|
|
Staff Purpose
|
 ResearchPROJECT 1- Catalyzed Hydrolysis of the Carbon-Oxygen Bond in Ethers Many of the fuels and commodity chemicals developed and used during the 20th century were derived from oil. Although this has led to strong economic growth, there is a need to consider alternate feedstock in order to guarantee that this growth continues. One such feedstock is biomass, which is a renewable resource that represents a useful involvement of solar energy in the production of chemicals. A significant difference between oil and biomass is that the former is assembled with carbon-carbon bonds, but the latter has both carbon-carbon and carbon-oxygen bonds. If biomass is to be used as a source for both commodity and speciality chemicals, selective reactions for the cleavage of carbon-oxygen bonds will be required. The hydrolysis of ether bonds is a potentially green method for using plants as a source of chemicals. Hydrolysis of ethers results in the formation of alcohols (equation 1). This reaction has, however, received little study, and for simple aliphatic and aromatic ethers the reaction is close to being thermoneutral. For aliphatic ethers the electronegativity of the oxygen center results in
(1)
the carbon directly bonded to it having carbonium ion character. A strategy to effecting ether hydrolysis therefore is to target nucleophilic attack at this carbon center in conjunction with increasing the electrophilicity at this carbon by coordinating a metal cation to the ether oxygen. Since there is precedent for ether hydrolysis in the conversion of ethylene oxide into ethylene glycol in the presence of molybdate as nucleophile, this gives a point where to begin. The arylalkyl ether anisole can be hydrolyzed to a ixture of phenol and methanol in the presence of aluminum chloride. This reaction, however, has the serious disadvantage that it produces large amounts of alumina as by product. Since the carbon atoms of ethers that are directly bound to the oxygen have carbonium ion character, nucleophilic attack is a possible route to inducing cleavage of these carbon-oxygen bonds. The hydrolysis of the carbon-oxygen bond in ethers is induced by molybdate as nucleophile, or by aluminum phosphate in combination with ultrasound. High yields are obtained with epoxides, and significantly lower yields with trimethylene oxide and tetrahydrofuran. The molybdate or phosphate oxoanion acts as the nucleophile, and possibly aluminum(III) as a cation. Calculations have been carried out to determine patterns affecting the enthalpy of the hydrolysis of ethers to alcohols. These show that the reaction is close to being thermoneutral, but that both ether hydrolysis and ether ammonolysis are enthalpically favored, especially for ethers that have sterically bulky or fluorinated substituents.
A Menschutkin Type Amine Alkylation Involving Methyl Transfer to Benzylamine from Palladium(II) Chelate Complexes of o-Diphenylphosphinothioanisole, A. Benefiel, D. M. Roundhill, W. C. Fultz and A. L. Rheingold, Inorg. Chem., 23, 3316 (1984).
Kinetics and Mechanism of the Reaction of Palladium(II) Complexes of o-Diphenylphosphino-thioanisole and o-Diphenylphosphinoselenoanisole with the Nucleophiles Thiocyanate and Iodide. Carbon NMR Spectroscopy of the Methyl-Heteroatom Complexes, X-ray Structural Characterization of Diiodobis(o-Diphenylphosphinobenzenethiolato)dipalladium(II), D. M. Roundhill, S. G. N. Roundhill, W. B. Beaulieu, and U. Bagchi, Inorg. Chem., 19, 3365 (1980).
Kinetics and Mechanism of the Reaction of Palladium(II) Complexes of o-Diphenylphosphino-thioanisole and o-Diphenylphosphinoselenoanisole with the Nucleophiles Thiocyanate and Iodide, D. M. Roundhill, W. B. Beaulieu, and U. Bagchi, J. Amer. Chem. Soc., 101, 5428 (1979).
References
Nucleophile Assisted Hydrolysis of Carbon-Oxygen Bonds in Ethers, C. Polydore, H.-Q. Liu and D. M. Roundhill, J. Mol. Catal. A: Chemical, 186, 65 (2002).
PROJECT 1- CATALYZED HYDROLYSIS OF THE CARBON-OXYGEN BOND IN ETHERS
Introduction
For the long term health of the chemical industry it is necessary to develop environmentally benign reactions involving renewable resources. For practical purposes, many of these reactions will need to be catalytic in order to eliminate waste products. Furthermore, oil is not a renewable resource, and another source of carbon will be needed to achieve sustainability. One possible source of renewable carbon is plants and biomass. Conversion of such compounds into useful chemicals represents a useful application of photochemistry to convert carbon dioxide into carbohydrates. The challenge is to use catalytic reactions to selectively convert such compounds into small molecules. This proposal is focused on studying reactions that involve the metal induced hydrolysis of ether bonds. The impetus to studying these reactions is the recognition that at some point of time, the economics of chemical production may make if feasible to competitively produce commodity or fine chemicals from plant products. Potentially useful plant products are biomass, starch, cellulose, or lignin. A difference between these precursors and those from derived from oil is that these materials are bound together with carbon-oxygen bonds in addition to carbon-carbon bonds. In the production of chemicals from oil products it is common practice to crack the hydrocarbon oligomers into small alkenes such as ethylene and propylene, and then use these molecules as building blocks to assemble the desired bonds and functional groups. In considering the question of chemicals from plant products, a different strategy can be considered where specific sites are targeted for cleavage to generate smaller molecules. From the proposed structural types of lignin and humus materials shown in the following pages it is apparent that there are a significant number of O-C (aliphatic) and O-C (aromatic) ether linkages throughout the structure. A potential approach therefore to the systematic degradation of plant products is the selective hydrolysis of ether linkages to alcohols as shown in equation 1.
In order to obtain some preliminary information about reaction 1 we have carried out a series of calculations to provide guidelines as to what substituents on the functional groups R and R1 will favor this hydrolysis reaction. From these data collected in Table 1 it appears that this reaction is favored, and especially
Table 1. Computational Enthalpies (kcal mol -1) for the Hydrolysis of Ethers
(DH1, semi empirical (PM3) methods. DH2, density functional theory) if electron withdrawing groups or sterically bulky groups are present on the functional groups R or R1. The hydrolysis reaction is also favored if the substituents R and R1 are part of a ring system. We propose to employ a strategy to effect the hydrolytic cleavage of carbon-oxygen ether bonds that involves nucleophilic attack at the carbon center that is directly bonded to the oxygen. This strategy will involve targeting an increase in the electrophilicity of this carbon by complexation of the ether oxygen to an electron-poor metal center. Causing the alkyl group of the ether to become more susceptible to nucleophilic attack is an important aspect of this proposed research, and metal centers having different coordination characteristics and charge densities will be used. This strategy of targeting the ether for nucleophilic attack makes the choice of nucleophile an important one. Good nucleophiles such as cyanide, iodide, or thiolate will likely be effective for the heterolytic cleavage of the oxygen-carbon bond in ethers, but subsequent hydrolysis of the alkyl-nucleophile bond will be less easily accomplished. Since we are targeting reactions that lead to hydrolysis of this carbon-oxygen ether bond, the nucleophile will need to be one that can undergo hydrolysis under mild conditions so that a catalytic cycle can be achieved. This strategic approach to ether hydrolysis forms the basis of this research.
Background
There is precedent for metal activation of ether carbons to nucleophilic attack, especially if the concept is extended to thioether complexes. In our own work we have prepared the compounds Ph2PC6H4(SMe-2) and Ph2PC6H4(OMe-2), and used them to prepare P, S and P, O-chelate complexes of platinum(II), palladium(II) and ruthenium(II) [1-3]. For the former compound coordination to the metal center leads to the metal center leads to the heterolytic cleavage of the carbon-sulfur bonds by the nucleophiles iodide or thiocyanate (equation 2). The reaction occurs in solvents of high dielectric constant such as acetonitrile. The P, O-chelate complex with ruthenium(II) has been prepared, although
little has been done to try and effect the nucleophilic cleavage of the carbon-oxygen bond. Additional examples attest to the generality of these nucleophilic cleavage reactions of coordinated thioethers [4-6]. The heterolytic cleavage of phenyl ethers induced by coordination of the arene moiety to an
(3)
electrophilic metal center has also been realized. An example of such a reaction is shown in equation 3, whereby an anisole moiety bound in a cationic metal complex undergoes nucleophilic attack by the tert-butoxide ion to give tert-butyl methyl ether, and an uncharged complex having a coordinated phenolate anion [7]. This reaction occurs at ambient temperature in THF solvent. Another approach to effecting carbon-oxygen bond cleavage is to initiate nucleophilic attack at the electrophilic carbon center. Subsequent hydrolysis can be used to regenerate the nucleophile (scheme A). Such reactions can be facilitated if the cation introduced with the nucleophile binds to the
ether oxygen, thereby increasing the electrophilicity of the ether carbons. Precedence for such a pathway is found in the catalyzed hydrolysis of ethylene oxide to ethylene glycol. Anionic catalysts such molybdate and sulfate are used, with the PPN salts being used for phase transfer catalysis [8]. The proposed pathway (scheme B) involves initial nucleophilic attack by the molybdate anion at the carbon
center. An alternative final step could involve closure to give a five-membered metallocycle that can undergo reductive elimination, which is the proposed pathway. Precedent for such a pathway is found in platinum(IV) chemistry [9]. The catalytic reaction proceeds at ambient temperature and pressure. Aluminosilicates have also been used for the ring opening of epoxides [10].
Preliminary Results
We have begun some initial preliminary research on the catalyzed hydrolysis of carbon-oxygen bonds in ethers [11]. We have investigated three systems. We find that the molybdate nucleophile PPN2[MoO4]
also catalyzes the ring opening hydrolysis of butylene oxide and trimethylene oxide to 1,2-butanediol and 1,3-propanediol respectively, although the yields are low. These two hydrolyses, in addition to the hydrolysis of tetrahydrofuran to 1,4-butanediol, dioxane to ethylene glycol, and anisole to a mixture of phenol and methanol, also occurs in low yield in a heterogeneous system with aluminum phosphate. Higher yields for the hydrolysis of anisole are obtained using aluminum trichloride in a Friedel-Craft like reaction. The reaction conditions of this latter reaction involve hydrolysis of the product obtained between aluminum trichloride and anisole. Although this reaction is unlikely to develop into a useful one, it does however show that the hydrolysis of anisole is a viable reaction. The aluminum phosphate reaction may merit further study. The PPN2[MoO4] catalyzed hydrolysis correlates with ring strain whereby the more strained 3- and 4-membered ring ethers undergo hydrolysis, but the 5- and 6-membered rings are unreactive. Higher nucleophilic strength, or higher electrophilicity of the ether carbon induced by metal ion complexation, may be required to achieve the hydrolytic cleavage of these rings. Also, a different nucleophile other than molybdate may be necessary. The PPN salt was used because it gave the molydate ion a higher solubility in an organic medium [8], but the PPN cation cannot bind to the ether oxygen to increase the electrophilicity of the carbons. Since hydrolytically stable molybdate esters are not available, this is not a plausible route to using the alkali metal salts of molybdates as hydrophobic nucleophiles.
Proposed Research
This proposed research is targeted toward understanding factors that facilitate the hydrolytic cleavage of carbon-oxygen bonds. The proposed strategy builds on systems that have been used for the hydrolysis of carbon-oxygen bonds in epoxides to yield alcohols. Such systems have involved initial nucleophilic attack at the ether carbon, followed by hydrolysis of the alkyl ester bond. In particular, catalytic systems will be sought that target a synergistic binding of a metal cation to the ether oxygen in order to induce attack by the nucleophilic anion at the ether carbon.
Simple Nucleophiles
Our initial approach will be to follow a strategy that builds on the catalyzed hydrolysis of ethylene oxide. An alternative epoxide that can be reacted with water in the presence of a molybdate salt as catalyst is butylene oxide. Since both this compound and the 1,2-butanediol that is formed (equation 4) are liquids, they will be easier to handle in our laboratory than is ethylene oxide. Other anionic catalysts such as sodium sulfate, sodium nitrate and sodium phosphate will be used, or the PPN+ or
n-butylammonium salts if the biphasic nature of the reaction mixture becomes a problem. Less reactive carbon-oxygen bonds will be investigated. Two potential targets are the hydrolysis of tetrahydrofuran and anisole to give 1,4-butanediol, and a mixture of phenol and methanol respectively. Another target molecule is methyl tert-butyl ether (MTBE). This compound is chosen because our computational data in Table 1 suggest that the hydrolysis of ethers having tertiary butyl substituents is more favored than the methyl derivative (equation 5). However, since steric factors may negate this small advantage, MTBE
has the advantage of having both a methyl and a tertiary butyl carbon substituents on the ether, thereby having both steric extremes. If hydrolysis is observed, isotopic labeling of the water oxygen should allow for identification of the site of attack at the ether carbons. We anticipate attack at the methyl ether oxygen. The mechanism of the hydrolysis of epoxides is proposed to involve the formation of a macrocycle that undergoes subsequent hydrolysis to the diol [8]. An alternate pathway could involve the formation of only one molybdenum alkoxide bond. Either sequence involves hydrolysis of such bonds. This hydrolytic cleavage of the bond between the alkyl carbon and the oxygen atom of the nucleophile completes the catalytic cycle. For many stronger nucleophiles, this hydrolytic cleavage is less facile.
Nucleophile Enhancement
Another facet of designing catalysts for ether hydrolysis is the nature of the nucleophile. In one respect, the oxoanions are a good choice because the alkylated nucleophile formed in the first step of the cycle can readily undergo hydrolysis to yield the alcohol and regenerate the oxoanion. In another respect, however, oxoanions are a poor choice because they are only weak nucleophiles (Table 2), and therefore their reactions
Table 2. Relative Nucleophilicities of Oxoanions
a. For nucleophilic reactions of para-nitrophenyl acetate in water at 25°C
with carbonium ions are likely to be rather slow [12, 13]. Because of the advantage of ready hydrolysis, we propose to target the enhancement of these nucleophilic reactivities. One approach to accomplish this is to bind the sodium ion into a complexant, thereby reducing ion pairing with the oxoanion. From literature precedent this can be achieved by the addition of a crown ether that has the correct cavity size to strongly bind the sodium cation [10]. Another approach is to use a calixarene complexant. Calixarenes have several advantages in that within the same molecule they can have separate oxoanion and sodium ion binding sites, and also they are good phase transfer catalysts. The proposed catalysts in this family of compounds is 1, which can be prepared in good yield using the sequence of reactions in scheme C. In this compound the methoxy phenyl ether site is a complexant for the sodium ion, leaving the phosphonate site as a deprotonated anion.
Phase Transfer Aspects
Other approaches to designing catalytically active nucleophiles will be followed. These involve making modifications that will influence both the nucleophilicity and the phase transfer properties of the oxoanion. We plan to focus these efforts on molybdate, sulfonates and phosphonates. For molybdate, we will focus on the phase transfer aspects because introducing substituents that are stable to hydrolytic cleavage on such inorganic oxoanions is not a realistic option. Two media that may allow sodium molybdate itself (rather than its PPN salt) to be used as an effective catalyst for carbon-oxygen bond cleavage are poly(ethylene glycol) and fluorocarbons. These media both show biphasic behavior. Interconversion between two phases and a single phase can be accomplished with poly(ethylene glycol) by the addition of salts [14-18], and with fluorous phase compounds by changes in temperature [19-21]. Another alternative is to use long chain alkylammonium cations such as the butyl, octyl, dodecyl, or hexadecyl derivative to increase the hydrophobicity of the molybdate salt. Similar considerations would
apply to other oxoanions such as tungstate, vanadate, or nitrate. For sulfonates and phosphonates, however, substituents can be introduced to change their character. The simplest way to accomplish this is to introduce long chain alkyl groups at the sulfur and phosphorus centers. Such compounds having both a hydrophobic and a hydrophilic end are nucleophiles that can also act as phase transfer catalysts. Examples of two such compounds that will be investigated are 2 and 3. Each of these compounds with the heteroatom in its highest oxidation state is oxidatively stable. One difference, however, is that the alkylsulfonic acid 2 is a stronger protonic acid than is the alkylphosphonic acid 3, therefore 2 will be dianionic in solutions of higher acidities. Although our initial choice of substituents will be the n-octyl, the desired phase transfer properties may required a longer alkyl chain substituents such as the n-octadecyl (n-C18H37) derivative. This choice will need to be made by experiment.
Cooperativity
Another approach will be to try and induce cooperativity between the oxoanion nucleophile and its cation in the salt. If the cation can be complexed to the ether oxygen, it will increase the electrophilicity of the carbon atom covalently bound to this oxygen. The nucleophile system used in scheme B has the disadvantage that by using the PPN+ salt to achieve solubility, there is no possibility of cooperative binding of the cation to the ether oxygen. Sodium is a good choice for the cation because it can bind to the ether oxygen. Phase compatibility must then be achieved in a different manner, and since water is present in the catalytic system, it will be competitive with the ether oxygen for binding to the cation. Lithium and magnesium are alternate choices because both have higher cationic charge densities than does sodium. Higher charge densities will result in greater induced electrophilicities at the alpha carbon of the ether if these ions bind to the oxygen. A limitation, however, is that salts of the higher charged cations are more likely to be insoluble. Our choice of cation will be a combination of solubility properties and the function of the cation to induce electrophilicity at the ether carbon.
Organometallic Approach
Another approach to inducing the cleavage of a carbon-oxygen bonds in an ether is to use an organometallic strategy. Among the possible nucleophiles that may be methylated by anisole are the platinum(0) complexes Pt(PEt3)3 and Pt(PCy3)2, and the sodium salts of cpMo(CO)2(PPh3)-. These organometallic salts have the advantage that cooperativity is possible between the sodium cation and the organometallic anion. Identification of the methylmetal complex that is formed can be readily achieved by a combination of hydrogen-1 and carbon-13 NMR spectroscopy. A disadvantage of using an organometallic approach is the difficulty of achieving a catalytic system, but it is useful to investigate it as a stoichiometric reagent because it will provide information about the reactivity that is required to induce ethers to react as electrophilic alkylating agents to metal centers.
Reaction Pathways
Although we have assumed that these cleavage reactions will occur by a simple nucleophilic displacement reaction, there are other possible pathways. Another alternative is an electron transfer pathway. Such a pathway may be the predominant one when the nucleophile is sterically blocked from directly approaching the electrophilic carbon on the ether. An electron transfer pathway leading to the nucleophile induced cleavage of a carbon-oxygen bond in an ether is shown in Scheme 2. Again, the alcohol ROH is the hydrolysis product.
Nuc- + ROR ® Nuc + ROR-
ROR- ® RO- + R
RO- + H2O ® ROH + HO-
Nuc + R ® NucR
NucR + H2O ® NucH + ROH
NucH + Base ® Nuc- + BaseH+
Scheme 2
Ammonolysis
Although the catalyzed hydrolysis of the carbon-oxygen bond is the major focus of this proposed research, we also plan to investigate whether ammonolysis can be achieved (Table 3). Similar reactions
Table 3. Computational Enthalpies (kcal mol -1) for the Ammonolysis of Ethers
(ΔH1, semi empirical (PM3) methods. ΔH2, density functional theory) a. Ammonolysis to PhOH and MeNH2 b. Ammonolysis to PhNH2 and MeOH
conditions to the hydrolysis studies will be used, except that aqueous ammonia will be used in place of water (equation 6). The higher nucleophilicity of ammonia over water may lead to ammonolysis being competitive with hydrolysis.
Thioethers
A second hydrolysis and ammonolysis system that will be investigated is thioethers. Our computational enthalpy data show that the hydrolysis of sulfides is either enthalpically favorable, or that the ones that are endothermic can possibly be made thermodynamically favorable by the entropic term (Table 4). Table 4. Computational Enthalpies (kcal mol -1) by Density Functional Theory for the Hydrolysis of Thioethers
The targeted hydrolysis reactions is shown in equation 7. In targeting thioether hydrolysis a slight variation to our strategy will be made. This variation will be in the cation activation step. For the cationic activation of ethers to nucleophilic attack at carbon we proposed using cations such as sodium,
lithium, and magnesium. These are “hard” cations that bind to oxygen donors. For thioethers we need a cation that will bind to the sulfur center. Since sulfur is a “soft” donor, we will need to use a “soft” cation to selectively bind to it. Of the soft cations that we propose to use as cations are lead(II) and cadmium(II), although we would prefer a less toxic metal such as palladium(II).
Analytical Methods
For this research project we will analyze our hydrolysis and ammonolysis products by a combination of chromatography (thin layer, gas, and liquid) and spectroscopy (IR, NMR, and mass), against authentic samples if possible. If we are successful in achieving catalytic carbon-oxygen bond catalysis we will try our system on biopolymers such as starch, humus material, and lignin, and we will also investigate methods to prepare solid state materials that have the same or better properties.
References
[1] Platinum Metal Complexes of Amine and Ether Substituted Phosphines, T. B. Rauchfuss, F. T. Patino and D. M. Roundhill, Inorg. Chem., 14, 652 (1975). [2] Kinetics and Mechanism of the Reaction of Palladium(II) Complexes of o_Diphenylphosphino-thioanisole and o_Diphenylphosphinoselenoanisole with the Nucleophiles Thiocyanate and Iodide, D. M. Roundhill, W. B. Beaulieu and U. Bagchi, J. Am. Chem. Soc., 101, 5428 (1979). [3] Menschutkin Type Amine Alkylations Involving Ethyl Transfer from Platinum(II) Chelate Complexes of o_(Diphenylphosphino)thiophenetole, A. Benefiel and D. M. Roundhill, Inorg. Chem., 25, 4027 (1986). [4] Characteristics of Nickel (0), Nickel (I) and Nickel (II) in Phosphino Thioether Complexes: Molecular Structure and S-Dealkylation of (Ph2P(o-C6H4)SCH3)2Nio, J.S. Kim, J.H. Reibenspies and M.Y. Darensbourg, J. Am. Chem. Soc., 118, 4115 (1996). [5] Synthesis and Structural Investigation of Polyoxomolybdate Coordination Compounds Displaying a Tetranuclear Core. Crystal and Molecular Structures of [n-Bu4N]2[Mo4O10(OMe)4X2] (X = -OMe, -Cl) and Their Relationship to the Catecholate Derivative [n-Bu4N]2[Mo4O10(OMe)2(OC6H4O)2] and to the Diazenido Complexes of the o-Aminophenolate and the Naphthalene-2,3-diolate Derivatives [n-Bu4N]2[Mo4O6(OMe)2(HNC6H4O)2(NNC6H5)4] and [n-Bu4N]2[Mo4O6(OMe)2(C10H6O2)2(NNC6H5)4]. Comparison to the Structure of a Binuclear Complex with the [Mo2(OMe)2(NNC6H5)4]2+ Core, {Mo2(OMe)2(H2NC6H4O)2(NNC6H5)4, H. Kang, S. Liu, S.N. Shaikh, T. Nicholson, and J. Zubieta, Inorg. Chem. 28, 920 (1989). [6] Hexanuclear Complexes of Molybdenum(V) containing [Mo6O12(OCH3)4(acac)3]- anion, M. Cindric´, G. Pavlovic´, V. Vrdoljak, B. Kamenar, Polyhedron, 19, 1471 (2000). [7] Fecp+ Induced Heterolytic Cleavage of Phenyl Ethers and Transetherification, L. Djakovitch, F. Moulines and D. Astruc, New J. Chem., 20, 1071 (1996). [8] Selective Ethylene Oxide Hydrolysis Catalysed by Oxo-Molybdenum Species, J. R. Briggs, A. M. Harrison and J. H. Robson, Polyhedron, 5, 281 (1986). [9] Direct Observation of C-O Reductive Elimination from Pt(IV), B. S. Williams, A. W. Holland and K. I. Goldberg, J. Am. Chem. Soc., 121, 252 (1999). [10] Organic Syntheses using Aluminosilicates, Y. Izumi and M. Onaka, Adv. In Catal. 38, 245 (1992). [11] Nucleophile assisted hydrolysis of carbon-oxygen bonds in ethers, C. Polydore, D.M. Roundhill, H.-Q. Liu, J. Mol. Catal. A: Chemical, 186, 65 (2002). [12] General Base and Nucleophilic Catalysis of Ester Hydrolysis and Related Reactions, S. L. Johnson, Adv. Phys. Org. Chem., 5, 237 (1967). [13] The Mechanisms of Reactions of ß-Lactam Antibiotics, M. I. Page, Adv. Phys. Org. Chem., 23, 165 (1987). [14] Manipulation of Nucleophilic Displacement Reactions by Host-Guest Complexes, H.-J. Scheider, R. Busch, R. Kramer, U. Schneider, I. Theis, Adv. in Chem. Ser., 215, 457 (1987). [15] Poly(ethylene Glycol) Chemistry: Biotechnical and Biomedical Applications, J. M. Harris, ed., Plenum, New York, 1992. [16] Aqueous Biphasic Separations: Biomolecules to Metal Ions, R. D. Rogers, A. H. Bond, C. B. Bauer, J. Zhang, M. L. Jezl, D. M. Roden, S. D. Rein and R. R. Chomko, Plenum, New York, R. D. Rogers and M. A. Eiteman, eds., 1995, pp. 1-20. [17] Ion Exchange and Solvent Extraction, R. D. Rogers and J. Zhang, Dekker, New York, J. A. Marinsky and Y. Marcus, eds., 1997, Vol. 13, Chapter 4, pp. 141-193. [18] Metal Ion Separations in Polyethylene Glycol-Based Aqueous Biphasic Systems, R. D. Rogers, A. H. Bond and C. B. Bauer, Sep. Sci. Technol., 28, 1091 (1993). [19] Synthesis and Characterization of Calix[4]arene Functionalized Poly(ethylene glycol) Derivatives, J. Shen, H. F. Koch and D. M. Roundhill, J. Incl. Phenom. Macrocycl. Chem. 38, 57 (2000). [20] Facile catalyst Separation without Water: Fluorous Biphase Hydroformylation of Olefins, I. T Horváth and J. Rábai, Science, 266, 72 (1994). [21] Fluorous Biphase Chemistry, I. T Horváth, Accts. Chem. Res., 31, 641 (1998). [22] Molecular Engineering in Homogeneous Catalysis: One-Phase Catalysis Coupled with Biphase Catalyst Separation. The Fluorous-Soluble HRh(CO){P[CH2CH2(CF2)5CF3]3}3 Hydroformylation System, J. Am. Chem. Soc., 120, 3133 (1998).
PROJECT 2-Liquid-Liquid Extractions of Oxoanions from Aqueous Phase
The liquid-liquid extraction of metals from an aqueous into organic phase has mainly been focused on cations. More recently, however, anions have begun to be targeted. Two particular ones that are receiving attention are chloride and phosphate. Phosphate presents an interesting challenge because, being a tetrahedral oxoanion it has both shape and peripherical oxygens that can lead to be used to induce selectivity. Three similar tetrahedral, or pseudotetrahedral oxoanions that present environmental problems at nuclear facilities are pertechnetate, chromate, and aluminate. In order to selectively extract these anions from mixed wastes it would be useful to have available hosts for these anions, and especially ones that can discriminate between them. Calixarenes have found important applications as phase transfer extractions for metal ions, particularly those of Group I. Calixarenes are especially useful extractants because they are stable in air and in both acidic and basic solutions. In addition, they can be made soluble in water or organic solvents, or be bound to a solid support. Recent calculations have shown calixarenes to be effective liquid-liquid extractants because they have a tendency to aggregate at the phase boundary. The selective extraction of metal cations and anions from aqueous solution into an organic phase is an important goal, especially if the particular ions are toxic and present to a ligating group on the host, but for anions or uncharged molecules there may be no direct interaction between the metal center and host. Although numerous molecules act as hosts and complexants for cations, fewer act as hosts for anions. Recently, however, chemically modified calixarenes have been synthesized that are hosts for simple anions. Calixarenes are cyclic oligomers obtained by condensation reactions between p-tert-butyl phenol and formaldehyde. By judicious choice of base, reaction temperature, and reaction time, calixarenes having different ring sizes can be prepared in good yield. Calixarenes are like crowns in that they are pre-organized complexants, yet, unlike crowns, they can be readily synthesized in large quantities. Unlike porphyrins, calixarenes are not fully conjugated, and the three-dimensional structure leads to cavities. The name calixarene derives from the Greek word for a vase, which aptly describes their molecular structure. The structures and abbreviations used for designating the widely used calix[4]arenes are shown in 1-3. The structures of the other calix[n]arenes (n = 5, 6, 7, 8, etc) 4 differ only in the size of the ring oligomer. Other ring sizes are available, but few metal ion complexes have been isolated with them. These calix[4]arenes have both a wide (upper) and a narrow (lower) rim that can be chemically modified to produce complexants that are selective for particular metal ions. In the simple calixarene framework the wide rim has hydrocarbon functionalities, and the lower rim phenolic groups. Calixarenes are conformationally mobile, and the extreme structures for the calix[4]arenes have been termed the cone, partial cone, 1,3-alternate and 1,2-alternate conformations 5.
Because of the conical geometry of the calix[4]arene structure, the cavity size of the wide rim is larger than that of the narrow rim. In the cone conformation, which is often the one having the highest stability, the pre-organization puts all of the functional groups in positions where they can bind to a substrate. As a result, functionalized calix[4]arenes can be designed that have a large number of donor atoms that can bind to substrates. Calix[n]arenes with n > 4 have lower barriers to conformational change than do the calix[4]arenes. Several books have been written on calixarenes (1-6), a review on metal complexes of calixarenes has been published (7), and a recent book includes references to their applications in liquid-liquid extraction and as sensors (8).
1. C. D. Gutsche, Calixarenes, Royal Society of Chemistry: Cambridge, 1989. 2. C. D. Gutsche, Calixarenes Revisited, Royal Society of Chemistry: Cambridge, 1998. 3. J. Vicens, V. Böhmer, Eds., Calixarenes: A Versatile Class of Macrocyclic Compounds, Kluwer Academic, Dordrecht, 1991. 4. J. Vicens, Z. Asfari, J. B. Harrowfield, Eds.; Calixarenes: 50th Anniversary, Kluwer Academic, Dordrecht, 1994. 5. J. Vicens, V. Böhmer, Calixarenes: A Versatile Class of Macrocyclic Compounds, Kluwer, Dordrecht, Netherlands, 1990. 6. Z. Asfari, V. Böhmer, J. B. Harrowfield, J. Vicens, Eds.; Calixarenes 2001, Kluwer Academic, Dordrecht, 2001. 7. G. J. Lumetta, R. D. Rogers, A. S. Gopalan, Eds., Calixarenes for Separations, ACS Sympos. Series 757; American Chemical Society: Washington, DC, 2000. 8. L. G. Sillen, A. E. Martell, Stability constants, Special Publication No. 17, The Chemical Soceity, London, 1964. 9. M. J. Taylor, Comprehensive Coordination Chemistry, G. Wilkinson, ed., Pergamon, 1987, Vol. 3, Chapter 25.1. 10. D. M. Roundhill, Metal Complexes of Calixarenes, Prog. Inorg. Chem., 1995, 43, 533. 11. D. M. Roundhill, Extraction of Metals from Soils and Waters, Plenum, New York, 2001. Recently we have synthesized a series of calix[4]arene amides and amines. In a survey of their potential use as phase transfer extractant, we have discovered one that is effective for chromium(VI), and another for rhenium(VII), a surrogate for technetium(VII). We have not investigated aluminum(III). Following our synthesis of a series of calix[4]arene amides and amines we surveyed a series of oxoanions to determine whether any were extracted from an aqueous to an organic layer. The oxoanions were introduced as the salts sodium chromate and potassium perrhenate. Extraction data were collected at both pH 0.85 and 7. Our results are summarized Table 1 for the compounds 1-7. An important aspect of developing chelate extractants for metals is that they are kinetically rapid phase transfer agents.
|
