Calixarenes: An Introduction (Monographs in Supramolecular Chemistry, Volume 10) - Hardcover

Gutsche, C David

Hardcover

ISBN 10: 085404258X ISBN 13: 9780854042586

Publisher: Royal Society of Chemistry, 2008

Synopsis

Calixarenes was the first book to be published in the `Monographs in Supramolecular Chemistry' series and is also the first complete survey available of this rapidly developing field. It provides a fascinating and lively account of the history, development and applications of calixarenes, which are probably the world's most readily available synthetic molecular baskets. These basket shaped compounds possess the ability to hold metal ions, as well as molecules, in their interior and as a result of their extraordinarily easy synthesis from phenols and aldehydes are receiving increasingly wide attention. This book is a must for advanced undergraduates and post-graduates studying bio-organic and supramolecular chemistry.

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From the Back Cover

Calixarenes was the first book to be published in the `Monographs in Supramolecular Chemistry' series and is also the first complete survey available of this rapidly developing field. It provides a fascinating and lively account of the history, development and applications of calixarenes, which are probably the world's most readily available synthetic molecular baskets. These basket shaped compounds possess the ability to hold metal ions, as well as molecules, in their interior and as a result of their extraordinarily easy synthesis from phenols and aldehydes are receiving increasingly wide attention. This book is a must for advanced undergraduates and post-graduates studying bio-organic and supramolecular chemistry.

Excerpt. © Reprinted by permission. All rights reserved.

Calixarenes

An Introduction

By C. David Gutsche

The Royal Society of Chemistry

Copyright © 2008 C. David Gutsche
All rights reserved.

ISBN: 978-0-85404-258-6

Contents

Chapter 1 From Resinous Tar to Molecular Baskets, 
Chapter 2 Making the Baskets: Synthesis of Calixarenes, 
Chapter 3 Proving the Baskets: The Characterization and Properties of Calixarenes, 
Chapter 4 Shaping the Baskets: Conformations of Calixarenes, 
Chapter 5 Embroidering the Baskets: Introduction of Functional Groups, 
Chapter 6 Combining the Baskets: Multi-Calixarenes, 
Chapter 7 Filling the Baskets: Complex Formation with Calixarenes, 
Chapter 8 Using the Baskets: Calixarenes in Action, 
Appendices, 238, 
Author Index, 244, 
Subject Index, 261, 

CHAPTER 1

From Resinous Tar to Molecular Baskets

"A man is wise with the wisdom of his time only and ignorant with its ignorance. Observe how the greatest minds yield in some degree to the superstitions of their age"

Henry David Thoreau, Journal, 1951

1.1 The Resinous Tar

The path of scientific research is seldom straight, often taking twists and turns quite unexpected at the outset of an odyssey. Such is the case with phenol-formaldehyde chemistry, which began over a century ago in the laboratories of Adolph von Baeyer. It has developed in ways that could not have been foreseen by this eminent scientist but which would certainly provide him with amusement and delight were he still alive to enjoy the passing scene of twenty-first-century chemistry.

Johann Friedrich Wilhelm Adolph von Baeyer was one of the great organic chemists of the nineteenth century and received the Nobel Prize in Chemistry in 1905. Born in Berlin in 1835, he received his training in chemistry from Bunsen and Kekulé in Heidelberg. His first professional appointment was at the University of Ghent with Kekulé, but his career as an independent investigator really began in 1860 when, at the age of 25, he joined the staff of the Technical Institute at Berlin. Twelve years later he moved to Strasbourg as director of the chemical laboratories of the university where his talents as a teacher and researcher blossomed, leading in 1875 to an appointment as successor to the great Justin Liebig at the University of Munich. He held this position until his retirement many years later. Although Baeyer was best known for his elucidation of the structure and synthesis of the naturally occurring dye indigo as well as his work on carbocyclic ring compounds, he also delved into many other areas of organic chemistry. Among these was a brief foray into the reaction of phenols with formaldehyde. Two short papers' were published from Berlin in 1872 followed by a somewhat longer one later that year after he had moved to Strasbourg. These appeared in Chemische Berichte and describe the results of mixing aldehydes and phenol in the presence of strong acids.

In the first of the three papers Baeyer simply reported that a thickening of the reaction mixture of aldehydes and phenols occurs with the formation of a "kittartige Substanz" ("a cement-like substance"). Specific examples of these reactions were discussed in more detail in the second paper, including the reaction of benzaldehyde and pyrogallol (benzene-1,2,3-triol), which gave a red-brown, resin-like product. Not until the third paper, however, was the reaction with formaldehyde introduced, heralding the beginning of the field of phenol-formaldehyde chemistry. This delay can probably be attributed to the fact that in 1872 formaldehyde was a rare chemical not easily available. Baeyer had to prepare it by reducing iodoform (CHI3) with HI and red phosphorus to methylene diiodide (CH2I2) and then replacing the iodine atoms with oxygen moieties by treatment with silver acetate in acetic acid. This yielded the liquid which Baeyer formulated as

(i.e. the adduct of HCHO and CH3CO2H), recognizing with great insight the proclivity of the C=O group of formaldehyde to add proteated nucleophiles. Thus, acquiring formaldehyde in sufficient quantity for use in reactions was a major undertaking, in marked contrast to today where it is one of the world's cheapest and most readily available chemicals (current production exceeds 2.5 million tons per year).

Using formaldehyde synthesized as described above, Baeyer showed that it reacts with phenol in a fashion similar to that of the larger aldehydes to produce a resinous material. However, he was unable to isolate pure substances from any of these reactions and, therefore, could not obtain elemental analyses which might have allowed him to propose possible structures. In retrospect we can appreciate how formidable a problem he faced, for even today with our impressive array of analytical techniques the structure of the phenol-formaldehyde product is not known in complete detail. However, Baeyer did manage to provide some structural insights by proposing a dimesitylmethane structure for the more tractable product obtained from mesitylene and formaldehyde. So, even though he was unsuccessful in characterizing the phenol- formaldehyde product, Baeyer had, nevertheless, given birth to phenol-formaldehyde chemistry.

The next chapter in this nascent field occurred in 1894 when two German chemists, L. Lederer and O. Manasse, independently studied the base-induced reaction of formaldehyde with phenol and succeeded in isolating o-hydroxy-methylphenol and p -hydroxymethylphenol as well-defined crystalline solids, viewing the reaction as a dehydration process in which formaldehyde reacts in its hydrated form, viz.

o-Hydroxymethylphenol was of particular interest, because it had already been isolated a half century earlier as a naturally occurring compound present in nature as a glucoside. The success of the Lederer-Manasse reaction, as it came to be called, depended on the use of mild and well-controlled conditions. Under more strenuous conditions the base-induced reaction, like its acid-catalyzed counterpart, yields a resinous tar of ill-defined structure.

At the dawn of the twentieth century phenol-formaldehyde chemistry, introduced a quarter of a century earlier, remained a rather intransigent and largely unattended area of investigation that seemed to hold little reward for explorers intrepid enough to set foot within its confines. But, explorers there were, such as Blumer, Storey, Luft and others, all of whom tried to tame these resinous tars and find practical applications for them. For one reason or another, however, all failed to produce materials with marketable qualities.

Success was to go to another explorer, Leo Hendrik Baekeland, who was born in 1863 in Ghent, Belgium. From early childhood Baekeland's brilliance as a scholar was evident, and by 1884 at the youthful age of 21 he had already acquired a Doctor of Science degree. After several years of restless associations with chemistry departments at Ghent and Bruges in Belgium, Cambridge and Oxford in England, and Edinburgh in Scotland, Baekeland and his young wife sailed to America in 1889, there to continue his research on photographic papers.

This eventually led to a commercially successful material that he called "Velox", a name still known to an older generation of pre-digital photographers. So successful was this product that the Velox process was purchased by George Eastman in 1900 for one million dollars, making Baekeland a wealthy man at the age of 37. Constrained by the terms of the sale from continuing to carry on his photographic experiments, yet not wanting to abandon his passion for research, Baekeland set up a laboratory in his home, hired a number of assistants, and proceeded to explore an amazing diversity of projects. Among these was an investigation starting in 1902 that dealt with the reaction of phenol and formaldehyde. Although it had already occurred to a number of others that the hard cement-like substance described by Baeyer might have utility as an item of commerce, only after several years of careful and painstaking work was Baekeland able to prove this premise and show that by using a small and controlled amount of base an appealing material could be obtained. On February 18 1907 he filed for a patent on this process for making a material that he eponymously called Bakelite. With this, the age of modern synthetic plastics had begun.

The Bakelite process, ultimately described in over 400 patents issued to Baekeland, constituted the first large-scale production of a synthetic plastic. Like most new things, it took time to gain acceptance, but once the induction period had come to a close an exponential growth phase ensued that brought great wealth to Baekeland and many others and that inspired a flood of research. One of the earliest reviews of the research on the chemistry of what were called "teerphenole" (tar phenols) was written in 1912 by Raschig, who came to the conclusion that "über die Chemie des Bakelits tappen wir noch vollständig im Dunklen". Today, almost a century later we continue to be at least somewhat in the dark about the precise details of the Bakelites. The progress that has been made and the problems that remain unresolved are recounted in a variety of books and articles. As had already been realized by Baeyer in 1872, however, it is CH 2 and CH2OCH2 groups that are the most likely linkages between pairs of aromatic rings in a formaldehyde-phenol condensation product. Thus, the dominant structural diaryl moieties in a typical resin are those shown in Plate 1-6; viz. resoles, novolaks and dibenzyl ethers.

When resoles are heated they undergo conversion to novolak-like structures, and it is the changes that occur during this "curing" process that have attracted most of the attention not only from the process engineers in the production plant but also the research chemists in the laboratory. The former are interested in discovering how varying conditions affect the physical attributes of the final product; the latter are interested in discovering the nature of the chemical transformations that are occurring.

1.2 Glistening Crystals: The Zinke Products

It is the outcome of a study of the "curing" phase of the phenol-formaldehyde process that provided the next episode in this story, bringing to the fore the central subject of this book. In 1942 Alois Zinke, a professor of chemistry at the University of Graz in Austria, and his coworker Erich Ziegler decided to "simplify" the problem by looking not at phenol itself but at p- substituted phenols in the condensation reaction with formaldehyde. Whereas phenol reacts at both the ortho- and para- positions to form a highly cross-linked polymer in which almost all of the phenolic units are attached to three other phenolic residues, viz.

a para-substituted phenol can react only at the two ortho- positions, thereby reducing the cross-linking possibilities, viz.

It was anticipated that with this more limited reaction a more tractable product would be obtained that might provide insight into the curing process. A typical experiment in the Zinke and Ziegler study is described in the 1944 paper as follows: "When 50 g of the resin which is obtained by heating 100 g of p-tert- butylphenol, 100 mL of 3 N NaOH, and 97 g of a 35% formaldehyde solution is heated with stirring with 200 g of linseed oil, it dissolves at 100–120 °C. At 140–160 °C there is a vigorous foaming, and turbidity appears which increases greatly upon further heating at 200–220 °C. The resulting brown waxy paste is stirred with ethyl acetate, washed thoroughly, and reprecipitated from CCI4 with alcohol to give a crystalline product as platelets or rosettes that decompose above 300 °C." A product of presumably identical composition with the empirical formula C11nH14O had already been described in less detail by these authors three years earlier. The lack of reactivity of this compound with HBr indicated the absence of ether linkages, but no structure was proposed in the 1941 paper. By 1944, however, the idea of a cyclic structure had sprung to the minds of several investigators and was "in the air", so to speak. Joseph Niederl and his coworker Heinz Vogel. at New York University, for example, had proposed a cyclic tetrameric structure for compounds obtained by the acid-catalyzed treatment of aldehydes and resorcinol (cf. next section), so it was a propitious time for the Austrian workers to propose the cyclic tetrameric structure 1.1 for the base-induced product.

Alois Zinke was born in Bärnbach, a region of Voitsberg, Austria, in 1892. His early schooling was acquired in Voitsberg, but in 1912 he went to Graz for the completion of his training for the Doctorate from the University of Graz in 1915 under the tutelage of Roland Scholl. He remained in Graz for the rest of his life, starting his professional career at the University of Graz, transferring his allegiance to the Technische Hochschule for a brief four years and then returning to the university as a professor of pharmaceutical chemistry. In 1941 he was appointed director of the Institute für Organische und Pharmazeutische Chemie. During his distinguished career, which extended to his death in 1963, he carried out research in several areas, but his best known work is with the phenol-formaldehyde resins. When he started this work in 1936 there already were several well-established investigators in the field, including Koebner, Megson, v. Euler, Hultzsch and Adler. Undaunted by this competition, Zinke initiated his own program and went on to make numerous and important contributions including the seminal discovery of the cyclic oligomers.

The product that Zinke and Ziegler described in their first experiment was a crystalline acetate melting at 314 °C, which had the surprisingly high molecular weight of 1725. On the premise that the compound from which the acetate was derived was certainly a cyclic oligomer, this value would appear to indicate the presence of 8 or even 9 p-tert-butylphenol units in the cyclic array. However, this seemed so unlikely that they desisted from drawing any structure in 1941 and waited until 1944 before proposing the intuitively more appealing cyclic tetramer structure.

Zinke's choice of a cyclic tetrameric structure for the product obtained from the base-induced condensation of p-tert-butylphenol and formaldehyde seemed quite logical, and the high molecular weight value for the acetate was dismissed as a complication caused by mixed crystal or molecular compound formation. Additional examples of the condensation reaction, which Zinke refers to as one involving "nicht alkalifrei gewaschenen Resols in Leinöl ("resoles, not washed entirely free of base, in linseed oil"), were reported in a short paper four years later in which p -phenylphenol, p-cyclohexylphenol and p-benzylphenol are all described as giving high melting, organic solvent-insoluble materials (cf. structures 1.2). Cyclic tetrameric structures were assigned to all of these products, although no experimental data were provided. Not until 1952 was further information forthcoming in a paper that is the most detailed of all of Zinke's publications on the cyclic oligomers. Published with R. Kretz, E. Leggewie and K. Hössinger, it provided additional examples of phenols reacting with formaldehyde to give high melting, organic solvent-insoluble substances. More importantly it provided additional evidence in support of the cyclic tetrameric structure. Zinke had recognized that an unequivocal proof of structure had not been given in his previous publications, and he implicitly acknowledged that the molecular weight of the acetate of the product from p-tert-butylphenol and formaldehyde constituted a puzzling complication. Thus, it must have been a great relief to him to find that the acetate of the product from p-1,1,3, 3- tetramethylbutylphenol and formaldehyde, isolated as needles with mp 333 °C, had a cryoscopic molecular weight of only 876, in perfect agreement with that calculated for a cyclic tetramer. Satisfied with this piece of evidence, Zinke concluded that all seven of the products he had obtained were cyclic tetramers, viz.

The four papers by Zinke that are discussed above represent only a small fraction of his total scientific output, but in the context of the present book they are his most important. This pioneering work gave glimpses of the treasures that are to be found in the resinous tar of a phenol-formaldehyde condensate, and they provide an important part of the scaffolding on which calixarene chemistry has been built. No matter how original one's contributions may appear to be, however, antecedents can usually be found lurking in the shadows. The case of the cyclic tetramer is no exception, for Raschig had suggested such a structure back in 1912 in the paper cited earlier in this chapter, although his idea must be regarded as a lucky guess rather than a rational choice based on experimental evidence. To Zinke should go the honor of true parentage. But, as we shall see in later sections of this chapter, the complete character of his progeny was yet to be revealed.

1.3 More Crystals: The Niederl Products

At this point we must retrace our steps, look once again at Baeyer's experiment in 1872 and follow another trail of events that flowed from these beginnings. Resorcinol was among the reactants that Baeyer used in his investigations with phenols and aldehydes, and he discovered that it reacts with aldehydes such as acetaldehyde and benzaldehyde under acidic conditions to produce crystalline, high-melting compounds. However, the products lacked the dye-stuff properties he was seeking, so he decided not to pursue their characterization but simply to conclude that they were 1:1 condensation products. The reaction was re-investigated a decade later by Michael, who succeeded in isolating a pair of crystalline materials for which he postulated cyclic dimeric structures. Similar experiments were carried out in 1894 by Möhlau and Koch and again in 1904 by Liebermann and Lindebaum, but after that the compounds remained unattended until 1940 when Niederl and Vogel reinterpreted their chemistry. These workers isolated solid, high melting condensation products from the reaction of resorcinol and aldehydes (e.g. acetaldehyde, propionaldehyde, isovaleraldehyde) and concluded, primarily on the basis of molecular weight determinations, that the products are best represented as cyclic tetramers 1.3. In contrast to the Zinke tetrols in which four OH groups are intraannular (also designated as "endo-annular"), the eight OH groups in the Niederl octols are extraannular (also designated as "exo-annular").

(Continues...)

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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2008
• Language: English
• ISBN 10: 085404258X
• ISBN 13: 9780854042586
• Binding: Hardcover
• Edition number: 2
• Number of pages: 282