Showing papers in "Pure and Applied Chemistry in 1968"

Thermally stable polymers

Abstract: Most organic polymeric materials melt below 200 °C and most of them begin to degrade rapidly at temperatures only slightly above 200°G. Thermally stable polymers are generally considered to be those which will withstand much higher temperatures without loss of strength or change of structure. In general we expect these materials to withstand at least 300°C in air and up to 500°C or higher in inert atmospheres. Polymers which show these properties are usually highly aromatic in structure, often with heterocyclic units, high melting, sometimes infusible and usually with low solubility in all solvents. This makes their fabrication very difficult and as a consequence limits their usefulness.

Electron-transfer and complex formation in the excited state

Abstract: FLUORESCENCEt Some years ago, while investigating fluorescence quenching of aromatic hydrocarbons (A) by typical electron donors (D), like anilines, we observed1 a broad structureless emission band about 5000 cm1 to the red of the fluorescence of the aromatic hydrocarbon of normal structure. This anomalous fluorescence, as shown in Figure 1, increases in intensity with increasing donor concentration at the expense of the fluorescence intensity of the hydrocarbon, thereby following the same Stern—Volmer-type relation as does the well known excimer fluorescence, e.g. in the case of pyrene2. Extrapolation to infinite donor concentration gives the dashed spectrum (cf. Figure 1) which

Nucleic acid synthesis

Abstract: INTRODUCTION Synthesis in the field of nucleic acids has been a major and continuing interest in my laboratory ever since we knew the structures of the nucleic acids. Clearly the foundations of synthetic work in the nucleotide field were laid by the prolonged and intensive efforts of Professor Todd's laboratory (Cambridge, England) and it was my good fortune to be associated with this laboratory before our own work in the field got going. With the elucidation of the internucleotidic linkage in the nucleic acids, polyiiucleotide chemistry entered a new area of complexity in. which the major problems to be posed were those of sequence and synthesis of poiynucleotides. In principle, organic chemists could attempt to put together nucleosides and nucleotides to form short chains of polynucleotides which would be identical in every way with the naturally-occurring nucleic acids. It was very reminiscent of the developments of protein chemistry at the turn of the century and into the twenties and thirties which followed the proposal of the peptide theory of protein structure. There was a strong and clear case for sustained effort in developing synthesis in the nucleic acid field, especially when one looked around at that time at the level of sophistication that many other areas of natural product chemistry had attained. However, in addition to the intrinsic interest in developing the organic chemistry of nucleic acids, there was the hope that the availability of synthetic polynucleotides of known size and structure would provide many opportunities for chemical, physico-chemical, and enzymatic studies of nucleic acids. So we began to undertake work on the organ.o-chemical synthesis of polynucleotides in the middle fifties. While our, early work proceeded hand-in-hand with a broader study of the problems of phosphate chemistry, e.g., methods of phosphorylation of nucleosides and of carbohydrates, properties of phosphate esters, specific activation of mononucleotides to form nucleotide coenzymes, polynucleotide synthesis became the most serious commitment in due course of time. Looking back over the years, three phases of our work in this field can be discerned. Up till the early sixties, the main preoccupation was with the organo-chemical synthesis and chemical and enzymatic characterization of polynucleotides. While even at present, organo-chemical methods demand further investigation and refinement, nevertheless, synthesis of short chains of deoxypolynucleotides with known and controlled sequences became possible in the early sixties. A second phase of our work then started and a central question which was posed at this time was "Can chemical synthesis make a contribution to the study of the fundamental process of biological information flow, DNA-÷ RNA—protein ?".

Chemistry of bioluminescence

Abstract: THE STRUCTURE OF THE LUCIFERINS With their fascinating phenomena we recognize many bioluminescent organisms, such as firefly, luminous bacteria, luminous fungi, and luminous deep-sea fish. Some eighty years ago, Dubois found that the bioluminescence of the luminescent beetle Pyrophorus is due to the luciferin—luciferase reaction. Bioluminescence1—9 was observed when a hot aqueous extract ("luciferin") of the insect was mixed with a cold aqueous extract ("luciferase") after the initial luminescence had disappeared. This observation led to numerous studies on the luciferin—luciferase reaction9' 10 many bioluminescent organisms (Table 1). It is now known that hot water extracts luciferin and destroys luciferase; the cold-water extract contains luciferase, and also luciferin at first, but the latter changes into an inactive oxidation product on standing in air.

Recent advances in the chemistry of natural products

Abstract: It gives me great pleasure to exercise this opportunity to present to you some of the results of a collaborative investigation which has as one of its objectives the synthesis of vitamin B12. These studies have been pursued in my laboratory at Cambridge and in that of Albert Eschenmoser at Zurich. They have been prosecuted on a very broad front, and in the limited time available here it will be possible to describe only a small portion. of the work. I have chosen to present those results which seem to us at present most surely destined to constitute established stages in the achievement of the specific objective of synthesizing vitamin B,2. To make this choice is not to derogate the interest and fascination of other portions of the investigation. Indeed, before embarking on today's story, I permit myself a parenthesis. The history of organic chemistry provides in abundance instances of the major role played by the study of natural products in revealing, extending, and shaping the fundamental bases of the science. Time and again the penetration of a new sector of the vast, often surprising and always beautiful panorama of natural products has led to new insights which could hardly have been achieved by more self-conscious fundamental investigations. This role of natural product studies is in no way diminished in our day, and it will certainly continue in the future; the proposition cannot be better illustrated than by my alluding to the fact that the principle of orbital symmetry conservation arose directly from our studies on vitamin B12 synthesis. But that story has been outlined in another place', and I shall have nothing further to say about it today. Let us commence by scrutinizing the structure of the vitamin B12 molecule (I). That intricate and fascinating array, revealed to us by the beautiful x-ray crystallographic studies of Dorothy Hodgkin, exhibits certain resemblances to other natural substances. Thus, like the blood pigment, haem, and the leaf pigment, chlorophyll, it contains a metal atom embedded within a macrocyclic nucleus containing four five-membered heterocyclic rings— A, B, C and D. But while in the blood and leaf pigments these five-membered rings are held together by single-carbon bridges, in the case of vitamin B12 two of them, rings A and D, are directly linked. Above all, the most striking contrast between vitamin B12 and its more simply constituted relatives, haem and chlorophyll, lies in the realm of stereochemistry. The porphyrin nucleus of the haem molecule is constituted entirely of trigonal carbon atoms and presents no points of stereochemical interest; the dihydroporphyrin, or chlorin, nucleus of chlorophyll presents only the relatively trivial stereochemical problem associated with the presence of two unsymmetrically substituted tetrahedral carbon atoms. In striking contrast, the periphery

Photochemical reactions with non-conjugated dienes

Abstract: INTRODUCTION Electronic interactions between classically non-conjugated, by one or several sp3-hybridized carbon-atoms insulated centres, have been studied intensively. Today, homoallylic ions and homoaromatic compounds, to mention a few, are experimentally well documented and theoretically well understood'. Far less settled is the state of affairs when we talk about interactions in the excited states. Such interactions of nominally unconjugated, supposedly simple chromophores have been observed in a number of chemically distinct systems, but physical interpretation of these observations turned out to be rather difficult, even for the simple case of two C=C double bonds2. In this talk I would like to present some further examples where light-induced interactions within homodieneand within homovinyl-cyclopropane systems have been used for the synthesis of some valuable, by other means hardly accessible, products. In the first part we will start from a well known photoisomerization reaction. Following the original report by Cristol and Snell3 the intramolecular cycloaddition reaction norbornadiene—quadricyclane (I) — (II)

The use of photochemical reactions in organic synthesis

Abstract: INTRODUCTION In the last decade photochemistry has grown from a minor to a major activity in organic chemical laboratories. Many striking and (thermally) unusual processes can be observed and the mechanistic problems posed in their interpretation demand new dimensions in organic chemical thought. However, in spite of the large number of new photochemical reactions which have been discovered in recent years, rather few have yet found use in organic synthesis and fewer still have been of economic value in industry. Some years ago' we adumbrated a new rearrangement reaction of alkoxyl radicals in general and of alkoxyl radical derived from the photolysis of nitrites in particular. This photochemical reaction has found a number of applications and it has value in the technical synthesis of aldosterone2.

Five coordination in 3 d metal complexes

Abstract: INTRODUCTION In recent years a good number of five-coordinate complexes of metals of the first transition row have been described, so that now this type of coordination is no longer exceptional'. We are going to discuss the properties of these complexes, paying particular attention to their thermodynamic, magnetic and spectroscopic aspects. In this contribution five-coordinate complexes of 3d metals with zero or negative oxidation states and compounds which have been known for a long time are not considered.

Photoaddition and photoisomerization reactions of the benzene ring

Abstract: INTRODUCTION The thermal chemistry of benzene and its derivatives has been extensively studied for well over a century, and constitutes one of the best-trodden fields of organic chemistry: its characteristic feature is the strong tendency for substitution reactions leading to products in which the aromatic ring survives. In marked contrast, the photochemistry of the benzene ring had been scarcely investigated at all prior to the present decade. About twenty different types of photochemical reactions involving the benzene ring have now become known, and nearly all of these lead to non-aromatic products. Herein lies a major difference between the thermaland photo-chemistry of the benzene ring, a difference which provides a powerful stimulus for research into the nature of the photochemical processes involved, and an opportunity for the synthesis of molecules not readily accessible through thermal chemistry. Indeed, some of these molecules have quite exotic architecture. Our own interest in this field stems from the observation that liquid benzene can undergo photoisomerization to fulvene via the first optical transition1. The purpose of this lecture is to summarize both some new and previously known results on the photochemistry of benzene and its simple derivatives, and to present some mechanistic proposals which appear to collate much of the present information and may serve as a basis for future discussions and predictions.

Some problems in aromatic photosubstitution

Abstract: INTRODUCTION In this paper we wish to summarize results in the field of (heterolytic) aromatic photosubstitution and especially bring into focus a few problems that arise from recent investigations. To this end the following plan is adopted: (A) Review and interpretation of results as available 1—2 years ago. A few fundamental problems will be indicated. (B) Presentation of some new reactions that in at least one respect seem to differ from those discussed under (A). (C) Summary of present situation and of problems for discussion.

Some studies in the biosynthesis of terpenes and related compounds.

Abstract: Cyclic terpenes as a class of natural products display a bewildering array of structural types. Well over a hundred different carbon frameworks have been detected so far, many new substances are being currently added to an already impressive list of compounds, and even several representatives of the long-sought group of sesterterpenes have at last made their appearance on the chemical scene14. In spite of this almost inexhaustible prodigality of Nature, it had been recognized at an early stage that the biological formation of cyclic terpenes can be traced back to the operation of a unifying and, in principle, rather simple biosynthetic scheme5. In the light of present knowledge three main phases can be discerned. In the first phase (Figure 1), regularly built pyrophosphate esters of aliphatic terpene alcohols are formed through successive elongation of a basic

Progress in the synthesis of polycyclic natural products: The total synthesis of lycopodine

Abstract: Numerous alkaloids have been isolated over the years from the genus Lycopodium. Among these, Lycopodine first isolated in 18811, seems to be the most widely distributed2. Structural work, ranging over a number of years, culminated in the structure and absolute stereochemistry shown in (I) for the alkaloid2. The identical expression (II) can be written as in the twodimensional representation (III).

Some Fundamental Aspects of Polymer Reactions

Abstract: In discussing the reactivity of macromolecules, it is generally assumed that a functional group attached to a high polymer has a chemical reactivity similar to that which would be observed for such a group in small molecules1. This generalization may essentially be true, but we often find reactions in which a functional group attached to a high polymer exhibits widely different reactivity to that in small molecules.

Photooxydation des dérivés aromatiques

Abstract: Dans le cas des dienes conjugues cycliques, cette "autoxydation photosensibilisee" conduit a des endoperoxydes transannulaires et de nombreux exemples de cette reaction ont ete decrits dans les series du cyclopentadiene, du cyclohexadiene-1 ,3 ou du cycloheptadiene-1 ,31 Elle s'effectue de meme regulierement avec certains dienes heterocycliques, en particulier avec Jes furannes2 et les pyrroles N-substituCs3, aboutissant alors a des endoperoxydes peu stables dont on n'isole le plus souvent que les produits de transformation (Figure 1).

Nuclear magnetic resonance and optical studies of polypeptide chain conformation

Abstract: Everyone is aware of the intense effort that has been devoted to the study of the structure of proteins and polypeptides during the last two decades, culminating in the complete x-ray determination of the structures of myoglobin, hemoglobin, lysozyme, and ribonuclease. Few problems in natural science have been subjected to such a heavy assault with such a variety of weapons. The most important of these, of course, has been x-ray diffraction. One cannot imagine at present any other technique capable of providing the thousands upon thousands of individual parameters necessary to describe a complete protein structure. In view of the spectacular triumphs of this method, why do many of us who are concerned with polypeptide and protein structure continue to employ such relatively humble techniques as optical rotation, circular dichroism, and nuclear magnetic resonance, which are capable of giving at most only a handful of structural parameters ? I think there are good answers to this question, and I would suggest a few as follows: (a) If our main interest is in polypeptides of a single repeating residue, then only a relatively limited number of molecular parameters may be adequate for a good description. (b) X-ray studies are limited for the most part to crystalline solids and, except in certain special circumstances, cannot inform us what bio-polymers are doing in solution. (One such exception is the x-ray study of polypeptides in solution by Brady, Salovey and Reddy1 in our laboratories; this requires labelling with heavy atoms such as bromine.) (c) Optical and n.m.r. methods have an intrinsic interest in their own right. This may be the most important reason for using them.

Photochemistry of cyclic ketones in solution

Abstract: ion from the solvent by the excited cyclocamphanone to give (XXV), cleavage of the cyclopropane ring to give (XXXVI), and hydrogen atom transfer and ketonization. This postulated route for the isomerization of cyclocamphanone to (XXXIV) is based on the proposal by Srinivasan for the route of isomerization of camphor to the ketone (XIV)'°. The photolysis of (XXXIV) to give (XVI) finds close analogy in work on the photolysis of dehydronorcamphor (XXXVII a) which has shown that cleavage occurs to give cyclopentadiene and ketene'5. A similar reaction has also been observed in the case of dehydrocamphor with a 5-aryl substituent (XXXVII b) which gives a 3-aryl-1 ,5,5-trimethylcyclopentadiene (XXXVIII b)'6. Formation of cyclic acetals by irradiation of other cyclic ketones Since our original preliminary report on the novel photochemical ring expansion of cyclic ketones to cyclic acetals'7 many other examples have come to light. Hostettler18 found that irradiation of the bicyclic ketone (XXXIX) gave two major products. One of these is the cyclic acetal (XL), which is the analogue of the acetals obtained from cyclocamphanone and nortricyclanone. The other is the ester (XLI), which is considered to be formed from the corresponding ketene, itself formed by photocleavage of the four-membered ring in a well-known fashion that was first observed in the vapour phase photolysis of cyclobutanone itself19. Hostettler20 has also observed cyclic acetal formation in the photochemical reactions of a series of 3-substituted-2,2,4,4-tetramethylcyclobutanones (XLII). Irradiation in alcohols gave a mixture of the corresponding ring-expanded cyclic acetal

Fate of the excitation energy in the quenching of fluorescence by conjugated dienes

Abstract: INTRODUCTION Recently we have reported that the fluorescence of many aromatic hydrocarbons, and notably that of anthracene and naphthalene derivatives, is strongly quenched by conjugated dienes in solutionl3. This phenomenon has been discussed in rather general terms and the particular interactions leading to singlet quenching are not as yet fully understood. Our attention was first directed to this problem when measured quantum yields in certain photoreactions sensitized by aromatic hydrocarbons fell short of the values which we anticipated. Inspection of representative data,

Catalytic effects of metal chelate compounds.

Abstract: This paper is essentially a progress report on the developments in the field of metal chelate catalysis since the first review of this subject fifteen years ago'. For the purpose of defining the scope of this field, metal chelate catalysis is considered to include any reaction that is altered or modified by metal ions through chelate ring formation, as well as reactions of metal chelates themselves. The relatively narrow definition of catalysis, in which the catalytic substance remains unaltered at the end of each reaction cycle or sequence thus constitutes oniy a part of the overall topic under discussion. The catalytic reactions which do not fit this specific definition may be considered as being "promoted" or "inhibited" by metal chelate ring formation.

Some recent results on hydrolytic equilibria

Abstract: INTRODUCTION Berzelius (1812)1 defined a neutral salt as one that contains equivalent amounts of acid and base. He pointed out that neutral salts of sulphuric acid with bases with a weak affinity, like alumina and Fe203, still have an acid reaction to taste and to plant dyestuffs. Werner (1895)2 stated that a metal salt hydrate like C12Cu(0H2)2 may also be an acid, and split off H, and that this idea can explain the fact that the aqueous solutions of so many metal salts are strongly acidic. Bjerrum (1910) used formulae such as Cr(H2O)5OH2 for the products.

Mass spectrometry of natural and synthetic peptide derivatives

Abstract: INTRODUCTION Preliminary studies on the use of mass spectrometry for the determination of amino acid sequences in peptides have been published by Biemann a al.1' 2 who reduced the peptide bonds and the terminal carboxyl by LiA1H4 or LiA1D4 to obtain polyamino-alcohols or the corresponding polyamines. For the purpose of obtaining sufficiently volatile derivatives, all later authors have used N-acyl derivatives of oligopeptide methyl (or ethyl, or t-butyl) esters. Thus, Stenhagen3 and Weygand et al.4 have obtained mass spectra of N-trifluoroacetyl methyl esters of oligopeptides up to pentapeptides and Manusadzhyan et al.5 have used N-acetyl oligopeptide ethyl esters. With a direct inlet system, Heyns and Grutzmacher6'7 have studied N-acetyl peptides and have obtained a fairly satisfactory sequential splitting, the highest peptide measured being a pentapeptide derivative of mass 539. Detailed studies of the fragmentation of peptides and their derivatives have been performed by Weygand et al.8, and Shemyakin et al.'°, the latter having especially studied depsipeptides and cyclic peptides'1' ha, The usefulness of mass spectrometry for the determination of the sequence of amino acid residues was clearly recognized in 1984, due to structural studies on natural peptidolipids12. Since then mass spectrometry has become a very helpful tool for correcting12' 13, or confirming'4 the structure of oligopeptide derivatives established by classical analytical methods; the most interesting application of mass spectrometry in this field is, however, the determination of the sequence of amino acids in oligopeptides and their derivatives. The minute quantities (10—100 g) needed for obtaining a mass spectrum and the rapidity of the measurements as well as the possibilities of automation compete favourably with other well established methods. We shall not go into details of fragmentation mechanisms, our principal aim being to try to show how the "normal" splitting of the peptide bond can be used for sequence determination. For recent reviews see references 15—19.

Infrared spectroscopy and polymer structure

Abstract: The power of infrared spectroscopy as a tool in studying the structure of small molecules is well known. This insight into structure results largely from our ability to analyse the spectrum theoretically by means of a normal coordinate analysis. This technique consists of calculating the normal vibration frequencies of a molecule from information on its structure and internal force field. From such a calculation we obtain the values of the frequencies and the forms of the vibrations of the molecule which can be observed in infrared absorption and in Raman scattering. By comparing such calculated frequencies with the observed spectrum it is possible to evaluate our assumptions concerning the structure and force field of the molecule.

Über nickelorganische Verbindungen

Abstract: In der Hoffnung, daß Sie mir eine kurze Reminiszenz erlauben, möchte ich Sie zunächst in das Jahr 1953 zurückführen, denn in diesem Jahr nahm im Mülheim.er Institut eine Entwicklung ihren Anfang, die wenig später weltweite Folgen haben sollte und in deren allererstem Anfang das Übergangsmetall Nickel bereits eine entscheidende Rolle spielte. Bei der Beschäftigung mit Aluminiumalkylen hatten Ziegler und Gellertl in den Jahren 1950/51 die sogenannte \"Aufbaureaktion\", d.h. die vielfache Addition der Al--CBindungen von Aluminiumtrialkylen an die C=C-Doppelbindungen des Äthylens entdeckt.

Optical activity and optical rotatory dispersion in synthetic polymers

Abstract: Among the large number of methods proposed for the investigation of macromolecular conformation in solution, optical activity is in principle one of the most powerful since, in general, optical activity is strongly dependent on conformational equilibria1. Its use, however, is seriously hindered by the difficulty in calculating the optical activity even of small molecules theoretically. This difficulty can be partially solved in some cases by using a semi-empirical calculation of the optical rotation2; furthermore some information concerning the relationships between optical activity and conformation in polymers can be obtained by comparing the optical activity of polymers with that of low-molecular-weight models in which intermolecular interactions between the chromophoric systems responsible for the optical rotation can often be excluded.

Recent progress in chlorophyll and porphyrin chemistry

Abstract: Following the isolation and elucidation of the structure of a natural compound there remain two additional goals: total synthesis and last, but not least, a deeper understanding of the molecule at hand by means of a closer study. Chlorophyll, the green pigment of the plant kingdom, being in the first instance responsible for transforming light energy into chemical reactivity with the ultimate production of starch, and haeme, the red blood pigment which secures the transport of oxygen for maintaining respiration, have aroused the interest of scientists for more than 100 years. The names of Borodin, Willstätter, H. Fischer, Stoll, Conant, Linstead, and others are associated with isolation, structural problems, and a vast amount of primary chemical knowledge in this fieldl. Woodward and his team2 achieved the enormous task of the total synthesis of chlorophyll a in 1960. And lately the final question of stereochemistry at the carbons 7 and 8 has been settled by Fleming3. For about 10 years scientists have tried with renewed intensity to learn more about this fascinating compound, naturally with the help of modern chemical and physical methods. To understand better the properties of chlorophyll and its derivatives a closer knowledge of its conjugated electron system seems important.

Primary structure determination of peptides and proteins by mass spectrometry

Abstract: A pressing problem in modern organic chemistry is the determination of the structure of polypeptides and proteins, for it is here that the needs and interests of biochemistry have far surpassed present chemical possibilities, while elucidation of many a biological problem on the molecular level requires a clear understanding of the relation between structure and biological function. I should like to survey here the studies we have carried out in recent years that have led to the proposal of a new, general method for determining the amino acid sequence in peptides and proteins by means of mass spectrometry. As is well known the primary structure of proteins determines the architecture of the molecule as a whole, including its conformational specificities and in the long run its biological properties. Therefore, determination of the amino acid sequence is one of the basic requisites for further progress in physicochemical and biological investigations of proteins. What means do we actually possess for making such a determination? At present the selective chemical or enzymatic degradation of the protein molecule into polypeptide fragments and their separation presents no basic difficulties. However, determination of the amino acid sequence in these polypeptide fragments rapidly and with unequivocal results is not always an easy task, particularly in view of the fact that in the majority of cases one has to deal with microquantities of substance. All methods used for this purpose are based on the stepwise chemical or enzymatic cleavage of the amino acid residues, beginning with the Nor C-terminus. Usually thereby the positions of not more than 1—3 residues can reliably be established. The only exception is Edman's phenylthiohydantoin method which allows determination of 4—6 N-terminal amino acid residues. Possibly, using Edman's automatic "sequenator" the method will permit determination directly in the protein chain of much larger fragments1. However, the sequenator has not left the experimental stage. At the same time the problem of the primary structure elucidation of proteins is so important and the aims are so varied that it is necessary to have available several fundamentally differing methods, not only for the sake of mutual control, but also for the solution of the particular problems. For instance, the sequenator cannot be used to determine the primary structure

Structure and absolute configurations of cobalt(III) complexes

Abstract: INTRODUCTION Although the existence of chelate rings and the general features of the structure of metal chelates have been established by chemical methods of investigation, our knowledge of chelation was greatly strengthened by the confirmation of the correctness of their structures by x-ray crystal analysis. X-ray crystal analysis introduced a metrical element into the understanding of coordination by revealing the lengths and angles of chemical bonds and other structural details that could not be ascertained by other physical and chemical methods. Normal x-ray methods do not tell us whether the optically active complex has a particular configuration or one related to this as its mirror image. In other words, it is not possible to assign the absolute stereochemical configurations to enantiomorphically related pairs of the complexes. However, if an x-ray wavelength a little shorter than the absorption edge of the heavy atom in the crystal is used, the anomalous phase shift on scattering by the heavy atom has the effect of advancing the wave from it relative to the waves from the rest of the atoms. This will give rise to a slight but discernible change in intensities of pairs of reflections, which are normally equal in intensity because of the requirement of the space group. Bijvoet' first pointed out that this effect can be used to determine the absolute configuration of the molecules. The intensities can be calculated by assuming a particular enantiomorphic configuration for the complex and the result can be compared with the observation. If the intensity relations are the reverse of those observed, then the inverted configuration represents the correct absolute stereochemical configuration. No other physical method is, at present, capable of determining the absolute configuration unambiguously. The knowledge of the absolute configuration of optically active complex ions will not only deepen our understanding of the mechanism of the interaction by which the rotatory power and its close associate, circular dichroism, are exerted, but also enable us to rationalize various chemical phenomena

Cyclic ketones: Photolytic eliminations and reductions

Abstract: INTRODUCTION The atomic orbital resonance model (I a) ÷—(I b) of the it, ii excited carbonyl group suggests that, on one hand, both carbonyl oxygen and carbon should possess radical-like reactivities and, on the other hand, the oxygen should approach the chemical properties of an electrophilic and the carbon those of a nucleophilic site. This frequently quoted, simple concept has served as a useful illustration of the major primary processes which prevail in the solution photochemistry on non-conjugated ketones. A serious restriction of its predictive value for many cases, however, derives from its failure to correlate reactivity with additional, partly inter-dependent ±

Ion-exchange membranes - Correlation between structure and function

Abstract: The study of a complex system such as an ion-exchange membrane can be approached from two directions, which are not mutually exclusive. We can make use of the theoretical framework of irreversible thermodynamics as first applied by Staverman1 to membrane systems, and as amplified by a number of other investigators, particularly Kedem and Katchalsky2–4. Here, we measure fluxes and generalized forces (the latter all too often involving the estimation of thermodynamic activity coefficients) and compute the phenomenological coefficients. This is essentially a ‘‘black box” approach which must give us correct and exact answers, provided the requirement of linearity between flux and force is obeyed, as they seem to be under the conditions which obtain in these systems.

Cyclobutadiene and its metal complexes

Abstract: The practical and theoretical problems associated with the cyclobutadiene molecule have concerned organic chemists for over one hundred years and, as is well known, this compound has played a particularly interesting role in the development of organic chemistry. The importance of the system to organic chemists stems mainly from the fact that the molecule is a cyclic conjugated system for which, seemingly, two equivalent structures (Ia and Tb) could be written, just as two such structures (ha and JIb) can be written for benzene. Kekule and many of his contemporaries associated the peculiar

Substitution reactions of metal complexes

Abstract: INTRODUCTION Dating back to the time of the coordination theory, there has been a continued interest in the mechanisms of reactions of metal complexes. For example, Werner' called attention to the fact that some reactions of cobalt(ni) complexes take place with rearrangement of configuration. In an attempt to explain these observations he suggested that stereochemical change during reaction was determined by the orientation of the entering group in the second coordination sphere relative to the leaving groupin the first sphere. He envisioned these two groups trading places between the first and second coordination spheres, a picture which we will see is much the same as the current view of an interchange mechanism for substitution. The degree of sophistication of the experiments designed, and of the interpretations given to the results obtained in investigations of mechanisms of reactions of metal complexes has been most impressive during the past decade. Details of these advances can be found in several books2-5 that have been written on the subject. This paper will attempt only to summarize some of the general aspects of substitution reactions of metal complexes and will not give an exhaustive coverage of the literature. The discussion will be limited to reactions of inert metal complexes, because the reactions of labile systems were covered by Eigen6 at the VII International Conference on Coordination Chemistry. This would appear to be an appropriate time for a plenary lecture on substitution reactions of metal complexes for there is wide-spread agreement on the gross aspects of the mechanisms involved. It is these aspects that will be discussed. Further investigations and deliberations will be necessary to attain a better understanding of the intimate details of the reaction processes.