Noteworthy Chemistry
March 24, 2008
Polyimide–diamond hybrids have enhanced properties
Highly crystalline mesoporous transition-metal oxides
Gold complexes promote allyl–allyl coupling
Aggregation induces phosphorescence in organoiridium complexes
Order of addition matters in producing mixed metal catalysts
An efficient aerobic oxidation of primary amines to oximes
Synthesis of mesoporous materials with crystalline zeolite pore walls
Polyimide–diamond hybrids have enhanced properties. Incorporating diamond into composite materials would be expected to improve properties such as hardness and scratch resistance. However, diamond may not be compatible with the host polymers, so Q. Zhang, Y. Kagawa, and coauthors at the University of Tokyo and the National Institute for Materials Science (Tsukuba Japan) tried growing a polymer from the diamond.
The authors oxidized nanodiamonds to obtain surface carboxylic acid groups, which they then converted to the corresponding acid chloride groups. Treatment with 4,4’-(m-phenylenedioxy)dianiline grafted the diamine moiety onto the diamond. Reaction with 3,3’,4,4’-benzophenonetetracarboxylic acid anhydride produced the polyamic acid, and thermal imidization resulted in the polyimide–diamond nanocomposite (1). The diamond particles (1 wt%) were well distributed throughout the matrix, and surface hardness was greater than that of the diamond-free system. (Macromolecules 2008, 41, 536–538; David A. Schiraldi)
Highly crystalline mesoporous transition-metal oxides have been synthesized by J. Lee and co-workers at Cornell University (Ithaca, NY). Mesoporous materials with crystalline pore walls are desirable in many technologies and materials, such as photocatalysis, sensors, and fuel cells. Although ordered mesoporous materials synthesized in the presence of surfactant micelles were first reported more than two decades ago, the synthesis of crystalline mesoporous materials, especially mesoporous transition-metal oxides, has been limited. It is a challenge to convert amorphous pore walls to crystalline forms while retaining the mesostructure.
The authors report that combined assembly by soft and hard (CASH) chemistries works well for synthesizing crystalline mesoporous transition-metal oxides. They combined an amphiphilic diblock copolymer, poly(isoprene-block-ethylene oxide) (PI-b-PEO), which contains a hydrophilic block and an sp2-hybridized-carbon-containing hydrophobic block, with a sol of group IV or group V transition-metal oxides. The sol selectively swells the hydrophilic PEO block of the copolymer. When heated in an inert environment, PEO decomposes, and PI is converted to amorphous carbon, which acts as a rigid support to prevent collapse. The subsequent high-temperature treatment crystallizes the pore walls.
Small-angle X-ray scattering, powder X-ray diffraction, and selected area electron diffraction patterns showed that the final materials have an ordered mesoporous structure and a highly crystalline pore wall. Transmission electron microscopy verified the highly ordered mesoporous structure. Nitrogen physisorption characterization indicated that the prepared metal oxides have a typical mesoporous structure with a large mesopore size of ~24 nm. Raman spectra and thermogravimetry–differential thermal analysis confirmed the presence of the carbon formed in situ during inert atmosphere heat treatment, which is believed to stabilize the metal oxide mesostructures during high-temperature treatment. (Nat. Mater. 2008, 7, 222–228; George Xiu Song Zhao)
Gold complexes promote allyl–allyl coupling.
A. M. Echavarren and coauthors at the Institute of Chemical Research of Catalonia (Tarragona, Spain) and the Autonomous University of Madrid note that some transformations are based on selectively activating alkynes, allenes, or alkenes with Au(I); however, coupling allyl fragments with Au(I) has not yet been accomplished. They demonstrate a highly efficient intramolecular cyclization of allyl acetates with allylstannanes and allylsilanes in the presence of cationic gold complex catalyst 1. This reaction also proceeds with excellent stereoselectivity in several cases. Normally efficient rhodium and ruthenium catalysts, such as [{Rh(cod)}2]BF4 and [RuCl2(CO)2(PPh3)2], do not catalyze this cyclization.
When the authors used catalyst 1, several substrates reacted satisfactorily producing the desired cyclic structure in every case. It appears that the substrate structure requires electron-withdrawing substituents such as CO2R, SO2Ar, or CH2OBn on the central carbon atom to promote the reaction; but the authors did not discuss the function of these moieties. They suggest that Au(I) acts as a mild, selective Lewis acid to promote the formation of an allyl cation from the allyl acetate, which then reacts with the allylstannane or allylsilane. This efficient cyclization procedure should be applicable to many synthetic sequences. (Angew. Chem., Int. Ed. 2008, 47, 1883–1886; W. Jerry Patterson)
Aggregate formation induces phosphorescence in organoiridium complexes.
Light emission from chromophoric molecules is usually quenched by aggregate formation, but scientists have recently found that light emission from some molecules can be induced by aggregate formation. The emissions from almost all of these molecules, however, have been fluorescence. F. Li, Z. Liu, C. Huang, and co-workers at Fudan University (Shanghai) have discovered two organoiridium complexes (1 and 2) whose phosphorescence is induced by aggregate formation.
Solutions of 1 and 2 in effective solvents (e.g., MeCN) do not emit, but their aggregates in poor solvents (e.g., MeCN–H2O mixtures with high water content) are luminescent. The authors attribute this phenomenon of aggregation-induced phosphorescence to intermolecular packing, which switches the nonemissive ligand-centered (3LX) excited states of the complexes to their emissive metal-to-ligand–ligand charge-transfer (3MLLCT) transitions. (Chem. Commun. 2008, 685–687; Ben Zhong Tang)
Order of addition matters in producing mixed metal catalysts. While preparing a series of Pd–Ag–Au catalysts, C. M. Cobley, D. J. Campbell, and Y. Xia* at the University of Washington (Seattle) focused on how the order in which metals are added influences catalyst structure and performance. To produce mixed metal systems, the authors treated silver nanocubes with the following sequences of metal salts: gold alone, palladium alone, gold followed by palladium, and palladium followed by gold.
The initial addition of palladium caused the silver cubes to dealloy, creating porous structures; adding gold first did not lead to these porous materials. Once gold was applied to the system, palladium lost its ability to access the silver and create porous structures. Gold was readily added to porous Ag–Pd nanocubes.
Because initial addition of palladium created more porous structures, catalysts produced by treating the silver cubes with palladium and then gold had higher surface areas and were more active as hydrogenation catalysts. If addition of gold was taken to an extreme, however, the palladium could be blocked by gold atoms, preventing palladium-catalyzed chemical reactions. (Adv. Mater. 2008, 20, 748–752; David A. Schiraldi)
Here’s an efficient aerobic oxidation of primary amines to oximes. Oximes are useful intermediates for the synthesis of fine chemicals and bioactive materials, yet the selective catalytic oxidation of primary amines to oximes has not been achieved. Furthermore, there has been no efficient way to promote this reaction via air oxidation.
K. Suzuki*, T. Watanabe, and S.-I. Murahashi* at Okayama University of Science (Japan) and Asahi Kasei Chemicals (Kurashiki, Japan) initially tried transition metals as catalysts for the reaction without success. Similarly, attempts with organocatalysts such as those derived from phthalimides failed. They then focused on a combination of 1,1-diphenyl-2-picrylhydrazyl (1) and WO3/Al2O3 that was an excellent system for converting primary amines to oximes.
In a typical reaction, the authors used a mixture of oxygen with a small amount of nitrogen (for safety) at a total pressure of 5 MPa. This gave impressive conversions with oxime yields in the range 72–90%. Cyclic and acyclic amine substrates worked equally well in this reaction, and polar aprotic solvents such as MeCN or DMF were critical for high activity.
The value of this process is demonstrated in the production of cyclohexanone oxime (see figure). This compound is an important precursor of ε-caprolactam, the cyclic monomer used in the production of nylon 6. Catalyst 1 could be reused at least three times without loss of activity or selectivity. The authors are studying the extension of this catalytic system to other oxidation reactions. (Angew. Chem., Int. Ed. 2008, 47, 2079–2081; W. Jerry Patterson)
The synthesis of mesoporous materials with crystalline zeolite pore walls is demonstrated by M.-O. Coppens and coauthors at Delft University of Technology (The Netherlands), the University of St. Andrews (U.K.), and Rensselaer Polytechnic Institute (Troy, NY). Zeolites are important heterogeneous catalysts used in oil refining, fine chemicals production, and environmental applications. Most of the world’s gasoline is produced by fluid catalytic cracking using zeolite catalysts. However, blockage of the small micropores in zeolites can reduce yields and selectivities.
Introducing mesopores into the microporous zeolite is a promising solution to this problem. Although a variety of attempts have been made to synthesize mesoporous–microporous hierarchical zeolites, additional complex and expensive structure-directing agents must be used. In an easy one-pot synthetic method, the authors used a single organic molecule as the template to prepare a hierarchical ZSM-5 zeolite composite.
The basic synthesis procedure involves two steps: zeolite crystal formation under hydrothermal conditions and mesostructure formation by controlled evaporation. X-ray diffraction and transmission electron microscopy results show that the composite is assembled by embedding the ZSM-5 nanocrystals in a mesoporous matrix. Nitrogen adsorption isotherms indicate that the composite is a mesoporous structure. The authors propose a scaffolding mechanism to explain the growth process of the hierarchical zeolite. (J. Mater. Chem. 2008, 18, 468–474; George Xiu Song Zhao)
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