Asymmetric reactions on chiral catalysts entrapped within a mesoporous cage

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Synthesis and characterization of [Cu(salen)]2 coordination nano-assembly by a green and simple method

Hierarchical structured microplates consisting of nanoparticles were grown based on a self-assembly approach via a hydrothermal synthesis route without using any additive. Experimental parameters, such as reaction temperature, time and solvent play crucial roles in determining the morphology of final product. The nanoscale shape and macroscopically assembled architecture of [Cu(salen)]2 {salen = N,N′-bis(salicylidene)1,2-ethyelenediimine} crystals were totally controlled by preparation conditions for crystal growth. The synthesized [Cu(salen)]2 hierarchical architectures were characterized with UV-Vis, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetric (TGA) analyses. The thickness of the fabricated plates changes from 25 nm to 3.5 μm. The as-prepared nanoplates are actually composed of nanoparticles with diameters under 100 nm. Furthermore, the formation mechanism of the microplates decorated with nanorods has also been discussed, based on the influence of the kinetics of nucleation and crystal growth.

Current advances in the utility of functionalized SBA mesoporous silica for developing encapsulated nanocatalysts: state of the art

The cavities of SBA mesoporous silica materials can be used as nanoreactors for embedding catalytic species such as nanoparticles, complexes and heteropolyacids etc.

An NMR study on the mechanisms of freezing and melting of water confined in spherically mesoporous silicas SBA-16

Thermodynamic and dynamic properties of water confined in mesoporous silica glass SBA-16 were investigated by DSC, and (1,2)H NMR spectroscopy and (2)H NMR spin-lattice relaxation time (T1) as a function of pore size. SBA-16 possesses the main spherical pores, interconnecting channels and micropores (corona). Water in the characteristic spherical pores of SBA-16 freezes at the homogeneous nucleation temperature of water. Between room and freezing temperatures, the correlation time of the isotropic rotation of water in the pores of SBA-16 followed the Vogel-Fulcher-Tammann (VFT) relation, which reflects the formation and growth of clusters of fragile water for changing to the strong water. The vitrification of water in micropores around 200 K was observed by (2)H NMR. Above 200 K, the correlation time of the rotation of water in micropores exhibited non-Arrhenius behavior, which is correlated with the gradual decrease in the mobility of water due to the growth of hydrogen bonding, forming low density water before vitrification. After vitrification, the activation energy of the rotation of water in micropores was 25-33 kJ mol(-1), which was similar to that in ice Ih for all samples. The freedom of cluster formation and water rotation increased with the increasing the pore size.

Asymmetric hydrogenation by RuCl2(R-Binap)(dmf)n encapsulated in silica-based nanoreactors

RuCl₂-(R-Binap)(dmf)_n encapsulated in silica-based nanoreactors: a solid catalyst as a whole, but a homogeneous catalyst on the nanoscale.

Piperazinylpyrimidine modified MCM-41 for the ecofriendly synthesis of benzothiazoles by the simple cleavage of disulfide in the presence of molecular O2

Synthesis of benzothiazoles by metal free cleavage of disulfide bond.

Radical-involved photosynthesis of AuCN oligomers from Au nanoparticles and acetonitrile

We show here the first radical route for the direct photosynthesis of AuCN oligomers with different sizes and shapes, as evidenced by TEM observations, from an Au nanoparticle/benzaldehyde/CH(3)CN ternary system in air under UV-light irradiation. This photochemical route is green, mild, and universal, which makes itself distinguishable from the common cyanidation process. Several elementary reaction steps, including the strong C-C bond dissociation of CH(3)CN and subsequent •CN radical addition to Au, have been suggested to be critical in the formation of AuCN oligomers based on the identification of •CN radical by in situ EPR and the radical trapping technique, and other reaction products by GC-MS and (1)H NMR, and DFT calculations. The resulting solid-state AuCN oligomers exhibit unique spectroscopic characters that may be a result of the shorter Au-Au distances (namely, aurophilicity) and/or special polymer-like structures as compared with gold cyanide derivatives in the aqueous phase. The nanosized AuCN oligomers supported on mesoporous silica showed relatively good catalytic activity on the homogeneous annulation of salicylaldehyde with phenylacetylene to afford isoflavanones employing PBu(3) as the cocatalyst under moderate conditions, which also serves as evidence for the successful production of AuCN oligomers.

Synthesis and anti-oxidant activity of (5E,9E)-16-(substituted)-7,8-dihydro-16λ(5) -dibenzo[d,l][1,3,7,10,2]dioxadiazaphosphacyclotridecine-16-ones

A new class of biologically active 13-membered phosphorus-macroheterocycles (6a-l) were conveniently synthesized from 1,2-bis(salicylidene amino)-phenylene (1), by treating with phosporusoxychloride (3) and followed by reacting with various aromatic thiols and amines (5f-l) in one path, and in another path 1 was directly treated with various phosphorodichloridates (2a-e) in the presence of triethylamine at 0-10°C under N(2) atmosphere in THF. All the title compounds were confirmed by analytical and spectral data (IR, (1) H-, (13) C-, (31) P-NMR, and mass spectra) and screened for anti-oxidant activity. Among these compounds, 6k, 6e, and 6l containing nitro, fluoro, and chloro groups as substituents on the phenyl ring exhibited high anti-oxidant activity with effective inhibitory concentration (IC(50) ) values.

Retracted Article: A series of novel types of immobilized chiral salen Mn(iii) on different organic polymer-inorganic hybrid crystalline zinc phosphonate-phosphate act as catalysts for asymmetric epoxidation of unfunctionalized olefins

The immobilized catalysts displayed superior catalytic ability both for experimental scale and for large-scale reactions.

Epoxidation of alkenes catalyzed by phenyl group-modified, periodic mesoporous organosilica-entrapped, dimeric manganese-salen complexes

A series of reusable, recoverable, diamine-bridged dimeric manganese-salen complexes were prepared by the encapsulation of homogeneous dimeric Mn(salen) complexes into nanocages of a 3D periodic mesoporous organosilica (PMO) support followed by silylation of the support with organosilane. The composition, structure, morphology, and textural properties of the prepared PMO-entrapped dimeric Mn(salen) complexes were characterized, and their catalytic performances were tested in the epoxidation of alkenes (styrene, cyclohexene, and 1-phenylcyclohexene), with NaClO as an oxygen source and 4-phenylpyridine-N-oxide as an axial ligand. Furthermore, the influences of the textural and morphological properties of the entrapped dimeric Mn(salen) complexes and the key reaction parameters on the catalytic activity and selectivity are discussed. Finally, the reusability of the supported dimeric Mn(salen) complexes was evaluated over three catalytic runs.

Opportunities offered by chiral η⁶-arene/N-arylsulfonyl-diamine-RuII catalysts in the asymmetric transfer hydrogenation of ketones and imines

Methods for the asymmetric transfer hydrogenation (ATH) of ketones and imines are still being intensively studied and developed. Of foremost interest is the use of Noyori's [RuCl(η⁶-arene)(N-TsDPEN)] complexes in the presence of a hydrogen donor (i-PrOH, formic acid). These complexes have found numerous practical applications and have been extensively modified. The resulting derivatives have been heterogenized, used in ATH in water or ionic liquids and even some attempts have been made to approach the properties of biocatalysts. Therefore, an appropriate modification of the catalyst that suits the specific requirements for the reaction conditions is very often readily available. The mechanism of the reaction has also been explored to a great extent. Model substrates, acetophenone (a ketone) and 6,7-dimethoxy-1-methyl-3,4-dihydroisoquinoline (an imine), are both reduced by this Ru catalytic system with almost perfect selectivity. However, in each case the major product is a different enantiomer (S- for an alcohol, R- for an amine when the S,S-catalyst is used), which demanded an in-depth mechanistic investigation. Full-scale molecular modelling of this system enabled us to visualize the plausible 3D structures of the transition states, allowing the proposition of a viable explanation of previous experimental findings.

Hoveyda-Grubbs catalyst confined in the nanocages of SBA-1: enhanced recyclability for olefin metathesis

Via a simple adsorption, the second generation Hoveyda-Grubbs catalyst was successfully immobilized on a mesoporous material SBA-1, leading to a highly recyclable solid catalyst for olefin metathesis.

Synthesis of ultralarge-pore FDU-12 silica with face-centered cubic structure

Ultralarge-pore FDU-12 (ULP-FDU-12) silicas with face-centered cubic structures (Fm3m symmetry) of spherical mesopores were synthesized at low initial temperature (∼14 °C) using commercially available PEO-PPO-PEO triblock copolymer Pluronic F127 as a micellar template and xylene as a micelle expander. Xylene was selected on the basis of its predicted higher swelling ability for the Pluronic surfactant micelles in comparison to 1,3,5-trimethylbenzene that was used previously to obtain large-pore FDU-12. The optimization of the synthesis conditions afforded as-synthesized ULP-FDU-12 materials with unit-cell parameters up to 56 nm, which is comparable to the highest reported values for Fm3m structures templated by custom-made surfactants. Calcined silicas were obtained with unit-cell parameters up to 53 nm and pore diameters up to ∼36 nm (for N(2) adsorption at 77 K, the capillary condensation relative pressure was up to 0.938). The preferred silica source was tetraethylorthosilicate, but tetramethylorthosilicate was also found suitable. The pore diameter was dependent on the unit-cell size of the as-synthesized material, but was further tuned by adjusting the time and temperature of the treatment in the HCl solution. If the synthesis was performed at low temperature only, highly ordered closed-pore silicas were obtained at calcination temperatures as low as 450 °C. On the other hand, the hydrothermal treatments, including the acid treatment at 130 °C, afforded silicas with large pore entrance sizes. The present synthesis constitutes a major advancement in the synthesis of ordered silicas with very large open and closed spherical mesopores.

Sol gel-derived SBA-16 mesoporous material

The aim of this article is to review current knowledge related to the synthesis and characterization of sol gel-derived SBA-16 mesoporous silicas, as well as a review of the state of the art in this issue, to take stock of knowledge about current and future applications. The ease of the method of preparation, the orderly structure, size and shape of their pores and control, all these achievable through simple changes in the method of synthesis, makes SBA-16 a very versatile material, potentially applicable in many areas of science and molecular engineering of materials.