摘要:

P E R S P E C T I V E S G112 Green Chemistry August 1999 Palladium-catalysed reactions Over the past 20 years or so, the use of palladium as a highly efficient catalyst for a range of reactions has provided many vitally important methods for the production of chemicals. The following examples represent valuable green additions to palladium’s repertoire. Non-natural amino acid production The first comes from Matthias Beller’s group at the University of Rostock, and follows on from earlier work on homogeneous Pd catalysts (Angew.Chem. Int. Ed. Engl., 1997, 36, 1494). Beller has developed a novel heterogenous catalyst system which is capable of the addition of three components—an amide, carbon monoxide and an aldehyde—to give an amidocarboxylic acid (Tetrahedron Lett. 1999, 40, 4523): The products from the reaction are nonnatural amino acids, and find uses as structural units in peptoids, and in a range of other applications. Current methodology involves the Strecker reaction (aldehyde + cyanide, followed by hydrolysis) followed by acylation, and generates considerable amounts of salt. The new method uses Pd/C as the catalyst with small amounts of LiBr, and either N-methylpyrrolidone or acetonitrile as solvent.Phosphines are not required. The amide product can be secondary or tertiary, with aryl or alkyl substituents being possible. Yields are high and turnover numbers are good (54 to 144). Most impressively, the tertiary products are produced in much higher yield and with far better turnover numbers than can be achieved with the homogeneous palladium bromide–triphenylphosphine system.The authors claim that the catalyst is more thermally robust and more easily recovered than the homogeneous versions. Carbonylation of chloroquinones A second, related piece of work is the Pd catalysed carbonylation of chloroquinones, as described by Carpentier, Castanet and co-workers from the CNRS in Lille (Tetrahedron Lett, 1999, 40, 3719).Their target class of compounds are potentially valuable as anti-malarials. They found that the desired esters could be produced by insertion of carbon monoxide into the C–Cl bond of the substrate selectively at the 4-position (selectivity up to 99%). Trapping of the product with ethanol or methanol gave the desired ester cleanly: Attempts to extend this selectivity to dichloropyridines were less successful, although good selectivity could be achieved in some cases. 1,4-Oxidation of dienes Palladium often works in tandem with other metals to effect selective catalytic reactions, examples being the Wacker and the Sonagashira reactions (Pd and Cu). Jan-E Bäckvall and Jens Woltinger of the University of Uppsala and Agnes Zsigmond of Jozsef Attila University in Szeged, Hungary, have combined forces to develop a powerful technique for the Pd–Co catalysed 1,4-oxidation of dienes (Eur.J. Chem., 1999, 5, 1460). One of the most elegant parts of this work is the combination of a heterogeneous cobalt catalyst with a homogeneous palladium species. This allows for ready separation of the metals after reaction, as well as slowing down the degradation of the cobalt catalyst, a salophen catalyst with oxidisable p-electron periphery, and a propensity to form non-catalytically active bridged dimers.The approach taken to immobilise the Co–salophen catalyst was ‘ship-in-bottle’ encapsulation in a zeolite: The oxidative cycle involves aerobic activation of the cobalt by oxygen, and subsequent oxidation of hydroquinone by the activated complex, liberating water.The Pd cycles from oxidation state 0 to oxidation state +2 by reaction with the quinone, with the Pd(II) finally converting the diene to a 1,4-diacetate. The combination of immobilised Co and homogeneous Pd was found to be more active than the wholly homogeneous version of the system. Perspectives C G R NHR¢ O R¢¢ H O + R N OH O O R¢¢ R¢ CO, Pd/C LiBr, H+ N Cl Cl N Cl CO2Me CO, MeOH PdCl2(PPh3)2 OAc OAc Pd(ll) Pd(0) OH OH O O Con–1 ship in a bottle CoII ship in a bottle water oxygen N N O Co OGreen Chemistry August 1999 G113 P E R S P E C T I V E S Aerobic oxidations Some more aerobic oxidations have been described in two other papers.Alcohols to aldehydes The first comes from John Osborn’s group in Strasbourg (Tetrahedron Lett., 1999, 40, 3723) who have developed a method for the catalytic oxidation of alcohols to aldehydes using catalytic amounts of osmium tetroxide and copper(I) chloride: While copper was not absolutely necessary, its use did result in improvements to the system.Benzylic alcohols were oxidised most efficiently, with allylic and aliphatic alcohols being relatively less prone to oxidation (selectivity was very high in all cases though).Turnover numbers of several hundred were reported, meaning that very small amounts of osmium are required. Asymmetric dihydroxylation A second osmium-mediated aerobic oxidation was reported by Kriel et al. (Tetrahedron Lett., 1999, 40, 4189). This paper describes the asymmetric dihydroxylation of a-methylstyrene by air, using a combination of cinchonaderived ligands, a selenium species and light.The latter two serve to reoxidise the osmium from VI to VIII, and replace the large amounts of potassium ferricyanide normally used for this purpose: The oxidations proceed at 12 °C, and after 24 hours, the product, the chiral diol is isolated. Yields are excellent (87–93%) and ee’s are equally impressive (93–97%).Thus, this method represents a significant step towards a green asymmetric dihydroxylation. Mesoporous catalysts Vitamin K3 Mesoporous catalysts have found uses in oxidation chemistry too. Sorokin and Tuel have recently published details of their studies on immobilising metallophthalocyanines on silicas or MCMs (New J. Chem., 1999, 23, 473).This work is aimed at new methods for the clean production of vitamin K3. Current technology is still based on stoichiometric Cr(VI), but methods to carry out hydroxylation of 2-methylnaphthalene are slowly improving. Sorokin and Tuel prepared immobilised iron(III) phthalocyanines and used them to catalyse the hydroxylation of 2-methylnaphthalene with tert-butyl hydroperoxide. Their results indicate that the selective oxidation of the methyl-substituted ring is possible with a 4:1 ratio of 1,4-hydroxylation to 5,8-hydroxylation.These results are promising from the point of view of developing a further, more selective catalytic system, but also for some of the details about the catalysts. Unusually, the µ-oxo bridged dimer form of the catalyst is significantly more active than the monomeric species—in such species the dimeric form is normally considered to be inactive.Alkyl glucosides A further example of the use of MCM-41 materials comes from the group led by Avelino Corma in Valencia (J. Catal., 1999, 183, 76). This work describes the use of Al-MCM-41 as a mild acid catalyst for the preparation of alkyl glucosides—useful as surfactants and derived from renewable resources (Green Chem., 1999, 1, 38).The authors have carefully examined a number of factors relating to catalyst performance, and shown that pore size and hydrophobicity (controlled by the Si/Al ratio) play important roles in determining the activity of the catalysts. Reuse was shown to be possible after washing the catalyst with methanol, and then water.This was successful, although slight loss of activity was seen after a few cycles—this was attributed to slow degradation of the catalyst structure. Making tin catalytic Organotin reagents are amongst the most versatile in organic chemistry. Unfortunately, they are also difficult to separate from products, and are toxic. They are also typically used in stoichiometrically quantities.Recently, various research groups have begun to find solutions to this problem. The problem of separation has been tackled successfully by McCluskey (Green Chem., 1999, 1(3), 167) by the development of an aqueous reaction system, where the tin by-products separate easily owing to solubility differences. Maleczka and Terstiege (J. Org. Chem., 1998, 63, 9622) have developed a one-pot Pd-catalysed Stille coupling of acetylenes with bromoalkenes, which is C G OH R O R OsO4 (1 mol%) O2 CuCl (1.5 mol%) OH OH 96% yield 97% ee HO H OH OH H H OH H OH O HO H OH OH H H OH OBu BuOH Al-MCM-41P E R S P E C T I V E S G114 Green Chemistry August 1999 catalytic in tin.The overall scheme is depicted below, and revolves round the in situ generation of tributyltin hydride from the oxide and a reducing agent, poly(methylhydrosiloxane) (PMHS).The tin hydride then adds to the acetylene, under Pd catalysis, to form a vinylstannane. This is then coupled with the bromoalkene via Pd(0) catalysis, generating the tin bromide. Aqueous sodium carbonate and PMHS is sufficient to regenerate the tin hydride. This procedure is remarkable in that it surmounts a variety of difficulties: e.g.the requirements for the two Pd catalysed steps are opposing, strong s-donor ligands are necessary for the first step, weak donors for the second, and the complexity of the catalytic sequence requires compatibility of a range of different reagents. Yields are comparable with step-wise reactions, and although the turnover numbers are relatively modest, a reduction in the amount of tin required of up to 80% is achievable.Another paper by the same group (J. Org. Chem., 1999, 64, 342) describes a similar procedure based on the combination of PHMS and KF, with the active species possibly being a hypervalent Si–F centre. This method was applied to both dehalogenations, and to C–C bond forming reactions. Turnover rates indicate that the tin centre can be used between 2 and 10 times.An alternative approach can be found in Tetrahedron, 1999, 40, 2415, where John Murphy’s group at Strathclyde University have shown that hypophosphorous acid and its salts are capable of carrying out some transformations associated with tributyltin hydride, thus avoiding the difficulties associated with tin waste. The methodology works in both organic and aqueous environments.Stephen Graham, the student who carried out the work, recently won two prizes at the Pfizer National Poster Symposium in London for this work. Reactions in water High-temperature water The Heck reaction is a versatile and popular synthetic procedure. Typical solvents include dipolar aprotics and acetonitrile. In an attempt to move towards more environmentally friendly methodology Liz Gron and Amanda Tinsley of Hendrix College, Arkansas, have developed a novel version of the Heck reaction which uses high temperature water (HTW) as solvent (Tetrahedron Lett., 1999, 40, 227).HTW is becoming a popular choice of solvent for organic transformations—it is much less aggressive than supercritical water, but is still much less polar than ‘normal’ water, making it more able to interact with organics.Reactions studied involved iodobenzene and cycloalkenes. The observed reactivity trend—faster rates as ring size increases—differs from that seen in organic systems. For example, reactions run in dimethyl formamide give relative reactivity of cyclopentene > cyclooctene > cycloheptene > cyclohexene. The different trend may be due to differing hydrophobicity.Reactions are also much faster in HTW, being complete in minutes, as opposed to several days for conventional systems (Green Chem., 1999, 1, 65). In another study, Manabe and Kobayashi have shown that certain combinations of scandium salts and surfactants can enhance the activity of simple Brønsted acids (Tetrahedron Lett., 1999, 40, 3773).They studied the Mukaiyama aldol reaction in water at room temperature, and found that the combination of surfactant and Brønsted acid (tosic acid or HCl) resulted in large rate enhancements, with little hydrolysis of the silyl thioketone acetals. (See this issue for a further paper by Kobayashi on scandium salts and surfactant combinations.) Selective photooxygenations of alkanes in water The ability to functionalise alkanes is an important target, and one which has received a great deal of attention.The oxidation of many alkylaromatics takes place at high temperature and uses corrosive mixtures of metals, bromide and carboxylic acids. Such conditions are harsh, and novel chemistry is needed to produce the many important products accessed via this method. Water as a reaction medium, coupled with air as oxidant, and light as a catalyst is a particularly green combination, which has been put to good use by a group led by Michael Gonzalez from the USA (J. Catal., 1999, 183, 159). They have managed to oxidise toluene and ethylbenzene to the corresponding alcohols and aldehyde/ketone products in moderate yields and selectivity. Reactions run at room temperature in water and the photocatalyst is provided by a combination of a quartz lamp and titanium dioxide. While the results are preliminary, this technique does represent a potentially valuable and clean route to useful products. C G O Br O O O O I R O R R1 R2 R2 R1 H3PO2 AlBN I + HTW Pd catalyst CHO + OSIMe3 R R = aryl, aralkyl, thioalkyl R OH O + OH O water, light air, TiO2 + Br Ph Ph Pd-catalyzed hydrostannylation

ISSN:1463-9262    

DOI:10.1039/a906803a

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

SEPTEMBER 1999 The 2nd Post-Graduate Summer School September 6–12 on Green Chemistry Venice, Italy. (http://hydra.unive.it/inca/summer/index2.htm) Biotrans ‘99 September 26 Giardini Naxos-Taormini, Sicily, Italy –October 1 (http://dept.chem.polimi.it/biotrans) One-day Symposium on Green Chemistry September 28 for Young Chemists University of York, UK (greennet@york.ac.uk) OCTOBER 1999 Catalysis Technology Car Boot Sale October 7 Weston Building, Manchester Conference Centre, Manchester, UK (http://www.chemsoc.org/bootsales/home.htm) 3rd European Biofuels Forum October 11–13 Palais des Congres, Brussels, Belgium (http://www.europoint-bv.com) NOVEMBER 1999 Sustainability 2000 (S2K): November 1–12 The online global conference for sustainable development World Wide Web, organised by Institution of Chemical Engineers (http://www.sustainability2000.org) GRIF‘99 November 2–3 (Gateway to Renewable Industrial Feedstocks) John Innes Centre, Norwich, UK (http://www.actin.co.uk) Environment China ‘99 November 2–5 Guangzhou, China (environment@gima.de) International Congress of Chemistry November 27–30 and Environment Indore, India (http://www.chemenviron.com) December 1999 Clean Processes and Environment: December 6–8 The Catalytic Solution ESCPE-Lyon Campus de la Doua, Villeurbanne, France (http://www.cpe.fr/lcoms/cartier) Conference Diary January 2000 2nd Asia-Pacific Congress on Catalysis January 31 Sydney, Australia –February 2 (http://www.chemsoc.org/events/_events/ 00001018.htm) APRIL 2000 Green-Tech® 2000 April 3–5 Royal Dutch Jaarbeurs, Utrecht, Netherlands (http://www.europoint-bv,com) CAPoC5—5th International Congress on April 12–14 Catalysis and Automotive Pollution Control Université Libre de Bruxelles, Belgium (http://www.ulb.ac.be/sciences/surfcat/CAPoC5/) 9th International (and 4th European) April 13–14 Symposium on Supercritical Fluid Chromatography and Extraction.In cooperation with Analytica Conference 2000 Munich, Germany (sfc2000@mx.uni-saarland.de) MAY 2000 16th Canadian Symposium on Catalysis May 23–26 Banff, Alberta, Canada (http://www.gch.ulaval.ca/~sayari/16csc/) JUNE 2000 R'2000 Recovery, Recycling, Re-integration.June 5–9 5th World Congress with Trade Show Toronto, Ontario, Canada (barrage@peak.ch) JULY 2000 IEX 2000: Ion exchange at the Millenium July 16–21 Organised by the SCI. Churchill College, University of Cambridge, UK (http://sci.mond.org/conference/meetings/IEX.HTM) AUGUST 2000 ACS 220th National Meeting including a August 20–24 symposium on Environmental Issues Washington, DC, USA (http://www.acs.org/meetings/future/washdcacp.htm) Green Chemistry August 1999 G115 D I A R Y C G

ISSN:1463-9262    

DOI:10.1039/a906804j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Summary Faujasites such as HY, CaY and MgY are employed as solid acid catalysts for the clean and less hazardous preparation of phenylhydrazone, 2,4-dinitrophenylhydrazone as well as 1,3-dithiane derivatives of carbonyl compounds. The reactions proceed very smoothly and the yields of the derivatives are excellent. Introduction Zeolites are microporous, crystalline aluminosilicates made up of corner-sharing SiO4 and AlO4 tetrahedra.By virtue of their structure, crystallinity and variable stoichiometry, zeolite catalysts have well defined pore size distributions, high and adjustable acidity, very high surface area and good thermal stability.1 The range of available pore dimensions permits chemists to choose suitable zeolites which can match the size of organic molecules enabling one to successfully include these molecules within the cavities and carry out selective transformations.Due to the presence of high and tunable acidity,2–5 zeolites can potentially replace conventional corrosive liquid acids in many of their applications.6 Protection of carbonyl groups is often a necessary requirement in reactions involving substrates with multifunctional groups. Usually the carbonyl groups are protected as 1,3-dithianes while their oximes, phenylhydrazones and 2,4-dinitrophenylhydrazones are other useful derivatives. The formation of these derivatives involves Brønsted as well as Lewis acid catalysis.7 As the conventional acids used in the preparation of these derivatives are highly corrosive, they pose severe environmental hazards.Solid acids like zeolites8 and clays9 with high and adjustable acidity can overcome this problem. They enjoy advantages over conventional liquid acid catalysts by giving good and higher yields, easier separation of products, reusability, milder reaction conditions, etc. Herein we report a smooth and facile conversion of carbonyl compounds to phenylhydrazones, 2,4-dinitrophenylhydrazones and 1,3-dithianes (Scheme 1) in the presence of acidic zeolites as Scheme 1 O R1 R2 NNHAr R1 R2 S S R1 R2 HY/CaY/MgY NNHPh R1 R2 HS(CH2)3SH HY/CaY/MgY HY/CaY/MgY ArNHNH2 PhNHNH2 III I II Ar = 2,4-dinitrophenyl R1 = Ph, R2 = CH3 R1 = R2 = Ph R1 = R2 = -(CH2)5- R1 = Ph, R2 = H Zeolite mediated protection of carbonyl groups Appaswami Lalitha,a Kasi Pitchumani*b and Chockalingam Srinivasan*a a Department of Materials Science and b School of Chemistry, Madurai Kamaraj University, Madurai - 625 021, India Received 29th March 1999 catalysts.The reaction involves the addition of an acidic zeolite to a solution of a carbonyl compound and the reagent used for derivatization and refluxing for a short period. The reaction is general and has been successfully demonstrated with a representative example of each of an alkyl aryl ketone (acetophenone), a diaryl ketone (benzophenone), a cyclic ketone (cyclohexanone) and also with an aromatic aldehyde (benzaldehyde).To the best of our knowledge, this is the first report in which divalent cationexchanged zeolites are used for derivatisation. Results and discussion When the nonacidic zeolite NaY is employed as the catalyst in the carbonyl protection reactions no derivative is isolated.This rules out any significant role of the basic zeolite framework in derivatization and indicates the need for generation of acidic sites in the framework. However, when acidic zeolites such as HY, CaY and MgY are used, the formation of derivatives of carbonyl compounds is clean, smooth and excellent (Table 1).The acidic sites present in the zeolites are primarily responsible for the formation of the derivatives. Replacement of Na+ by H+ in NaY leads to the formation of acidic HY zeolites. Acidity in CaY and MgY can be explained by the dissociation of coordinatively bound water molecules under the action of the electrostatic field associated with the divalent cations.It is known that CaY and MgY activated at 500 °C under aerated conditions possess a large number of Brønsted acidic sites,10 which are employed11,12 in generating carbocationic intermediates. Thus the present work highlights the utility of HY and divalent cation exchanged acidic zeolites in protecting the carbonyl groups in a simple, clean and efficient way. The experimental methods are easier and superior compared to conventional Green Chemistry August 1999 173 C G The adage that the best protecting group is no protecting group is very true.However, there is still a need to protect reactive functional groups as a method of improving selectivity, and it is therefore imperative that the protection and deprotection steps generate as little waste as possible.This contribution describes the efficient formation of dithianes as protecting groups for carbonyl compounds using zeolite catalysis. Several other derivatives can also be prepared using such catalytic systems. Yields are excellent under mild conditions. The development of a similarly clean deprotection system will complete the cycle of protection/deprotection nicely. DJM Green Contextmethods.Also this method is advantageous as it employs heterogeneous catalysts which can be reused (zeolites after washing thrice with dichloromethane are activated again at 500 °C before reuse. The catalytic efficiency is essentially retained; however, the yield decreases significantly after using the zeolites four or five times) and are attractive alternatives to hazardous reagents such as strong corrosive liquid acid catalysts.Also while earlier studies employing conventional acid catalysts have been carried out in polar solvents,6 derivatisation with acidic zeolites can be performed in nonpolar solvents. Absence of any tetrahydrocarbazole formation from the phenylhydrazone of cyclohexanone indicates that the reaction is also selective and does not proceed beyond derivatization.Experimental Acidic zeolites CaY and MgY were obtained by exchanging NaY (Aldrich) powder by stirring with the respective nitrate solution (10%) at 70 °C for about 12 h. The exchange was repeated at least four times. After each exchange, the zeolite powder was washed repeatedly with distilled water and then dried. The procedure is essentially the same as that employed previously by one of the authors.11 All these zeolites were activated at 500 °C for about 10h prior to use.To a solution of carbonyl compound with the respective reagent in hexane/methanol as solvent, activated acidic zeolites were added. The reaction mixture was refluxed with stirring for a specified time. The zeolite was filtered off and the filtrate was evaporated to give the products which were identified by GC analysis (Netel Chromatographs with OV1GC column packed with 10% phenyl methyl silicone and using an FID detector) and also by their melting points.The derivatization was also confirmed by coinjecting with the derivatives prepared by conventional methods. Formation of phenylhydrazones and 2,4-dinitrophenylhydrazones For the preparation of phenylhydrazones and 2,4-dinitrophenylhydrazones, the carbonyl compound (0.3 g) and the reagent (0.5 g) were dissolved in methanol to which zeolites (0.3 g) were added and warmed for 10 min over a water bath.The zeolite was filtered off and the derivative separated out from the filtrate. Formation of 1,3-dithianes To a solution of carbonyl compound (0.6 g) and 1,3- propanedithiol (0.6 g) in hexane, activated acidic zeolite (0.6 g) was added and refluxed with stirring for 1 h.The zeolite was filtered and the filtrate was extracted with an aqueous solution of NaOH to remove excess thiol and the organic solvent was evaporated to give the pure 1,3-dithiane derivative. The yields are quantitative. Acknowledgement Financial assistance from UGC (Major Project No.F.12-57/93, SR-I), New Delhi is gratefully acknowledged. A. L. thanks CSIR for the award of a Senior Research Fellowship. References 1 D. W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974; A. Dyer, An Introduction to Zeolite Molecular Sieves, Wiley, Chichester, 1988. 2 Y. Izumi, K. Urabe and M. Ohaka, Zeolite, Clay and Heteropolyacids in Organic Reactions, VCH, Kodansha, Tokyo, 1992. 3 E.Armengol, A. Corma, H. Garcia and J. Primo, Appl. Catal. A: General, 1997, 149, 411. 4 A. Corma and A. Martinez, Adv. Mater., 1995, 7, 137. 5 W. E. Farneth and R. J. Gorte, Chem. Rev., 1995, 95, 615. 6 W. Hoelderich, M. Hesse and F. Naumann, Angew. Chem., Int. Ed. Engl ., 1988, 27, 226; J. M. Thomas, Angew. Chem., Int.Ed. Engl., 1988, 27, 1673. 7 H. J. E. Lowenthal, Protective Groups in Organic Chemistry, ed. J. F. W. McOmie, Plenum Press, New York, 1973; T. W. Greene, Protective Groups in Organic Synthesis, John Wiley & Sons, New York, 1st edn., 1981; T. W. Greene and P. G. M. Wuts, Protective Groups in Organic Synthesis, John Wiley & Sons, New York, 2nd edn., 1991. 8 P. Kumar, R. S.Reddy, A. P. Singh and B. Pandey, Tetrahedron Lett., 1992, 33, 825; P. Kumar, R. S. Reddy, A. P. Singh and B. Pandey, Synthesis, 1993, 67. 9 B. Labiad and D. Villemin, Synth. Commun., 1989, 19, 31; R. Miranda, H. Cervantes and P. Joseph-Nathan, S y n t h . Commun., 1990, 20, 153. 10 J. W. Ward, in Zeolite Chemistry and Catalysis, ed. J. A. Rabo, American Chemical Society, Washington DC, 1976, p.118; M.L. Poutsma, in Zeolite Chemistry and Catalysis, ed. J. A. Rabo, American Chemical Society, Washington DC, 1976, p. 437. 11 K. Pitchumani and V. Ramamurthy, Chem. Commun., 1996, 2763. 12 K. Pitchumani, A. Joy, N. Prevost and V. Ramamurthy, Chem. Commun., 1997, 127. Paper 9/02533B 174 Green Chemistry August 1999 Table 1 Zeolite catalysed derivatisation of carbonyl compoundsa, b Carbonyl Ic IIc IIId compound Zeolite (% yield) (% yield) (% yield) Acetophenone HY 93 (105) 87 (249) 92 (97) CaY 91 85 90 MgY 80 82 83 Benzophenone HY 92 (137) 95 (238) 90 (136) CaY 92 91 90 MgY 85 87 86 Cyclohexanone HY 87 (77) 89 (162) 87 (145)* CaY 83 85 89 MgY 85 82 80 Benzaldehyde HY 90 (158) 92 (237) 96 (69) CaY 87 92 91 MgY 81 83 86 a Numbers in the parentheses are the melting points/boiling points* of the derivatives in °C. b For structures I to III refer to Scheme 1. c Warmed for 10 min. d Refluxed for 1 h. e Yields refer to isolated pure products.

ISSN:1463-9262    

DOI:10.1039/a902533b

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Summary Catalytic asymmetric aldol reactions in water without the use of organic solvents have been carried out by using a combination of copper bis(dodecyl sulfate), a chiral bis(oxazoline) ligand, and a carboxylic acid to afford the desired products in high yields with good enantiomeric excesses. Introduction The use of water as a solvent in organic synthesis is of great current interest in relation to environmental consciousness.1,2 In such reactions, tedious procedures to remove water from the substrates (water of crystallization, etc.) and solvents are not necessary and, moreover, the use of harmful organic solvents can be avoided.While Lewis acid-catalyzed aldol reactions of aldehydes with silyl enolates (Mukaiyama aldol reaction)3–5 are powerful tools for carbon–carbon bond formation in modern organic synthesis to produce diverse b-hydroxy carbonyl compounds, strictly anhydrous conditions are needed when typical Lewis acids such as TiCl4 and BF3·Et2O are used.On the other hand, we have found that metal triflates such as Sc(OTf)3 and Cu(OTf)2 6–10 can be used as water-stable Lewis acids for organic reactions in water-containing solvents (water–THF, etc.).11 Furthermore, we have recently developed a new reaction system in which Sc(OTf)3 catalyzes the aldol reactions in water without using organic solvents in the presence of a small amount of a surfactant such as sodium dodecyl sulfate (SDS).12 More recently, a new type of Lewis acid, scandium tris(dodecyl sulfate), has been introduced.13,14 This ‘Lewis acid–surfactant-combined catalyst (LASC)’ forms stable colloidal dispersion systems with organic substrates in water and efficiently catalyzes the aldol reactions.In the course of our investigations on stereoselective reactions in aqueous media, chiral copper(ii)-catalyzed asymmetric aldol reactions have been successfully carried out in a water–EtOH solution.15 These reactions have great advantages over the previously reported catalytic asymmetric aldol reactions16–18 because our system does not require strictly anhydrous conditions and extremely low temperature such as –78 °C.In order to develop this system further, we planned to use LASCs for asymmetric aldol reactions in water without using any organic solvents. Here we report that a combination of copper bis(dodecyl sulfate) [Cu(DS)2]19,20 and chiral bis(oxazoline) ligands21 has achieved the catalytic asymmetric aldol reactions in water with the aid of catalytic amounts of carboxylic acids as additives.The use of water as a solvent has a range of obvious advantages as a replacement for traditional organic solv e n t s . Work is being carried out on two main fronts, ‘normal’ c o nditional (see for example A.McCluskey, Green Chemistry, 1999, 1, 167) and supercritical/close to critical (‘hot’ water see e . g. M. Poliakoff, Green Chemistry, 1999, 1, 65). Despite the intuitive keep-water-out feeling that most of us have about organic reactions, a surprising number of organic reactions run very well in water. This article describes the early stages in the development of a useful procedure for the asymmetric aldol reaction in water, using a combined Lewis acid-surfactant catalyst system. This paper demonstrates that it is possible to obtain good enantioselectivity in such a system, although for this to be genuinely useful will require some further development.DJM Green Context Catalytic asymmetric aldol reactions in water using a chiral Lewis acid–surfactant-combined catalyst Sh–u Kobayashi,* Yuichiro Mori, Satoshi Nagayama and Kei Manabe Graduate School of Pharmaceutical Sciences, The University of Tokyo, CREST, Japan Science and Technology Corporation (JST), Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.E-mail: skobayas@mol.f.u-tokyo.ac.jp Received 3rd June 1999 Results and discussion We chose the reaction of benzaldehyde with 3-trimethylsiloxypent- 2-ene (E :Z = 6:94) in water as a model.After several trials, a combination of Cu(DS)2 as a LASC and bis(oxazoline) 12 2 – 2 6 as a chiral ligand was found to give the corresponding aldol adduct in 23% yield with 58% enantiomeric excess (ee) (Table 1, run 1). Quite recently, we have reported that addition of a small amount of a Brønsted acid, especially HCl, dramatically acceler - ated LASC-mediated aldol reactions in water.27 To improve the low chemical yield, therefore, we tested various Brønsted acids as additives for the present asymmetric aldol reactions.Although HCl slightly increased the yield of the product (Table 1, run 2), benzoic acid gave the product in better yields (runs 3, 4 and 5). It should be noted that not only the yield but also the enantioselectivity of the major diastereomer was improved by addition of the acid.Neither a stronger Brønsted acid, camphorsulfonic acid (run 6), nor a weaker one, phenol (run 7), was better than benzoic acid. Among the carboxylic acids tested, lauric acid gave the best result as far as both the yield and the ee of the syn isomer were concerned (run 11).As for the substituents of the bis(oxazoline) ligand, isopropyl groups were found to give better results than isobutyl, tert-butyl, phenyl or benzyl groups. Green Chemistry August 1999 175 C GThe present reaction conditions could be applied to other substrates affording the products with the comparable enantioselectivities (eqns. 1 and 2). Although the yields and the selectivities are still not yet optimized, it is noted that these enantioselectivities have been attained at ambient temperature in water.The precise mechanism of the effects of the Brønsted acids on the yields and the enantioselectivities is not clear at this moment. We assume that the Brønsted acids raise the Lewis acidity of the Cu cation, producing a more active catalytic species.It is noteworthy that the hydrophobic and hydrophilic structure as well as the acidity of the Brønsted acids affects the results of the reactions. 28 A general experimental procedure is as follows. Cu(DS)2 (0.10 mmol) and 1 (0.12 mmol) were added to water (1.5 mL) at 23 °C. A Brønsted acid (0.05 mmol), an aldehyde (0.50 mmol) and then a silyl enolate (0.75 mmol) were successively added.After stirring the reaction mixture for 20 h at 23 °C, satd. aq. NaHCO3 and brine were added. The mixture was extracted with ethyl acetate three times, and concentrated. This was dissolved in 1 m aq. HCl–THF (1: 20) at 0 °C, and the whole was stirred for 1 h. After evaporation of the solvents, saturated aq. NaHCO3 and brine were added. The mixture was extracted with ethyl acetate three times, dried over Na2SO4, concentrated, and purified by silica gel chromatography to give the desired product.The diastereoselectivity was determined by 1H NMR, and the enantioselectivity was determined by HPLC analysis. Conclusion Catalytic asymmetric aldol reactions of aldehydes with silyl enolates in water have been achieved by using a combined catalytic system of the CuII-based LASC, chiral ligand 1 and a carboxylic acid.This is the first example of Lewis acid-catalyzed asymmetric aldol reactions in water without using organic solvents.29 It is noted that this reaction system does not need strictly anhydrous conditions and low temperature in contrast with the conventional examples of chiral Lewis acid-catalyzed asymmetric aldol reactions in organic solvents.In addition, the present reaction system may be applied to precise enzymatic reactions in flasks in terms of asymmetric reactions occurring at a hydrophobic reaction site created in water. Further investigations to improve the yield and the selectivity and to clarify the detailed reaction mechanism in these reactions are now in progress. Acknowledgements This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture, Japan.Y. M. and S. N. thank the JSPS fellowship for Japanese Junior Scientists. References 1 P. A. Grieco, in Organic Synthesis in Water, London, 1998. 2 C.-J. Li, Chem. Rev., 1993, 93, 2023. 3 T. Mukaiyama, K. Narasaka and T. Banno, Chem.Lett., 1973, 1011. 4 T. Mukaiyama, K. Banno and K. Narasaka, J. Am. Chem. Soc., 1974, 96, 7503. 176 Green Chemistry August 1999 Table 1 Effect of Brønsted acids on asymmetric aldol reactions in water Run Brønsted acid t/h Yield(%) syn:anti ee (syn)b 1 — 20 23 3.2 :1 58 2 HCl 12 31 2.7 :1 61 3 PhCO2H 12 48 2.7 :1 65 4 PhCO2H 20 76 2.7 :1 63 5 PhCO2Hc 20 70 2.8 :1 66 6 (+)-camphorsulfonic acid 20 34 3.0 :1 63 7 PhOH 20 15 2.7 :1 60 8 p-O2NC6H4CO2H 20 63 2.4 :1 64 9 p-MeOC6H4CO2H 20 73 3.0 :1 61 10 C5H11CO2H 20 72 3.3 :1 58 11 C11H23CO2H 20 76 2.8 :1 69 12 C17H35CO2H 20 68 2.8 :1 63 aDS = dodecyl sulfate (O3SOC12H25).b Determined by HPLC analysis using DAICEL CHIRALPAK AS. c 5 mol%.5 T. Mukaiyama, Org. React., 1982, 28, 203. 6 S. Kobayashi, Chem. Lett., 1991, 2087. 7 S. Kobayashi and I. Hachiya, Tetrahedron Lett., 1992, 33, 1625. 8 S. Kobayashi and I. Hachiya, J. Org. Chem., 1994, 59, 3590. 9 S. Kobayashi, S. Nagayama and T. Busujima, Chem. Lett., 1997, 959. 10 S. Kobayashi, S. Nagayama and T. Busujima, J. Am. Chem. Soc., 1998, 120, 8287. 11 J. B. F. N. Engberts, B. L. Feringa, E. Keller and S. Otto, Recl. Trav. Chim. Pays-Bas, 1996, 115, 457. 1 2 S. Kobayashi, T. Wakabayashi, S. Nagayama and H. Oyamada, Tetrahedron Lett., 1997, 38, 4559. 13 S. Kobayashi and T. Wakabayashi, Tetrahedron Lett., 1998, 39, 5389. 14 K. Manabe and S. Kobayashi, Synlett, 1999, 547. 15 S. Kobayashi, S. Nagayama and T. Busujima, Chem. Lett., 1999, 71. 16 H. Groger, E. M. Vogl and M. Shibasaki, Chem. Eur. J., 1998, 4, 1137. 17 S. G. Nelson, Tetrahedron: Asymmetry, 1998, 9, 357. 18 T. Bach, Angew. Chem., Int. Ed. Engl., 1994, 33, 417. 1 9 Y. Moroi, K. Motomura and R. Matsuura, J. Colloid Interface Sci., 1974, 10, 3455. 20 S. Otto, J. B. F. N. Engberts and J. C. T. Kwak, J. Am. Chem. Soc., 1998, 120, 9517. 21 A. K. Ghosh, P. Mathivanan and J. Cappiello, Tetrahedron: Asymmetry, 1998, 9, 1. 22 A. S. Gokhale, A. B. E. Minidis and A.Pfaltz, Tetrahedron Lett., 1995, 36, 1831. 23 D. A. Evans, K. A. Woerpel, M. M. Hinman and M. M. Faul, J. Am. Chem. Soc., 1991, 113, 726. 24 D. A. Evans, S. J. Miller and T. Lectka, J. Am. Chem. Soc., 1993, 115, 6460. 2 5 D. A. Evans, C. S. Burgey, M. C. Kozlowski and S. W. Tregay, J. Am. Chem. Soc., 1999, 121, 686. 26 D. A. Evans, M. C. Kozlowski, J. A. Murry, C. S. Burgey, K. R.Campos, B. T. Connel and R. J. Staples, J. Am. Chem. Soc., 1999, 121, 669. 27 K. Manabe and S. Kobayashi, Tetrahedron Lett., 1999, 40, 3773. 28 Several examples of cooperative catalysis of Brønsted acids and Lewis acids have been reported. (a) M. V. Deaton and M. A. Ciufolini, Tetrahedron Lett., 1993, 34, 2409; (b) K. Ishihara, M. Miyata, K. Hattori, T. Tada and H. Yamamoto, J.Am. Chem. Soc., 1994, 116, 10520; (c) K. Ishihara, H. Kurihara, M. Matsumoto and H. Yamamoto, J. Am. Chem. Soc., 1998, 120, 6920; (d) V. K. Aggarwal, E. Anderson, R. Giles and A. Zaparucha, Tetrahedron: Asymmetry, 1995, 6, 1301; (e) H. C. Aspinall, N. Greeves and E. G. McIver, Tetrahedron Lett., 1998, 39, 9283. 29 For Lewis acid-catalyzed asymmetric Diels–Alder reactions in water, see: S.Otto, G. Boccaletti and J. B. F. N. Engberts, J. Am. Chem. Soc., 1998, 120, 4238. Paper 9/04439F Green Chemistry August 1999 1775 T. Mukaiyama, Org. React., 1982, 28, 203. 6 S. Kobayashi, Chem. Lett., 1991, 2087. 7 S. Kobayashi and I. Hachiya, Tetrahedron Lett., 1992, 33, 1625. 8 S. Kobayashi and I. Hachiya, J. Org. Chem., 1994, 59, 3590. 9 S. Kobayashi, S. Nagayama and T.Busujima, Chem. Lett., 1997, 959. 10 S. Kobayashi, S. Nagayama and T. Busujima, J. Am. Chem. Soc., 1998, 120, 8287. 11 J. B. F. N. Engberts, B. L. Feringa, E. Keller and S. Otto, Recl. Trav. Chim. Pays-Bas, 1996, 115, 457. 1 2 S. Kobayashi, T. Wakabayashi, S. Nagayama and H. Oyamada, Tetrahedron Lett., 1997, 38, 4559. 13 S. Kobayashi and T. Wakabayashi, Tetrahedron Lett., 1998, 39, 5389. 14 K. Manabe and S. Kobayashi, Synlett, 1999, 547. 15 S. Kobayashi, S. Nagayama and T. Busujima, Chem. Lett., 1999, 71. 16 H. Groger, E. M. Vogl and M. Shibasaki, Chem. Eur. J., 1998, 4, 1137. 17 S. G. Nelson, Tetrahedron: Asymmetry, 1998, 9, 357. 18 T. Bach, Angew. Chem., Int. Ed. Engl., 1994, 33, 417. 1 9 Y. Moroi, K. Motomura and R. Matsuura, J. Colloid Interface Sci., 1974, 10, 3455. 20 S. Otto, J. B. F. N. Engberts and J. C. T. Kwak, J. Am. Chem. Soc., 1998, 120, 9517. 21 A. K. Ghosh, P. Mathivanan and J. Cappiello, Tetrahedron: Asymmetry, 1998, 9, 1. 22 A. S. Gokhale, A. B. E. Minidis and A. Pfaltz, Tetrahedron Lett., 1995, 36, 1831. 23 D. A. Evans, K. A. Woerpel, M. M. Hinman and M. M. Faul, J. Am. Chem. Soc., 1991, 113, 726. 24 D. A. Evans, S. J.Miller and T. Lectka, J. Am. Chem. Soc., 1993, 115, 6460. 2 5 D. A. Evans, C. S. Burgey, M. C. Kozlowski and S. W. Tregay, J. Am. Chem. Soc., 1999, 121, 686. 26 D. A. Evans, M. C. Kozlowski, J. A. Murry, C. S. Burgey, K. R. Campos, B. T. Connel and R. J. Staples, J. Am. Chem. Soc., 1999, 121, 669. 27 K. Manabe and S. Kobayashi, Tetrahedron Lett., 1999, 40, 3773. 28 Several examples of cooperative catalysis of Brønsted acids and Lewis acids have been reported. (a) M. V. Deaton and M. A. Ciufolini, Tetrahedron Lett., 1993, 34, 2409; (b) K. Ishihara, M. Miyata, K. Hattori, T. Tada and H. Yamamoto, J. Am. Chem. Soc., 1994, 116, 10520; (c) K. Ishihara, H. Kurihara, M. Matsumoto and H. Yamamoto, J. Am. Chem. Soc., 1998, 120, 6920; (d) V. K. Aggarwal, E. Anderson, R. Giles and A. Zaparucha, Tetrahedron: Asymmetry, 1995, 6, 1301; (e) H. C. Aspinall, N. Greeves and E. G. McIver, Tetrahedron Lett., 1998, 39, 9283. 29 For Lewis acid-catalyzed asymmetric Diels–Alder reactions in water, see: S. Otto, G. Boccaletti and J. B. F. N. Engberts, J. Am. Chem. Soc., 1998, 120, 4238. Paper 9/04439F Green Chemistry August 1999 177

ISSN:1463-9262    

DOI:10.1039/a904439f

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Summary The scope and utility of fluorous (perfluorinated) fluids as reaction media for Lewis acid catalyzed Friedel–Crafts reaction are described. Introduction The Friedel–Crafts reaction is one of the most important synthetic reactions, especially in industrial production.1 Usually, this reaction is carried out in toxic and/or harmful organic reaction media like CH2Cl2, CS2, etc.2 Ogawa and Curran reported that benzotrifluoride (BTF) is a useful alternative solvent to CH2Cl2 and can be used in Friedel–Crafts acylation.3 However, BTF reacts with AlCl3 which is typically used in Friedel–Crafts reactions (Scheme 1).4 In addition, BTF is sensitive to some reducing Scheme 1 conditions and hydrolyzed by aqueous acid at high temperature.5 These reactivities clearly limit its utility as a reaction medium.Recent studies of fluorous (perfluorinated) fluids as new alternative reaction media are having an important impact on organic reactions.6 These fluids all have very unusual properties, such as high density and high stability, low solvent strength, and extremely low solublity in water and organic materials.6 However, the scope and limitations of the utility of fluorous media are still unclear.In our previous paper,7 we reported the ease of handling and re-use of fluorous fluids. In this paper, we describe the utility of fluorous fluids as the reaction medium for Friedel–Crafts reactions. Results and discussion As shown in Table 1, acetylation proceeds smoothly in every fluorous reaction medium tested as well as in CH2Cl2, and negligible differences in solvent effect were observed.The catalyzed acetylation of benzene and p-xylene using an equimolar amount CF3 + AlCl3 CCl3 + AlF3 Friedel–Crafts reaction in fluorous fluids Hirofumi Nakano and Tomoya Kitazume* Department of Bioengineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku Yokohama 226- 8501, Japan. E-mail: tkitazum@bio.titech.ac.jp Received 4th May 1999 of AlCl3 was performed at room temperature by employing perfluorotriethylamine as the solvent, resulting in 89 and/or 55% yields, respectively; however the reaction in hexane did not proceed.We have reported that fluorous solvents can be more than 90% recovered by three phase extraction,7 and it is possible to reuse them in the same reaction system.These results show that fluorous liquids possess the possibility to be good reaction media, especially for Friedel–Crafts reaction. Common problems in the use of stoichiometric amounts of AlCl3 are its instability and the disposal of the stoichiometric amount of Al(OH)3 after aqueous work-up. In view of ‘Atom Economy’,8 catalytic reactions are preferable. Hence, catalytic acylation with ZnCl2 was next investigated (Table 2).This benzoylation was performed at reflux temperature in highly toxic sym-tetrachloroethane (entry 11).9 The same reaction was carried out in perfluoro-2-butyltetrahydrofuran under the same conditions (entry 12) and the benzoylated product was isolated in comparable yield. In entry 13, the same reaction was successfully carried out in perfluorotriethylamine at a lower reflux temperature. These results show that fluorous media with lower reflux temperatures and non-flammability are good substitutes in Friedel–Crafts acylation for conventional organic liquids.For economical and environmental reasons, recycling of catalysts is favorable. Thus far, the use of catalysts bearing perfluorinated ligands and their recovery after reaction using a fluorous biphase system6 has been of great concern to chemists.However, such ‘fluorous catalysts’ are not commercially available and their synthesis often requires tedious steps and expensive starting materials. Therefore, before pursuing perfluorinated catalysts, we Green Chemistry August 1999 179 C G One of the main issues in green chemistry is the choice of solvent.Many solvents such as benzene and carbon tetrachloride are now considered as unusable due to toxicity problems, and others such as hydrocarbons are problematic due to difficulties with flammability and volatility. Ionic liquids and supercritical fluids are some of the newer choices available to the chemist (see e.g. Green Chemistry, 1999, 1 23, 65 and 91). A third class of solvents which may be of some benefit are the fluorous materials —highly fluorinated molecules with exceptional stability.The work in this paper describes the use of these solvents in the Friedel–Crafts reaction, a particularly aggressive reaction medium, involving strong Lewis acids such as aluminium chloride, and also a very important reaction industrially. DJM Green Contexttried to utilize commercially available catalysts which can be recycled easily.Sc(OTf)3 has been shown to be a good Friedel–Crafts catalyst and can be recovered quantitatively after extractive work-up in an aqueous phase separated from the organic products.10 As shown in Scheme 2, Sc(OTf)3 effectively catalyzes the acetylation of anisole in perfluorinated solvents and only the p-adduct was obtained in 69%, 3 times the average isolated yield.Moreover, benzaldehyde dimethylacetal was also reacted with anisole to produce a disubstituted material in 57% yield (Scheme 3). Further, Sc(OTf)3 can be recovered free from organic products by simple extractive work-up. Successive reuse of the recovered Sc(OTf)3 and solvent in the same reaction without further purification yielded the product in 40% yield (Fig. 1). In conclusion, we have shown that perfluorinated liquids are good alternative reaction media for use in the Lewis acid catalyzed Friedel–Crafts reaction. 180 Green Chemistry August 1999 Table 1 Acetylation reactions using different solvents Entry Arene Solvent Product Yield (%)a 1 Benzene Perfluorotriethylamine 89 2 Perfluorotri-n-butylamine 81 3 Perfluoro-2-butyltetrahydrofuran 87 4 p-Xylene Perfluorotriethylamine 55 5 Perfluorotri-n-butylamine 57 6 Perfluoro-2-butyltetrahydrofuran 46 7 Perfluoro-n-hexane 63 8 CH2Cl2 78 9 Anisole Perfluorotriethylamine 89b 10 Mesitylene Quantitative a Isolated yield.b Only p-adduct was obtained.O MeO O O O R O Cl + R O rt, overnight AlCl3 (1.0 eq.) Table 2 Catalytic acylation with ZnCl2 Reflux Yield Entry Solvent temp./°Ca (%)b 11 sym-Tetrachloroethane 138 66c 12 Perfluoro-2-butyltetrahydrofuran 99–107 62 13 Perfluorotriethylamine 70 64 a Bath temperature.b Isolated yield. c Ref. 9. OMe But O Ph Cl + OMe But Ph O reflux, 40 h ZnCl2 (10 mol %) Scheme 2 OMe Ac2O + OMe O Sc(OTf)3 (20 mol %) perfluorotriethylamine rt, 4 h cycle 1: 69% cycle 2: 40%Scheme 3 Fig. 1 Notes and references 1 G.A. Olah, Friedel–Crafts Chemistry, Wiley, New York, 1973. 2 Review: P. H. Gore, Chem. Rev., 1955, 55, 229. 3 A. Ogawa and D. P. Curran, J. Org. Chem., 1997, 62, 450. 4 A. L. Henne and M. S. Newman, J. Am. Chem. Soc., 1938, 60, 1697; R. K. Ramchandani, R. D. Wakharkar and A. Sudalai, Tetrahedron Lett., 1996, 37, 4063. 5 R. E. Banks, B.E. Smart and J. C. Tatlow, Organofluorine Chemistry, Plenum, New York, 1994. 6 Selected examples: I. T. Horváth and J. Rábai, Science, 1994, 266, 72; S. G. DiMagno, P. H. Dussault and J. A. Schultz, J. Am. Chem. Soc., 1996, 118, 5312; A. Studer, S. Hadida, R. Ferritto, S.-Y. Kim, P. Jeger, P. Wipf and D. P. Curran, Science, 1997, 275, 823; I. Klement, H. Lütjens and P. Knochel, Angew.Chem., Int. Ed. Engl., 1997, 36, 1454. 7 H. Nakano and T. Kitazume, Green Chemistry, 1999, 1, 21. 8 B. M. Trost, Angew. Chem., Int. Ed. Engl., 1995, 34, 259. 9 M. Kulka, J. Am. Chem. Soc., 1954, 76, 5469. 10 A. Kawada, S. Mitamura and S. Kobayashi, Synlett, 1994, 545; T. Tsuchimoto, K. Tobita, T. Hiyama and S. Fukuzawa, Synlett, 1996, 557; T. Tsuchimoto, T. Hiyama and S.Fukuzawa, Chem. Commun., 1996, 2345; T. Tsuchimoto, K. Tobita, T. Hiyama and S. Fukuzawa, J. Org. Chem., 1997, 62, 6997; H. Kotsuki, T. Oshisi and M. Inoue, Synlett, 1998, 255. 11 Typical procedure is as follows. To a solution of acetyl chloride (0.356 ml, 5 mmol), aluminium chloride (667 mg, 5 mmol) and perfluorotriethylamine (3 ml) at 0 °C, benzene (0.444 ml, 4 mmol) was added. After completion of the addition, the ice bath was removed and the reaction mixture was three-phase extraction aqueous layer organic layer fluorous reaction medium Sc(OTf)3 organic product recycle recycle OMe OMe Ph Sc(OTf)3 (20 mol %) perfluorotriethylamine rt, overnight 57 % OMe Ph OMe + OMe stirred overnight at room temperature. The reaction was quenched with 4 ml of water, and stirred for a few minutes. Then, the fluorous solvent was recovered by three phase extraction, and the organic layer was extracted with ethyl acetate. The organic layer was dried over anhydrous Na2SO4 and concentrated in vacuo. Purification of the residue by silica gel chromatography afforded the product. Paper 9/03544C Green Chemistry August 1999 181

ISSN:1463-9262    

DOI:10.1039/a903544c

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Summary The reaction of carboxylic acids with propadiene or propyne catalysed by [Ru(m- O2C H ) ( C O )2( P P h3) ]2 affords isopropenyl esters via regioselective addition of the carboxylate to the central carbon atom of the allene. Introduction The C3 mixture resulting from cracking of hydrocarbon feedstocks mainly contains propene and propane, together with propyne and propadiene. The possibility of direct utilization of this C3 mixture without separation of its constituents to produce high added-value compounds via a clean and selective catalytic transformation would increase the synthetic value of this hydrocarbon fraction and be of major interest from an economical point of view.The ability of ruthenium complexes to activate terminal alkynes in a stoichoimetric manner is well documented1 and new ruthenium-catalysed transformations of alkynes with high regioand stereoselectivity and atom economy have been developed.2–4 By contrast, the analogous catalytic activation of allenes by ruthenium catalysts has not been performed yet.Isopropenyl esters5 and diisopropenyl diesters,6 respectively prepared from carboxylic acids and oxalic acid, and rutheniumactivated propyne have been shown to be useful acylating and 1,2-diacylating reagents under very mild and neutral conditions releasing only acetone as by-product. The presence of allene together with propyne in the industrial hydrocarbon C3 mixture provided impetus to study the addition of carboxylic acids to allene in the presence of a ruthenium catalyst, as we had previously shown that the addition of acids to alkenes did not take place under these catalytic conditions. The palladium-catalysed additions of carbonucleophiles,7 amines,8 tosylhydrazine9 and carboxylic acids10 to unactivated allenes have recently been reported, all of them afford allylic derivatives resulting from an intermediate allylpalladium species generated via insertion of the terminal double bond of the allene into a Pd–H bond.We report here that ruthenium catalysis offers the first example of metal-catalysed activation of propadiene towards the addition of carboxylic acids to selectively produce isopropenyl esters v i a a d d ition of the carboxylate to the central C-2 carbon atom (Scheme 1). Results and discussion In the presence of (p-cymene)RuCl2(PPh3) as catalyst, which is known to give the Markovnikov addition of carboxylic acids to The conversion of small hydrocarbon feedstocks to more functional materials is of vital importance to the chemical industry.The ability to carry out such transformations without the need for pre-separation of components in a complex mixed feedstock is an additional benefit, as it reduces the complexity of the process and eliminates wasteful and energy-intensive separations.This article describes the conversion of a C3 feedstock containing a range of compounds (including propadiene and propyne) to isopropenyl esters. Reaction involves catalytic hydrocarboxylation of both propyne and propadiene with high regioselectivity to C-2. The products are excellent acylating agents.DJM Green Context Utilization of an industrial feedstock without separation Ruthenium-catalysed hydrocarboxylation of propadiene and propyne Christian Bruneau,* Muriel Neveux-Duflos and Pierre H. Dixneuf* UMR 6509: CNRS - Université de Rennes, Organométalliques et Catalyse : Chimie et Electrochimie Moléculaires, Campus de Beaulieu, 35042 Rennes, France Received 29th April 1999 Scheme 1 terminal alkynes,3–5 the addition of benzoic acid to propyne at 100 °C for 20 h led to 87% of esters containing 91% of isopropenyl benzoate 1 (R = Ph).Under similar conditions, the activation of allene itself in the presence of (p-cymene)RuCl2(PPh3) was less efficient as only 44% of the acid was converted after 20 h at 100 °C and the total conversion required 40 h of heating.[Ru(m-O2CH)(CO)2(PPh3)]2 11 appeared to be a much better catalyst and the treatment of 20 mmol of propadiene with 10 mmol of benzoic acid in 10 mL of toluene at 100 °C for 15 h in the presence of 0.5 mol% of [Ru(m-O2CH)(CO)2(PPh3)]2 as catalyst precursor led to the complete conversion of the carboxylic acid into propenyl benzoates. An overall yield of 81% of ester was isolated by distillation under reduced pressure, which contained 90% of isopropenyl benzoate 1 and 10% of the two stereoisomers of prop-1-en-1-yl benzoate PhCO2CHNCHMe.Moreover, when a 1+1 mixture of propyne and allene was reacted with benzoic acid at 100 °C for 20 h in the presence of the same catalyst, complete conversion was observed and after distillation 86% of benzoates were collected containing isopropenyl benzoate as the major Green Chemistry August 1999 183 C Gcompound (>90%) with a small amount of prop-1-en-1-yl benzoate.Isopropenyl benzoate has a boiling point of 80 °C under 2 mm Hg and is easily separated from the gaseous starting C3 reactants. These first results clearly showed that in the presence of the binuclear ruthenium complex as catalyst, the regioselective addition of the carboxylate to the central carbon atom of both the allene and propyne leading to isopropenyl ester could be achieved.Among the C3 components, as only allene and propyne—and not propene and propane—are activated towards the addition of carboxylic acids in the presence of a ruthenium catalyst and as the reaction is complete, this catalytic transformation followed by a classical distillation allows an easy separation of the enol esters from the other hydrocarbons.From another viewpoint, this catalytic reaction actually makes possible the elimination of allene and propyne from the C3 mixture. The ruthenium-catalysed addition to allene was extended to other carboxylic acids. p-Chlorobenzoic acid led to the corresponding isopropenyl aromatic ester 2 (R = p-ClC6H4) in 80% yield and more than 90% regioselectivity after reaction for 20 h at 100 °C in toluene in the presence of [Ru(m- O2CH)(CO)2(PPh3)]2 as catalyst.Under the above conditions, the addition of N-protected Z-alanine gave the optically pure isopropenyl amino ester 3 in 86% yield with a high regioselectivity of 90% (Scheme 2).Scheme 2 Similarly, mandelic acid led to 77% of the dioxolanone 5 in the presence of [Ru(m-O2CH)(CO)2(PPh3)]2. We have shown11 that the dioxolanone 5 resulted from the formation of the intermediate isopropenyl mandelate 4 without racemization, and that the ruthenium catalyst promoted as well the intramolecular addition of the OH group to the activated CNC bond of 4 to produce 5 with a high stereoselectivity (Scheme 3).Scheme 3 It must be noted that the presence of a ruthenium catalyst precursor had no influence on the reaction of carboxylic acid with an activated allene such as methoxyallene. Indeed, the addition of benzoic acid to methoxyallene at 100 °C for 20 h with or without catalyst led to a mixture of the allylic esters 5 and (E) -6 in the ratio 5 7+43 (84% total yield) resulting from the addition of the carboxylate to both C-1 and C-3 carbons of the allene (Scheme 4).Scheme 4 The results in Table 1 show that the reactivity of allene in the presence of [Ru(m-O2CH)(CO)2(PPh3)]2 which provides the most efficient catalytic system is very similar to that of propyne. The addition of carboxylic acids to propadiene takes place with the same regioselectivity as the addition of thiols to allenes in the presence of Pd(OAc)2 12 but dramatically contrasts with the Pd0-catalysed addition of other pronucleophiles which leads to allylic derivatives via (p-allyl)Pd species resulting from hydroor carbopalladation of the allene.7–10 The reactivity of ruthenium catalysts corresponds to an electrophilic activation of one CNC double bond of the allene, whereas protonation of the precursor to give an allylruthenium intermediate is not favoured as is the insertion of allenes into a Pd–H bond (Scheme 5).Scheme 5 In conclusion, we have shown that the catalytic activation of propadiene towards the addition of carboxylic acids to form isopropenyl esters is very efficient in the presence of the binuclear ruthenium catalyst [Ru(m-O2CH)(CO)2(PPh3)]2.As propyne and propadiene present the same reactivity towards carboxylic acids under similar catalytic conditions, the above results have potential for the direct utilization of the industrial C3 mixture without previous separation of its constituents for the catalytic preparation of isopropenyl esters. The use of a recoverable and reusable ruthenium catalyst such as [Ru(O2CH)(CO)2(Ph2P(CH2CH2)nCH2CH3)]2 which is also very efficient in this type of catalytic addition13 might present advan - tages from economical, industrial and environmental viewpoints. The overall process requires the catalytic transformation followed by a simple distillation and can be achieved on the spot where C3 is produced, thus avoiding inflammable products transportation.References 1 (a) M. I. Bruce, Chem. Rev., 1991, 91, 197; (b) H. Le Bozec, K. Ouzzine and P. H. Dixneuf, Organometallics, 1991, 10, 2768; (c) D. Touchard, P. Haquette, S. Guesmi, L. Le Pichon, Table 1 Ruthenium-catalysed formation of isopropenyl esters from propyne and propadienea Yield from Yield from Me–CáCH H2CNCNCH2 cat. [Ru]1 cat. [Ru]2 Acid Ester (%) (%) Benzoic acid 1 873 90 p-Chlorobenzoic acid 2 583 80 Z-Alanine 3 723 76 (R)-Mandelic acid 5 7011 77 a Allene or propyne (30 mmol), carboxylic acid (20 mmol), toluene, 100 °C, 20 h.Z = PhCH2OCO; [Ru]1 = (pc y m e n e ) R u C l2( P P h3); [Ru]2 = [Ru(m- O2C H ) ( C O )2( P P h3) ]2. 184 Green Chemistry August 1999A. Daridor, L. Toupet and P. H. Dixneuf, Organometallics, 1997, 16, 3640. 2 (a) B. M. Trost, Chem. Ber., 1996, 1 2 9, 1313; (b) F. Kakiuchi, Y. Yamamoto, N. Chatani and S. Murai, Chem. Lett., 1995, 681; (c) T. Kondo, N. Suzuki, T. Okada and T. Mitsudo, J. Am. Chem. Soc., 1997, 1 1 9, 6187; (d) S. Dérien, D. Jan and P. H. Dixneuf, Tetrahedron, 1996, 52, 5511; (e) Y. Wakatsuki and H. Yamazaki, J. Organomet. Chem., 1995, 500, 349. 3 C.Bruneau, M. Neveux, Z. Kabouche, C. Ruppin and P. H. Dixneuf, Synlett, 1991, 755. 4 C. Bruneau and P. H. Dixneuf, Chem. Commun., 1997, 507. 5 C. Ruppin, P. H. Dixneuf and S. Lécolier, Tetrahedron Lett., 1988, 29, 5365. 6 M. Neveux, C. Bruneau, S. Lécolier and P. H. Dixneuf, Tetrahedron, 1993, 49, 2629. 7 (a) Y. Yamamoto, M. Al-Masum and N. Asao, J. Am. Chem. Soc., 1994, 116, 6019; (b) B.M. Trost and V. J. Gerusz, J. Am. Chem. Soc., 1995, 117, 5156; (c) L. Besson, J. Goré and B. Cazes, Tetrahedron Lett., 1995, 36, 3853; (d) Y. Yamamoto, Pure Appl. Chem., 1996, 68, 9. 8 (a) L. Besson, J. Goré and B. Cazes, Tetrahedron Lett., 1995, 36, 3857; (b) M. Al-Masum, M. Meguro and Y. Yamamoto, Tetrahedron Lett., 1997, 38, 6071. 9 S. Kamijo, M. Al-Masum and Y. Yamamoto, Tetrahedron Lett., 1998, 39, 691. 10 M. Al-Masum and Y. Yamamoto, J. Am. Chem. Soc., 1998, 120, 3809. 1 1 M. Neveux, B. Seiller, F. Hagedorn, C. Bruneau and P. H. Dixneuf, J. Organomet. Chem., 1993, 451, 133. 1 2 A. Ogawa, J.-I. Kawakami, N. Sonoda and T. Hirao, J. Org. Chem., 1996, 61, 4161. 13 O. Lavastre, P. Bebin, O. Marchaland and P. H. Dixneuf, J. Mol. Catal. A, 1996, 108, 29 Paper 9/03452H Green Chemistry August 1999 185A.Daridor, L. Toupet and P. H. Dixneuf, Organometallics, 1997, 16, 3640. 2 (a) B. M. Trost, Chem. Ber., 1996, 1 2 9, 1313; (b) F. Kakiuchi, Y. Yamamoto, N. Chatani and S. Murai, Chem. Lett., 1995, 681; (c) T. Kondo, N. Suzuki, T. Okada and T. Mitsudo, J. Am. Chem. Soc., 1997, 1 1 9, 6187; (d) S. Dérien, D. Jan and P. H. Dixneuf, Tetrahedron, 1996, 52, 5511; (e) Y.Wakatsuki and H. Yamazaki, J. Organomet. Chem., 1995, 500, 349. 3 C. Bruneau, M. Neveux, Z. Kabouche, C. Ruppin and P. H. Dixneuf, Synlett, 1991, 755. 4 C. Bruneau and P. H. Dixneuf, Chem. Commun., 1997, 507. 5 C. Ruppin, P. H. Dixneuf and S. Lécolier, Tetrahedron Lett., 1988, 29, 5365. 6 M. Neveux, C. Bruneau, S. Lécolier and P. H. Dixneuf, Tetrahedron, 1993, 49, 2629. 7 (a) Y. Yamamoto, M. Al-Masum and N. Asao, J. Am. Chem. Soc., 1994, 116, 6019; (b) B. M. Trost and V. J. Gerusz, J. Am. Chem. Soc., 1995, 117, 5156; (c) L. Besson, J. Goré and B. Cazes, Tetrahedron Lett., 1995, 36, 3853; (d) Y. Yamamoto, Pure Appl. Chem., 1996, 68, 9. 8 (a) L. Besson, J. Goré and B. Cazes, Tetrahedron Lett., 1995, 36, 3857; (b) M. Al-Masum, M. Meguro and Y. Yamamoto, Tetrahedron Lett., 1997, 38, 6071. 9 S. Kamijo, M. Al-Masum and Y. Yamamoto, Tetrahedron Lett., 1998, 39, 691. 10 M. Al-Masum and Y. Yamamoto, J. Am. Chem. Soc., 1998, 120, 3809. 1 1 M. Neveux, B. Seiller, F. Hagedorn, C. Bruneau and P. H. Dixneuf, J. Organomet. Chem., 1993, 451, 133. 1 2 A. Ogawa, J.-I. Kawakami, N. Sonoda and T. Hirao, J. Org. Chem., 1996, 61, 4161. 13 O. Lavastre, P. Bebin, O. Marchaland and P. H. Dixneuf, J. Mol. Catal. A, 1996, 108, 29 Paper 9/03452H Green Chemistry August 1999 185

ISSN:1463-9262    

DOI:10.1039/a903452h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Henry reactions catalysed by modified Mg–Al hydrotalcite: an efficient reusable solid base for selective synthesis of b-nitroalkanols† B. M. Choudary,*a M. Lakshmi Kantam,*a Ch. Venkat Reddy,a K. Koteswara Raoa and F. Figuerasb a Indian Institute of Chemical Technology, Hyderabad-500007, India. E-mail: choudary@iict.ap.nic.in b Institut de Recherches sur la Catalyse, 2, Avenue Albert Einstein, 69626 Villeurbanne cedex, France Received 20th May 1999 Summary Henry reactions have been performed for the first time with suit - ably activated Mg–Al hydrotalcite as catalyst in quantitative yields in the liquid phase under mild reaction conditions and with low reaction times.Exclusive synthesis of b-nitroalkanols (3) is realised by the compatible basic Brönsted hydroxy sites of the modified hydrotalcite.Reusability with consistent activity and selectivity for a number of cycles is established indicating that the hydrated hydrotalcite employed here is a potentially ecofriendly catalyst which can replace soluble bases in commercial synthesis. Introduction The design and development of environment-friendly solid base catalysts to replace soluble bases for C–C bond formation in organic transformations widely employed in the bulk and fine chemical industries in order to achieve atomic selectivity of the desired product and reduce the salts formed as a result of neutralisation of soluble bases is of intense research activity.1–3 2-Nitroalkanols, the products of the Henry reaction and important representatives of C–C bond formation, are ubiquitous materials extensively used in many important syntheses.4 The greatest challenge in the selective synthesis of 2-nitroalkanols among the multiple product options such as polymerization, aldol olefins and Cannizzaro products, is the selection of the appropriate base to achieve the most sensitive operation in the organic synthesis.Further, 2-nitroalkanols formed from aryl aldehydes tend to eliminate water to form nitroalkenes5 which readily polymerise.The classical methods for this important transformation involving the use of bases such as alkali metal hydroxides, carbonates, bicarbonates, alkoxides, barium and calcium hydroxide, magnesium and aluminium ethoxides, a rhodium complex, potassium exchanged zirconium phosphate and also organic bases such as primary, secondary and tertiary amines predominantly give dehydrated products.4 Thus, careful control of the basicity of the reaction medium is crucial to achieve better yields of b-nitroalcohols.Such efforts require longer reaction times but only give moderate yields.6,7 Heterogeneous catalysis induced by solid catalysts such as basic alumina,8 alumina-KF,9 amberlyst6 and phase transfer catalysis with surfactants10 are the two divergent approaches applied in an attempt to obtain improved selectivity.The former approach requires longer reaction times and in some cases affords condensed olefins.5,11 Although good selectivity has been achieved in phase transfer reactions, the reaction still requires a soluble base, which tends to give salts upon neutralisation at the end of the reaction.10 With this background, we envisaged the use of solid bases in the form of layered double hydroxides (LDHs) or hydrotalcite like compounds (HTLCs) in view of their potential usefulness as adsorbents, anion-exchangers and most importantly as basic catalysts.3,12 LDHs upon thermal decomposition at ca. 450 °C give highly active homogeneous mixed oxides, which are potential basic catalysts used for a variety of organic transformations.13–17 Recently, we reported a modified method2 for the activation of a hydrotalcite catalyst the basicity of which was tuned for base catalysed aldol, Knoevenagel and Michael reactions18,19 in the liquid phase under very mild reaction conditions.The significance of 2-nitroalkanols in organic synthesis4 and our continued interest involving hydrotalcites prompted us to explore the Henry reaction.Green Chemistry August 1999 187 C G The Henry reaction is an important class of C–C bond forming reaction. It operates under base catalysis conditions to give b-nitro alkanols, important intermediates for a variety of useful compounds such as amino alcohols. Difficulties which lead to poor selectivity and thus waste include dehydration (and possible subsequent reaction of the nitroalkene formed) and Cannizzaro reaction of the aldehydic component.This paper describes the application of Mg–Al hydrotalcites to the Henry reaction. Hydrotalcites are mixed oxides, in some ways similar to clays, but basic rather than acidic, and have found uses as medium strength base catalysts in recent years.They are readily prepared, and catalyse several reactions selectively. In the cases described here, they produce a selective and clean reaction under mild conditions. Catalyst reuse is also demonstrated. A fully optimised procedure would require further optimisation of the solvent used to wash the catalyst, and recovery and recycling of the excess nitroalkane.DJM Green Context † IICT Communication no. 4021.Experimental Here, we report a convenient and selective synthesis of 2- nitroalkanols via the Henry reaction,20 Scheme 1, affording 100% selectivity to b-nitroalcohols (3) in quantitative yields using modified Mg–Al hydrotalcite.18 Modified Mg–Al hydrotalcite (Mg/Al ratio = 2.5) was synthesized as follows: an aqueous solution (0.221 l) containing Mg(NO3)2·6H2O (0.2213 mol) (Aldrich) and Al(NO3)3·9H2O (0.0885 mol) (Aldrich) was added slowly to a second solution (0.221 l) containing NaOH (0.7162 mol) and Na2CO3 (0.2084 mol) in a 1.0 l round-bottom flask under vigorous stirring.The addition took nearly 3 h. Then the contents were heated to 338 K for 16 h. The precipitate formed was filtered off and washed with hot distilled water until the pH of the filtrate was 7.The precipitate was dried in an oven at 353 K for 15 h. The XRD pattern shows the presence of pure hydrotalcite, with lattice parameters corresponding to those reported in the literature.3 Scheme 1 Henry reactions between nitromethane and various substituted aromatic and aliphatic aldehydes. Table 1 Henry reactions of various substituted aromatic and aliphatic aldehydes with nitromethane catalysed by MHT a Determined by 1H NMR,based on starting aldehyde.b Isolated pure product. c Yield after fifth cycle. The synthesised Mg–Al hydrotalcite catalyst was first activated by calcination at to 723 K in a flow of air. The temperature was raised at a rate of 10 K min21 to 723 K and maintained at this temperature for 8 h.The solid was then cooled in dry nitrogen and rehydrated at room temperature under a flow of nitrogen gas (6 l h21) saturated with water vapour for c a . 6 h before use in reactions. Results and discussion Results of Henry aldol product synthesis are compiled in Table 1 with modified hydrotalcites showing the best catalytic performance. No dehydrated product (4) was observed even after continuing the reaction for prolonged periods when we deliberately chose aryl aldehydes as one of the reactants.The reused catalyst exhibited consistent activity and selectivity. Taking the Henry reaction of benzaldehyde with nitromethane as a model reaction we compared the activity and selectivity of the modified Mg–Al hydrotalcite catalyst with a variety of soluble and solid bases such as sodium hydroxide, neutral aluminium oxide, magnesium oxide and diamino-functionalised MCM-4121 under similar reaction conditions (Table 2).The reaction of benzaldehyde with nitromethane using magnesium oxide or neutral aluminium oxide catalyst required longer reaction times and gave poor yields, while the reaction with diamino-functionalised MCM-41 yields the nitroalkene as the sole reaction product.The Henry reaction was slightly sluggish under phase transfer conditions10 using sodium hydroxide as a soluble base. In summary, the results show that the modified Mg–Al hydrotalcite catalyst is more active and selective than the other catalysts. The Henry reaction usually requires Brönsted hydroxy sites to abstract a proton from an active methylene group of the nitro compound.X-Ray diffraction studies of the activated hydrotalcite show that the layered structure is removed upon calcination but is restored by rehydration and a meixnerite-like structure is formed in which OH2 are the compensating anions. These anions are mobile and therefore basic and catalyse several reactions proceeding through basic mechanisms such as aldolisation,2,22,23 Knoevenagel18 or Michael reactions.19 From their catalytic properties it is usually accepted that LDH has a pKb of ca. 11.4–12 and therefore is a base of moderate strength comparable to piperidine. 23 The lack of activity of calcined hydrotalcites reiterates that the reaction of Henry requires Brönsted bases of mild strength.The classical mechanism of the Henry reaction6 can therefore be applied here. The abstraction of a proton from the active methylene group of the nitro compound gives a carbanion which can be stabilised by the cationic charge of aluminium in the lattice of hydrotalcite as suggested in Scheme 2. In this scheme it is sug- Table 2 Henry reaction between benzaldehyde and nitromethane using various base catalysts Yield Entry Catalyst t/h (%)a 1 Modified Mg–Al hydrotalcite 0.5 95b 2 Aluminium oxide 12.0 37 3 Magnesium oxide 8.0 51 4 Sodium hydroxide with PTC 2.0 70c 5 Diamino-functionalised MCM-41 3.0 97d a Determined by 1H NMR, based on starting aldehyde, unless otherwise stated. b Isolated pure product.c Isolated yield, using sodium hydroxide with surfactant.10 d Using diaminofunctionalised MCM-41, nitroalkene is the sole product.21 188 Green Chemistry August 1999Scheme 2 A plausible mechanism for the Henry reaction between nitromethane and benzaldehyde catalysed by modified Mg–Al hydrotalcite.gested that this carbanion further adds to the carbonyl compound to form an intermediate A which, in turn, removes a proton from water to give the final nitro aldol product.It is of interest that this aldolisation reaction is very specific towards the nature of the Brönsted base.2,23 In spite of the large number of basic sites determined by calorimetric adsorption of CO2, decarbonated hydrotalcites are not active, as they do not have any Brönsted hydroxy groups. Conclusion Compatible Brönsted basic hydroxy sites present in the modified hydrotalcite give rise to selective nitroaldol reactions in excellent yields over short times more efficiently than methodologies described earlier.The following advantages are: (a) high catalytic activity under very mild liquid phase conditions, (b) easy separation of the catalyst by simple filtration, (c) excellent yields and 100% selectivity of b-nitroalkanols at high rates of reaction, (d) use of non-toxic and inexpensive materials, (e) recycling of the catalyst and (f) zero emission of pollutants.The present catalytic system is thus a potential alternative to soluble bases. Acknowledgements This work was carried out as part of the Indo-French co-operative programme, funded by IFCPAR (Project No. IFC/1106- 2/96/2460). References 1 Y.V. Subba Rao, D. V. De Vos and P. A. Jacobs, Angew. Chem., Int. Ed, Engl., 1997, 36, 2661. 2 K.Koteswara Rao, M. Gravelle, J. Sanchez and F. Figueras, J. Catal., 1998, 173, 115. 3 F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 1991, 11, 173. 4 G. Rosini, in Comprehensive Organic Synthesis, vol. 2, ed. C. H. Heathcock, ch. 1.10 p. 321, ed. B. M. Trost and I. Fleming, Pergamon Press, Oxford, 1991, and references therein. 5 B. P. Bandager, M. B. Zirange and P. P. Wandgaokar, Synlett, 1996, 149; G. Rosini, R. Ballini, M. Petrini and P. Sorrenti, Synthesis, 1985, 515. 6 R. Ballini, G. Bosica and P. Forconi, Tetrahedron, 1996, 52, 1677. 7 U. Contantino, M. Curini, F. Marmottini, O. Rosati and E. Pisani, Chem. Lett., 1994, 2215. 8 G. Rosini, R. Ballini and P.Sorrenti, Synthesis, 1983, 1014. 9 J.-M. Melot, F. Texier-Boullet and A. Foucaud, Tetrahedron Lett., 1986, 27, 493. 10 R. Ballini and G. Bosica, J. Org. Chem., 1997, 62, 425. 11 R. Ballini, R. Castagnani and M. Petrinini, J. Org. Chem., 1992, 57, 2160. 12 W. T. Reichle, J. Catal., 1985, 94, 547; J. G. Nunan, P. B. Himelfarb, R. G. Herman, K. Klier, C. E. Bogdan and G. W. Simmons, Inorg.Chem., 1989, 28, 3868; C. Busetto, G. Delpiero, G. Manara, F. Trifiro and A. Vaccari, J. Catal., 1984, 85, 260. 13 E. Suzuki and Y. Ono, Bull. Chem. Soc. Jpn., 1988, 61, 1008. 14 E. Suzuki, M. Okamoto and Y. Ono, J. Mol. Catal., 1990, 61, 283. 15 C. Cativiela, F. Figueras, J. I. Garcia, J. A. Mayoral and M. Zurbano, Synth. Commun., 1995, 25, 1745. 16 C. Cativiela, F.Figueras, J. M. Fraile, J. I. Garcia and J. A. Mayoral, Tetrahedron Lett., 1995, 36, 4125. 17 W. T. Reichle, US Pat., 4 458 026, 1984 to Union Carbide. 18 M. Lakshmi Kantam, B. M. Choudary, Ch. Venkat Reddy, K. Koteswara Rao and F. Figueras, Chem. Commun., 1998, 1033. 19 B. M. Choudary, M. Lakshmi Kantam, Ch. Venkat Reddy, K. Koteswara Rao and F. Figueras, J. Mol. Catal., in press. 20 Representative procedure: To a mixture of nitromethane (10 mmol) and benzaldehyde (2 mmol), 0.2 g of catalyst was added at room temperature and stirring was continued until completion of the reaction, as monitored by TLC. The catalyst was filtered off, washed with dichloromethane (10 ml 3 3) and the filtrate concentrated under reduced pressure. The crude product was purified by column chromatography (silica gel, 60–120 mesh; hexane–ethyl acetate 98+2 v/v) to give PhCH(OH)CH2NO2 (0.317 g, 95% yield).The product was characterised by comparison of its NMR and IR spectra with those reported in the literature. 10 21 M. Lakshmi Kantam and P. Sreekanth, Catal. Lett., 1999, 57, 227. 22 D. Tichit, M. H. Lhouty, A. Guida, B. Chiche, F. Figueras, A. Auroux and E. Garrone, J.Catal., 1995,151, 50. 23 F. Figueras, D. Tichit, M. Bennani Nasiri and R. Ruiz, in Catalysis of Organic Reactions, ed. F. E. Herkes, Marcel Dekker, New York, 1998, pp. 37–49. Paper 9/04075G Green Chemistry August 1999 189Scheme 2 A plausible mechanism for the Henry reaction between nitromethane and benzaldehyde catalysed by modified Mg–Al hydrotalcite. gested that this carbanion further adds to the carbonyl compound to form an intermediate A which, in turn, removes a proton from water to give the final nitro aldol product.It is of interest that this aldolisation reaction is very specific towards the nature of the Brönsted base.2,23 In spite of the large number of basic sites determined by calorimetric adsorption of CO2, decarbonated hydrotalcites are not active, as they do not have any Brönsted hydroxy groups. Conclusion Compatible Brönsted basic hydroxy sites present in the modified hydrotalcite give rise to selective nitroaldol reactions in excellent yields over short times more efficiently than methodologies described earlier.The following advantages are: (a) high catalytic activity under very mild liquid phase conditions, (b) easy separation of the catalyst by simple filtration, (c) excellent yields and 100% selectivity of b-nitroalkanols at high rates of reaction, (d) use of non-toxic and inexpensive materials, (e) recycling of the catalyst and (f) zero emission of pollutants.The present catalytic system is thus a potential alternative to soluble bases. Acknowledgements This work was carried out as part of the Indo-French co-operative programme, funded by IFCPAR (Project No.IFC/1106- 2/96/2460). References 1 Y. V. Subba Rao, D. V. De Vos and P. A. Jacobs, Angew. Chem., Int. Ed, Engl., 1997, 36, 2661. 2 K.Koteswara Rao, M. Gravelle, J. Sanchez and F. Figueras, J. Catal., 1998, 173, 115. 3 F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 1991, 11, 173. 4 G. Rosini, in Comprehensive Organic Synthesis, vol. 2, ed. C. H. Heathcock, ch. 1.10 p. 321, ed. B. M. Trost and I. Fleming, Pergamon Press, Oxford, 1991, and references therein. 5 B. P. Bandager, M. B. Zirange and P. P. Wandgaokar, Synlett, 1996, 149; G. Rosini, R. Ballini, M. Petrini and P. Sorrenti, Synthesis, 1985, 515. 6 R. Ballini, G. Bosica and P. Forconi, Tetrahedron, 1996, 52, 1677. 7 U. Contantino, M. Curini, F. Marmottini, O. Rosati and E. Pisani, Chem. Lett., 1994, 2215. 8 G. Rosini, R. Ballini and P. Sorrenti, Synthesis, 1983, 1014. 9 J.-M. Melot, F. Texier-Boullet and A. Foucaud, Tetrahedron Lett., 1986, 27, 493. 10 R. Ballini and G. Bosica, J. Org. Chem., 1997, 62, 425. 11 R. Ballini, R. Castagnani and M. Petrinini, J. Org. Chem., 1992, 57, 2160. 12 W. T. Reichle, J.Catal., 1985, 94, 547; J. G. Nunan, P. B. Himelfarb, R. G. Herman, K. Klier, C. E. Bogdan and G. W. Simmons, Inorg. Chem., 1989, 28, 3868; C. Busetto, G. Delpiero, G. Manara, F. Trifiro and A. Vaccari, J. Catal., 1984, 85, 260. 13 E. Suzuki and Y. Ono, Bull. Chem. Soc. Jpn., 1988, 61, 1008. 14 E. Suzuki, M. Okamoto and Y. Ono, J. Mol. Catal., 1990, 61, 283. 15 C. Cativiela, F.Figueras, J. I. Garcia, J. A. Mayoral and M. Zurbano, Synth. Commun., 1995, 25, 1745. 16 C. Cativiela, F. Figueras, J. M. Fraile, J. I. Garcia and J. A. Mayoral, Tetrahedron Lett., 1995, 36, 4125. 17 W. T. Reichle, US Pat., 4 458 026, 1984 to Union Carbide. 18 M. Lakshmi Kantam, B. M. Choudary, Ch. Venkat Reddy, K. Koteswara Rao and F. Figueras, Chem. Commun., 1998, 1033. 19 B. M. Choudary, M. Lakshmi Kantam, Ch. Venkat Reddy, K. Koteswara Rao and F. Figueras, J. Mol. Catal., in press. 20 Representative procedure: To a mixture of nitromethane (10 mmol) and benzaldehyde (2 mmol), 0.2 g of catalyst was added at room temperature and stirring was continued until completion of the reaction, as monitored by TLC. The catalyst was filtered off, washed with dichloromethane (10 ml 3 3) and the filtrate concentrated under reduced pressure. The crude product was purified by column chromatography (silica gel, 60–120 mesh; hexane–ethyl acetate 98+2 v/v) to give PhCH(OH)CH2NO2 (0.317 g, 95% yield). The product was characterised by comparison of its NMR and IR spectra with those reported in the literature. 10 21 M. Lakshmi Kantam and P. Sreekanth, Catal. Lett., 1999, 57, 227. 22 D. Tichit, M. H. Lhouty, A. Guida, B. Chiche, F. Figueras, A. Auroux and E. Garrone, J. Catal., 1995,151, 50. 23 F. Figueras, D. Tichit, M. Bennani Nasiri and R. Ruiz, in Catalysis of Organic Reactions, ed. F. E. Herkes, Marcel Dekker, New York, 1998, pp. 37–49. Paper 9/04075G Green Chemistry August 1999 189

ISSN:1463-9262    

DOI:10.1039/a904075g

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Summary Adsorbed on kaolinite, aromatic amines are readily condensed with formaldehyde to give the corresponding diaminodiphenylmethanes. The methodology is novel in its simplicity, selectivity and ecofriendly nature. Introduction Diaminodiphenylmethanes find use in a variety of applications including curing agents and chain extenders in polymers.1 The conventional preparation is cumbersome involving specific use of mineral acids and alkalis at various stages.Polymerization is unavoidable resulting in poor selectivity. 2 Heterogeneous catalysis on clays under solvent conditions with ultrasound irradiation and also in dry media under microwave conditions have been extensively studied compared to catalysis over clays in aqueous media.3–14 Here we report the synthesis of 4,4A-diaminodiphenylmethanes employing natural kaolinite as catalyst in aqueous media for the first time.It is known that kaolinites intercalate with molecules as polar as water, formamide, N,N-dimethylformamide, dimethyl sulfoxide and dimethylselenoxide within their layers.15 Nevertheless, the catalytic activity of kaolinitic clays have been seldom exploited contrary to their smectite counterparts.Commercial catalysts like K10, KSF, Filtrol, etc. are derived from the latter variety. Results and discussion The experimental procedure involves the following steps: (a) mixing of catalyst and water; (b) addition of amine to the agitated slurry, and (c) dropwise addition of formaldehyde to the catalyst–water–amine mixture and continuing stirring for the completion of the reaction.Work-up involves simple filtration followed by dissolution of the product in hot alcohol from which it is recrystallised. The method employed is simple, efficient, reproducible and avoids the use of hydrocarbons, acid, alkali etc. unlike the conventional procedure.2 It is noteworthy that no conversion takes place when the reaction is performed on commercial silica or alumina (chromatographic grade).Moreover, the catalyst employed is regenerable by washing with hot acetone. The catalytic performance studied up to 5 cycles of the same reaction is found to be steady. Results are presented in Table 1. The condensation of aniline with formaldehyde is typical for the general procedure. In a typical experiment, the catalyst (1 g) was stirred with 200 ml of water.Aniline (9.3 g, 0.1 mol) was added to this solution under stirring. To this formaldehyde solution (37%, 4.5 ml, An easy synthesis of 4,4A-diaminodiphenylmethanes on natural kaolinites Damodaran Bahulayan, Rugmini Sukumar, Kuzhunellil Raghavanpillai Sabu and Malathy Lalithambika* Regional Research Laboratory [CSIR], Thiruvananthapuram - 695 019, India.E-mail:lali@csrrltrd.ren.nic.in Received 29th July 1999 0.05 mol) was added slowly and stirring was continued. The precipitated 4,4A-diaminodiphenylmethane was extracted into hot alcohol from which it is recrystallised: mp 98 °C [lit, mp 96–98 °C].2 Conclusion To summarise, we have developed an efficient method of synthesis of 4,4A-diaminodiphenylmethane catalyzed by an ecofriendly catalyst, kaolinitic clay, which is abundantly available.The conversion is significantly high with product selectivity in the range 68–98%. Besides, the operational ease makes the process attractive and cost effective. Acknowledgements We thank Dr Vijay Nair, Director, RRL, Trivandrum, for his wholehearted cooperation during the progress of this work. Fellowships to D. B. and K.R. S. by the CSIR, Government of India is also acknowledged. Green Chemistry August 1999 191 C G The polymer industry requires large quantities of 4,4A- diaminodiphenylmethanes. These compounds find use in various applications as polymer additives. Current synthetic methods involve the condensation of anilines with formaldehyde under acidic conditions. As is common for such reactions, a substantial amount of waste acid needs to be neutralised, generating substantial aqueous salt waste which is likely to be contaminated with residual anilines and/or formaldehyde, both compounds of considerable toxicity.While a completely green solution to this problem might involve the replacement of this chemistry with an inherently less toxic combination of reagents, the replacement of the current homogeneous catalysts with a heterogeneous catalyst might ease the problems of a waste aqueous phase.Such a catalyst switch might improve the situation by allowing the easy separation of catalyst from product, without the need for neutralisation. Thus, the aqueous waste stream can be avoided, and the catalyst can be recovered. This article demonstrates a feasible process for this chemistry, using a readily available and reusable clay catalyst.Yields of a range of diamines are excellent, and the method avoids the use of problematic solvents. DJM Green ContextNotes and references The catalyst used is natural kaolinitic clay. SiO2 = 47.05%, Al2O3 = 36.98%, Fe2O3 = 0.34%, TiO2 = 0.34%, Na2O = 0.12%, K2O = 0.08%, Loss on ignition = 14.34%, BET surface area = 13.0 m2 g21, Hammett acidity function, Ho = 23.0 (0.03 mmol g21).The 4,4 A-diaminodiphenylmethanes obtained were characterized on the basis of 1H NMR (300 MHz), HPLC, GC-MS and elemental analysis. Selected data for 4,4A-diaminodiphenymethane: Anal. Calc. for C13H14N2+C 78.78, H 7.07, N 14.14%. Found: C 78.64, H 7.11, N 14.08%. 1H NMR (CDCl3, 300 MHz): d 3.4 (s, 2H, –CH2–), 3.7 (s, 4H, 2NH2), 6.4–6.8 (m, 8H, Ar).HPLC: CLC-NH2 (M) column, 1 ml min–1 CH3CN, Rt 3.07 min. GCMS +OV-101, m/z 298 (M·+). 1 Reaction Polymers, ed. W. F. Gum, W. Riese and H. Ulrich, Hanser, New York, Oxford University Press, 1992. 2 J. T. Scanlan, J. Am. Chem. Soc., 1935, 57, 890. 3 B. K. G. Theng, The Chemistry of Clay-Organic Reactions, Adam Hilger, London, 1974, p. 261. 4 W. G. Duben, J. M. Cogan and V. Behar, Tetrahedron Lett., 1990, 31, 3241. 5 D. Villemin, M. Hammadi and B. Martin, Synth. Commun., 1996, 26, 2895. 6 M. Hammadi and D. Villemin, Synth Commun., 1996, 26, 2901. 7 S. Chalais, P. Laszlo and S. Mathey, Tetrahedron Lett., 1986, 27, 2627. 8 P. G. Gassman and D. A. Singleton, J. Am. Chem. Soc., 1984, 106, 7993. 9 R.S. Verma and R. Dahia, Tetrahedron Lett., 1997, 38, 2043. 192 Green Chemistry August 1999 Table 1 Condensation of aromatic amines with formaldehyde, formation of 4,4A-diaminodiphenylmethanes Entry Amine Product Reaction time/min Isolated yield (%) 1 60 96 2 60 99 3 60 68 4 60 98 5 30 100 6 60 79 7 60 97 8 60 78 9 60 71 aAmine+formaldehyde = 2+1, ambient temperature.10 R. S.Verma, Green Chemistry, 1999, 1, 43. 11 K. R. Sabu, R. Sukumar and M. Lalithambika, Bull. Chem. Soc. Jpn., 1993, 66, 3535. 12 M. Lalithambika, Rugmini Sukumar, D. Bahulayan and K. R. Sabu, Bull. Catal. Soc. India , 1999, 9, 156. 13 A. Cornelis and P. Laszlo, Synthesis, 1985, 909. 14 D. Ponde, H. B. Borate, A. Sudalai, T. Ravindranathan and V. H. Deshpande, Tetrahedron Lett., 1996, 37, 4605. 15 C. Collet, A. Delville and P. Laszlo, Angew. Chem., Ind. Ed. Engl., 1990, 29, 535. Paper 9/06165G Green Chemistry August 1999 19310 R. S. Verma, Green Chemistry, 1999, 1, 43. 11 K. R. Sabu, R. Sukumar and M. Lalithambika, Bull. Chem. Soc. Jpn., 1993, 66, 3535. 12 M. Lalithambika, Rugmini Sukumar, D. Bahulayan and K. R. Sabu, Bull. Catal. Soc. India , 1999, 9, 156. 13 A. Cornelis and P. Laszlo, Synthesis, 1985, 909. 14 D. Ponde, H. B. Borate, A. Sudalai, T. Ravindranathan and V. H. Deshpande, Tetrahedron Lett., 1996, 37, 4605. 15 C. Collet, A. Delville and P. Laszlo, Angew. Chem., Ind. Ed. Engl., 1990, 29, 535. Paper 9/06165G Green Chemistry August 1999 193

ISSN:1463-9262    

DOI:10.1039/a906165g

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Summary The versatility of the one-pot synthesis of organically modified silicas is shown by the preparation of materials with very high loadings (up to 4.85 mmol g– 1) and by the synthesis of bifunctional catalysts. The synthesis of these materials is shown to be very atom efficient, with all the Si species being incorporated into the product, and 98% of the template being recovered, and successfully reused.Catalytic activity is excellent, and the new catalysts outperform both the traditional silica-based catalyst, and the initial versions of these materials. The incorporation of non-polar phenyl groups has a particularly pronounced positive effect on the rate of reaction, the yield and the catalyst lifetime. Introduction The development of a simple one-pot method for the preparation of organically modified hexagonal mesoporous silicas (HMS)1 – 3 h a s made available a range of different hybrid materials.These materials exhibit pore sizes which are in the mesoporous range and have a very narrow pore size distribution, making them excellent potential candidates for catalytic applications. We have published initial evaluations of the catalytic activity of two different amino-substituted HMSs. 3-Aminopropyl-HMS is an effective catalyst for the Knoevenagel reaction, giving good yields and excellent selectivity,2 whereas 3-N , N-dimethylaminopropyl-HMS is active in the Michael r e a c t i o n .3 One of the great strengths of the one-pot preparative method is that the loadings achieved can be much higher than those achieved by other routes. We have found that 2.5 mmol g21 l o a d e d materials are, not surprisingly, more active than the corresponding 1.1 mmol g21 systems in both the Knoevenagel and Michael systems.Loadings > 2.5 mmol g21 have been prepared for certain m a t e r i a l s ,4 and are effective in oxidation reactions, but do not exhibit the highly uniform pore system of the true HMSs.We present here results relating to the clean preparation of these materials, to the use of high loading materials in the Knoevenagel reaction, and to the development of highly active surface modified materials which display further advantages of activity, yield and lifetime in this reaction. Results and discussion 2.1 Preparation of the catalysts Detailed results on the preparation of the catalysts have already been published.1,5 To summarise, a solution of long chain amine Important organic transformations, including the Knoevenagel and Michael reactions, proceed under base catalysed conditions, and there is continuing interest in heterogeneous, reusable catalysts for these systems.This report describes the use of hexagonal mesoporous silicas modified with organic amine groups as base catalysts.The catalysts themselves are prepared in a highly atomefficient manner, and with effective recovery of the templating agent, to ensure that the environmental impact of catalyst manufacture AND the organic synthesis are minimised. Stewart J. Tavener, University of York Green Context Organically modified hexagonal mesoporous silicas Clean synthesis of catalysts and the effect of high loading and non-catalytic second groups on catalytic activity of aminederivatised materials Duncan J.Macquarrie Department of Chemistry, University of York, Heslington, York, UK YO10 5DD. E-mail:djm13@york.ac.uk Received 14th June 1999 (in this case, n-dodecylamine) is prepared in aqueous ethanol (46 ml ethanol–53 ml water). To this is added tetraethoxysilane (TEOS) and 3-aminopropyl(trimethoxy)silane (AMP-silane); the triethoxy derivative can also be used. After reaction is complete, the solid product is filtered off, and the template extracted with hot ethanol.One of the prerequisites for the incorporation of organic groups into the synthesis gel is that the template can be removed without damaging the incorporated groups.Thus, the use of quaternary ammonium templates, as described in the preparation of the MCMs,6 is not suitable for a one-step process as their removal requires the use of high temperatures. Neutral templates can be easily and completely removed by washing with e.g. ethanol. One of the key goals for us was to develop a synthetic method for the preparation of the catalysts which itself generated minimal quantities of waste.In order to do this, it is necessary to recover and reuse the template. This is in fact very easily achieved. After the reaction is complete, and the solid filtered off, one is left with a solid containing template, and a solution containing ethanol, water and amine—if the trimethoxy silane is used, then a small amount of methanol is also found in solution.Extraction of the solid recovers the remaining template, leaving a solution of amine in ethanol. Combination of both solutions gives a solution resembling the original templating solution. This contains a larger quantity of ethanol than necessary. Complete evaporation of this solution gave a 98% recovery of the Green Chemistry August 1999 195 C Gtemplate of a comparable purity to the original amine.No siliconcontaining material was found in the recovered amine, indicating complete copolymerisation during the formation of the catalyst. Re-dissolution of the amine in the templating solvents, and repeated reaction gave product with essentially identical physical and catalytic characteristics. Thus, a completely atom efficient synthesis can be achieved— all the silicon species are incorporated into the catalyst, and all the template can be recovered and reused.The only losses are potential solvent losses during solvent recovery. It should be possible to develop a procedure where the appropriate amount of ethanol is removed from the recovered combined solution and this should reduce processing losses even more.This has not yet been tested, since the relatively small scale makes it difficult to produce the exact ethanol–water ratio required for synthesis. Nonetheless, the process as it stands is a remarkably clean route to these catalysts. 2.2 Preparation and evaluation of high loading catalysts The preparation of the high loading peracids by Elings et al.4 has shown that materials with organic contents up to ca. 4.0 mmol g21 can be prepared by this route. This contrasts with post-modified silicas, where ca. 1.0 mmol g21 is achievable for amine containing materials, and typically < 0.5 mmol g21 is the maximum possible for other silanes. We have therefore prepared a series of catalysts varying the ratio of TEOS to AMP-silane from 9:1 to 1:1. The results are shown in Table 1.As can be seen, the materials prepared are structured up to 2.7 mmol g21; at higher loadings than this, the materials are amorphous, but still display respectable surface areas, with the highest loading being 4.85 mmol g21. Higher loadings have not yet been investigated, owing to the results obtained in the catalytic reactions, but no drop in yield was seen in the range tried. Catalytic activity was evaluated in the Knoevenagel reaction of ethyl cyanoacetate and cyclohexanone.This reaction was chosen as a reasonably challenging example of the Knoevenagel reaction. The usual ethyl cyanoacetate–benzaldehyde test reaction is much more facile with most base catalysts, but is anomalously slow with aminopropyl-HMS materials.2,7 Results for this reaction (in toluene at reflux, with continuous removal of water) with aminopropyl-HMS of different loading are presented in Table 2.As can be seen, the activity increases with loading up to ca. 4.0 mmol g21 and then drops off somewhat. The reasons for this behaviour are likely to be a complex interplay of many factors, Table 1 Properties of catalysts investigated Composition/mmol g21 (Sa/) Pore Catalyst AMP Ph Me m2 g21 size/nm 1 1.2 0 0 756 3.6 2b 1.2 0 0 748 3.7 3 2.7 0 0 745 3.7 4 4.0 0 0 132 10 (broad) 5 4.85 0 0 249 10 (broad) 6 1.15 1.15 0 947 2.0 7 1.1 0 1.1 790 2.1 8c 1.1 0 0 715 1.9 9 1.0 AMP-silica 254 8.0 (broad) a Specific surface area.b Repeat preparation of 1. c Prepared with n-octylamine template. such as the physical nature of the catalysts (pore size distribution, surface area, pore shape), the relative polarity of the catalyst sur face (which is expected to drop with increasing organic content) and the proportion of amine groups which are available to react.Indeed, in the higher loading, amorphous materials, it is likely that many aminopropyl groups are embedded within the bulk structure and cannot take part in catalysis, making the effective catalyst loading lower than the total nitrogen content would predict. 2.3 Catalysts containing two functional groups Since one of the most important criteria controlling the rates of heterogeneously catalysed reactions is sorption of substrates/ products to and from the surface, the ability to control the nature of the surface is important. Corma et al.8 have recently published details of improvements which are made possible by increasing the hydrophobicity of catalyst surfaces by post-treatment with trimethylsilylating agents.Mann et al.9 have recently reported the direct formation of organically modified MCMs containing two different groups. We have prepared and evaluated the catalytic activity of materials containing the aminopropyl catalytic centre, and a second catalytically inert group, designed to modify the polarity of the catalyst surface.This approach is extremely simple, and involves a synthesis mixture consisting of TEOS, the AMP-silane and a third alkyl or aryl substituted silane. The aminopropyl/phenyl and the aminopropyl/methyl materials 6 and 7 were tested in the Knoevenagel reaction and compared to two simple aminopropyl-HMSs 1 and 8.The physical characteristics of the catalysts are summarised in Table 1. The first of these (1) is a 3.6 nm pore diameter material with a surface area of 756 m2 g21, and has been used as an efficient catalyst for the Knoevenagel reaction.2 The second (8) has smaller pore size (1.9 nm) but similar surface area. Results are given in Table 3.Comparison is hindered somewhat by the differences in pore size between 1, 6, 7 and 8, but the relative rates of 1 and 8 indicate that initial rates of reaction are essentially independent of pore size for the reactants under investigation. The only significant difference between the two is that the lower pore sized material becomes poisoned much more rapidly.2 As can be seen, the methyl-substituted material displayed the same activity as the simple aminopropyl system 1.The phenyl containing material was, however, substantially more active than the simple materials. For example, the reaction between ethyl cyanoacetate 13 and cyclohexanone 10 took 2 h to go to completion with 1 (loading 1.2 mmol g21), and 1.5 h with 2.7 mmol g21 3; with 6 (1.15 mmol g21 aminopropyl and 1.15 mmol g21 Ph) the same reaction Table 2 Knoevenagel reaction (10 + 13)a catalysed by AMP-HMS materials with different loadings of amine Catalyst Loading/mmol g21 Yield (%) t/h 1 1.2 92 2 3 2.7 94 1.5 4 4.0 96 1.0 5 4.85 94 1.5 a 10 = Cyclohexanone; 13 = ethyl cyanoacetate. 196 Green Chemistry August 1999was complete in 30 min. Thus, reaction rates are much faster with 4 than with a catalyst with a higher loading of catalytic sites. (All reactions were carried out with the same mass of catalyst.) Little effect was seen with the methyl-substituted material, presumably due to the methyl groups being too small to exert any significant effect on the reaction space. Similar results were obtained with ethyl cyanoacetate and pentan- 3-one, where 6 again outperformed even the 2.5 mmol g21 aminopropyl catalyst 3.The very challenging substrate acetophenone is also converted to the corresponding Knoevenagel product in 86% yield within 24 h, significantly better than the results obtained with the simple aminopropyl catalysts. These results are consistent with the polarity of the surface being a major determining factor in the activity of the catalysts.A particularly important finding is that the turnover number (TON) is also substantially increased by having phenyl groups present. Increases in TON lie in the range of 2.5–4 compared to the larger pore materials, and by an order of magnitude compared to the silica-based catalyst 9. TONs were not measured for the 1.9 nm pore size material 8, since it became deactivated very rapidly.However, it can be estimated that TONs for the new catalyst 1 compared to 8 are higher by a factor of ca. 25–40 for most substrates. This represents a second major advantage of these catalysts over the simple materials. While we are not yet sure of the deactivation mechanism of the HMS catalysts, it does appear to be related to the preferential adsorption of the ketone component to the exclusion of the carbon acid.10 Such a mechanism should be very dependent on the surface chemistry of the catalyst, and the results here are again consistent with altered adsorption characteristics owing to the presence of the phenyl groups.Experimental All chemicals were obtained from Aldrich and were used as received. Porosimetry was carried out on a Coulter SA2100 porosimeter using dinitrogen as adsorbate.IR spectra were measured on a Bruker Equinox spectrometer fitted with an Environmental Chamber diffuse reflectance unit. 3.1 Preparation and recycling of template To a solution of n-dodecylamine (5.08 g, 27.5 mmol) in water–ethanol (53 ml water, 46 ml ethanol) were added, at room temperature, separately but simultaneously, TEOS (18.8 g, 0.09 mol) and AMP-silane (1.79 g, 0.01 mol).The mixture was initially clear, but became increasingly cloudy, and after 18 h was a thick white paste. Filtration of the mixture gave a white solid (catalyst + template) and filtrate (water + ethanol + template). The solid was then extracted with ethanol using a Soxhlet extractor (8 h). The solid was collected and dried at 110 °C.The yield of solid was 10.6 g. The template can be recovered by combination of the filtrate and the extract solutions and removal of solvent by evaporation. The resultant material is n -dodecylamine, pure by IR, NMR and melting point. Yield of recovered template, 4.99 g, 98%. Reuse of the template resulted in an identical reaction with the same yield (10.4 g) and physical characteristics (See Table 1, entries 1 and 2) . 3.2 Preparation of high-loading materials Higher loading materials were prepared by the above procedure, but using ratios of TEOS : AMP-silane of 4 : 1, 2 : 1 and 1 : 1. Total molar quantities of Si remained 0.1 mol. Yields were 9.86 g, 10.32 g and 9.95 g, respectively. Physical characteristics are given in Table 1. 3.3 Preparation of bifunctional materials Organically modified HMSs containing two organic groups were easily prepared by an extension of the original method. 3 Tetraethoxysilane (TEOS; 16.8 g, 0.08 mol), 3-aminopropyl (trimethoxy)silane (AMPS, 1.79 g, 0.01 mol) and phenyl(triethoxy) silane (2.40 g, 0.01 mol) were added separately but simultaneously and rapidly to a solution of n-dodecylamine (5.09 g) in a water–ethanol mixture (53 ml–46 ml) at 20 °C.The clear solution initially became turbid, then milky (after a few minutes), and finally solid began to appear (ca. 30 min). The suspension was allowed to age for 18 h. The solid was filtered off, and washed with ethanol. The amine template was then removed by Soxhlet extraction with ethanol (15 h). The resultant material was then dried overnight at 110 °C, to yield 8.95 g of a fine white powder 6.Elemental analysis indicated a loading of aminopropyl groups of 1.15 mmol g21, but phenyl loading could not be calculated from elemental analysis due to residual RO groups in the material. However, analysis of the liquid from the reaction indicated that no silicon-containing species were present, and thus the loading of phenyl groups is likely to be similar to that of aminopropyl groups.Green Chemistry August 1999 197 Table 3 Knoevenagel reactions of various aminopropyl–containing catalysts Composition Catalyst AMP Ph Me Reaction Yield (%) t/h Solvent TON 1 1.2 0 0 10 + 13 92 2 Toluene 2400 3 2.5 0 0 10 + 13 94 1.5 Toluene 2550 6 1.15 1.2 0 10 + 13 98 0.5 Toluene 5900 7 1.1 0 1.1 10 + 13 93 2 Toluene 2520 8 1.1a 0 0 10 + 13 49 2 Toluene 9 1.0 AMP-silicab 10 + 13 98 1 Cyclohexane 650 1 1.2 0 0 11c + 13 49 72 Toluene 55 3 2.5 0 0 11 + 13 48 36 Toluene 47 6 1.15 1.2 0 11 + 13 86 24 Toluene 215 9 1.0 AMP-silicab 11 + 13 68 72 Cyclohexane 250 1 1.2 0 0 12d + 13 95 18 Toluene 1127 3 2.5 0 0 12 + 13 97 4 Toluene 305 6 1.15 1.1 0 12 + 13 96 2.5 Toluene 3600 9 1.0 AMP-silicab 12 + 13 97 2 Cyclohexane 265 a 1.9 nm pore size catalyst: rate to ca. 40% conversion was identical to that from the larger pore catalysts X. b Cyclohexane was found to be the optimum solvent for AMP-silica; toluene the best for AMP-HMS catalysts. c 1 1 = Acetophenone. d 1 2 = Pentan-3-one.Fig. 1 The diffuse reflectance IR spectrum of the phenyl/aminopropyl spectrum is shown in Fig. 1. As expected, v(C–H) bands from the phenyl groups can be clearly seen at 3060 cm21, as well as v(C–H) vibrations from the aliphatic chains of the aminopropyl groups. A vibration at 1600 cm21 is also present and the invariance of this band with temperature (150–325 °C) suggests that it is due to N–H deformations rather than surface water. Aminopropyl/methyl HMS 7 was prepared in the same way, using methyl(triethoxy)silane in place of phenyl(triethoxy)silane. Again, aminopropyl loading was approximately the expected value of 1.1 mmol g21.Pore size distributions and surface areas were measured for both the phenyl and the methyl containing materials. In both cases the materials displayed isotherms characteristic of HMS materials with small pores and high surface areas (2.0 nm and 947 m2 g21 for 6; 2.1 nm and 790 m2 g21 for 7). 3.4 Reaction studies All reactions were carried out as described in ref. 4. A typical experiment involved the reaction of ethyl cyanoacetate (20 mmol) with the ketone (20 mmol) in the presence of 0.25 g catalyst in 25 ml of toluene at reflux. Water was separated by a Dean and Stark trap. Turnover numbers were calculated by reaction, decantation of supernatant, and addition of further substrate, until no more conversion was achieved.The reactions did not proceed with the reagents alone, nor did they proceed with unfunctionalised HMS or with phenyl-HMS. Conclusion High loading and bifunctional organically modified hexagonal mesoporous silicas can be easily prepared by a simple extension of the existing methodology.The materials prepared display significant advantages over the simple mono-functional materials in terms of rate, yield and catalyst lifetime. It is thought that the reasons for both these improvements lie in the more advantageous sorption of material to and from the surface. Acknowledgement D. J. M. thanks the Royal Society for a University Research Fellowship. References 1 D. J. Macquarrie, Chem. Commun., 1996, 1961. 2 D. J. Macquarrie, and D. B. Jackson, Chem. Commun., 1997, 1781. 3 D. J. Macquarrie, J. E. G. Mdoe and J. H. Clark, Synlett, 1998, 625 4 J. A. Elings, R. Ait-Meddour, J. H. Clark and D. J. Macquarrie, Chem Commun., 1998, 2707. 5 D. J. Macquarrie, D. B. Jackson, J. E. G. Mdoe and J. H. Clark, New J. Chem., 1999, 23, 539. 6 C. T. Kresge, M. E. Leonowicz, W. K. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 1992, 359, 710. 7 D. J. Macquarrie, A. Priest, A. Lambert, J. E. G. Mdoe and J. H. Clark, React. Funct. Polym., 1997, 35, 153. 8 A. Corma, M. Domine, J. A. Gaona, J. L. Jorda, M. T. Navarro, F. Rey, J. Perez Pariente, J. Tsuji, B. McCulloch and L. T. Nemeth, Chem. Commun., 1998, 2211. 9 S. R. Hall, C. E. Fowler, B. Lebeau and S. Mann, Chem Commun., 1999, 201. 10 D. J. Macquarrie, D. B. Jackson and J. H. Clark, unpublished results. Paper 9/04692E 198 Green Chemistry August 1999

ISSN:1463-9262    

DOI:10.1039/a904692e

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Summary A clean method for the rapid and solventless preparation of herbicides based on 2,4-dichlorophenoxyacetic acid (2,4-D esters) is reported. Micro-particulate inorganic materials such as silica, clays and zeolites are able to act as supports for the 2,4-D esterification reactions under mild conditions in the absence of organic solvents. The synthesis takes place with elevated yield and the conversion rate of the process is strongly enhanced when the reaction is microwave-assisted.Not only can well-defined supports be used, but soil can also be used for this synthesis, except when iron oxides are present which lead to extensive oxi - dation under microwave irradiation. The 2,4-D ester remains adsorbed on the solid support as a bioactive product. Therefore the resulting powder is a partially formulated compound that can be directly applied in the field avoiding the use of solvents and with minimal hazardous and pollutant effects.Introduction 2,4-Dichlorophenoxy acetic acid (2,4-D) esters are extensively used around the globe as effective hormonal herbicides with high selectivity receiving wide application to e . g . cereals, grazing land and sugar cane plantations.1 These herbicides are also used in domestic gardening in Western Europe and in the USA.Unfortunately, as is well known, the manufacture and use of pesticides often results in environmental damage. Besides, more than 40 years of experience in the continuous and increasing use of such herbicides shows that their usage provokes only minor changes in the soil when they are employed with caution.There is thus the opportunity to study strategies to mitigate their harmful environmental impact in their use and preparation. New approaches to 2,4-D ester preparation and use have been developed with the aim to make these herbicides more ecologically acceptable. Thus, bioactive polymers based on 2,4-D and cross-linked acrylamide gels have been recently prepared in order to obtain controlled-release formulations enhancing the efficacy of the herbicide and reducing any environmental problems.2 The conventional esterification of 2,4-D and related compounds following standard methods consists in the treatment of these acids with the corresponding alcohol, the mixture being refluxed in benzene for long periods of time (in general more Synthesis of 2,4-D ester herbicides New routes using inorganic solid supports Laura Lami,b Blanca Casal,a Luis Cuadra,c Jesús Merino,a Amauri Alvarezb and Eduardo Ruiz-Hitzky*a a Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid, Spain.E-mail: eduardo@icmm.csic.es b Instituto Cubano de Investigaciones de los Derivados de la Caña de Azúcar, ICIDCA Cuba c Centro de Ciencias Medioambientales, CSIC, c/ Serrano 115 dpdo., E-28006 Madrid, Spain Received 29th March 1999 than 10 hours) in the presence of an acid catalyst (e.g.H2SO4) and using the 2,4-D in excess with respect to the alcohol.3 It is necessary to control the amount of alcohol present in the reaction media as the esterification progresses. Thus, a progressive addition of alcohol is needed, the reaction yield being critically determined by the quantity of water produced in the reaction.Alternative procedures for 2,4-D esters synthesis are based either on (i) direct reaction of the 2,4-D in the acid chloride form with the appropriate alcohol,4 or (ii) use of polymer-supported reagents involving the preparation of intermediate Amberlist 2,4- dichlorophenoxy acetate resins.5 Although this last method appears a laborious procedure, it produces under mild experimental conditions (2–9 h at room temperature), different 2,4-D esters in high yield (85–97%).However, all these routes involve the use of large quantities of chemicals and generate a significant volume of waste. This paper reports the synthesis of 2-4-D esters using microparticulate inorganic solids such as silica, clays and zeolites as reaction media, without organic solvents, i.e. in so-called dry media conditions,6–10 either by conventional heating or using microwaves.8,11,15 In the last case, the esters of 2,4-D are obtained in high yield in short reaction times (Scheme 1).In addition, the resulting product can be directly applied to the field because they are semi-formulated compounds which could find application in controlled release processes.Green Chemistry August 1999 199 C G The use of microwaves on a solid support has led to many very useful synthetic methods. Rapid reaction times and high conversions and selectivities are some of the benefits of this approach (see e.g. R. S. Varma, Green Chemistry, 1999, 1, 43.This paper deals with the preparation of a herbicide, and the authors have shown that the solid support can actually be a soil. This could allow the possibility of a preparative method for a soil–pesticide composite which could be applied to the land with fewer handling difficulties than traditional methods. DJM Green ContextScheme 1 Results and discussion The preparation of the 2,4-D ester in dry media conditions consisted of heating the 2,4-D and the alcohol in almost equi-molecular quantities (1+1 molar ratio) with both co-adsorbed on an inorganic powder.Heating can be either conventional (100–150 °C, 2–6 h) or, better, by microwave (MW) irradiation (150–350 W, 3–10 min). The 2,4-D esterification yield, using different alcohols, in experiments carried out on silica under microwave irradiation (3–5 min) always occurs with high conversion rates (Table 1).Table 1 2,4-D esterifications on silica gel using MW irradiation Reaction Yield of Alcohol:acid Alcohol time/min ester (%) 1.5:1 ethyl 3 98 1.5:1 isopropyl 3 99 1.5:1 n-octyl 3 98 1.5:1 isooctyl 3 99 1:1 ethyl 5 98a 1:1 isopropyl 5 95a 1:1 n-octyl 5 96a 1:1 isooctyl 5 97a 1:1 isopropyl 5 96b a 50% of full power MW irradiation.b 75% of full power MW irradiation. Cl Cl O CH2 C O OH + ROH Cl Cl O CH2 C O OR + H2O 2,4-D ester (yield>90%) 2,4-D acid heat Solid support Fig. 1 shows, as an example, the IR spectra of the reaction products after MW heating of the 2,4-D and isooctanol mixture co-adsorbed on a silica support. The intense band around 1725 cm21 is clearly observed and is assigned to the n(CNO) stretching vibrations of the ester functional group, whereas the 342 cm21 band associated to the 2,4-D acid is absent, confirming that the esterification reaction is almost quantitative. Typical n(O–H) bands of possible residual alcohol or acid reagents is overlapped by the broad intense n(O–H) absorption band of silanol groups of the silica support (3300–3700 cm21).The bands observed in the 1500–1650 and 750–1000 cm21 regions correspond to the characteristic vibrations of the 1,2,4-substituted aromatic ring, and the stretching vibrations corresponding to the C–H groups, appears in the 2800–3100 cm21 region. The organic reaction products are easily extracted by an organic solvent such as methanol or acetone, and analysed by GC-MS and GC-FTIR.The mass spectra (MS) of the resulting products show typical fragmentation of the 2,4-D esters. As an example, the MS of the reaction product with isooctanol shows the main fragments corresponding to the 2,4-D isooctyl ester at (m/z): 332 [M+], 220 [M+ 2isooctyl (2,4-D acid)], 164 [M+2acetate (2,4-dichlorophenol)], 145, 111 and 71 [2,4-dichlorophenol fragmentation]. High conversion rates (>90%) were also observed when other solids were used as the support, (Table 2).In particular, silicates of relatively high So specific surface area (So > 100 m2 g21) (zeolites, sepiolite and saponite), and therefore solids with high adsorptive capacity, were particularly efficient reaction supports.By contrast, the kaolinite aluminosilicate of low specific surface area So = 8.5 m2 g21, gives much lower reaction yield (<50%) than the other silicates used, in experiments carried out under similar experimental conditions (Table 2). The water content of the supports must also be considered in order to explain the effectiveness of the microwave activation. In fact, the kaolinite is practically water-free because only its external surface contains physically adsorbed water molecules (<1% w/w).In contrast, zeolites and saponites are tecto- and layeredsilicates respectively, containing water molecules (5–10% w/w) that belong to the hydration shell of the exchangeable cations which are located either on the external surfaces and/or in the intracrystalline region of these silicates.Sepiolite is a hydrated magnesium silicate of large specific surface area (>300 m2 g21) containing about 15% (w/w) water. As is well known, the MW action in heating materials is directly related to their dielectric polarisation capacity and, therefore, with the presence of associ- 200 Green Chemistry August 1999 Fig. 1 Infrared spectra of the 2,4-D isooctyl ester synthesised on silica gel (5 min of MW irradiation).ated water molecules with the solids.Thus, the action of the alternating electromagnetic field on the water molecules produces rotational energy which is transferred to the surrounding environment, activating the supported reactions.15 The temperatures reached by the solids supports after 2–5 min of MW irradiation are in the 150–180 °C range, using sample amounts and MW powers as employed in this work.The use of metal oxides, such as alumina and iron(iii) oxides, produces a strong decrease in the reaction yield (<5% for the 2,4- D isooctyl ester synthesis) (Table 3). Alumina has been largely used as an efficient support for dry media synthesis.1 6 Nevertheless, in this case it is probable that the carboxylic acid remains chemisorbed on the alumina surface preventing further reaction between the CO2H and the OH group of the alcohol.Such explanation implies the existence of symmetric bi-anchored carboxylate anions (Fig. 2), as reported elsewhere.17 The incorporation of an additional amount of water to those systems increases the conversion rate. In this way, the addition of 5% (w/w) of water to the alcohol increases the ester yield from 5 to about 15% (Table 3).In agreement with other authors17 this behaviour is probably due to the enhancement of reactivity produced by the presence of water on the alumina surface inducing the capture of protons. Besides this effect, the role of the MW–water interactions increasing the heating of the system should be considered although it is necessary to take into account the fact that the presence of water could disfavour the esterification reaction (Scheme 1).At present, it is difficult to ascertain a balance between these factors. When the experiments were carried out using a-Fe2O3 as support, the reaction yield drops practically to zero due to the oxidative degradation of the adsorbed organic compounds (Table 3).As occurs in general for MW-assisted organic phases,9,10,13 the activation by MW irradiation compared to conventional heating (oven) is always more efficient in producing strong acceleration of reactions. In the case of 2,4-D esterification, some representa - tive results are shown in Table 4, showing the potential interest in the use of such a procedure as a promising clean method.The potential advantages of using clay minerals are well known because the herbicide interacting with such micro-particulate solids could be slowly released resulting in a sustainable activity. Large amounts of clay minerals are used as pesticide carriers (around 200 000 tons per year both in the USA and in Western Europe). This means that 2,4-D supported esters can be directly used as obtained in the procedure involving the use of natural silicates operating in dry media conditions.In this way we have carried out bioassays consisting in the application of such compounds as aqueous dispersions containing the same amount of the herbicide as in conventional formulations, revealing good activity towards young plants of the Amaranthaceae family (Amaranthus albus).Such bioactivity assayed in the greenhouse (30 °C at daylight and 5 °C at night), corresponds to herbicide amounts that could be extrapolated to around 600 g a.i. ha21, i.e. almost the same quantity that is currently used in field application. The observed pre-emergent activity in soils afforded by the silicate/2,4-D isooctyl ester is also a novelty because the activity of 2,4-D esters has been always described to have a post-emergent character.It is also noteworthy, that in place of the more or less pure silicates described above, it is possible to use soil to prepare the 2,4- D esters. Thus, we have selected two agricultural soils (calcium Luvisol from Toledo, Spain, and ortic ferralsol from Havana, Cuba).Both were finely sieved and used under the same conditions as the silicates (Table 5). Thermal treatment in the same Green Chemistry August 1999 201 Table 2 2,4-D isooctanol esterification using silicates as supports in dry media microwave-assisted synthesis Alcohol: acid molar ratio (support : reagents Irradiation Yield of Inorganic support w/w, %) Power (%) time/min ester (%) Na–Y zeolite 1:1 (50) 50 10 90 Na–Y zeolite 1.5:1 (55) 50 5 92 Zeolite (natural)a 1:1 (33) 50 5 93 Zeolite (natural)b 1:1 (50) 50 5 97 Kaolinite 1:1 (50) 75 5 47 Saponite 1.5:1 (55) 100 10 100 Sepiolite 1.5:1 (55) 50 10 97 Sepiolite 1:1 (50) 50 15 93 Sepiolite (Pangel) 1:1 (50) 50 7 100 a From San Ignacio.b From La Pita. Table 3 2,4-D isooctanol esterification using Al2O3 and a-Fe2O3 metal oxides as supports in dry media microwave-assisted synthesis Alcohol+acid molar ratio Inorganic (support+reagents Irradiation Yield of support w/w %) Power (%) time/min ester (%) Al2O3 1+1 (50) 100 10 <5 Al2O3 a 1+1 (50) 75 5 14 a-Fe2O3 1+1 (50) 50 2 0 a-Fe2O3 1+1 (50) 50 5 0 a-Fe2O3 1+1 (50) 50 10 0 a Experiments carried out adding 5% (w/w) of water with respect to the alcohol.Fig. 2 Schematic representation of carboxylate anions adsorbed onto alumina. way as pure silicates in a conventional oven gives the supported esters which also exhibit good activity in the corresponding bioassays. The only negative result detected was with ferralsols when the activation of the esterification reaction was carried out using MW irradiation. In this case, the iron content (identified by X-ray diffraction as a-Fe2O3) of the soil used as reaction support induces organic decomposition which can be explained in terms of oxidation enhancement imposed by the electromagnetic field interacting with the strongly paramagnetic iron oxide particles.The addition of small amounts of a-Fe2O3 (i.e. 5% w/w) to the Toledo soil, or to a selected clay mineral, also reduces the reaction yield (Table 5) corroborating the role of such oxide.Finally, Fig. 3 resumes the steps needed in the use of soil minerals to obtain semi-formulated 2,4-D ester compounds. We have reported here a profitable approach to a selected herbicide which could be extended to other pesticide formulations. It should also be noted that this approach could lead to a lower dosage based on the slow release of the bio-active agents associated with the inorganic materials that have been used as supports in the preparation. Experimental Reagents 2,4-Dichlorophenoxyacetic acid (2,4-D) as well as ethyl, isopropyl, n-octyl and isooctyl alcohol (i.e. 2-ethylhexanol) were purchased from Fluka (reactive quality) and were used without further purification.Mineral substrates and metal oxides Sepiolite This mineral is a hydrated magnesium silicate, Si12O30Mg8(OH,F)4(H2O)4x· 8H2O, which is structurally formed by the alternation of blocks and tunnels along the c-axis.18 Sepiolite from Yunclillos (Toledo, Spain) (<200 mesh), purchased from TOLSA S.A., with 99% pure mineral, was used. The specific surface area (N2, B.E.T.) is 340 m2 g21 and the cationic exchange capacity is close to 0.15 meq g21.Micronised sepiolite purchased from TOLSA S.A. with the trade name of Pangel was also used as a reaction support. Other characteristics of sepiolite and Pangel are described in ref. 19. Saponite This clay mineral belongs to the layered 2+1 charged silicates, with its octahedral sites mainly occupied by magnesium ions and the electrical charge located in the tetrahedral layers (Si/Al substitutions).The mineral used here, also purchased from TOLSA S.A., is from Vicálvaro (Madrid, Spain) deposits, that contains about 15% of sepiolite. The hydrated exchangeable cations located in the interlayer space are mainly Mg2+ and Ca2+. Other characteristics are described elsewhere.20 Kaolin From Asturias (Spain), with a high content in pure kaolinite mineral (>90%).The chemical composition (%) of this 1+1 layer aluminosilicate is: SiO2 = 48.68; Al2O3 = 38.40; Fe2O3 = 0.15; CaO = 0.012; MgO = 0.049; TiO2 = 1.05; K2O = 1.08; Na2O = 0.011. Specific surface area (BET, N2): 8.5 m2 g21. Quartz content: <5%. Zeolites Synthetic (Y-zeolite) in its Na+ form, was purchased from Union Carbide.Chemical composition (%): SiO2 = 67.9; Al2O3 = 20.6; Na2O = 8.49. Two natural zeolites: from San Ignacio (zeolite-1) and from La Pita (zeolite-2), Cuba, were also used, and consisted of a mixture of minerals: zeolite-1 (heulandite-clinoptilolite 56%; mordenite 30%; quartz and montmorillonite <5%) and zeolite-2 (heulandite-clinoptilolite 26%; mordenite 10%; montmorillonite 22%; calcite 5,4%).Oxides S i l i c a+silica gel 60 (Merck), for column chromatography (220–440 mesh). Alumina (Fluka) for chromatography, type 507 C neutral (100–125 mesh). Iron oxide identified as hematite (a-Fe2O3). Soils The raw mineral fraction (<200 mesh) of the two soils used was (i) from Toledo (Spain), a calcium luvisol containing 67.5% of sand, 20.1% of slime, 12.4% of clays, and (ii) from Havana (Cuba), an ortic ferralsol containing 18.9% of sand, 22.0% of slime and a 59.1% fraction of clays and iron oxide which is mainly a-Fe2O3 hematite representing about 38% of this fraction.Synthesis of 2,4-D esters The standard procedure consists in the preparation, as the first step, of a homogeneous mixture of the reagents, i.e. 2,4-D and the 202 Green Chemistry August 1999 Table 4 Comparison between conventional thermal treatment and microwave-assisted synthesis of 2,4-D esters on inorganic solid supports (dry media conditions) MW Inorganic Alcohol+acid ratio Conventional irradiation Time of Yield of support (support+reagent, %) heating (T/°C) (power) (%) treatment ester (%) Silica gel 1+1 (50) — 100 3 min 97 Silica gel 1.5+1 (50) — 100 5 min 93 Silica gel 1.5+1 (50) 150 — 4 h 94 Silica gel 1+1 (50) 100 — 6 h 90 Zeolitea 1+1 (50) — 50 5 min 97 Zeolitea 1+1 (50) 100 — 4 h 93 a Natural zeolite (from San Ignacio, Cuba).alcohol (in general isooctanol was used, i.e. 2-ethylhexanol), with the inorganic solid as support (support/reagent: 50% w/w; typical experiments involve the use of 1 g of solid support).This operation could also be carried out with the assistance of an organic solvent common for both the acid and the alcohol which after impregnation is completely removed in a rotary evaporator. The reaction mixture is heated over variable periods of time (2–6 h) in an oven (100–150 °C). Alternatively, the heating could be carried out by MW irradiation (150–350 W, 3–10 min). The resulting products that remain impregnated on the solid support are extracted with a solvent (methanol, acetone, etc.), filtered over microporous alumina and analysed by GC-MS and FTIR.The yields (%) are deduced from the amount of unreacted material remaining adsorbed on the inorganic support. Apparatus The experiments using MW irradiation (2450 MHz) were carried out in a domestic Moulinex FM 460 oven.Characterisation of the extracted phases (2,4-D esters) was carried out by GC-MS using a Hewlett Packard 5890 series II spectrometer coupled to a selective mass detector series 5971 equipped with a capillary column (25 m length and 0.20 mm internal diameter), with a stationary phase thickness (methylsilicone) of 0.33 mm. FTIR Nicolet 20SXC spectrophotometer was used (Nujol dispersion or KBr pellets).Acknowledgements We gratefully acknowledge Professor R. González-Ponce for bioassays facilities, Dr P. Aranda for revising the manuscript and ICIDCA (Cuba), CSIC and CICYT (Spain) for financial support. We are also indebted to the Spanish Ministry of Foreign Office (Programa Fondo de Expertos, AECI) for the facilities given to develop this work. References 1 (a) Pesticide Manual.Basic Information on the Chemicals used as Active Components of Pesticides, ed. H. Martin and C. R. Worthing, British Crop Protection Council, Nottingham, 1977, 5th edn.; (b) C. Barbera, Pesticidas Agrícolas, Omega S. A., Barcelona, 1989, 4th edn. 2 E. R. Kenawy, React. Funct. Polym., 1998, 36, 31. 3 M. S. Newman, W. Fones and M. Renoll, J.Am. Chem. Soc., 1947, 69, 718. 4 C. R. Wagner, C. L. Hamner and H. M. Shell, J. Am. Chem. Soc., 1953, 75, 4861. 5 M. M. Shalunkhe, M. T. Thorat, R. B. Mane and P. P. Wadgaonkar, Eur. Polym. J., 1989, 25, 1091. 6 Z. Cohen, E. Keinan, Y. Mazur and T. H. Varkony, J. Org. Chem., 1975, 40, 2141. 7 G. Bram and A. Loupy, Preparative Chemistry Using Supported Reagents, ed. P. Laszlo, Academic Press, San Diego, 1987, p. 387. 8 E. Gutiérrez, A. Loupy, G. Bram and E. Ruiz-Hitzky, Tetrahedron Lett., 1989, 30, 945. 9 E. Ruiz-Hitzky, B. Casal, L. Lami and A. Alvarez, Spanish Pat., P9601359, Appl. Jun. 1996. 10 R. S. Varma, Green Chem., 1999, 43. Green Chemistry August 1999 203 Table 5 Esterification of 2,4-D with isooctyl alcohol using soil as reaction support (1+1 alcohol+2,4-D acid molar ratio and 50% w/w soil+reagents) Yield (2,4-D Soil Heating method isooctyl ester) (%) Luvisol (Toledo) Conventional heating, 125 °C, 2 h 98 Luvisol (Toledo) MW irradiation, 7 min 97 Ferralsol (Havana) Conventional heating, 125 °C, 2 h 95 Ferralsol (Havana) MW irradiation 5–10 min <5 Luvisol + 5% Fe2O3 MW irradiation 2–10 min <50 Fig. 3 Methodology proposed for 2,4-D ester synthesis using soil as reaction support.11 A. Ben Alloum, B. Labiad and D. Villemin, J. Chem. Soc., Chem. Commun., 1989, 386. 1 2 G. Bram, A. Loupy, M. Majdoub, E. Gutiérrez and E. Ruiz-Hitzky, Tetrahedron, 1990, 46, 5167. 13 M. P Mingos and D. R Baghurst, Chem. Soc. Rev., 1991, 2 0, 1. 1 4 R. Alajarín, J. J. Vaquero, J. L. García Navío and J. Alvarez-Builla, Synlett, 1992, 297. 1 5 Microwave Enhanced Chemistry. Fundamentals, Sample Preparation and Applications, ed. H. M. Kingston and S. J. Haswell, American Chemical Society, Washington, 1997. 16 G. H. Posner, Angew. Chem., Int. Ed. Engl., 1978, 17, 487. 17 H. Ogawa, T. Chihara and K. Taya, J. Am. Chem. Soc., 1989, 107, 1365. 18 K. Brauner and A. Preisinger, Miner. Petr. Mitt., 1956, 6, 120. 1 9 A. Alvarez, J. Santaren, R. Perez Castell, B. Casal, E. Ruiz-Hitzky, P. Levitz and J. J. Fripiat, Proc. Int. Clay Conference, Denver 1985, ed. L. G. Schultz, H. Van Olphen and F. A. Mumpton, The Clay Minerals Society, Bloomington, 1987, pp. 370. 20 (a) B. Casal, J. Merino, E. Ruiz-Hitzky, E. Gutiérrez and A. Alvarez, Clay Miner., 1997, 32, 39; (b) S. Moreno, R. Sun Kuo and G. Poncelet, J. Catal., 1996, 162, 198. Paper 9/02531F 204 Green Chemistry August 1999

ISSN:1463-9262    

DOI:10.1039/a902531f

出版商:RSC    

年代:1999

数据来源: RSC