to mediate the conversion of suitable substrates in the presence of CO2 into a variety of value-added commodities with advantageous features combining cost-effectiveness, metal-free transformations and general substrate activation profiles. In this review, the latest developments in the field of CO2 catalysis are discussed with a focus on organo-mediated conversions and their increasing importance in serving as practicable alternatives for metal-based processes. Also a critical assessment of the state-of-the-art methods is presented with attention to those features that need further development to increase the usefulness of organocatalysis in the production of organic molecules of potential commercial interest.

Docetaxel loaded oleic acid-coated hydroxyapatite nanoparticles enhance the docetaxel-induced apoptosis through activation of caspase-2 in androgen independent prostate cancer cells

Docetaxel (Dtxl) remains the preferred choice of improving the survival of patients with hormone refractory prostate cancer (HRPC), but many patients suffer from modest drug response and significant toxicity. In the present study, we investigated the efficiency of novel Dtxl loaded-[1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-carboxy(polyethylene glycol)]2000 (DSPE-PEG-COOH) stabilized-oleic acid (OA) coated hydroxyapatite (HA) nanoparticles (Dtxl-NPs) and gained insights into the molecular mechanism of the apoptosis induced by these novel Dtxl-loaded nanoparticles. The drug encapsulation efficiency of Dtxl was 83.6% and the sustained drug release was observed over 30days. The Dtxl-NPs exhibited significantly more cytotoxicity in both prostate cancer cell lines (PC3 and DU145) compared with Dtxl in vitro and increased the Dtxl-induced apoptosis in the PC3 cells. Cell cycle analysis showed that the PC3 cells treated with Dtxl-NPs exhibited significant arrest in the G2-M phase but a higher sub-G(0)/G(1) population when compared with Dtxl. The enhanced apoptosis induced by Dtxl-NPs in the PC3 cells was associated with the changes in mitochondrial membrane potential (MMP) and seemed to involve the activation of caspase-2. The kinetic studies of caspases demonstrated an early activation of caspase-2 in Dtxl-NPs-induced apoptosis in PC3 cells, which differs from Dtxl-induced apoptosis. The inhibition of caspase-2 activation by small interfering RNA (siRNA) knockdown resulted in the significant inhibition of Dtxl-NPs-induced disruption of MMP and Dtxl-NPs-induced apoptosis, indicating that the activation of caspase-2 was the critical event before the mitochondrial depolarization in the PC3 cells. Our findings showed that nanoparticles, more than simple drug carriers, may play an active role in mediating the biological effects.

A Metal-Free Synthesis of N-Aryl Carbamates under Ambient Conditions.

The first chemo- and site-selective process for the formation of N-aryl-carbamates from cyclic organic carbonates and aromatic amines is reported. The reactions proceed smoothly under extremely mild reaction conditions using TBD (triazabicyclodecene) as an effective and cheap organocatalyst, thus providing a sustainable and new methodology for the formation of a wide variety of useful N-aryl carbamate synthons in good to excellent yields. Computational investigations have been performed and show the underlying reason for the observed unique reactivity as related to an effective proton-relay mechanism mediated by the bicyclic guanidine base.

Highly Chemoselective Catalytic Coupling of Substituted Oxetanes and Carbon Dioxide

The chemoselective coupling of oxetanes and carbon dioxide to afford functional, heterocyclic organic compounds known as six-membered cyclic carbonates remains a challenging topic. Here, an effective method for their synthesis relying on the use of Al catalysis is described. The catalytic reactions can be carried out with excellent selectivity for the cyclic carbonate product tolerating various (functional) groups present in the 2- and 3-position(s) of the oxetane ring. The presented methodology is the first general approach towards the formation of six-membered cyclic carbonates (6MCCs) through oxetane/CO2 coupling chemistry. Apart from a series of substituted six-membered cyclic carbonates, also the unprecedented room-temperature coupling of oxetanes and CO2 is disclosed giving, depending on the structural features of the substrate, a variety of five- and six-membered heterocyclic products. A mechanistic rationale is presented for their formation and support for the intermediary presence of a carbonic acid derivative is given. The presented functional carbonates may hold great promise as building blocks in organic synthesis and the development of new, biodegradable polymers.

Stereoselective and Versatile Preparation of Tri- and Tetrasubstituted Allylic Amine Scaffolds under Mild Conditions

Significant progress has been observed in recent years in the synthesis of allylic amines, which are important building blocks in synthetic chemistry. Most of these processes are effective toward the preparation of allylic amines, with limited potential to introduce three or four different substituents on the olefinic unit in a stereocontrolled fashion. Therefore, the discovery of a mild and operationally simple protocol allowing such challenging stereoselective synthesis of multisubstituted allylic amines remains an inspiring target. Herein, we report the first general and practical methodology for the stereoselective synthesis of tri- and tetrasubstituted allylic amines based on Pd-catalyzed conversion of allyl surrogates readily obtained from cyclic vinyl carbonates. These rare conversions are characterized by excellent stereoselectivity, operational simplicity, mild reaction conditions, and wide scope in reaction partners. DFT studies were performed to rationalize the stereocontrol in these allylic amine formation reactions, and evidence is provided that the formation of a six-membered palladacyclic intermediate leads toward the formation of (Z)-configured allylic amine products.

Highly Efficient Catalytic Formation of (Z)‐1,4‐But‐2‐ene Diols using Water as a Nucleophile

The first general catalytic and highly stereoselective formation of (Z)-1,4-but-2-ene diols is described from readily available and modular vinyl-substituted cyclic carbonate precursors using water as a nucleophilic reagent. These 1,4-diol scaffolds can be generally prepared in high yields and with ample scope in reaction partners using a simple synthetic method that does not require the presence of any additive or any special precaution unlike the stoichiometric approaches reported to date. Control experiments support the mechanistic view that hyperconjugation within the catalytic intermediate after decarboxylation plays an imperative role to control the stereoselective outcome of these reactions.

Stereodivergent Carbamate Synthesis by Selective in Situ Trapping of Organic Carbonate Intermediates

Trans carbamates have been prepared in a diastereoselective approach by a judicious one-pot combination of organic carbonates, prepared in situ, and suitable amine reagents under appropriate reaction conditions. This unprecedented approach allows for stereodivergence from a single oxirane substrate with easy access to both cis and trans carbamate isomers with high stereoselectivity (>19:1 d.r.). Key to the control of the diastereoselective nature of the conversions that lead to the trans carbamates is the in situ formation of trans-configured oligo/polycarbonates through Al catalysis, which provides the targeted products after aminolysis. The present results demonstrate the valorization of a renewable carbon-based reagent (CO2) into new valuable scaffolds and an unusual stereocontrol exerted through carbonate intermediates. A series of control experiments support the proposed mechanistic rationale towards the trans carbamate products, which is based on the trapping of an in situ formed trans-configured oligo/polycarbonate.

Water-Soluble Gold Nanoparticles: From Catalytic Selective Nitroarene Reduction in Water to Refractive Index Sensing

Water-soluble gold nanoparticles (Au NPs) stabilized by a nitrogen-rich poly(ethylene glycol) (PEG)-tagged substrate have been prepared by reduction of HAuCl4 with NaBH4 in water at room temperature. The morphology and size of the nanoparticles can be controlled by simply varying the gold/stabilizer ratio. The nanoparticles have been fully characterized by TEM, high-resolution (HR) TEM, electron diffraction (ED), energy-dispersive X-ray spectroscopy (EDS), UV/Vis, powder XRD, and elemental analysis. The material is efficient as a recyclable catalyst for the selective reduction of nitroarenes with NaBH4 to yield the corresponding anilines in water at room temperature. Furthermore, the potential ability of the Au NPs as a refractive index sensor owing to their localized surface plasmon resonance (LSPR) effect has also been assessed.

A Domino Process toward Functionally Dense Quaternary Carbons through Pd-Catalyzed Decarboxylative C(sp3)–C(sp3) Bond Formation

An efficient protocol was developed to construct functionally dense quaternary carbons with concomitant formation of a new Csp3-Csp3 bond via Pd-catalyzed decarboxylative transformation of vinyl cyclic carbonates. This redox-neutral catalytic system features stereocontrolled formation of multisubstituted allylic scaffolds with an aldehyde functionality generated in situ, and it typically can be performed at room temperature without any additives. DFT calculations provide a rationale toward the selective formation of these compounds and reveal a complex mechanism that with the help of microkinetic models is able to reproduce the nontrivial dependence of the identity of the product on the nature of the substituents in the substrate.

Pd-Catalyzed Enantio- and Regioselective Formation of Allylic Aryl Ethers

A general methodology for the synthesis of enantioenriched tertiary allylic aryl ethers through Pd-catalyzed decarboxylative reactions of vinyl cyclic carbonates and phenols is presented. Switching of the regioselectivity toward the formation of linear products by a judicious choice of the ligand is also reported.