摘要:

Sugar catalysts for the production of biodieselThe transformation of diesel from vegetable oil suffers from the lack of an ecologically friendly catalyst. In its present form the process for the esterification of higher fatty acids uses H2SO4, which is both highly energy consumptive and chemically wasteful. Other materials such as Nafion or sulfonated carbonized naphthalene are either very expensive and have low activity or they are too soft and leach out during the liquid-phase reactions, leading also to a significant loss of activity. A collaboration of researchers from Tokyo, Yokohama and Ibaraki (Toda, Takagaki, Okamura, Kondo, Hayashi, Domen and Hara) recently investigated the possibility of carbonizingd-glucose at low temperatures, followed by sulfonation to generate an alternative catalyst for biodiesel production.1The material obtained consists of sheets of amorphous carbon with hydroxyl and carboxyl and also SO3H groups in high density. As anticipated the solid is an appropriate catalyst for the esterification of the vegetable-oil constituents oleic acid and stearic acid, as was deduced from the high activity (more than half of the activity of liquid sulfuric acid) outperforming many other known solid catalysts for this reaction. Neither did the catalyst lose its activity nor was a leaching of SO3H detectable in repeated reactions at temperatures between 80 and 180 °C. Apart from the production of biodiesel the material might have potential for other acid catalyzed reactions that await a more sustainable process than the ones in use currently.

ISSN:1463-9262    

DOI:10.1039/b516605p

出版商:RSC    

年代:2005

数据来源: RSC

摘要:

Changes to the Editorial BoardAt the end of 2009 Janet Scott and Alexei Lapkin finished their terms on our Editorial Board. We would like to thank them for their input into the development of the Journal during their time on the Editorial Board. We also welcome Peter Dunn onto the Editorial Board, who will provide a valuable industry perspective to editorial discussions.2010 marks the launch of our new Advisory Board for the Journal. This very much enlarged Advisory Board brings together green chemists from across the world to help shape the Journal for the future. We welcome all the new members of the Advisory Board and look forward to working with them.

ISSN:1463-9262    

DOI:10.1039/b924137j

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

Maggel DeetlefsMaggel joined the QUILL Research Centre as a post-doctoral researcher in 2001 and has since advanced to the position of Assistant Director, with the dual-purpose of managing the Centre and maintaining her own research portfolio. Her research interests focus on the green synthesis and applications of ionic liquids, including purpose-specific design and developing simple methods to predict their physical properties.

ISSN:1463-9262    

DOI:10.1039/b915049h

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

Since the discovery of carbon nanotubes, nanotubular materials have received great attention, both on their fundamental and industrial aspects, due to their superior properties and applications.1Unfortunately, reports on their synthesis, especially of metal oxides, by environmentally benign approaches are scarce,2,3as they are extremely unstable and need organic supports. Among the various metal oxides, titania has attracted much attention, owing to its utility in the photochemical splitting of water,3as an electrode material in dye-sensitized solar cells,4as a photocatalyst for removal of harmful organic compounds,5in UV-protective and self-cleaning coatings6and as a catalyst support.7Recently, much interest has been paid to the shape-controlled synthesis of anisotropic titania nanorods or nanowires.8These nanowires are found to play an important role as both interconnections and active components in fabricating nanoscale electronic, photonic devices and as sensors.9Templating is commonly employed for the controlled production of materials with an ordered structure and with desired properties. In the past, a variety of templates, including aluminium oxide,10carbon nanotubes,11surfactants,12polymer fibers,13supramolecular compounds14and egg shell membranes15, has been widely used. Recently, Kunitakeet al.have used natural cellulosic substances as templates for the synthesis of titania nanotubes with varying outer diameter, as a replica of the initial cellulose fibers, by a repeated filtration–deposition cycle using a nanocoating technique.16Synthesis of nanomaterials in solvents has the advantage of uniform coating of the materials on the surface of the template. In this context ionic liquids (ILs) are considered to be a green solvent for the synthesis of organic substances. Only in recent years, due to the environmental knowledge, has attention been focused on the synthesis of nanomaterials by the use of ionic liquids as solvents.17Hollow TiO2microspheres have been prepared in hydrophobic ionic liquids, and it was reported that the size was influenced by the counter ion, the rate of stirring and the reaction temperature.18Rogerset al.have shown that ILs can be used as non-derivatizing solvents for cellulose, and regeneration of the cellulose fibers can be achieved by the addition of water or ethanol solution without a change in its morphology.19It is important to note that natural cellulose fibers possess surface hydroxyl groups, and can be a suitable binder for immobilization and stabilization of metal oxide nanoparticles.20In continuation of the environmentally benign methods for the synthesis of nanoparticles,3herein we report a green and simple method for the synthesis of TiO2nanowires on cellulose fibre by taking advantage of the dissolution of cellulose in an ionic liquid, and thereby obtaining a uniform TiO2layer over cellulose.The TiO2/cellulose composite wires were prepared by the addition of Ti(OBu)4(0.5 mg) over a period of 30 min to a dissolved lint free cellulose sheet (0.5 mg) (Adventec, Japan) in 5 ml of 1-butyl-3-methylimidazolium chloride ([C4mim]Cl). To this stirred solution, 20 ml of ethanol was added, and centrifuged to remove the IL and the unreacted metal alkoxide. Complete removal of IL was done by washing the TiO2composite for several times. Finally, the material was dried in flowing air at ambient temperature.The field emission scanning electron microscopy (FESEM)FESEM images were recorded on a JEOL JSM-6330FS Fluorescent Emission Scanning Electron Microscope, along with energy dispersion X-ray (EDX) analysis. X-Ray powder diffraction (XRD) patterns were recorded on a Japan Rigaku D/max-c rotation anode X-ray diffractometer, using CuKα radiation (l≈ 1.54178 Å), with a scanning rate of 2° s−1. HRTEM images were recorded on a Hitachi H-800 TEM at an operating voltage of 200 kV.image inFig. 1(a)shows a network of TiO2layered fibers with the morphology of the cellulose template and their replica.Fig. 1(b)shows the individual TiO2/cellulose composite nanowires, separated from the assembly, which is 100–500 nm in length. It is important to note that the cellulose sheet used in the present study is composed of long uniform cellulose fibers in a network morphology, and the regenerated cellulose fibers will have a uniformity from the interior to the exterior surface.21Fig. 1(c)shows the uniform TiO2layer, with wall thickness of 30–100 nm. However these nanowires were found to be fragile. The thickness of the TiO2layer can be controlled by changing the concentration of the Ti(tOBu)4.FESEM image of TiO2/cellulose composite wire. (a) Titania replicas of cellulose, (b) individual nanowires separated from the assembly, (c) FESEM image of the boxed area in (b).As expected, the energy dispersive X-ray (EDX)analysis of TiO2/cellulose composite shows that the composition present is C, O and Ti. The X-ray diffraction analysisof TiO2/cellulose composite showed no peaks, indicating that the composite is amorphous. On the other hand, calcined TiO2(500 °C, 2 h) displayed crystalline reflection peaks that are characteristic to the anatase TiO2.Fig. 2(a)shows the FESEM image of the calcined nanowires, retaining the morphology of the cellulose sheet, but with a porous crystalline formation due to the higher temperature of calcination. A typical HRTEMimage of the titania nanowires obtained from the calcined TiO2/cellulose composite at 500 °C for 2 hours is shown inFig. 2(b). The image revealed the presence of many crystallites showing clear anatase lattice fringes. The nanotubes consist of anatase nanocrystals, which was further confirmed from the XRD pattern.(a) FESEM image of the calcined TiO2/cellulose composite, (b) HRTEM image of calcined TiO2nanowires constituting networks of the cellulose template.The specific surface area was measured by physisorption of nitrogen according to Brunauer–Emmett–Teller (BET), and for the nanotubes obtained after calcination was found to be 14.65 m2g−1, higher than observed by Suet al.for anatase nanoparticles calcined at 700 °C.22Reflectance UV–Vis spectroscopic data of the calcined TiO2wire shows an absorption edge of 360 nm that corresponds to a size of 3.0 nm for the anatase nanoparticles, which was in agreement with the previous reports.23In conclusion, we demonstrate herein an efficient and green chemical pathway to TiO2/cellulose nanowire composites and titania nanotubes by the surface sol-gel process, using an IL as a green solvent medium and cellulosic substances as templates. This method is advantageous as it gives a uniform surface of TiO2around the template and the IL can be reused. This approach may provide a pathway to synthesis of hybrid metal–titania nanotubes and metal nanowires, which is under investigation.

ISSN:1463-9262    

DOI:10.1039/b609887h

出版商:RSC    

年代:2006

数据来源: RSC

摘要:

Phenyl-2-propanone, commonly referred to as P2P, is probably the most popular intermediate for the manufacture of amphetamine and methamphetamine,1and represents a versatile intermediate for the synthesis of pharmaceuticals, agrochemicals and fragrances. Due to the relatively simple structure of the compound and because of its common use,1a number of synthetic routes for its production have been developed. Most of these methods require the presence of a catalyst based on organometallic complexes,2metal acetates,3metal halides,4or Grignard reagents5to give overall yields of up to 70%. There remains however scope for greener and cleaner methods based, for example, on electrochemical technology to be more effectively exploited. In 1977, Shono6described a novel electrosynthesis process based on the reduction of benzyl chlorides in the presence of carboxylic acid chlorides in acetonitrile orN,N-dimethylformamide media, and using a conventional two compartment cell with a ceramic diaphragm and 1 M supporting electrolyte. Yields varied between 29 and 73% depending on the starting materials. In 1986, a patent7areported a process for the synthesis of P2P by electrochemical reduction of benzyl chlorides in the presence of acetic anhydride using an undivided electrolysis cell equipped with a sacrificial anode (i.e.Mg, Al, Zn), organic solvent (N,N-dimethylformamide, acetonitrile, tetrahydrofuran), and supporting electrolyte to give yields of 55% to 64%. In 1994, this process was further modified to use the electrochemical arylation of α-chloroketones with arylhalides in the presence of a catalytic nickel complex.7cAll of these cited electrochemically based processes suffer, however, from complicated work up and generate only modest yields.Micro reactor methodology has been shown to have numerous practical advantages (when compared with batch reactors),8including a safe operating environment, good process control, and the capability to scale-up for industrial production. In addition electrosyntheses in micro reactors has been shown to offer higher yields, in the absence of a supporting electrolyte,9which reduces costly work up and purification steps.In this present study we describe a simple and clean process for the synthesis of P2P based on a one-step electrochemical acylation reaction by direct electroreductive coupling of benzyl bromides and acetic anhydride in a micro-flow electrolysis cell equipped with micro-gap Pt electrodes. The reaction occurs in DMF solvent without supporting electrolyte to generate excellent yields of the products when compared with conventional synthetic methods. Notable benefits of this novel electrochemical process include (i) simple operation, (ii) no need for electrolytes, (iii) minimum product work-up, and (iv) high yield and selectivity of products.Initially, the acylation reaction of benzyl bromide with acetic anhydride was studied by cyclic voltammetry at conventional Pt-disc (diameter 0.5 mm) and micro Pt-disc electrodes (diameter 25 µm) to establish the reaction mechanism. The electroreduction of benzyl bromide is chemically irreversible, leading to the formation of either tolueneviaa two-electron reduction10or dibenzyl formallyviaa one-electron reduction.Fig. 1shows cyclic voltammograms obtained in DMF for (i) the reduction of acetic anhydride, (ii) the reduction of benzyl bromide, and (iii) the reduction of benzyl bromide in the presence of excess acetic anhydride.Cyclic voltammograms (scan rate of 0.1 V s−1) obtained at a 0.5 mm diameter platinum disc electrode immersed in 0.1 Mn-Bu4NBF4–DMF for (i) 60 mM acetic anhydride, (ii) 3 mM benzyl bromide, and (iii) 3 mM benzyl bromide in the presence of 60 mM acetic anhydride.The irreversible reduction of benzyl bromide (Process 1) occurs as a two-electron process (see ESI). In the presence of acetic anhydride, a new reductive peak appears (Process 2) at more positive potential position. The peak current for Process 2 increases with increasing amounts of acetic anhydride (for 15 mM to 60 mM) whilst the peak current for Process 1 gradually decreases. The overall mechanism remains a two-electron transfer, with Process 2 being observed only at platinum electrode surfaces and not at glassy carbon or gold (see ESI). The ratio of peak currents for Processes 1 and 2 is scan rate dependent, consistent with a fast preceding chemical step coupled to electron transfer at the platinum surface. Acetyl from acetic anhydride is likely to act as a “trap” for a benzyl anion intermediate formed at the platinum electrode surface.Preparative micro reactor electrolysis was conducted in a rectangular cavity micro-flow cell (seeFig. 2) with products being determined off-line by using GC/MS and1H-NMR. The reaction medium, containing 5 mM benzyl bromide in DMF with varied amount of acetic anhydride, was continuously pumped through the cell, in which two platinum electrodes with a working area of 45 mm2each were positioned with an inter-electrode gap of 160 µm to produce a 7.2 µl cell volume.Schematic representation of the acylation reaction during micro reactor electrosynthesis. A flow of reagents through a rectangular cavity with working and counter electrode facing each other results in the formation of products.The electro-acylation reactions were conducted galvano-statically and product samples were collected for a 30 minute period.Table 1summarizes the conversion and product distribution for the range of conditions employed in this study.Data for preparative electrolysis of benzyl bromides (BB) in the presence of acetic anhydride (AA) in DMF in a micro-flow cell without intentionally added supporting electrolyteEntryCurrent/mAAA/BB(mol/mol)Conv. (%)Distribution (%)R1R2P2PDBr5 mM benzyl bromides, acetic anhydride concentration as shown in the Table, electrode gap is 160 µm, electrode area 45 mm2, flow rate 10 µl min−1corresponding to 43 s contact time.Molar concentration ratio.The conversion was determined based on the quality of benzyl bromide before and after reaction usingn-decane as an internal standard.P2P represents P2P or its derivatives.DBr is debromination yield for benzyl bromides.Side products include debromination of benzyl bromide (16%) and dimer formation (10%).10.8HH1087612620.8HH2085622331.1HH2092662641.1HH409081951.1CH3H409387661.1HCH3409896271.3CH3OH4099831681.1BrH40735122FromTable 1it can be seen (Entry 1–4) that the conversion and product distribution are dependent on the molar ratio of benzyl bromide to acetic anhydride and the applied current (or potential). Voltages between 5–5.4 V were required to obtain sufficiently high levels of conversion (>85%) in most cases. The best result obtained was 81% of phenyl-2-propanone with 9% of toluene at a flow rate of 10 µl min−1(corresponding to 43 s contact time, see Entry 4). Lower ratios of acetic anhydride to benzyl bromide led to the formation of more toluene (Entry 2–3), and even the formation of the dibenzyl product (Entry 1).Other benzyl bromide derivatives such as 1-phenylethyl bromide, 4-methylbenzyl bromide, 4-methoxybenzyl bromide, and 4-bromobenzyl bromide were also examined (see Entry 5–8 inTable 1) for the acylation reaction with acetic anhydride. It is noted that the presence of Br– and CH3O– groups on the benzyl bromides promote the formation of the debromination products (see Entry 7–8), compared to CH3– and H– groups. In contrast, in the presence of an electron donating group (see Entry 5–6) yields are improved. The formation of bromine due to oxidation of bromide (as a follow up anodic process) was not observed, presumably due to the limited overlap of diffusion layers within the flow cell. The diffusion layer thickness for the process can be estimated based oneqn (1).1In this equation the diffusion layer thickness is obtained based on the diffusion coefficientD, the electrode areaA, the half height of the cellh, and the volume flow rateVf. For a diffusion coefficient of 10−9m2s−1and under conditions employed here, the diffusion layer thickness is estimated asδ= 120 µm, which is approaching the inter-electrode distance.Current efficiencies for all processes are typically around 20–25%, consistent with an overall transfer of 4 electrons per benzyl bromide, but background currents in the presence of acetic anhydride are likely to be responsible for the low yield (seeFig. 1). This observation is consistent with literature reports.7a,bThe electrochemical process was also scaled-up by connecting four identical micro electrochemical cells in parallel. In this case, a similar level of product yield was obtained with a four-fold increase in the quality of the product formation. In summary, micro-flow electrosynthesis offers a surprisingly simple and low waste access to phenyl-2-propanone derivatives which is readily optimized and can be scaled-up. It is very likely that in future a wider range of chemical processes will be identified to be suitable for this kind of simple and clean micro-reactor technology.

ISSN:1463-9262    

DOI:10.1039/b610415k

出版商:RSC    

年代:2006

数据来源: RSC

摘要:

Lactones are frequently occurring structural subunits in many biologically active natural products, for example natural flavors and pheromones. Functionalized chiral lactones have found extensive use as building blocks for the stereoselective preparation of alkaloids, antibiotics, pheromones and flavor components.1It is therefore of importance to develop simple and reliable methods of preparing lactones, and a variety of methods have indeed been introduced, the most common being cyclization of hydroxyacids or haloacids.2On the other hand, hydroxynitriles should be highly useful as precursors of lactones because of the ease with which various nitriles can be prepared, for example by cyanide ion nucleophilic substitution at electrophilic carbons. Nevertheless, this approach towards lactones has been severely hampered by the harsh conditions usually required. A recent attempt to convert hydroxynitriles into lactonesviamild, enzymatic hydrolysis of the cyano group was of limited value for synthetic purposes.3Thus, a useful method for the direct conversion of hydroxynitriles into lactones is a desirable yet unfulfilled goal. Also, because of environmental concerns and increased restrictions on the use of hazardous organic solvents it has recently become of significant interest to develop reactions in water, which is a cheap, safe and non-toxic solvent.4If, in addition, aqueous reactions can be efficiently mediated by heterogeneous catalysts that can be recycled and reused many times, the result will be nearly ideal processes in terms of both greeness and simplicity. Herein we present the direct, high-yielding conversion of hydroxynitriles into lactones using water as solvent and a cationic exchange resin as a recyclable, heterogeneous catalyst.In an ongoing synthetic project we needed to convert a γ-hydroxynitrile into the corresponding lactone. The few reported experiments that exist on this transformation are two-step procedures.5First, cyclization of the starting hydroxynitrile is achieved by an intramolecular Pinner reaction to give a cyclic imidate (Scheme 1). This step has been shown to require strong acid catalysis in organic media (e.g.HCl/EtOH or HCl/ether),5d,eexcept in highly favorable cases where aqueous basic,5aor even neutral5bconditions have been used. The second step, hydrolysis of the cyclic imidate into the lactone, typically involves highly acidic aqueous conditions at elevated temperatures.5a,b,d,eThe two-step Pinner-cyclization/hydrolysis reaction sequence has been of limited use in the preparation of lactones due to the harsh conditions usually required.Not surprisingly, the harsh conditions caused severe problems with side reactions in our synthetic study and it was necessary for us to develop an alternative method. Recently, Mizuno and co-workers reported an efficient procedure for the hydration of nitriles to amides in water using an alumina-supported ruthenium catalyst (Ru(OH)x/Al2O3).6The reaction was proposed to proceedviacoordination of the nitrile to the ruthenium metal. The activated nitrile then undergoes facilitated hydrolysis. We reasoned that if a hydroxynitrile was used, an intramolecular attack of the hydroxyl group could take place to form a cyclic imidate rather than an open chain amide. In a first attempt we heated hydroxynitrile1aat 125 °C in water in the presence of Ru(OH)x/Al2O3(5% Ru). This led to complete conversion of starting material, but only after 27 h, and three products were detected. These were identified as the expected cyclic imidate2aand, gratifyingly, lactone3ain a 3 ∶ 2 ratio, along with minor amounts of the amide corresponding to the hydrolysis product of nitrile1a(Table 1, entry 1).Optimization of conditions for the cyclization of hydroxynitrile1ainto lactone3aEntryCatalystX−Time/hT/°C3a∶2a5% Ru was used in entries 1–4.Also contained minor amounts of amide corresponding to the hydrolysis product of1a.1Ru(OH)xOH−271252 ∶ 32RuCl3OH−421359 ∶ 13RuCl3/acetic acid (100%)AcO−421307 ∶ 34RuCl3/Dowex 50 × 8 (H+)RSO3−61351 ∶ 05Dowex 50 × 8 (H+)RSO3−11351 ∶ 06—OH−61351 ∶ 3The formation of significant amounts of lactone was encouraging at this stage. The presence of amide in the product mixture was not surprising in view of the work by Mizuno and co-workers described above. However, according to their mechanistic proposal, the hydroxide that is added to the cyano group does not originate from the surrounding water but from Ru(OH)xviaan intramolecular transfer. We therefore decided to test the use of RuCl3a reagent which, although inferior to Ru(OH)x/Al2O3, also converted nitriles to amides but presumablyviaa different mechanism where the hydroxide must be delivered from a source external to the RuCl3–CN complex. Indeed, heating1awith RuCl3(5%) in water in a sealed tube at 135 °C for 42 h yielded no detectable amide, but cleanly converted starting material into lactone3aand cyclic imidate2ain a 9 ∶ 1 ratio (entry 2). No other products were observed.With hopes of reducing reaction time, we decided to add a Brønsted acid as co-catalyst. An initial attempt using acetic acid as additive had no significant effect on the rate of the reaction (entry 3). On the other hand, a remarkable improvement was found when we ran the reaction in the presence of a solid-supported acid catalyst. Heating1awith RuCl3(5%) in water with acidic Dowex 50 W × 8–200 (1.7 meq mL−1) cation exchange resin at 135 °C afforded lactone3aexclusively in only 6 h (entry 4).7At this point we became interested in deducing the roles played by the metal and the acid catalyst in mediating lactonization, and to initiate such a study we ran the reaction without RuCl3. Surprisingly, the reaction proceeded efficiently also in the absence of metal! In addition, we were able to reduce the reaction time drastically to just one hour (entry 5). No other products were detected and the lactone was obtained in spectroscopic purity (1H-,13C-NMR) after filtration and removal of the solvent under reduced pressure. When we ran the reaction in neat water,without catalyst, we found to our further surprise that significant amounts of the lactone was formed after 6 h reaction at 135 °C. No starting material was observed and the imidate was the major product (3a/2a1 ∶ 3, entry 6). From this result we draw the conclusion that the resin plays an important part in accelerating the hydrolysis of the intermediate imidate. A plausible mechanistic rationale is that the charged cyclic imidate becomes localized at the ionic surface of the catalyst, where hydrolysis to lactone should be facilitated because of the highly acidic environment.The high purity of the crude product can in part be explained by efficient removal of the stoichiometric ammonium ion byproduct from the reaction mixture through salt formation with the sulfonic acid groups of the catalyst resin, which also accounts for the need for more than a stoichiometric amount of catalyst. Unfortunately, attempts at running the reaction at lower temperature were not successful. At temperatures below 100 °C the reaction became extremely sluggish. On the other hand, higher temperatures than 140 °C led to thermal decomposition of the resin.After having established that the use of a cationic exchange resin in water efficiently promotes the projected Pinner cyclization/hydrolysis reaction sequence, we proceeded to investigate the scope of this method by applying it to other hydroxynitriles. As can be seen inTable 2, various γ-hydroxynitriles (1a–1f) cyclize to the corresponding γ-lactones (3a–3e) in good to excellent yields (65–99%) under the described conditions (entries 1–5). Gratifyingly, we were also able to apply this method to the preparation of a δ-lactone. Cyclization of δ-hydroxynitrile1funder the described conditions gave dihydrocoumarin,3f, thus suggesting an efficient entry to the attractive coumarin derivatives. A considerably longer reaction time (47 h) was required, however, in order to get a good yield (79%, entry 6). In general, the procedure for isolating the products from the aqueous reaction mixture was limited to removal of the cation exchange resin by filtration and evaporation of the solvent.Cyclization of hydroxynitriles1a–1finto lactones3a–3fusing acidic cation exchange resin in water at 135 °CEntryHydroxynitrileLactoneTime/hYield (%)For a typical experimental procedure, seeref. 8.Yields refer to crude products of >95% purity by NMR, except for3awhich was purified by flash chromatography.Work-up by extraction with diethyl ether. Quantitative recovery of unreacted imidate from the water phase.Performed with a mixture of diastereomers. Diastereomeric ratio retained in the product.1196269536>9943083516564779Alternatively, in the cyclizations of1band1d, where volatility was a potential problem, lactones3band3dwere extracted from the filtrate with diethyl ether (entries 2 and 4). In these cases, the lactones and the water-soluble unreacted imidate intermediates were efficiently separated into the organic and the aqueous phases, respectively, and the material was quantitatively recovered by concentration of the phases. Either work-up procedure led to >95% pure lactones (1H-,13C-NMR) thus obviating the need for costly and time-consuming chromatography for most synthetic purposes.Finally, in order to verify that the solid catalyst could be recycled, we recovered the resin from the cyclization of1c, reactivated it by treatment with a small amount of 1 M HCl and used it in subsequent cyclizations. The reaction was performed three times using the same resin and only a small decrease in the isolated yield of3cwas observed (Scheme 2).Recovery of the cation exchange resin for recycling and use in subsequent cyclization/hydrolysis reactions.In conclusion we have developed a useful method of preparing synthetically attractive γ- and δ-lactonesviathe cyclization of γ- and δ-hydroxynitriles. In comparison with reported protocols for preparing lactones from hydroxynitriles, the method presented herein avoids the use of concentrated mineral acids in organic and aqueous–organic media and instead employs a heterogeneous catalyst that is easily removed from the product mixture, and which can be recycled and reused. No metals or other hazardous chemicals are employed and the reactions are run in pure water, which is the cheapest and most harmless solvent available. Finally, lactones of >95% purity were simply and conveniently obtained by filtration followed by concentration of the filtrate. This is in part due to relatively short reaction times, but also because ionic byproducts can associate strongly with the catalyst itself and efficiently be removed from the reaction mixture, thus greatly facilitating purification procedures. These synthetic as well as green advantages should make cyclization of hydroxynitriles a more attractive route to various lactones.

ISSN:1463-9262    

DOI:10.1039/b513656c

出版商:RSC    

年代:2005

数据来源: RSC

摘要:

Asymmetric transfer hydrogenation (ATH) of prochiral ketones and imines is a pivotal reaction for the synthesis of chiral alcohols and amines owing to its ease of handling, lower cost and safety.1Among the various chiral catalysts reported, the most notable is the ruthenium catalyst Ru-TsDPEN [TsDPEN =N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine, (R,R)-1] developed by Noyori and Ikariya.2This catalyst has been applied to the asymmetric transfer hydrogenation of both ketone and imine, leading to good to excellent ees with 2-propanol2and HCOOH-NEt3,3respectively, as hydrogen donor as well as solvent.As a consequence of the increasing demand for efficient and environmentally friendly methods, there has been considerable interest in the development of water-soluble catalytic systems which allow catalytic reactions to occur in water.4We have reported a water-soluble catalyst based on the Noyori–Ikariya system, which has been successfully used in the asymmetric transfer hydrogenation of ketones and imines in aqueous media.5Although the ligand (R,R)-2in the catalyst can be easily synthesized, its purification was rather difficult. Herein, we report a novel water-soluble catalytic system with chiral vicinal diamine (S,S)-3as ligand for the ATH of ketones and imines in neat (i.e.organic solvent- and surfactant-free) water, which is easily prepared and purified from the nitro-precursorviacatalytic hydrogenation.6

ISSN:1463-9262    

DOI:10.1039/b611809g

出版商:RSC    

年代:2006

数据来源: RSC

摘要:

Green ContextThere is considerable interest in using ionic liquids as solvents and catalysts for organic reactions that normally require acid catalysts. Thus it is known that these are suitable media for Diels–Alder reactions but problems remain, notably moisture sensitivity, high cost and difficult extraction procedures. Here relatively inexpensive and water-insensitive ionic liquids with good activity in Diels–Alder reactions are described.JHC

ISSN:1463-9262    

DOI:10.1039/b108431c

出版商:RSC    

年代:2001

数据来源: RSC

摘要:

Recently it has been found thatanhydrousAuCl3catalyzes the formation of C–C and C–O bonds, behaving as an effective Lewis-acid catalytic system;1such as selective cross cycloisomerization/dimerization of propargyl or alkynyl ketones, benzoannulation betweeno-alkynylbenzaldehydes and alkynes or cyclizations of alkynyl furan.1aDue to the excellent alkynophilicity of gold; particular attention has been paid to the gold-based alkyne activation as an attractive strategy for developing new and efficient catalytic cyclizations. Yamamoto and co-workers recently reported a novel gold-catalyzed benzoannulation ofo-alkynylbenzaldehydes with alkynes involving [4 + 2] cycloaddition of a Au-pyrylium intermediate with dienophiles such as alkynes and enol ethers.2They also reported that ynals bearing a pendant alkyne group underwent [4 + 2] benzannulations intramolecularly.3Recently we reported that enynes bearing an aldehyde group underwent unprecedented Rh-catalyzed cyclization which could involve [3 + 2] cycloaddition of a Rh-carbenoid dipolar carbonyl ylide intermediate.4However, there were no reports on cycloaddition of such complexes to pendent allene group either inter or intramolecularly.Unique reactivity of two orthogonal π-bonds present in allenynes have made them to be of great interest in the field of organometallics5as well as for the construction of natural product6frameworks. Very recently, Qilong and Hammond reported that the formation of [2 + 2] cycloaddition product2from molybdenum catalyzed reactions of 1,7-allenynes1, where the metal complex was used catalytically.7A similar cycloaddition reaction under microwave irradiation was reported by Brummond and Chen,8and their reports on a stoichiometric Pauson–Khand reaction (PKR) with a similar substrate1using the same catalyst, gave3and4as products (Scheme 1).9We have also showed that allenynes5have different cyclization modes with different palladium catalysts. Cycloreduction occurred at the triple bond to give an alkenylpalladium species that underwent carbo-palladation to give six-membered cycles6, whereas rhodium catalyzed cyclizations of those allenynes5gave five membered rings7chemoselectively. In the case of 1,6-allenynes8, these gave five membered ring cycles9upon cycloreduction, whereas six-membered ring systems10were obtained on arylative cyclization under palladium catalysis (Scheme 2).10In continuation of our research interest in allenynes11and also in pursuit of previous investigations, a closer outlook prompted us to secure cycloisomerisation involving Huisen-type cycloaddition of Au-pyrylium intermediates with dipolarophiles.12Here we wish to report a highly economical as well as an environmentally benign methodology for intramolecular tandem [3 + 2] cycloaddition and cyclization ofo-1,6-allenynebenzaldehydes under gold (iii) chloride catalysis at room temperature. Also, the catalyst gold (iii) chloride was recovered as monodispersed spherical nanoparticles and reused for the same reaction. At first, we have taken 3 mmol% gold (iii) chloride and 1,6-allenynebenzaldehyde12(1 mmol) in a 10 mL test tube containing 2 mL of 1,2-dicholoroethane. Here we have found that compound12underwent a novel mode of cycloaddition–cyclization,13with the formation of gold spherical, monodispersed nanoparticles. The structure of product13was confirmed by IR, H1NMR, CMR, MS and HRMS.14The gold nanoparticles’ structure, surface properties and particle size were characterized by XRD, SEM and finally confirmed with TEM.In our experiments, we could isolate the product13exclusively, which is similar to our Rh-catalyzed reactions of enynals. Au-catalysed reactions of allenyne might also occurvia1,3-dipolar cycloaddition to form the electron deficient Au-carbene species which subsequently underwent sequential fragmentation to give product13and to generate AuCl3for the next cycle (Scheme 3). The fused polycyclic products obtained from this study were very stable during silica gel chromatography and prolonged storage at room temperature. We studied this reaction in 1,2-dichloroethane with different gold catalysts. AuCl3with/without combining AgOTf catalyzed [3 + 2] cycloaddition,15whereas a combination of AuCl3with PPh3did not catalyze any of these pathways (Table 1, entries 1–3). Triphenylphosphine might react with AuCl3to destroy its catalytic activity or Lewis acidity. Au (+1) also catalyzed [3 + 2] cycloisomerization but albeit in low yield (Table 1, entry 4). Among the reaction conditions we have tried, gold (iii) chloride with 3 mmol% in 1,2-dichloroethane solution at room temperature is the best suitable condition for the present study (Table 1, entry 1).Intramolecular [3 + 2] cyclization–cycloaddition under various reaction conditionsEntryReaction conditionsT/°C,t/hYield (%)1AuCl325, 8692AuCl3/AgOTf25, 8433AuCl3/PPh380, 10Trace4AuCl25, 24235AuBr325, 36466AuCl3nanoparticles25, 12637.AuCl3nanoparticles25, 1260A plausible mechanism for the present metal catalyzed cycloaddition–cyclization reaction is AuCl3-complexation ofo-1,6-allenylbenzaldehyde which is expected to form a zwitterion as proposed by Yamamotoet al.16This intermediate on successive tandem [3 + 2] cycloaddition with tethered allene will lead to the observed oxabicyclic product and addition of water can occur at any stage, regenerating Au (iii) and molecular hydrogen (Scheme 3).The gold nanoparticles formed were adsorbed onto a glass substrate of plate dimension 1 × 1 cm2and characterized by X-ray diffraction (XRD) (Siemens D-5005 diffractometer) using graphite-monochromatized Cu Kα radiation at 40 kV and 100 mA.Fig. 1ashows the X-ray diffraction pattern of gold nanoparticles recorded at room temperature. The location of planes corresponding to (111), (200), (220), (311) and (222) are in good agreement with the Joint Committee on Powder Diffraction Standards (JCPDS No. 040-784) reference diagrams for the corresponding bulk phases with lattice constanta= 4.087 Å. The satisfactory agreement among the ‘d’ (interplanar spacing) values confirms the presence of gold nanoparticles. In the XRD pattern the presence of the (111) reflection as a dominant peak along with the presence of reflections from planes of other types, indicates the formation of nanocrystallites with a moderate degree preferred orientation. Nothing other than a broad hump (15–40°), a characteristic peak due to the glass substrate, was detected in XRD spectrum. This observation reveals that the gold nanoparticles preferentially grew in 3D rather than 1D or 2D. An intense peak (111) was used to calculate the grain size using Scherrer's formula with a correction factor of 0.94 due to the specific geometry of grains and non-conducting nature of the substrate.17The 12.4 nm grain size was calculated for as-formed gold nanoparticles.(a) XRD profile of as-deposited gold nanoparticles onto a 1 × 1 cm2glass substrate, (b) SEM image, (c), (d), low and high magnified TEM images.In the SEM image18(Fig. 1b), good film substrate coverage of gold nanoparticles in the form of monodispersed spheres with considerable void spaces are clearly observed. The resolution limit of the SEM made it difficult for us to calculate the exact nanoparticle size, where we preferred to use transmission electron microscopy (TEM).19Representative TEM images of gold naoparticles for two different magnifications are shown inFig. 1c and d, indicating a controlled growth of spherical gold nanoparticles. It can be seen inFig. 1cthat there exists a large number of gold nanoparticles with sphere-like surface morphology. Observation of the magnified image inFig. 1d, shows that the gold nanoparticles are either isolated or in colony form. The average diameter of 12–14 nm was obtained which was found to be consistent with XRD data.The recovered valuable gold catalyst, as 12–14 nm range spherical nanoparticles, was reused in the same reaction (Table 1, entries 6 and 7) as well as its applications in various fields.20In summary, Au-catalyzed [3 + 2] intramolecular tandem cycloaddition–cyclization of 1,6-allenynebenzaldehydes with the formation of nanoparticles was shown. Thus we have demonstrated a very simple, eco-friendly, and more economical methodology for the synthesis of fused polycyclic ring systems from the correspondingo-1,6-allenynebenzaldehydes. We are currently pursuing the application of this methodology to the generation of a large library of polycyclic ring systems, which will be of interest to medicinal chemistry and in the construction of natural products.21

ISSN:1463-9262    

DOI:10.1039/b512034a

出版商:RSC    

年代:2005

数据来源: RSC

摘要:

IntroductionBrominated aromatic compounds are widely used as building blocks for fine chemicals.1They are also present in the structure of many natural compounds of pharmacological interest.2Most of the processes currently operating for the bromination of aryl compounds employ toxic, corrosive, and expensive molecular bromine, resulting in the formation of large amounts of HBr waste (eqn (1)). In view of the wide use of brominated aromatics, it is of interest to develop ecologically benign and economically attractive alternatives to these processes. In this context, oxybromination is safer and greener since it avoids the hazardous Br2, replacing it in most cases with a bromide salt in the presence of an oxidant agent under acidic conditions (eqn (2)).1Ar-H + Br2→ Ar-Br + HBr2The systems reported for oxybromination so far require strongly acidic conditions, volatile organic solvents (VOCs) and metal or other catalysts (e.g.vanadium, copper, zeolites).3The oxidizing equivalents are mostly provided by peroxides in the processes,4although molecular oxygen could be activated in some instances.5–7Kulkraniet al.reported a metal free system operating for the oxybromination of aniline and anisole derivatives in acetic acid as solvent.8Lianget al.recently performed the oxybromination of aromatics in CH3CN–aqueous HBr mixtures, mediated by NaNO2.7Increasing effort is currently devoted to the replacement of VOCs in modern organic synthesis and scCO2is a promising alternative, since it is non-toxic and non-flammable.9Moreover, it is cheap and readily available. In this article we describe an effective approach to the oxybromination of phenol and aniline derivatives performed without any metal catalyst or potential toxic/polluting acid addition in a biphasic system consisting of scCO2and H2O.

ISSN:1463-9262    

DOI:10.1039/b609992k

出版商:RSC    

年代:2006

数据来源: RSC