Noteworthy Chemistry
September 28, 2009
- Benzo[b]siloles from a simple cyclization route
- Aspirin derivatives that release nitrogen oxide
- Pt–Ru alloy catalyst with controlled internal structure
- Hierarchical alumina networks
- Product separation after enzymatic kinetic resolution
- The oxyfunctionalization of cubane
- Flexible electrodes from graphene–polyaniline composites
Generate benzo[b]siloles by a simple cyclization route. Silacyclopentadienes, or siloles, are a family of organosilicon molecules with exotic materials properties, such as fast electron transport and efficient light emission in the solid state. To explore the potential of this class of molecules as functional materials, versatile synthetic tools to generate new silole derivatives are needed.
L. Illies, H. Tsuji*, and E. Nakamura* at the University of Tokyo developed a synthetic route to a series of new benzosilole derivatives. A single-step, one-pot cyclization reaction of 2-alkynylsilanes (1) catalyzed by KH gives 2-substituted benzosiloles (2) in high yields (up to 99%). The reaction is versatile: The R2 substituent in the starting alkynylsilane can be aliphatic, aromatic, heteroaromatic, or silyl.
The benzosiloles can be further functionalized. A phenyl ring, for example, can be easily attached to the 3-position of cyclization product 3 by the iron-catalyzed C–H activation reaction developed by the same research group (Norinder, J., et al. J. Am. Chem. Soc. 2008, 130, 5858–5859).
All of the benzosiloles are luminescent in solution and in the solid state, with fluorescence quantum yields (ΦF) up to 99%. 3-Phenylated benzosilole 4 is almost nonfluorescent (ΦF = 0.4%) in solution, even though it would be expected to be more strongly emissive because it contains more phenyl rings than 3 and appears to be more highly conjugated. In the solid state, however, 4 becomes 130-fold more emissive. The active intramolecular rotations of the multiple phenyl rings in 4 may quench its emission in the solution state, whereas restricted rotation in the solid state may rejuvenate the luminescence process. (Org. Lett. 2009, 11, 3966–3968; Ben Zhong Tang)
These aspirin derivatives release nitrogen oxide. The nonsteroidal anti-inflammatory class of drugs is typified by aspirin, which is often considered a “wonder drug” because of its analgesic, anti-inflammatory, and antithrombotic properties. It protects against ischemic vascular disorders such as myocardial and cerebral infarctions. The benefits of aspirin result largely from its ability to irreversibly inhibit COX enzymes, preferentially the COX-1 isoform.
Aspirin’s strong gastrotoxicity, however, seriously limits its use in susceptible individuals. One strategy to decrease this toxicity is combining its structure with NO-donor groups. NO spares the gastrointestinal system from distress by several mechanisms.
A. Gasco and coauthors at the University of Turin (Italy) and Nicox Research Institute (Milan) expanded this approach by synthesizing a series of (nitrooxyacyloxy)methyl esters of aspirin and assessing their value as aspirin surrogates. The preparation of one of the more successful variants in this series is shown in the figure.
Most of the successful NO–aspirin surrogates were prepared by the reaction of chloromethyl 2-acetoxybenzoate (1) with the cesium salt of the appropriate acid (2) to give the target NO-releasing structure 3. The authors extended this synthetic method to the oxidation of 3 to give the corresponding sulfoxide and sulfone derivatives, which are also good aspirin-releasing compounds. The products that contain aromatic structures as part of their NO-donor moieties are significantly better aspirin-releasing compounds.
These materials behave as true NO-donor aspirins, from which aspirin release in serum medium follows pseudo–first-order kinetics. An important finding is that the amount of aspirin release depends on the nature of the acyloxy moiety. In the aromatic series, there is a good correlation between the amount of released aspirin and the efficacy of the products in inhibiting platelet aggregation. All products from this study also display in vitro vasodilator activity.
These new examples of NO-donor aspirins may provide improved alternatives to the use of aspirin in clinical settings. (J. Med. Chem. 2009, 52, 5058–5068; W. Jerry Patterson)
Here’s how to make a platinum–ruthenium alloy catalyst with controlled internal structure. Because of the significantly improved chemical stability and activity of alloyed Pt–Ru nanoparticles, there is a great deal of interest in synthesizing these particles with varying amounts of intermixed components. But a major obstacle to forming a well-mixed alloyed Pt–Ru catalyst is the difference in the reduction rates of the two metals. This can cause the formation of phase-separated entities or core–shell structures.
T. K. Sau, M. Lopez, and D. V. Goia* at Clarkson University (Potsdam, NY) may have solved this problem. They report a polyol method for depositing Pt–Ru nanoparticles with controlled size and internal composition on a carbon support. The key to precipitating a truly alloyed Pt–Ru catalyst is to ensure a similar reduction rate for both metals; this can be accomplished by properly selecting the polyol, pH, temperature, and the modality of combining the reactants. (Chem. Mater. 2009, 21, 3649–3654; George Xiu Song Zhao)
It’s all in the technique: hierarchical alumina networks. A. F. Lee, K. Wilson, and colleagues at the University of York (UK) and the University of Poitiers (France) developed an easy way to generate alumina with a tunable, hierarchical porous network. They used mesoporous (triblock copolymer) and macroporous (monodisperse polystyrene spheres) as structure-directing agents with Al(O-i-Pr)3 to yield bimodal, well-organized porous alumina in multigram quantities after calcination.
The mesoporous network, which exhibits hexagonal packing and long-range ordering, is affected only slightly by the macroporous ordering. The authors determined that the bimodal alumina lacks any crystalline reflections, which suggests an intermediate structure between hydroxide and oxyhydroxide. These hybrid alumina materials have high surface areas, thick mesopore walls, and high thermal stability (~800 °C). This type of dual-templated alumina platform may be useful as new catalyst supports and model systems for transport phenomena. (J. Am. Chem. Soc. 2009, 131, 12896–12897; LaShanda Korley)
Separate products easily after enzymatic kinetic resolution of secondary alcohols. The enzymatic kinetic resolution of alcohols can usually be achieved by transesterification with a hydrolase enzyme and a suitable acyl donor. T. S. Moody and co-workers at Almac Sciences (Craigavon, Northern Ireland) developed a new “trick” for this separation: Use the vinyl ester of N-Boc-β-alanine as the acyl donor.
At the end of the resolution (carried out in methyl tert-butyl ether), the enzyme is removed by filtration, and MeOH–5 M HCl is added to remove the tert-butoxycarbonyl group. The organic layer contains one enantiomer of the alcohol; the other enantiomer, which has been acylated, is in the acidic aqueous layer. (Org. Process Res. Dev. 2009, 13, 706–709; Will Watson)
How do you functionalize cubane? This highly strained molecule is the subject of much research, particularly in the area of cubane-functionalized derivatives that can be innovative hyperenergetic materials. It appears, however, that no effective methods have been reported for the direct hydroxylation of cubane.
P. E. Eaton, R. Curci, and coauthors at the University of Bari (Italy), the University of Pavia (Italy), the University of Chicago, and Brown University (Providence, RI) have remedied this shortcoming with direct mono- and bis-hydroxylation of cubane by using the powerful oxidant methyl(trifluoromethyl)dioxirane (TFDO). TFP is 1,1,1-trifluoro-2-propanone.
The authors had observed that, in the oxidation of other strained substrates such as adamantane, TFDO was more reactive than dimethyldioxirane by a factor of almost 1000 with no loss of selectivity. This extremely high reactivity and selectivity led to almost quantitative yields of cubanol (1) and its hydroxylation product, cubane-1,4-diol (2). Conversion in both reactions was >98%. Given the number of potential reaction sites on the cubane scaffold, it is remarkable that 2 is formed selectively as the only isolable product.
The mechanism for this oxidative process seems to be an almost concerted (although nonsynchronous) dioxirane O-insertion into the C–H bonds. The direct TFDO hydroxylation of cubane in high yield as well as a position-specific second hydroxylation represents a desirable alternative to other syntheses, which rely on more laborious procedures. (Org. Lett. 2009, 11, 3574–3577; W. Jerry Patterson)
Make flexible electrodes from graphene–polyaniline composites. Increased attention is being given to supercapacitors as clean power sources for portable electronic devices. Graphene is an intriguing 2-D carbon material that has superior physical and chemical properties. F. Li, H.-M. Cheng, and coauthors at the Chinese Academy of Sciences (Shenyang), the Australian Research Council Center of Excellence for Functional Nanomaterials (Brisbane), and the University of Queensland (Brisbane) show that freestanding electrodes made from the conducting polymer graphene–polyaniline have high capacitance and flexibility.
The authors prepared an electronic graphene “paper” by vacuum infiltrating a grapheme suspension through a cellulose membrane. They then electropolymerized aniline monomer as a thin film on the graphene paper. The electropolymerization was carried out at constant potential in H2SO4 solution. No nanoparticles or whiskerlike polyaniline structures were detected in the composite. The specific capacitance of the composite electrode was ~233 F/g, which is greater than capacitances obtained when carbon nanotubes and activated carbons were used as electrodes. The composite electrode showed good cycling stability. (ACS Nano 2009, 3, 1745–1752; George Xiu Song Zhao)