In the Laboratory
Calix[4]pyrrole: Synthesis and Anion-Binding Properties
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An Organic Chemistry Laboratory Experiment James A. Shriver* and Scott G. Westphal Department of Chemistry, Central College, Pella, IA 50219; *
[email protected] Over the past several years, the study of anion-binding events has received increased attention (1). With the realization that anions play important roles in myriad biological (such as the first X-ray determined chloride channel) (2) and ecological functions (3), the development of novel supramolecular architectures with anion-binding sites has flourished. Two diverging strategies emerged as more complex and selective anion-binding compounds were synthesized. The elder approach included cationic sites such as ammonium (4) or guanidinium (5) groups to promote electrostatic interactions with a targeted anion. This method has the advantage in that these electrostatic contact points promote increased affinity of anions and hence stronger binding. However, in many cases, this approach does not achieve the degree of selectivity required for certain systems and, more importantly, is typically not biocompatible. An alternative approach targets a rational geometric placement of hydrogen bonds to focus on a certain type of anion. A majority of these systems are biomimetic (6) and more appropriately mirror nature’s approach to anion complexation. The primary limitation of this strategy is that weaker affinity constants are seen and often studies of these molecules must be carried out in organic solvents as opposed to aqueous media. In general, the binding of anions such as phosphate, sulfate, nitrate, chloride, and fluoride have all been targeted using the “neutral molecule” method. It is postulated, and in some instances shown, that uses for these systems range from ion selective electrodes (7), to remediation of radioactive wastewater (8), and to medical applications such as dialysis (9) and treatment of illnesses like cystic fibrosis (10). The focus of this article is a system that has been explored as an anion-binding agent for fluoride and, to a lesser
Figure 1. Conformations of calixpyrrole.
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extent, chloride and acetate. Originally discovered by Baeyer in the 19th century (11), and named meso-octamethylporphyrinogen, this compound has been more appropriately dubbed meso-octamethylcalix[4]pyrrole (12) by leaders in this field owing its similarity to the widely studied class of compounds known as calixarenes. This is in contrast to porphyrinogens, which traditionally are the class of tetrapyrrolic macrocycles as they exist prior to their oxidation to porphyrin. Saturating the meso position with alkyl groups prevents this oxidation. Upon the introduction of an anion, four independent pyrroles concurrently donate a hydrogen bond to the guest molecule. This bound conformation gives the system its bowl shape typical of all calixarene systems. Examples of an unbound calixpyrrole shown in the typical 1,3-dialternate conformation and a bound calixpyrrole shown in its typical cone conformation are illustrated in Figure 1. Recently, this substrate has shown the remarkable ability to concurrently bind a cesium cation with an anion (13). This observation mirrors observations seen for standard calixarenes (14) and further supports the place of calixpyrroles as members of the calix-arene family. Given the explosion of research in this subfield over the last few years, the introduction of this science at an undergraduate level is warranted. Simple calixpyrrole macrocycles provide an easy-to-synthesize example of an anion-binding agent, which can be included in the undergraduate laboratory curriculum. Calixpyrrole-forming reactions are typically quick and proceed in less than an hour for all but the most unreactive pyrroles. For the latter, the reaction may take a few days (15). Subsequent to the initial submission of this publication, Sobral published a laboratory procedure for the direct synthesis of calixpyrrole from the condensation of acetone and pyrrole using acetone as the solvent (16). He obtained a nearly analytically pure sample of calix[4]pyrrole at 60% yield without purification. This sample was then characterized by a variety of methods in the context of its relationship to porphyrins. In contrast, our method focuses on the purification of the crude product by following the utilization of thin-layer chromatography (TLC) and recrystallization. This approach helps reinforce the utilization of these commonplace organic chemistry techniques with respect to a useful synthesis while remaining within the time constraints of a typical organic laboratory. We also look at the nature of the impurities inherent in the condensation of pyrrole and acetone. Additionally, our work focuses on the anion-binding properties of our formed product through additional experiments. In this way, we help bridge the gap between typical organic syn-
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In the Laboratory
thesis and the inclusion of analytical techniques that are commonplace in many research laboratories. Synthesis In the context of a standard laboratory session to test this procedure as a viable method, this protocol was performed by the second-semester organic chemistry class (31 students). All of the experiments were performed within a three-hour lab session by individual students. Detailed reaction conditions for the reaction shown in Scheme I, including the optimization of this procedure, can be found in the Supplemental Material.W Divergent to the work by Sobral, we desired to reinforce purification techniques and the importance of following a reaction by TLC. In light of this, TLC was performed on the crude and pure products affording the following information. Using standard normal-phase silica gel plates with a UV indicator, eluting the product in 7:3 hexanes:ethyl acetate leads to observation of the following: calixpyrrole Rf = 0.78, impurity Rf = 0.39, and polymer Rf = 0.00. The impurity is likely the N-confused calixpyrrole in which one of the four pyrroles has one attachment at a β position (17). The polymer spot is likely a remnant from photopolymerization of pyrrole and the intensity will vary depending on the purity of the starting pyrrole. While the synthesis, purification, and spectroscopic determination by standard means is targeting a student audience at the second-year organic chemistry level, the anion-binding properties of calixpyrroles offer a unique opportunity in advanced organic chemistry laboratories. Common methods for determining host–guest interactions in calixpyrroles, though substrate dependent, include standard spectroscopic tests such as fluorescence, NMR, and isothermal calorimetry (ITC). Additionally, colorimetric tests have also been developed to test the competitive binding between chromophores such as the 4-nitrophenolate anion and anions such as fluoride and chloride, for which calixpyrrole is very sensitive (18). Anion-Binding To introduce basic anion-binding concepts to an advanced organic lab, we decided to look at this colorimetric test to qualitatively explore the anion-binding capabilities of calixpyrrole. A small portion of calixpyrrole, which the student prepared in the prior experiment, was added to a solution of tetrabutylammonium 4-nitrophenolate. Immediately upon its addition to the solution, 4-nitrophenolate complexed with calixpyrrole as evidenced by the disappearance of the bright yellow color of the 4-nitrophenolate anion. With the addition of a small quantity of tetrabutylammonium fluoride, the bright yellow color immediately returned. This competitive titration demonstrated one example of how fluoride is preferred by calixpyrrole in comparison to other anions and introduced the concept of selectivity. The details of this procedure including the preparation of standard solutions and tetrabutylammonium 4-nitrophenolate are included in the Supplemental Material.W
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Scheme I. Synthesis of calixpyrrole.
Hazards None of the chemicals used for this procedure demonstrate a great danger to the students in the quantities used. Pyrrole, methanol, and acetone are all flammable liquids that can also be irritating or drying to the skin and should not be ingested. Care must be used when working with concentrated sulfuric acid as it is a corrosive liquid that may irritate or burn the skin, with potential problems scaling with quantity. We suggest setting out a minimal volume (with only two drops being needed for each experiment) to limit any potential dangers. For the anion-binding experiment, dichloromethane and benzene are used. Because they are volatile and are human carcinogens, all steps using these solvents should be performed under a hood. Conclusion We were pleased with the student results seen for this system. While the yields were lower than we achieved in the research laboratory, they still gave a range from 20–75% with a mean of 40% and a median yield of 35%, corresponding to an average recovery of 216 mg. This quantity is sufficient for thorough spectroscopic determination as well as any chemical tests. Higher yields were seen when the mother liquor was allowed to stand for longer than 30 minutes, though 75% is about the expected high-end yield for one crop. The inclusion of a test for anion binding also makes this lab procedure suitable for a more advanced audience and supplies a bridge for the introduction of supramolecular chemistry and a typical host–guest interaction. Acknowledgments The authors would like to thank Central College for start up funds providing support for this project. We also wish to thank the fall 2004 organic chemistry class at Central College for assistance in testing this procedure. W
Supplemental Material
A detailed lab procedure for both parts of this experiment, a data sheet, and notes to the instructor are available in the in this issue of JCE Online.
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