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Total Synthesis Of Complex Antifungal Agent Advances Falck divides the molecule mentally into four sections: a 2-methylcyclopropylmethyl, the string of four consecu tive cyclopropanes, a γ-substituted crotonyl, and the dihydrouridine. Synthe ses of three of these segments are known in the literature; the exception is the tetracyclopropyl group. Because Falck's target has the same configuration at all four cyclopropanes, 209th ACS National Meeting he has decided on a dimerization strat egy. In it, an enantiomeric 2-substituted cyclopropylcarbinyl ether dimerizes to ^ A N A H E I M a dicyclopropyl. Falck then adjusts the functionality and dimerizes the dicy clopropyl to a tetracyclopropyl. wo research groups came to This dimerization of dimers has an ad Anaheim to tell the Division of ditional advantage that Falck calls Organic Chemistry of their prog "dimerization-induced chiral enrich ress in the total synthesis of a natural ment/' As Falck explains, he makes the product that shows promise as an anti initial cyclopropyl compound in 88% en fungal drug. In addition to the medical antiomeric excess. If one calls one com interest, the compound is a synthetic pound R and the other S, and R is in great challenge. Known for now only as FRexcess, the dimer possibilities 900848, the molecule sports are RR, SS, RS, and SR. RR is 12 dissymmetric carbon at the desired enantiomer, SS is a oms and fairly bristles with Synthesis of polycyclopropane drug contaminant that lowers enanti cyclopropane rings. omeric excess of RR, and RS The agent shows promising and SR are diastereoisomers of activity against such filamen RR. The diastereoisomers can tous fungi as Aspergillus niger, be separated by ordinary nonMucor rouxianus, and Fiisarium enantioselective methods. The oxysiporum, with inhibiting diastereoisomers also serve as a concentrations as low as 0.05 sink to sop up some of the S at pg per mL. These microorgan the expense of some of the R. isms can infect patients who Thus the enantiomeric excess of have AIDS, diabetes, or im RR after dimerization is greater mune systems suppressed by than that of R. drugs. The agent has low tox . . . is broken down into component parts icity, with an LD50 (lethal dose So Falck came to Anaheim needed to kill 50% of a popu with l,2'"-dihydroxymethyltetlation) of 1 g per kg of body rapropyl and a plan to join it weight in mice. to the other segments. If he has guessed right about the stereo Structurally, the compound chemistry, he is home. If not, is a fatty acid amide of a uri he has a compound related to dine derivative. Fatty acids the real FR-900848 that may with one cyclopropane ring shed light on the source of its are known, but the fatty acid antifungal activity. in this compound has an un precedented five cyclopropane Meanwhile, organic chemis rings. And four of the rings are try professor Charles K. Zerchright together in a row. To top er at the University of New
• Two groups report progress in total synthesis of compound containing an unprecedented five cyclopropane groups
T
22
APRIL 17, 1995 C&EN
things off, nothing is known yet of the cis-trans, syn-anti stereochemistry of the cyclopropane rings. The compound was reported in 1990 by workers at Fujisawa Pharmaceuti cal, Osaka, Japan. They isolated it from fermentation broths of Streptoverticillium fervens and determined its overall structure. In the absence of stereochem ical information, a would-be synthesiz er has to make all possible combina tions or one lucky guess. As a result of postulating a biosynthetic route by which Streptoverticillium might make FR-900848, J. R. Falck of the University of Texas Southwest Medical Center in Dallas has chosen an all-trans all-syn configuration for the central string of four cyclopropanes. Falck, who is a professor of molecular genetics and pharmacology, is collaborating with postdoctoral fellows Belew Mekonnen and Jurong Yu.
Hampshire, Durham, has mounted a campaign to develop cyclopropane syntheses that he can modify easily to accommodate a cis-, trans-, syn-, or anticonfiguration at any step. He works with graduate students W. Scott McDonald and Cory R. Theberge to develop selective cyclopropanations of olefins. The technique the New Hampshire chemists use is one developed by organic chemistry professor André Charette and graduate student Hélène Juteau of the University of Montreal, Quebec. The technique uses the butaneboronate ester of either D- or L-tartaric acid bis-(N,Ndimethylamide) as a chiral template for cyclopropanation of olefins with methylene iodide and zinc-copper couple (the Simmons-Smith reaction).
Zercher's group has made enantiomeric irflHS-l,2-di(3-hydroxy-l-propenyl)cyclopropane with Charette's template. Further cyclopropanation of the compound with a r> or L-template yields either a syn- or anti-trans-tercyclopropyl. Further work in Durham includes extending the method to c/s-tercyclopropyls. Beyond that, Zercher faces the task of extending selectivity to tetracyclopropyls. AncJ although absent from Anaheim, Anthony G. M. Barrett of the Imperial College of Science, Technology & Medicine, London, is also on the trail of FR900848. Barrett, who is a professor of organic chemistry, recently reported on enantioselective syntheses of dicyclopropyls. Stephen Stinson
Fluoropolyol offers enhanced wetting resistance ι 209th ACS National Meeting
ΑΝΑΗΕΙΜ A team of researchers drawn from in dustry, government, and academe is close to commercial development of fluorinated monomers for wettingresistant, foulant-shedding coatings, ac cording to Tai Ho, a former chemistry professor at George Mason University, Fairfax, Va., who discussed the com pounds at a symposium sponsored by the Division of Polymer Chemistry. One immediate use for these coat ings, Ho says, may be paint that offers little or no adhesion to marine organ-
isms, such as barnacles, that attach themselves to ships or other marine vessels. The organisms would simply be wiped away. But the resins would not contain biocides whose nonselec tive toxicity has made them objection able in foulant-shedding coatings. Ho evaluated a fluorinated polyether polyol for polyurethanes that was in vented by GenCorp Aerojet, Sacramen to, Calif. The Aerojet chemist who in vented the polyol is Aslam A. Malik. The work was funded by the Office of Naval Research, Arlington, Va. Ho has since moved to Virginia Polytechnic & State University in Blacksburg. Beyond the Navy's interest in marine paints, the wetting-resistant resins might find use in medical devices such
as catheters, architectural coatings, re lease additives for rollers to avoid sticking, and humidity-resistant pot ting compounds to protect electronic devices. The inert resin surfaces also suggest use in medical implants. But lawsuits over silicone breast implants have led many producers of plastic res ins to refuse to sell their materials for de vices for implant that will be in the body for longer than 29 days. Ho makes a thermoplastic polyurethane rubber by reacting a fluorinated polyol, 4,4'-methylenediphenyl isocyanate (MDI), and 1,4-benzenedimethanol. The — N = C = 0 groups of MDI react with —OH groups of the other two compounds to form urethane linkages, —NH—COO—, catalyzed by dibutyltin dilaurate. The long, flexible alkyl chains of the polyol comprise what is called a "soft segment" in the polymer chain, while the rigid aromatic rings of MDI and benzenedimethanol form a "hard segment." The association of benzene rings in adja cent chains with one another produces a loose cross-linking effect. The cross-link ing at long intervals of the alkyl chains results in the stretchiness of the rubber. Because the cross-linking effect is physical association of aromatic rings rather than covalent bonding, the resin is soluble and castable into films. Films show good adhesion to steel, aluminum, hydrocarbon rubber, and graphite. Ho says fluorinated side chains of the poly ol probably orient to face outward to ward the surface, while the alkyl-arylurethane backbone lies closely associat ed with the substrate for good adhesion. Ho measures the hydrophobicity of
New process makes fluorinated polyols . . .
. . . which find use in foulant-shedding polyurethane coatings
APRIL 17,1995 C&EN
23
SCIENCE/TECHNOLOGY film surfaces by their contact angles. Contact angle is the angle that a water Vinyl alcohols arise from hydrolysis of acetals droplet makes lying on a surface. Hydrophilic surfaces let droplets spread OCHo ό over them with contact angles of less CH 3 C0 2 CH 3 + H O — C H = C H 2 CH? = C / than 90°. Hydrophobic surfaces cause OCH = CH2 droplets to bead up on them with con tact angles of greater than 90°. Ho's polyurethanes have contact angles of . . . or catalyzed rearrangement of allyl alcohols 116°, compared with 110° for polytetrafluoroethylene (DuPont's Teflon). CH 2 =CHCH 2 OH Rh-phosphine^ CH3CH = C H - O H The Aerojet synthesis of fluorinated catalyst polyols begins with trimethylolethane (2-hydroxymethyl-3-methylpropane1,3-diol). The company's proprietary . . . and last long enough to form copolymers process converts the compound to 3-bromomethyl-3-methyloxetane. Reac tion of the bromomethyl compound with the sodium salt of heptafluorobutanol, for example, puts a heptafluorobutoxymethyl chain on the oxetane ring. A Lewis acid catalyzes ring-open ing polymerization of the oxetane to a molecular weight of 20,000 daltons. OH CN For now, Aerojet is giving away samples of the polyols to others who CH CH> X want to evaluate them. Company CH2^ CH/ spokesmen say they hope to price polyols under $100 per lb when they come to market. Side chains available are trifluoroethyl, pentafluoropropyl, heptafluorobutyl, and pentadecafluoCH3CH = CHOH • rooctyl. Aerojet will also tailor molecu lar weights to users' needs. Ho is currently synthesizing fluorinated polyurethanes that are compatible with present commercial nonfluorinated ones. Thus, users may be able to blend costly fluorinated resins with inexpensive ones to tune desired properties and costs. But now polymer chemistry professor the isomerization of allylic alcohols by Stephen Stinson Bruce M. Novak and graduate student a ruthenium phosphine catalyst in dry Anna K. Cedarstav of the University of acetone to make 1-propenols, which Massachusetts, Amherst, shift that equi can be viewed as methyl-substituted librium by making vinyl alcohol in an vinyl alcohols. These also copolymerize hydrous acetone. The compound is sta with maleic anhydride, initiated by ble long enough under these conditions AIBN. And such 1-propenols are avail for the chemists to copolymerize it with able inexpensively from allyl or methalmaleic anhydride, maleimide, or acrylo- lyl alcohol. So it is this latter approach /i 1 209th ACS National Meeting nitrile. Novak described the research to a that is likely to be considered by poten symposium sponsored by the Division tial producers. Polymer Chemistry. To make ketene acetal, the Massachu β ·ΙΙ A N A H E I M i j of The Amherst scientists make vinyl setts chemists use a method developed alcohol by protonolysis of ketene meth by organic chemistry professor Brian The conventional wisdom about poly yl vinyl acetal. They copolymerize it by Capon of the University of Hong Kong. vinyl alcohol (PVA) is that you can't ultraviolet irradiation of a mixture with Transetherification of oc-chloroacetaldeget there from here. That is, the equilib the comonomer and the free-radical hyde dimethyl acetal with 2-chloroetharium lies so far away from vinyl alco initiator a,a'-azobis(isobutyronitrile) nol gives oc-chloroacetaldehyde 2-chlorohol toward acetaldehyde that direct po (AIBN) at -10 to 25 °C. Because the ethyl methyl acetal. And treatment with lymerization is impossible. So the 300 ketene acetal is costly, this route to vi potassium hydride/ferf-butoxide con million lb of U.S. annual PVA produc nyl alcohol copolymers is not commer verts that to ketene methyl vinyl acetal. tion comes from hydrolysis of polyvi cially viable. For isomerization of allylic alcohols, nyl acetate. But Novak and Cedarstav also use Novak and Cedarstav turn to a tech-
Polymerization of vinyl alcohol achieved
24
APRIL 17, 1995 C&EN
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nique developed by chemistry profes sor Brice Bosnien of the University of Chicago. The Bosnien catalyst is bis(l,2diphenylphosphinoethyl)dirhodium, which isomerizes allylic alcohols in stantaneously in dry acetone. The two researchers have not suc ceeded in homopolymerization of vinyl alcohol. This is in line with the known failure of free-radical polymerizations of vinyl ethers at the temperatures they use. And the chemists rule out cationic or anionic polymerizations, because such initiators might trigger ketonization of the enols. Stephen Stinson
New hydrofonnylation process developed J I |pi
209th ACS National Meeting | A N A H
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Researchers at Union Carbide have de veloped a new oxo process for hydroformylation of higher olefins. The pro cess permits catalyst recovery in a sin gle phase under moderate processing conditions and is based on rhodiumionic phosphine catalysts that are solubilized in both reactants and products
to yield single-phase homogeneous systems. The catalyst is recovered out side the reaction zone. Anthony G. Abatjoglou, a research chemist at Carbide's Corporate Techni cal Center in South Charleston, W.Va., described the process to a symposium sponsored by the Division of Industrial & Engineering Chemistry. "Catalyst/ product separation is a major limitation in existing hydroformylation processes/7 Abatjoglou pointed out. "Homogeneous hydroformylation has used vaporization in some commercial processes, but they are limited to those with volatile prod ucts and by-products." One alternative approach to hydro formylation is to "heterogenize" ho mogeneous catalysts on polymeric supports. Another uses semiperme able membranes for separation. Nei ther alternative has found commercial success. In recent years, Ruhr Chemie in Ger many and Rhône-Poulenc in France have developed a two-phase aqueous/ organic system for the catalytic hydroformylation of propylene to butyraldehyde. In this system, catalyst separation is by décantation. According to Abatjoglou, "The problem here is mat the process is applicable only to low molecular weight olefins that have limited water solubility." Phase-transfer agents have provided some improvement in this approach, but the loss of soluble rhodium
Carbide process allows catalyst recovery in a single phase Aldehyde product Water extractor Oxo reactor Olefin
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is significant and catalyst recovery remains difficult, he says. In the Carbide process, the catalysts achieve high reactivity and olefin selectivity greater than 90% in a single phase. The process design employs catalyst/product separation by décantation. Extreme catalyst recoveries are possible by staged adsorption/desorption techniques. According to Abatjoglou, "The key discovery is that alkali metal salts of monosulfated triphenylphosphine (TPPMS) become soluble in nonpolar organic media, if we use certain solubilizing agents." Solubilizing agents that have been used successfully include N-methylpyrrolidone (NMP), polyalkylene glycols, and materials with the generic formula R(OCH2CH?)nOH. Solubilization is a complex process. Research at Carbide suggests the ligand and the solubilizing agents in the new system make up surfactant/cosurfactant combinations that form reverse micelles in the nonpolar phase. This leads to formation of stable singlephase systems. The single-phase rhodium-ionic phosphine catalysts have the reactivity typical of conventional homogeneous hydroformylation catalysts. However, they can be easily induced to separate into nonpolar (product) and polar (catalyst) phases, thereby providing an effective means for catalyst recovery. In some systems, the separation is induced by raising the temperature or cooling to ambient temperature. In the case of NMP-solubilized systems, a sharp separation results from addition of excess water or other polar compounds such as methanol. The practical significance of this technology is that it permits the homogeneous hydroformylation of higher molecular weight and less volatile olefins such as octene, dodecene, styrene, and dienes. Upon phase separation, the catalyst is segregated in one phase and the products (aldehydes, olefins, and by-products) in another. Traces of catalyst are recovered by water extraction. The very high value of rhodium demands that catalyst losses be minimized. Water extraction reduces the catalyst concentration in the product to less than parts-per-million levels. Traces of rhodium in the products are complexed to the ionic ligand. The complexes can be recovered quantitatively and recycled by adsorption/desorption APRIL 17, 1995 C&EN
25
SCIENCE/TECHNOLOGY on silica gel and on some types of anion-exchange resins. Abatjoglou told the symposium that continuous olefin formylation based on Carbide's new chemistry was achieved in a small experimental unit. An 86-day run with TPPMS as the ligand yielded heptanal through pentadecanal. Olefin efficiencies were greater than 90%. Product changeovers were possible simply by changing the olefin feed, and product contamination at changeover was minimal and transient. The rhodi um concentration in the final effluent of the catalyst recovery train was less than 20 ppb. The reaction was run at 90 to 110 °C and at CO pressures from 5 to 30 psia. Joseph Haggin
Liquid-phase methanol process to be tested J I
209th ACS National Meeting
«Ρ [ A N A H Ε Ι Μ | The first commercial trial of a new liq uid-phase methanol process will be conducted in a joint venture between Eastman Chemical and Air Products & Chemicals. The methanol process, de veloped by Air Products, is being used at Eastman's Kingsport, Term., coal-tochemicals complex to improve produc tion of acetic anhydride. It can also be used to produce fuel-grade methanol. Steven L. Cook, a research engineer at Eastman's complex, told a symposium sponsored by the Division of Fuel Chemistry that there is a well-defined need for a more flexible methanol pro cess. The methanol currently used for methyl acetate production at Kingsport is produced in a gas-phase process. A problem with this process is that reac tion heat is difficult to remove, and over heating can damage the catalyst. Tem perature control by admission of cool synthesis gas (syngas)—a mixture of carbon monoxide and hydrogen—also limits the methanol production rate. Cook told the symposium that Air Products' new process appears to be the solution to these problems. Testing of the process is being partially funded by the Department of Energy. Eastman has been operating an array 26
APRIL 17, 1995 C&EN
of integrated plants at Kingsport based on syngas from coal gasification, Cook pointed out. Methanol in one plant is produced by the Lurgi, low-pressure, gas-phase process. The methanol is combined with recycled acetic acid to produce methyl acetate. Acetic anhy dride is then produced by reacting methyl acetate with additional carbon monoxide. Two high-pressure gasifiers produce the syngas for these plants. High-sulfur coal is ground and slurried with water before being reacted with oxygen from an air separation plant. The hot syngas is then scrubbed with water to remove ash and decrease the temperature. Part of the syngas stream is routed through a water gas-shift reactor to ad just the gas composition to that re quired by the gas-phase methanol reac tor. Hydrogen sulfide is scrubbed from both gas streams in a Claus plant and converted to elemental sulfur. In methanol plants, the catalytic re action is exothermic and strict temper ature control is required to prevent damage to the copper-zinc oxide cata lyst. Ideally, methanol would be pro duced by a liquid-phase process, which would allow for greater temperature control via more efficient heat removal from the reactor. The Air Products process meets this ideal, Cook told the symposium. It uses an inert oil to slurry the catalyst and serve as the heat-transfer medium. Isothermal operation is possible and per-pass con-
version to methanol is not limited as it is in the gas-phase process. According to Cook, an additional benefit includes elimination of the water gas-shift reactor because the process is less sensitive to the CO/C0 2 /H 2 stoichiometry of the syngas. The joint venture between Eastman and Air Products makes possible a rigorous test of the liquid-phase methanol plant because operation of the integrated plants in the complex will not depend on the operation of the new methanol plant alone. It will be possible to adjust the output volume of the plant, coproduce dimethyl ether, and produce fuel-grade methanol for testing in off-site locations. Cook said the technology of the new process would also be useful as a method for energy storage in fossil-fuelbased power plants. Efficient heat removal allows flexibility in the feed gas composition and permits the use of syngas produced by any commercial coal or residual oil gasification system without the need for a shift reactor. The liquid-phase process also allows methanol production rates to be adjusted without thermal damage to the catalyst. The methanol would be produced with excess syngas from the power plant and stored in tanks during off-peak hours. During peak hours, the methanol would be pumped from storage to fuel boilers. Any excess could be sold as transport fuel. Joseph Haggin
Salt hydrates hold promise as C02 absorbents
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209th ACS National Meeting
ïIANAHEIMI
Air Products & Chemicals has discovered a new type of reversible absorbent for selectively removing carbon dioxide and other industrial gases from gas streams. Robert Quinn, a senior principal research chemist at Air Products, told a symposium sponsored by the Inorganic Chemistry Division that the absorbent may challenge alkanolamines in gas cleanup processes and diminish the carbon dioxide discharged to the atmosphere. Carbon dioxide is usually removed from mixed gas streams by selective,
reversible absorption in alkanolamines or solutions of strong alkaline salts. In the course of seeking new gas separation techniques, including facilitated membrane transport and new absorbents, a research group at Air Products that included Quinn, senior chemist John P. Appleby, and chief scientist Guido P. Pez encountered a new type of gas absorbent—the salt hydrates. Salt hydrates contain bound water. They are not merely concentrated solutions, but rather salts containing the minimum water necessary to fill the primary hydration spheres of the ions. The melting points of the hydrates are much lower than those of the corresponding anhydrous salts and are often near ambient temperatures.
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APRIL 17,1995 C&EN
Because water in excess of the primary hydration spheres is absent, waterwater interactions are absent and water-ion interactions dominate. The differences between bound water in a molten hydrate and bulk water in an aqueous solution are profound, Quinn said. These differences are reflected in the thermodynamic and other properties of the hydrates. One property that is particularly striking is the ability of certain salt hydrates to absorb large quantities of carbon dioxide and several other gases. According to Quinn, this property has been unrecognized until now. The property is pronounced in hydrates of various salts containing fluoride and carboxylate ions. Two examples under study at Air Products are tetramethylammonium fluoride tetrahydrate [(CH3)4N]F · 4H 2 0 (TAFT), and tetraethylammonium acetate tetrahydrate [(CH3CH2)4N]CH3C0 2 · 4H 2 0 (TAAT). "The abilities of the hydrates to selectively absorb such gases as carbon dioxide and hydrogen sulfide exceed those of the alkanolamines, which are now used in industry/ 7 Quinn said. "Regeneration of the gas-free hydrate is expected to be more energy efficient than [it is] for typical alkanolamines." Molten TAFT at 50 °C and 100,000 pascal absorbed 0.28 mole of carbon dioxide per mole of salt. This corresponds to a concentration of about 1.9 M. In contrast, the solubility of carbon dioxide in water under the same conditions is lower by two orders of magnitude, about 0.02 M. The Air Products team determined absorption isotherms at 50 °C. Absorption and desorption data lie along the same curve, demonstrating the reversibility of the absorption. The shapes of the isotherms suggest that carbon dioxide is absorbed by chemical reaction with the salt hydrates. Some general rules for evaluating the absorption properties of the hydrates are also available. Compounds with the general formula A x m+ B y n " · rH 2 0 or salts in the presence of limited water revealed that those consisting of monovalent cations (A+) and fluoride or weak carboxylic acid anions (Bn~) reversibly absorb large amounts of carbon dioxide. The carbon dioxide absorption capacity of the hydrate also depends on the water content. Generally, the less
water present, the greater the carbon dioxide affinity and absorption capacity of the hydrate. Salt hydrates have been known for a long time, but their reactivity with carbon dioxide seems to have escaped notice, Quinn noted. Spectral data suggest that the reaction of carbon dioxide with TAFT yields bicarbonate and bifluoride: 2F- · nH 2 0 + C 0 2 -> HC0 3 - + HF2" · (2n-l)H 2 0 In fact, Quinn said, the chemistry is much more complex than the reaction indicates and probably involves clusters of hydrated ions. So far, Quinn and his colleagues have only the first glimpse of salt hydrate chemistry. The salt hydrates do not contain discrete water molecules or ions. Rather, the water is coordinated to the anions by strong hydrogen bonds, a detail that has been established by X-ray crystallography. The reaction of carbon dioxide with TAFT or TAAT seems to involve hydrated anions in an ordered liquid rather than the structures suggested by the spectral data. Quinn has no evidence of free hydroxyl ions in TAFT. The resulting bicarbonate salt is stable when the pressure on the salt is reduced to less than 0.1 mm Hg at 25 °C. Joseph Haggin
Inorganic hosts bind aromatic amino acids A research group at Lawrence Berkeley Laboratory (LBL) in California has reported what it believes to be the first example of recognition of aromatic amino acid guests by bioorganometallic hosts in aqueous media. The group, led by Richard H. Fish, found that amino acid guest molecules L-tryptophan (L-Trp) and L-phenylalanine (L-Phe) were recognized by several cyclopentadienyl rhodium-DNA/RNA (Cp*RhDNA/RNA) cyclic trimer complexes in water at neutral pH [/. Am. Chem. Soc, 117, 3631 (1995)]. Noncovalent interactions are important elements in catalysis and biological molecular recognition. Most of the hosts that have been employed to study such interactions have been strictly organic, Fish points out. Few studies have been made with inorganic or organometallic
