Showing papers in "Pure and Applied Chemistry in 1980"

Nickel and palladium complex catalyzed cross-coupling reactions of organometallic reagents with organic halides

Abstract: The discovery in 1972 by two research groups that dihalodiphosphinenickel(II) complexes exhibit extremely high catalytic activity for selective cross-coupling of Grignard reagents with sp2-carbon halides has aroused wide-spread interest in application in organic synthesis of this type and related reactions involving other organometallics than Grignard reagents and not only nickel but also palladium complexes as catalysts. In this lecture several aspects of the title subject will be described centering argument on the Grignard coupling using a variety of phosphine-nickel and -palladium complexes of the formula MC12L2 (M = Ni or Pd; L2 = diphosphine) as catalyst precursors.

Molecular shape selective catalysis

Abstract: Molecular shape selective catalysis in intracrystalline space of siliceous zeolites has opened up a spectrum of new opportunities in catalytic science and in industry. The mechanisms involved include: (a) selective, diffusional mass transport dynamics, and (b) steric modification of the intrinsic reaction rates.

Dinuclear cryptates: dimetallic macropolycyclic inclusion complexes: concepts - design - prospects

Abstract: Macropolycyclic structures possessing receptor sites for several substrates may present a variety of new properties compared to single site receptors. Inclusion of two metal cations yields dinuclear cryptates in which the geometrical arrangement and the properties of the cations depend on the features of the ligand: the nature of the binding sites, of the subunits by which the cations are complexed, of the framework which connects the subunits. Three main classes of macropolycycles are considered: bis—chelating macrocycles, macro— bicycles of either axial bis—tripodal or lateral chelating—macrocyclic type, cylindrical macrotricycles. The design of these structures is analyzed with respect to the choice and effect of binding sites, subunits and framework,on the properties of the cryptates foriied complexation and redox features, cation—cation interaction at short distances, formation of cascade complexes by fixation of substrates on the bound cations. Dinuclear cryptates of each type have been obtained, as well as several bridged species. Their structural, physical and chemical properties are discussed. The prospects of these type of complexes are outlined especially with respect to the design of polynuclear cryptates, catalysis of multicenter—multielectronic processes, biomimesis of metalloproteins and of biological redox processes. Macropolycyclic structures are able to provide suitable frameworks for the arrangement in space of several receptor sites, allowing the binding of several substrates or the multiple binding of a single polyfunctional substrate. They delineate molecular cavities into which the substrates penetrate, forming inclusion complexes, cryptates, whose geometry may be regulated via ligand design (1). In addition to the functions displayed by single site (monotopic) molecular receptors (1) (recognition, catalysis, transport), the simultaneous or successive participation of several sites to substrate binding by multisite (polytopic) receptors provides an entry into higher forms of molecular behaviour, cooperativity, allostery and regulation. Such co—systems may display successive binding of different substrates yielding \"cascade\" complexes (2—5). Polytopic receptors may be symmetric or dissymetric, contain equivalent or non—equivalent binding subunits; they may complex identical (homonuclear complexes) or different (heteronuclear complexes) substrates , inorganic metal cations, organic or biological molecular cations, inorganic or organic molecular anions as well as neutral molecules. Among the great variety of species which can be imagined, we shall limit our present discussion to dinuclear complexes formed by inclusion of two metal cations into a macropoly— cyclic ligand containing two binding subunits, i.e. the topic will be concerned with our work on dinuclear cryptates of metal cations. In these complexes, the distance and arrangement of the cations may be monitored via the design of the ligand. They provide a novelapproach to the study of cation—cation

Preparation and application of water-soluble polymer-metal complexes

Abstract: A series of water-soluble polymers and derivatives with chelating groups has been prepared and investigated for the complex formation with various metals. The complexation of basis polymers, such as poly(ethyleneiruine) , poly(vinylamine) , and poly(acrylic acid), as well as copolymers was carried out in homogeneous phase. Membrane filtration allowed continuous or discontinuous separation of the polymer chelates from metallic ions not bound to the polymer. The polymer complexes remained water-soluble independent of the chelated metal and the polymeric ligand. Studies on the dependency of the capacities of the polymers upon various parairters and methods for regenerating the polymeric ligands are described. The introduction of chelating groups, e.g. pyridine-2-aldimine, thiourea, iminodiacetic acid, 8-hydroxy quinoline, and hydroxyaniline, to the macromolecules by polymer-analogous reaction or by copolymerization improved significantly the selectivity for the metals cobalt, nickel, copper, cadmium, palladium, mercury, silver, gold, and platinum. By the use of membrane filtration in the preparation and application of water-soluble polymer—metal complexes it is possible to recover metal ions from solutions in a short time and in homogeneous phase. INTRODUCTION Water-soluble polymer complexes are. not only of theoretical importance but also of increasing practical interest, since they involve the potentiality of versatile applications in chemistry. Insoluble chelating resins, on the contrary, have considerable disadvantages, such as reaction in heterogeneous phase and long contact times. Up to present, many investigations have been carried out in this field, particularly on synthesis and properties of soluble polymer ligands and coordinating groups (Ref. 1-10). In general, there are three requirements with which polymers as chelating agents should comply: sufficient solubilizing power of the constitutional repeating unit which provides water-solubility of the polymer complexes, a great number of functional groups of the complexing agent for a high capacity, and a high molecular weight which allows an easy separation by usual methods from the metal not bound to the polymer. Preferable for these studies are molecular masses between 10 000 and 100 000. Molecular masses of polymers smaller than 10 000 are not suitable because of separation problems in membrane filtration. Therefore, all prepared macromolecular ligands consisted of molecular mass fractions greater than 10 000, which was verified by membrane filtration. Possibilities for the synthesis of polymer chelating agents are the following: 1) Preparation of polymers with chelating units, e.g. poly(vinylamine), poly (ethyleneimine). 2) Polymerization of monomers already bearing the complex forming group, e.g. acrylic acid, vinylmethyl ketone, vinylsulfonic acid. 3) Polymerization of monomers containing protected functional groups with following polymer-analogous deblocking reaction, e.g. poly(vinylalcohol) from poly(vinylacetate). 4) Conversion of side chains or functional units in order to form or to change complexing groups, e.g. hydrolysis of poly(1-vinyl-2-pyrrolidinone)

Light induced electron transfer reactions of metal complexes

Abstract: Properties of the excited states of tris(2,2'—bipyridine) and tris(l,lO—phenanthroline) complexes of chromium(III), iron(II), ruthenium(II), osmium(II), rhodium(III), and iridium(III) are described. The electron transfer reactions of the ground and excited states are discussed and interpreted in terms of the driving force for the reaction and the distortions of the excited states relative to the corresponding ground states. General considerations relevant to the conversion of light into chemical energy are presented and progress in the use of polypyridine complexes to effect the light induced decomposition of water into hydrogen and oxygen is reviewed.

Developments in the synthesis and coordination chemistry of macrocyclic Schiff base ligands

Abstract: The use of metal ions as templates in the synthesis of a range of macrocyclic Schiff base ligands varying in size (15-30 member atoms), in the number and nature of the potential donor atoms, and in flexibility is described. Factors controlling the course of the reactions are discussed. Open-chain intermediates have been isolated in several cases and mechanisms for the cyclizations are proposed. The structures, properties and reactions of the macrocyclic complexes, including the use of some binuclear copper complexes as receptors for small bridging molecules and ions (e.g. pyrazine, imidazolate ion), are reported.

From anthracene photodimerization to jaw photochromic materials and photocrowns

Abstract: The inter and intramolecular photochemical reactions of anthracenes in the absence of oxygen is discussed. The intermolecular photodimerization of 9-substituted anthracenes in fluid solution usually leads to head-to-tail photodimers; it will be shown that this trend can be obviated by non bonding attractive interactions between substituents or by mixed photodimerization. The mechanistic aspects will be examined, next, in connection with the problem of excimer intermediacy. Bis-9-anthracenes can form photocyclomers and, in certain cases, intramolecular excimers at room temperature. Those which display interesting photochromic properties, were called "jaw compounds"; some of them induce an unusual anthracene ring cyclomerization. A study of photophysical and photochemical processes by steady state and transient kinetic analysis of ,c)-bis-9-anthryl-n-alkanes (ethane to decane) and cC ,C) -bis-9-anthryl-polyoxaalkanes allows a deeper insight into the mechanism of photodimerization. By irradiation, the latter derivatives can generate "photocrowns"; this is the first photochemical synthesis of crown-ethers.

Scanning microcalorimeters for studying macromolecules

Abstract: Principles of scanning microcalorimeter design, their basic characteristics and microcalorimetric data processing in studying biological macromolecules in solution are discussed.

Effect of surface mobility on the dynamic behavior of thin liquid films

Abstract: The role of various factors such as bulk and surface diffusion of surfactant, surface viscosity and, in the case of emulsions, liquid flow in the drops, in determining the surface mobility and hence the velocity of thinning and the lifetime of liquid films are discussed. It is shown that: 1) With relatively low concentrations of surfactant sol— uble in the continuous phase surface diffusion plays a dominant role. 2) The velocity of thinning of a plane—parallel emulsion film depends strongly on the partition coefficient of the surfactant: when the sur— factant is soluble: in the drops rather than in the continuous phase it has no influence on the surface mobility and the velocity of film thinning is very high. 3) With surfactant free emulsions or with surfactant sol— uble in the drops the dissipation of energy in the film is negligible so that the velocity of thinning does not depend on film viscosity. 4) Sur— face mobility increases the amplitude of capillary waves in the films but does not affect the critical thickness of rupture when surface diffusion and surface viscosity can be neglected; the allowance for the latter two effects may help in explaining the experimentally observed dependence of the critical thickness, of rupture upon surfactant concentration.

Picosecond processes in the isomerism of stilbenes

Abstract: Following a review of the problem of stilbene isomerism an account is given of recent picosecond tinescale experiments on this system. Transient absorption methods are used to expose vibrational relaxation de— tails for molecules in solution and streak camera measurements of fluo— rescence decay are presented to demonstrate the irreversibility of the Si (trans) decay. A simple statistical mechanical model is presented to describe the effect of viscosity on the twisting dynamics of trans—stilbene. A summary of recent experiments involving stilbene in the gas phase is presented. The isolated molecules twist very rapidly (15 ps at 265 nm ex— citation) and display characteristics typical of systems with a potential barrier. Finally the solution and gas phase processes are contrasted.

Thermodynamics of metalloprotein electron transfer reactions

Abstract: Thin—layer spectroelectrochemical methods have been used to determine the formal reduction potentials of several electron transfer metalloproteins in the temperature range 5—40°C (5—26°C for plastocyanin). Measurements were made using OTTLE cells in a nonisothermal configuration. Electron transfer reaction entropies ('c ed 5x) at 25°C (pH 7, T 0.1 N) derived from these measurements are as follows: cytochrome c (horse heart), —14.9 ± 1.2; cytochrome c2 (Rhodospirillum rubrum), —9.6 ± 1.2; cytochrome c (Pseudomonas aeruinosa), —16.2 ± 1.2; HiPIP (Chro— matium vinosum), —.O ± 1.2; azurin (Pseudomonas aeruginosa), —16.1 ± 1.2; stellacyanin (Rhus vernicifera), —4.2 ± 1.2; plastocyanin (Phaseolus vulgaris), —2.4 ± 1.2 eu. In all cases the AH° values are negative. The fact that the A5c values are more negative than inorganic complexes with low inner sphere electron transfer barriers is discussed in terms of changes in protein—solvent interactions that could accompany reduction. Either increased solvent ordering in the protein interior or a more compact protein structure in which solvent is excluded could account for the observed loss in entropy in the reduced solution state.

A detailed description of the mechanism of reaction of grignard reagents with ketones.

Abstract: A detailed description of the reaction of Grignard reagents with ketones is presented based on several studies. The composition of Grignard reagents in Et20 and THF is described based on molecular association and NMR studies. The cause and mechanisms of formation of pinacol and hydrol in the reaction of "CH3MgBr" with benzophenone is also described. Several probes to detect the radical nature of the R group in RMgX and the radical anion nature of the ketyl produced in the reduction of the ketone, were used to study the electron transfer nature of Grignard reagent addition to ketones. The results of thesestudies show clearly that Grignard reagents react with ketones by both a polar and electron transfer pathway. Which of these two pathways involves the lowest energy depends on the nature of the R group of the Grignard, the purity of the magnesium used to prepared the Grignard reaaent. the nature of the ketone and the solvent.Grignard reagents that are difficult to oxidize (e.g. 1° R group), ketones that are difficult to reduce (e.g. acetone), solvents of low polarity (e.g., Et,O) and Grignard reagents made from pure magnesium metal favor a polar pathway, an usually only 1,2-addition product is formed. On the other hand, Grignard reagents that are easily oxidized (e.g. 3° R group), ketones that are easily reduced. (e.g., Ph2C=O) solvents of high polarity (e.g., THF, HMPA) and Grignard reagents made from magnesium containing 1st row transition metals, favor an electron transfer pathway. Usually 1,2and 1,6-addition products are formed as well as some pinacol. INTRODUCTION I am, of course, deeply honored to be invited to present our work carried out at Georgia Tech concerning the mechanism of Grignard compound addition to ketones which began in 1964, and which is still in progress (see note a). To have spent so many years studying the mechanism of a single reaction type, I believe, requires some justification. However, in this particular case, I believe such effort is justified in that the reaction of Grignard reagents with ketones is undoubtedly one of the most fundamental reactions in all of organic chemistry, and therefore should be understood in some depth. In addition, whatever has been learned in this study, hopefully can be applied in a broader way to other reactions involving main group organometallic systems, e.g. organolithium and organoaluminum, since detailed mechanistic studies of reactions in main group metal chemistry are rare indeed. We have also made several discoveries of a practical nature during these mechanistic studies which in themselves could easily justify the time and effort spent in this area. The years spent in this work have not been dull since every time we seemed to be coming closer to the end of our study, another major problem in understanding arose which necessitated more effort. There have been a number of times during the past 14 years that the thought of abandoning our efforts crossed our minds as we met what seemed like insurmountable problems. However, because of the unusual experimental skill and determination of my students, who did this work,I am happy to report that we have finally arrived at what we believe to be the beginning of a much better understanding of the mechanism of Grignard reactions. COMPOSITION OF GRIGNARD REAGENTS IN ETHER SOLVENTS In order to study the mechanism of Grignard reagent addition to any organic substrate, it is necessary first to understand the nature of Grignard reagents in solution. If Grignard reagents consist of a number of species in soltution, it is also important then to determine which of these species is the most reactive toward the organic substrate being studied. A Note a. I would like to dedicate this presentation in honor of a very prominent chemist who has contributed greatly to the field of Grignard chemistry, Professor Henri Normant on the occasion of his 72nd birthday.

Research on Enhanced Oil Recovery: Past, Present and Future

Abstract: Past and present research on Enhanced Oil Recovery is reviewed with emphasis on the surface phenomena involved. The nature of capillary pressure phenomena in porous media has been understood for some time, and much research has been devoted towards the alteration of the surface forces which prevent the efficient displacement of oil by water. Early work often treated surface active agents as wetting agents designed to remove the oil from the solid surface by classical detergent action. More recent work has recognized the strong influence of oil-water interfacial tension on the displacement of discontinuous oil blobs or ganglia. Therefore, surfactant systems are now being developed to produce the lowest possible oil-water interfacial tensions by adjusting the various components and thus, the phase behavior in the total system. In addition to inter-facial tension, the phase behavior itself can strongly influence the oil displacement. The surfactant work, current work in blob mechanics, current research in CO2 flooding, and past results in alcohol flooding all indicate that an expanding oil phase is very important for effective oil displacement. Therefore, much current research is directed toward methods which utilize materials (including gases such as CO2 and mobility control agents) to dislodge oil blobs, or to prevent their entrapment by maintaining a continuous oil phase and improving sweep efficiency during displacement. The general direction of future research on enhanced oil recovery is predicted.

The Rule of Schulze and Hardy

Abstract: In a brief historical survey various attempts to explain the rule of Schulze and Hardy are mentioned. It is argued that the inverse proportionality between the coagulation concentration and the sixth power of the charge number of the counterion, as it has been derived from the DLVO theory, cannot be the complete explanation, since as a rule potentials in the Gouy layer are not high enough. It is shown that a combination of van der Waals attraction and electrostatic repulsion can explain the rule of Schulze and Hardy, if adsorption of counterions in the Stern layer is taken into account. The adsorption potential must then increase fairly strongly with the charge of the counterion. Structural forces (due to the molecular structure of the solvent) may be important, but the quantitative theory of these forces has not yet been sufficiently developed to incorporate them in the theory of coagulation.

Three-membered phosphorus ring compounds

Abstract: The synthesis of novel homoand heterocyclic threemembered phosphorus ring compounds is described. Such strained ring systems are not in all cases hyperreactive substances, but may exist as isolable compounds. The structures are based on 31P NMR spectroscopic data and on single crystal X-ray structure analysis . INTRODUCTION In the past two decades, a great number of new cyclic phosphorus compounds could be synthesized. Nevertheless , monocyclic three-membered phosphorus ring systems without additional stabilization by coordination are almost unknown. This is particularly true for compounds with more than one phosphorus atom in a three—membered ring. Of all known elements, carbon shows, of course, the greatest 'ability to form cyclic compounds of ring size three. Cyclopropane was already detected nearly 100 years ago and the number .f its homoand heterocyclic derivatives is considerable. Although it is well known that a significant discontinuity in chemical properties and reactivity occurs between the first-row elements of the periodic table and those of subsequent rows, it had to be expected from the oblique relation between phosphorus and carbon that phosphorus also has a certain ability to form three—membered ring compounds with directly bonded phosphorus atoms. Besides, elemental white phosphorus consists of P4 molecules that are composed of four condensed threemembered rings. Therefore, it seemed a challenging problem to find out whether monocyclic three-menthered phosphorus ring compounds can also be sufficiently stabilized for chemical preparation and investigation. HOMOCYCLIC THREE-MEMBERED PHOSPHORUS RING COMPOUNDS The simplest phosphorus three-membered ring compound is the phosphorus hydride, cyclotriphosphane PH3 (Table 1). Already in 1965 we could detect small amounts of this compound together with other novel phosphorus hydrides in the volatile hydrolysis products of calcium phosphide by mass spectroscopy (Ref. 2). It may be seen that cyclotriphosphane is the first member of the homologous series of monocyclic phosphanes PnHn. In addition, there exist the open-chain phosphanes PnHn.2 and various series of cyclic phosphanes with lower hydrogen content and condensed ring systems. The two double underlined triphosphanes and tetraphosphanes could be separated in pure form by gas chromatography (Ref. 3). However until now, we were not able to isolate cyclo— triphosphane on a preparative scale because of its high reactivity. In contrast to the rather unstable phosphorus hydrides PnHn, their derivatives, in which the hydrogen atoms are substituted by organyl groups, are genrally much less reactive. Consequently, some of these organyl-cyclophosphanes have already been known for a long time (Ref. 4). The general procedures for their preparation are the reaction of a primary phosphine with an organyl-dichlorophosphine or the dehalogenation of a dichlorophosphine by an alkali metal and by lithiumhydride, respectively. s may be seen on Table 2, where the ring size n is given for various substituents R, mostly fiveor four-membered cyclophosphanes are formed in these reactions. for thermodynamic and kinetic reasons. In the case that R equals phenyl for instance, the five-membered ring compound Is the cyclophosphane with the highest stability. Besides, a cyclohexaphosphane of somewhat lOwer stability exists. The corresponding compounds where n equals four and lthree, however, were still unknown.

Molecular sieve zeolite technology - the first twenty-five years

Abstract: In twenty-five years molecular sieve zeolites have substantially impacted adsorption and catalytic process technology throughout the chemical process industries; provided timely solutions to energy and environmental problems; and grown to over a hundred million dollar industry worldwide. The evolution in zeolite materials with improved or novel properties has strongly influenced the expansion of their applications, and provided new flexibility in the design of products and processes.

New iron-porphyrin complexes with metal-carbon bond - biological implications

Abstract: A study of the reactions of ferroporphyrins, in the presence of an excess of a reducing agent, with various polyhalogenated compounds, has led to a general method of preparation of iron-carbene complexes, Fe(porphyrin) (CRR') . It has thus been possible to obtain iron complexes of the dihalogenocarbenes CC12, CBr2, CF2, CFBr and CFC1, which are thermally stable (up to 100°C) and not dissociated in solution, but which react irreversibly with dioxygen and nucleophiles such as pyridine or primary amines. The corresponding Fe(porphyrin) (Cl2) complex has not been isolated since it reacts rapidly with another FeH(porphyrin) to give a heme dimer bridged by a carbon atom, Fe=C=Fe. Fungicides of the RSCC13 type are reduced by ferroporphyrins with formation of Fe-CC1SR carbene complexes which are transformed into very stable thiocarbonyl-iron porphyrin complexes upon treatment by a Lewis acid. The insecticide DDT, (pC1C6H4)2CHCC13, forms, upon reaction with ferroporphyrins, very stable vinylidene carbene complexes Fe (porphyrin) [C=C (pC1C6H4) 2]. These results strongly support the formation of cytochrome P 450-Fe-carbene complexes during reductive metabolism of various polyhalogenated compounds, as proposed previously. Cytochromes P 450-carbene complexes are also formed by in situ oxidation of the methylene group of 1,3-benzodioxole derivatives. This is strongly indicated by the spectral properties of the model complex Fe(tetraphenylporphyrin) (1,3-benzodioxol-2-carbene) (nBuS) prepared by the general method here described, from 2, 2-dichloro1, 3-benzodioxole. The possible biological implications of the formation of these cytochrome P 450iron-carbon bonds, are discussed.

Aspects of cyclic conjugation

Abstract: (a) The usefulness of PMO theory as a guide to chemical behaviour is illustrated by its application to spiroconjugation. It is shown in this way that spiroconjugation can be chemically important only in the case of spiro [3,3] heptadieniyl which may be sufficiently stabilized to exist as a monomeric "nonclassical" species. (b) The resonance integral between twO sp3 AOs of a given carbon atom is at least as large as the pr:pir resonance integrals in conjugated hydrocarbons. Paraffins are thus a-conjugated molecules, isoconjugate with polyenes. Cyclopropane should, like benzene, be aromatic, and cyclobutane, like cyclooctatetraene, antiaromatic. The available evidence shows this to be the case. Cc) Consistent failure to reproduce the aromatic structures of higher annulenes, by a variety of procedures, suggests strongly that some factor must be operating other than those responsible for aromaticity and antiaromaticity in small rings. MNDO studies indicate this to be a special type of electron correlation, analogous to that postulated by thwdin in his alternant orbital theory.

Cryptate inclusion complexes, effects on solute-solute and solute-solvent interactions and on ionic reactivity

Abstract: — Cryptation of a suitable metal cation by inclusion into a macrobicyclic ligand leads to a large organic cation presenting much weaker interactions with the environment. As a result solute—solute and solute—solvent interactions are markedly decreased. The present account discusses the nature and properties of cryptate cations; the available information indicates that for a suitable cryptand— cation pair, the enclosed metal cation is effectively shielded from the environ— ment. Cryptate formation allows the stabilization of unusual species (metal anions, unstable oxidation states, etc) and markedly influences chemical reactivity, either inducing anion activation or inhibiting cation participation. It represents a tool for studying ionic reactions in solution, while a the same time strongly affecting the usual course of reactions like acid—base equilibria, phase transfer catalysis, alkylations, additions to carbonyl groups, nucleophilic substitutions, carbanion reactions, anionic polymerizations, etc. In condensed phase, liquid or solid, the interactions of a chemical species with its environment play a major role in determining its physical properties and its chemical reactivity. Marked changes may be brought about by introduction of a complexing agent, thus transforming a two—body interaction, solute—solute or solute—solvent, into a three—body interaction, substrate—ligand—solute or substrate—ligand—solvent. It is well known that such complexation greatly affects the properties of the bound species and that the resulting complex displays a wealth of new properties of its own, depending on the nature of tha substrate—ligand pair considered. Within this vast domain, the present discussion will bear on a specific combination: — ligands forming inclusion corirplexes, cryptates, in which the substrate is bound in a cavity inside an organic molecule and therefore more or less effectively shielded from its environment in the free state; — metal cation substrates, focusing mainly on alkali cations (AC's) which act as positive ions in many chemical phenomena involving ionic processes. Strong and selective complexes of AC's have only been obtained in rather recent times, with the discovery of new types of ligands. The most stable ones are found among the cmjptates formed by inclusion of the cation into a macropolycyclic ligand (cryptand) (1—3). Efficient complexation is also displayed by the macrocyclic polyethers, "crown" compounds (4), and by macrocyclic substances of natural origin (5). Within the first class of species, most studies have been performed to date with three macrobicyclic molecules, the cryptands I [2.1.1], II [2.2.1] and III [2.2.2] (6—7), which have been the most widely available. They contain a spheroidal cavity of size increasing

Old and new ylid chemistry

Abstract: A. The mechanism of the Wittig reaction is re-interpreted on the basis of new experimental and theoretical results. B. Reactions of trimethylsilylated phosphorus ylides are described. They allow a reversal of the stereochemistry of the Wittig reaction. C. The preparative possibilities of the reaction of N-phenyl-ketenylidenetriphenylphosphorane with carbon acids are presented and the synthesis of the tetramer of phenyl isonitrile is reported. D. The results of investigations into the reactions of Hexaphenylcarbodiphosphorane are presented. A. The Wittig Reaction Detailed investigations have led us to propose a new mechanism for the Wittig reaction.123 Ylides, (1), and aldehydes, (2), combine to give oxaphosphetanes, (3), in which the 0-atom occupies an apical position on the pentavalent phosphorus and in which the substituents on the four membered ring are B to one another. Cleavage of the original. ylid C-P bond, which is necessary for formation of the olefin, requires a ligand reorganisation process (pseudorotation) which brings this bond to an apical position, The opening of the C-P bond to give the betain (5) occurs during or after the conversion to the trigonal bipyramidal structure (u). The electronic nature of the substituents R1 and R2 in (5) is instrumental in determining the stereochemistry of the product olef in. When R1 phenyl electron donating R2 substituents cause a very fast elimination of the phosphine oxide, giving B olefins (7). Electron-withdrawing R2 groups (R1phenyl) extend the lifetime of (5), which can now isomerise to the thermodynamically more stable (6), from which phosphine oxide elimination gives E-olef ins (8). If the ligands R1 on phosphorus are electron-donating the rate of the phosphine oxide elimination is also decreased. In this case strongly increased formation of E-olefins is observed, compared to R1C6H5, even when R2 is an electron donor.

Cycloaddition mechanism and the solvent dependence of rate

Abstract: The dependence of the rate constant on solvent polarity is an important mechanistic criterion which should always be used in conjunction with other diagnostics tool. The slow step of 2 + 2 cycloadditions of tetracyanoethylene (TCNE) with enol ethers [1], thioenol ethers [2] and trans-fixed 1,3-dienes [3] is the formation of a zwitterionic intermediate. The log k2 values are linear functions of the Dimroth-Reichardt parameter ET. The rate accelerations of TCNE cycloadditions in going from cyclohexane to acetonitrile amount to 29,000 for anethole, 10,800 for 1-ethoxyisobutene, 2,600 for butyl vinyl ether, 17,000 for ethyl 1-propenyl sulfide and 54,000 for verbenene. The violation of stereo-specificity becomes greater with increasing solvent polarity. In contrast, the additions of TCNE to anthracene [4] and related Diels-Alder reactions show only a minute solvent dependence. The same is true for 1,3-dipolar cycloadditions of diazoalkanes [5], phenyl azide [6], C-phenyl-n-methylnitrone and azomethine imines to various dipolarophiles. These concerted cycloadditions are characterized by early transition states, i.e., there is hardly any change of solvation energy during the activation process. A dichotomy of reaction paths was found for diazocarbonyl compounds and enamines. Whereas dialkylaminocyclohexenes produce cycloadducts, the corresponding enamines containing the cyclopentene ring undergo azo coupling furnishes enamino-hydrazones via a zwitterionic intermediate. The contrasting dependence of rate on solvent polarity supports different mechanism [7]. Dimethylketene combines with N-isobutenylprrolidine to give a 3-pyrrolidinocyclobutanone as 1:1 adduct and a δ-methylene-δ-lactone as 2:1 adduct. The dependence of the product ratio on the concentration of dimethylketene allows to disent angle a concerted pathway and a reaction via a zwitterionic intermediate which can be intercepted by a second molecule of dimethylketene (see formula Scheme) [8]. The different solvent dependencies of the two reaction branches confirm the mechanistic divergence.

Is aromaticity outmoded

Abstract: A brief historical tabellar survey reveals two intensive research periods in aromaticity, closely following the two major theoretical breakthroughs Kekule's formula, and Hückei's rule. The post-war upsurge of interest in the Hackel theory was triggered by Dewar's insight of the tropolone structure. The limitations of the HUckel method, made evident by the introduction of the Dewar resonance energy, led to pace slackening in the last decade and to severe criticisms towards the notion of aromaticity. In defense of this concept, the paper argues that a reasonable balance of generality makes the concept of aromaticity extremely useful qualitatively; however, for quantitative determinations, narrower, more precisely defined, areas of aromaticity are to be secified. From the author's research areas, theoretical concepts are presented (graph—theoretical approach to predicting all possible monocyclic aromatic systems with heteroatoms, to enumerating cata-.condensed poy cyclic systems and valence isomers of annulenes) as well as experimen-. tal contributions (synthesis and reactions of pyrylium and boroxaropy— rylium salts, measments of relative ring currents using N-aryl-2,4,6..trimethylpyrylium salts, lack of aromaticity in 1,3-diketone chelatea), By means of graph theory new and better definitions and nomenclature may be obtained for some categories like cata/peri/corona-condensatior,, mesoionic compounds, valence isomers. The review closes with a few challenging problems in aromatic chemistr physical chemistry and biochemistry.

Colloidal behaviour of materials with ionizable group surfaces

Abstract: Classic DLVO descriptions of coagulation are cast in terms of mean interfacial potentials developed in self—consistent physical terms by a general surface charge. An alternative approach is to begin by identifying an array of well defined, chemically distinguishable surface groups. The charging of such groups is controlled through equilibrium constants by bulk aqueous solution parameters. Subsequent definition of the mean potential of the array of ionizable groups is then a first approximation towards the more difficult task of defining local potentials. Nevertheless, the combination of specific group charge plus a mean potential allows new approaches to be made towards understanding interaction of biological surfaces and model latex)cofloids, particularly in concentrated suspensions.

Heterogeneous electrochemical systems for solar energy conversion

Abstract: A survey is given on the various electrochemical systems which can be used for the conversion of solar light into electrical or chemical energy. They are compared with the similar processes in photosynthesis and their advantages or disadvantages are outlined. Systems based on the photovoltages obtained at semiconductorelectrolyte junctions have reached the highest efficiencies and are discussed in more detail. The theoretically possible conversion efficiencies are outlined and the difficulties to reach them are explained. Photodecomposition of the semiconductor electrode is the most serious problem, for which the thermodynamic and kinetic conditions are discussed. Finally, a brief outline of the prospects for application is given.

Geology of natural zeolites and zeolitic rocks

Abstract: Present status, particularly development in the latest decade, of the geology of natural zeolites and zeolitic rocks has been overviewed. Emphases are focused on the classification of genetical occurrences and the synthesis deduced from the nature..

Thermochemistry of aqueous micellar systems

Abstract: Surfactants tend to aggregate above a certain concentration, called the critical micelle concentration, into large clusters or micelles. With modern calorimetric and other related techniques it is now possible to measure precisely and rapidly the thermodynamic properties of liquid systems over a wide range of concentration and temperature. An overview is given of data obtained in our laboratory for heat capacities, enthalpies, entropies and free energies of aqueous sodium decanoate, octylammonium hydrobromide, nonyl and decyltrimethylammonium bromide. The thermodynamic functions of micellization are derived and interpreted with a phase—sepa— ration model. The closely related microemulsions are also briefly dis— cussed.