Reactivity and Structure: Concepts in Organic Chemistry – serie

This book was undertaken for the purpose of bringing together the widely diverse lines of experimental work and thinking which has been expressed but has often been unheard on the title question. It will be clear to the reader that a critical viewpoint has been maintained in assembling the material of this rapidly expanding area of concern to organic chemists. It should be clear, too, that the authors are not purvey­ ing a singular viewpoint and do not regard the discussions presented as the ultimate word on the subject. In fact, it should be anticipated that many ofthe viewpoints pre­ sented may have to be altered in the light of new developments. In recognition of this and to show the wayan appendix of recent results and interpretation has been included where an alteration in viewpoint on some of the material treated in the text has been necessitated by developments in the most recent literature. This ap­ pendix should be regarded as the reader's opportunity to maintain currency in all aspects of this subject ifit is kept abreast of the literature. The bibliography, from which most of the material of discussion has been drawn, is organized in a somewhat unusual manner which deserves some explana­ tion here. A reference citation can consist of (as much as) a six space combination of letters and numerals.

The field of phase transfer catalysis is a tribute to the chemists involved in process development research. Phase transfer catalysis is a solution to numerous cost and yield problems encountered regularly in industrial laboratories. In fact, much of the early work in this area was conducted by industrial chemists although the work was not labelled phase transfer catalysis at the time. We certainly do not intend to minimize the contributions of academic chemists to this field, but it is an unalterable fact that much of the early understanding and many of the early advances came from industrial laboratories. A special tribute is due to Dr. Charles Starks of the Continental Oil Company. By the mid sixties, Starks had formulated the principles of phase transfer catalysis and had applied for patents on many reactions that others were later to examine in somewhat greater detail. His mechanistic model of phase transfer catalysis still stands up well today and is a model for much of the thinking in this area. It is fitting that Starks suggested the name "phase transfer catalysis" by which the whole field is now known. We wish to thank a number of people who have aided us in many ways in the preparation of this volume. We very much appreciate the helpful discussions and insights provided by Drs. Henry Stevens and Andrew Kaman of PPG Industries in Barberton, Ohio. We also thank Dr. L. A.

Rotation about the Carbon-Carbon single bond is generallyregarded to be unrestricted. About 50 years ago, rotationalisomers were found to exhibit optical activity: substitutedbiphenyl derivatives. However, the author has investigatedother classes of compounds that have stable rotationalisomers at room temperature. These compounds can serve asmodels for studying very weak intermolecular interactions,that are difficult to observe. Even diastereotopic groupsshow different chemical reactivity.From the contents: The "Free Rotation Concept" - RotamerPopulations - Barriers to Rotation - Reactivity ofRotational Isomers.

Many organic chemists will agree with me that the old "electronic theory" has for a long time been inadequate for the interpretation of various new findings in chemistry, particularly for those of reactivity. Considering the outstanding progress which has been made during the past 20 years in the interpretation of these facts, aided by the molecular orbital theory, the time has finally come for a new book showing what is within and what is beyond the reach of quantum-chemical methods. It was therefore highly suitable that Dr. F. L. Boschke of the Springer­ Verlag suggested to me to make a contribution to a volume in the series "Topics in Current Chemistry" in February 1969. The article was published as Vol. 15, No 1 in June 1970. This new book is an expanded version of the article written in 1970. In this present volume several of the most up-to-date findings which have been gained in organic chemistry since then have been added. It is highly probable that a certain "theoretical" design in the experimenta­ lists' mind may have been the reason for these developments, whether they themselves are aware of it or not. Theory produces new experimental ideas and conversely, a host of experimental data add another vista to new theories. Due to the mutual beneficial effect of theory and experiment this book will always retain its value, although the quantum-chemical approach to the theory of reactivity is, of course, still in the develop­ mental stage.

In 1970 I wrote a review article for "Topics in Current Chemistry", surveying the general reaction patterns of transition-metal complexes from the standpoint of organic synthesis. The article seems to have evoked wide interest, and I therefore thought it appropriate to treat the subject more comprehensively in the form of a monograph. Organometallic chemistry is a rapidly growing field of intensive research. In this field, the application of organometallic compounds to organic synthesis is an important subject and many unique synthetic methods have been discovered, especially in the last decade. Syntheses using transition-metal com­ pounds are now in the foreground of organic chemistry. The purpose of this monograph is to give a bird's-eye view of this field to both organic and inorganic chemists through a mechanistic approach. A systematic unification of the voluminous data accumulated in this field is now urgently required. This subject is discussed by classifying various reactions into general patterns and by illustrating them with a limited number of pertinent examples.

Chemistry was at one time completely described in terms of collision theo­ ry, in which one molecule collided with another, sometimes producing reac­ tion. Then came the realization that enzymes which are highly efficient ca­ talysts, work by way of prior complexation, often stereospecific, which is then followed by chemical reaction. Thus, systems that exhibit "host-guest" relationships, i.e., that show complexing are being looked at an ever in­ creasing frequency. The cyclodextrins are the first and probably the most important example of compounds that exhibits complex formation. This is a book about the cyclodextrins. There are of course other compounds that exhibit "host-guest" relationships and thus bind other organic molecules, but so far they have not achieved the importance of the cyclodextrins. By their name it is obvious that cyclodextrins are cyclic compounds. The complexes that they form are therefore cyclic inclusion complexes. Because the complexes are cyclic in nature, complexation can be very strong, as op­ posed to 1t-complex, electrostatic, or apolar complexes in which complex formation is two-dimensional rather than three-dimensional. Cyclodextrins turn out to be excellent models of enzymes. This is proba­ bly not fortuitous because they were first sought since it was discovered that the principal binding in the enzyme chymotrypsin was a cyclic inclusion complex. Cyclodextrins can do more than form cyclic inclusion complexes, they can catalyze as well. But catalysis always occurs after complex formation.

Free Radical Chemistry has a comparatively long history. Before the accep- tance of the quadrivalence of carbon, various attempts were made to prepare trivalent carbon species like methyl and ethyl. Initial investigations involving reaction between alkyl iodides and zinc were thought to be successful \ but were subsequently branded as failures 2. In fact free methyl and ethyl were transient radicals involved in the reactions, but because of their short lifetimes the radicals were not detected, and only the hydrocarbons formed by dimeri- th sation of the radicals were isolated. At the beginning of the 20 century Gom- berg discovered the stable triarylmethyl radicals 3, and then in the 1920's Pa- neth 4 showed that free alkyl radicals could have a short lifetime in the gas phase. Free radicals as intermediates in reactions in solution were largely un- recognised until in 1937 Hey and Waters 5 interpreted a number of reactions, which did not fit into the then developing electronic theory of Organic Chem- istry6,7, by suggesting that they involved the intermediacy of free radicals.The other contemporary pioneer Kharasch 8 also proposed free radicals as interme- diates in certain reactions in solution, many involving organometallic species. However in the 1930 - 1950 period free radicals were not universally accepted as respectable reaction intermediates as evidenced by the initial rejection of the work of Koelsch 9 and the stand against free radicals made by Hodgson 10.

This book was written in the context of the daily confrontation with problems in the utilization of polymeric supports for the synthesis of pep tides. Therefore, views and experiences which usually are not mentioned in scientific journals are collected in these pages. The author has deliberately discussed in detail the possible influence of the polymer phase on the varying reaction conditions in the Merrifield synthesis; this aspect is neglected in most publications dealing with peptide synthesis. However, in view of the growing body of information on the chemistry of polymer-supported peptide syntheses, the international readership should regard the author's arguments as open to discussion. I am very much indebted to all of my colleagues with whom I have had the opportunity to cooperate in studying the potential of the Merrifield synthesis. Above all I like to express my gratitude to my teacher, Professor Dr. Theodor Wieland, Heidelberg, for his boundless encouragement and support in my ef­ forts in the field of peptide synthesis, particularly in its polymer phase bound version. Last but not at least I wish to thank Miss Hildegard Leyden. With in­ finite patience and great accuracy she typed the manuscript in addition to her daily duties. The work which influenced this book was performed under the auspices of the Max-Planck-Gesellschaft, of which the financial support is gratefully ac­ knowledged.

Metal Vapour Synthesis (MVS) can be defined as; "The use in synthesis of high temper­ ature gaseous species such as metal atoms by their reactions with themselves or other materials in a condensed phase. " This short book, covering the literature up to the middle of 1979, describes MVS in organic chemistry; i. e. the reactions of metal atoms with various, predominantly organic, substrates in the synthesis and reactivity studies of organic and organometallic compounds. In order to effectively describe all the underlying principles and to present a coesive picture of pertinent metal atom processes in condensed organic phases, some inorganic substrates such as rare gases, dinitrogen, dioxygen, dihalo­ gens, and inorganic halides have been included. For similar reasons, we have used, where relevent, information provided by the closely related technique of Matrix Isolation Spec­ troscopy (MIS). After an introductory chapter which gives the basis principles and includes a brief critique of the technique, the book is divided into three further chapters dealing respec­ tively with (a) experimental techniques, (b) behaviour of metal atoms in matrices, and (c) results of prep~rative experiments. While not being encyclopaedic the book describes or refers to all noteworthy areas if not in the deliberately short text in the many tables and figures.

More than a decade has passed since the appearance of the last edition of the book "Carbon Monoxide in Organic Synthesis". During the intervening period, the signi­ ficance of carbon monoxide chemistry has become even more evident. The oil crisis and the ever-present awareness of the constant depletion of the oil reserves have both contributed to a surge in activity. The fact that coal will once again replace oil in the near future as a feedstock source has been an impetus for new efforts in the field of carbon monoxide chemistry. Moreover, older almost neglected processes such as the Fischer-Tropsch synthesis have become a focal point for new activities. Therefore, the time was ripe to fundamentally revise "Carbon Monoxide in Organic Synthesis" and to complete its coverage by introducing the chapters "Homo­ logation" and "Carbon Monoxide Hydrogenation". However, other important sectors of carbon monoxide chemistry such as the con­ version of synthesis gas to methanol or higher alcohols were excluded. These are treated in the book "Chemierohstoffe aus Kohle", which devotes considerable attention to these syntheses. I would like to thank Drs. Bahrmann, Comils, Frohning, Mullen and Tummes, who had the task of compiling and critically evaluating the large volume of literature which has appeared during the last decade.

Around 30 years ago the transition metal chemistry received great impulses. In the focus have been reactions of nickel and cobalt and herein especially their carbonyls. Also industrial processes have been developed. When the technical oxidation of ethylene with palladium chloride had been discovered, and a great number oflaboratory reactions, many groups have turned towards this subject. Apart from two important industrial processes - acetaldehyde and vinylacetate from ethylene - a great number of conversions and catalytic reactions with palladium compounds have been researched. Their mechanisms have been cleared up and have con­ tributed to a better understanding of the complex chemistry of palladium. Last but not least these reactions have also served for more understanding of organic transition metal compounds and catalyses in general. Numerous conventional reactions appear today in a different light. The effects of co-

Although studies on synthetic dyes have been performed for more than 100 years, their detailed elucidation requires further extensive research. The discovery of novel high polymers, the necessity of supplying a whole range of shades and increasing require­ ments for dyestuffs of high fastness properties give rise to a permanent search for new dyes. Extensive investigations on dyes were also occasioned by various applications in the field of spectral sensitization and of staining of biological specimens. Another more recent development concerns the lasing properties of some organic dyes. Most of the progress, however, was only achieved by time-consuming, purely empirical approaches and theoretical understanding of the dye properties is only at its very beginnings. The color is the sine qua non of every dye. For this reason organic chemists and color chemists have looked for relations between the "color and constitution" of dye molecules for a long time. This knowlege as a whole is known as "theory of color". The classic theory of color was established abou t 100 years ago by Witt and was signi­ ficantly extended 50 years later by W. Konig.

It seems likely that no project of this magnitude is undertaken in the absence of several motivations. This book is no exception. There are, from our perspective, at least two rational reasons for conducting this cataloging and organizational effort. The first of these is the obvious fact that a very large number of macrocyclic polyethers have now been prepared and nowhere is there a comprehensive listing of them. A number of excellent reviews have been published dealing with one or more aspects of macrocycle chemistry, but we felt a great need for a single source listing known compounds. We have, the- fore, attempted to provide such a listing to assist ourselves in keeping track as well as, we hope, helping others in this field. We also felt that there was a need for a survey of methods which have been utilized in the syntheses of these cation binders so that the novice could establish some starting point. Wherever possible, we have also attempted to point to the motivations which en­ gendered the various syntheses. The somewhat irrational reason which propelled us into this endeavor was the ques­ tioning of Charlie Morgan who often asked if one or another obscure compound had been synthesized. As often as not, we were uncertain arid now at least have a reference which, if not absolutely complete, will at least save face.

The application of silicon reagents in organic synthesis has grown at an increasingly rapid rate over the last twenty years. This has been the result of truly international interest. Signifi- cant contributions have been made by Japanese, Russian, Ger- man, French, English, American, Swiss and Canadian as well as by chemists from many other countries. This monograph attempts to comprehensively cover this field. Some seventeen hundred articles reporting contributions by over eighteen hundred scientists are summarized. Nevertheless, I have no doubt that interesting and important work has been left out. I welcome comments about such results which should be in- cluded in any future editions of this monograph. I would like to thank Robert Damrauer who first stimulated my interest in organosilicon chemistry. In addition, I thank a number of chemists who have shared my enthusiasm for silicon chemistry over the years: A Chihi, M.E. Childs, R.A Felix, H. Firgo, T.Y. Gu, T.Llto, LN. Jung, K.E. Koenig, H.Okinoshima, M.M. Radcliffe, B.L Rosen, H.S.D. Soy sa, K.P. Steele, R.E. Swaim, D. Tzeng, P.B. Valkovich, AK. Will- ard, S. Wunderly, and present members of my research group.

The anomeric effect is a well-defined and much studied example of a deep- seated and very general effect in organic chemistry. It involves a fundamental interaction between the lone pair electrons of oxygen and the antibonding orbitals of adjacent bonds, which can control not only conformation but also reactivity in suitable systems. So the heart of this book is a consideration of the properties of non-bonding pairs of electrons, and the influence of orbital- orbital interactions on structure and reactivity. I have been lucky in recent months to be able to discuss various matters arising in this book with people who know more about them than I do. Parti- cularly Professors C. Altona, A. Eschenmoser, F. Hirshfeld, M. Lahav, R.A. McClelland, G.M. Sheldrick and A. Vos, and, nearer horne, I. Fleming, P.G. Jones, AJ. Stone (who helped with the rather theoretical Appendix) and I.H. Williams. I am also most grateful to Professors P. Deslongchamps, ID. Dunitz, D.G. Gorenstein, W.P. Jencks, G.A. Jeffrey, R.D. Lemieux and S. Wolfe, for very helpful correspondence, and information in advance of pUblication. I have covered the literature received in Cambridge up to early December 1981.

This book was prepared with the idea of collecting some of the multitudinous new literature on carbanions and pre- senting it along with the fundamentals of carbanion chemi- stry. Some 400 papers from the period 1976 to 1982 have been assimilated into the book along with about an equal number of references (many to reviews) from the earlier literature. The material is organized under the headings Structures (with emphasis on the results of X-ray studies since they unambiguously show the molecular geometry), Preparations, and Reactions. Reactions, the largest topic, is divided into reactions with electrophiles, eliminations, oxidations, and rearrangements. Under these headings, carbanions without resonance stabilization (0' carbanions) are generally discus- sed first, and are further subdivided into Sp3 (alkyl) followed by Sp2 (vinyl and aryl) followed by sp (acetylenic); resonance- stabilized (n) hydrocarbon anions such as allyl and benzyl follow; lastly, heteroatom-containing n carbanions such as enolates are considered. Some references to carbanion equivalents are included at the end. Tucson, Arizona U.S.A. Robert B. Bates May 1983 Craig A. Ogle Table of Contents I.Introduction 1 II. Structures . 3 1. Non-delocalized (0") 3 a. Sp3 ...3 b. Sp2 and sp ...7 2. Delocalized (n) . . 8 a. Hydrocarbon Anions 8 b. Heteroatom-containing Carbanions 10 III. Preparations...13 13 1. From Alkyl Halides 13 a. With Metals . . 15 b. With Organometallics . 17 2. From Alkanes by Proton Abstraction 17 a. Acidities of CH Protons. . 21 b. Base-solvent Systems ...25 3. From Unsaturated Compounds 25 a. By Reduction ...

The Diels-Alder reaction has long been a powerful tool in organic synthesis. In recent years, the Alder ene reaction has also achieved some prominence. From the beginning, it was apparent that the intramolecular variants of these reactions would be feasible. Many such have been reported, but the results are widely scattered in the chemical literature. This volume is an attempt to synthesize results observed to date, and to suggest directions for future development. One of the limiting factors in the application of the intramolecular Diels­ Alder reaction has been the development of methods for the preparation of the requisite trienes. The fIrst chapter of this volume summarizes methods for the preparation of dienes and dienophiles. Examples representative of every general approach to 1,3-dienes and to dienophilic functional group combinations have been included. There are two questions one might ask in considering the prospective cyclization of a given triene: what are the factors that govern the rate of cyclization? and, for cyclizations that lead to the creation of one or more new chiral centers, what are the factors that govern diastereoselectivity? These questions are addressed in Chapter Two. The third chapter is devoted to the all-carbon intramolecular Alder ene reaction. The tables in that chapter summarize all examples that could be found in the literature through 1981, with several additional examples from 1982. Leading references to heterocyclic ene reactions are also included in this chapter.

Chromium oxidation, well known and widely explored in organic chemistry since the very beginning of this science, is a topic of current interest for the organic chemist as evidenced by the continuous development of new techni­ ques and procedures reported in the literature. Chromium oxidation is a simple process which can be easily performed in the laboratory and scaled up in industry as well. Although almost every oxidizable organic functional group may undergo chromium oxidation, the most important fields of appli­ cation are the oxidation of alcohols, allylic and benzylic oxidation, oxidative degradation and oxidation of some organometallic compounds. A high degree of selectivity is often possible by choosing the most suitable reagent among those several ones now available. This book takes account of the various functional groups that undergo oxidation and the entire literature up to 1982. It has been written in the hope to help the synthetic organic chemist in his experimental work. For this purpose a number of tables comprising yields and references have been included; detailed descriptions of typical procedures are meant to show the experimental conditions and the scope of the reactions. We wish to thank Dr. Mario Orena for his valuable scientific and technical assistence and Prof. Bruno Camerino, who read the entire manuscript and corrected many of the errors. Bologna, February 1984 Gianfranco Cainelli Giuliana Cardillo Table of Contents I. Introduction . . . . . . . . . . .

Although the first electroorganic reaction used in organic synthesis is probably the famous Kolbe electrolysis published in 1849, no other remarkable reactions have been found until the reductive dimerization of acrylonitrile to adipo­ nitrile was developed by Dr. M. M. Baizer of Monsanto Co. in 1964. Since then, the electro organic chemistry has been studied extensively with the expectation that it is a new useful tool for finding novel reactions in organic synthesis. The purpose of this book is not to give a comprehensive survey of studies on electrochemical reactions of organic compounds but to show that the electro organic chemistry is indeed useful in organic synthesis. Thus, this book has been written under the following policies. (1) Since this monograph is mainly concerned with organic synthesis, only few studies carried out from the view­ point of electrochemical, theoretical, or analytical chemistry are mentioned. (2) Since electroorganic chemistry covers a great variety of reactions, the types of reactions described in this book are selected mainly with regard to their application in organic synthesis. Simple transformations of functional groups are only described in particular cases, and also some well established processes such as the Kolbe electrolysis, pinacolic coupling, and hydrodimerization are only briefly mentioned. (3) Since many reports have already been published for each type of these reactions, only a limited number of the relevant papers are cited in this book.

The field of organometallic chemistry has enjoyed explosive growth in recent years. During this time a rapidly increasing number of metals have found utility in organic synthesis as the corresponding organometallic compounds. The sub­ ject of "Organic Synthesis by Means of Transition Metal Complexes" was reviewed in the first volume of this series of monographs. This volume deals primarily with the appli­ cation of organomercury compounds in organic synthesis (exclusive of solvomercuration-demercuration reactions), but will of necessity involve a number of reactions of other organometallics as well. Organomercurials are among the oldest known organo­ metallics and were perhaps the first to have an entire book devoted to their chemistry, when Whitmore wrote an American Chemical Society monograph on the subject in 1921. Subsequently, two very detailed monographs on the subject have appeared. In 1967 "The Organic Compounds of Mercury", volume 4 in the series "Methods of Elemento­ Organic Chemistry" appeared and this was followed in 1974 by Houben Weyl's full volume, Band XIII/2b, devoted entirely to the organometallic compounds of mercury. These books cover the entire field of organomercury chemistry.

Titanium has been used to perform many kinds of reactions in organic and inorganic chemistry. The present book is concerned primarily with a new development in titanium chemistry which is useful in organic synthesis. In 1979/80 it was discovered that the titanation of classical carbanions using C1TiX leads to species with reduced basicity and reactivity. This increases 3 chemo-, regio-and stereo selectivity in reactions with organic compounds such as aldehydes, ketones and alkyl halides. Many new examples have been reported in recent times. Since the nature of the ligand X at titanium can be widely varied, the electronic and steric nature of the reagents is easily controlled. This helps in predicting the stereochemical outcome of many of the C-C bond forming reactions, but the trial and error method is still necessary in other cases. One of the ultimate objectives of chemistry is to understand correlations between structure and reactivity. Although this goal has not been reached in the area of organotitanium chemistry, appreciable progress has been made. A great deal of physical and computational data of organotitanium compounds described in the current and older literature (e. g. , Ziegler-Natta type catalysts) has been reported by polymer, inorganic and theoretical chemists. It is summarized in Chapter 2 of this book, because some aspects are useful in understanding reactivity and selectivity of organo­ titanium compounds in organic synthesis as described in the chapters which follow.

The appearance of the first review in 1965 [1] and the first monograph in 1968 [2] on chemiluminescence demonstrated the extent of the phenomenon of light emission from the reaction of organic compounds in solution. Since then the num­ ber of chemiluminescent compounds has greatly increased, although the advan­ ces in theory and, more recently, applications are probably more significant. The present work is written by two authors who, together with E. H. White, helped to bring the study of chemiluminescence into the modern era. However many investigators are making contributions to the subject, even if the number of enthusiasts still remains small. It is not our intention to write an exhaustive account of chemiluminescence, still less of bioluminescence, and we have concentrated on making the landmarks in the area familiar to a readership outside the circle of specialists. The emphasis is on the range of organic compounds showing light emission with very little description of the relatively few inorganic or the more numerous biological examples which have been discovered. We hope that some of the excitement of the striking demonstrations of chemiluminescence which can be made appears in the text, albeit in the form of intellectual satisfaction and interest. We thank Prof. Dr. J. Stauff, Frankfurt for his generous advice and his critical comments. The chapter dealing with Peroxy-oxalate chemiluminescence has been commented up on critically by Dr. M. M. Rauhut, Stamford, Connecticut which we gratefully acknowledge.

The subject of the book is electron transfer reactions in organic chemistry, with the emphasis on mechanistic aspects. The theoretical framework is that of the Marcus theory, well-known from its extensive use in inorganic chemistry. The book deals with definitions of electron transfer, theory of electron transfer reactions (Marcus' and Pross-Shaik's approach) experimental diagnosis of electron transfer reactions, examples from inorganic/organic reactants and purely organic reactants, electro- and photochemical electron transfer, electron transfer catalyzed reactions, connections between electron transfer and polar mechanisms, and applications of electron transfer, such as electrosynthesis of organic chemicals, photochemical energy storage, conducting organic materials and chemiluminescence. The approach is new in so far as no comparable book has been published. The book will be of value to anyone interested in keeping track of developments in physical organic chemistry.

New methods for the construction of condensed five-membered ring systems continue to be developed at an accelerated pace. The challenges underlying this tremendous current upsurge of interest arise from several directions. One stems from the desire to elucidate and resolve those special problems associated with the incorporation of added strain not present when six-membered rings are mutually fused. The many structurally interesting polyquinane natural products isolated and characterized in recent years have provided a particularly delightful forum for application of various new synthetic protocols, many of which must equally well accommodate the particular stereochemical demands of each indi­ vidual target. Synthetic elaboration of a marvellous array of new unnatural molecules also holds continued fascination. In the past, we have attempted to keep others abreast of developments in this rapidly burgeoning area by authoring a pair of comprehensive reviews in Topics in Current Chemistry that appeared in 1979 [1] and 1984 [2]. During this period, others have also surveyed the developments in cyclopentannulation [3] and the cyclopentanoid field in general [4]. In the last couple of years, the pace at which new synthetic facets have been reported has become more frenetic than ever before. Accordingly, a suitable updating of the exciting newer findings was deemed appropriate and the present overview, which extends approximately to mid - 1986, was written. Once again, our hope is that compilations of this type will serve to stimulate imaginative new scientific ventures that will propel the field forward to still greater maturity.

The effects of heat and light on chemical reactions have long been known and un­ derstood. Ultrasound has been known to promote chemical reactions for the past 60 years, but despite this, it did not attract the attention of synthetic chemists until recently. This arose historically from early studies which concentrated almost exclu­ sively on reactions in aqueous media and was also, in some measure, due to the availability of suitable technology. Since the early 1980s a plethora of literature has appeared of direct interest to synthetic chemists and the field has been developing rapidly. The aim of this book is to bring the background of this fascinating field to the atten­ tion of a wider audience. It explores the literature to date and attempts to indicate other areas in which ultrasound may be exploited. It also hopes to explode some of the myths surrounding this area which have hitherto been regarded by the synthetic community as a bit of a black art! Existing books and reviews have tended to concentrate on the physics of sonochem­ istry and to catalogue the instances in which ultrasound has proved useful in tack­ ling synthetic problems. Our aim has been to stress the relevance of this technique to synthetic chemists and we have included a section which deals with the practical aspects of carrying out these reactions.

This chapter provides abrief overview of organoborane reaction types. Reaction mechanisms are discussed, particularly with reference to whether they may or may not allow stereocontrol, but stereocontrolled applications are deferred to later chap- ters. Reactions that replace boron by atoms other than carbon are discussed first (Section 3. 2), followed by boron substituted carbanions (Section 3. 3) and reactions that have the net effect of replacing boron-carbon bonds by carbon-carbon bonds (Section 3. 4). However, ss-eliminations, which also create a carbon-carbon bond at the expense of a boron-carbon bond, are deferred to Section 4. 2. Finally, a variety of reactions of organoboranes that leave the boron-carbon bond intact and affect other functionality are described in Section 3. 5. 3. 2 Oxidative Replacement of Boron 3. 2. 1 Introduction Several oxidative boron-carbon bond cleavage reactions replace the boron atom stereospecifically with retention of configuration of the carbon atom, and therefore are particularly useful in asymmetric synthesis.These share the following general mechanistic features: (I) Coordination of an oxidizing agent having a nucleophilic site, (:Y_Z)-n (n = 0 or I), to the boron to form aborate complex (1). (2) Intramolecular migration of carbon from the electron-rich boron atom to the rela- tively electron-deficient atom Y with concurrent displacement of nucleofuge z-n, as illustrated by transition state 2 and product 3.

Considering aspects of symmetry rules in chemistry, one is faced with con­ tradictory terms as for example, "90 % concertedness" sometimes being used in literature. To accept conservation of orbital symmetry to be as controlled as inversion by alternative principles seems far more promising. The intention of this book is aimed at introducing a qualitative understanding of phase rela­ tions in electromagnetic interactions. Avoiding one-sided dogmatism we tried to demonstrate the importance of alternative principles as guidelines to the evolution of alternative order in chemical systems. Passing through the jungle of information it became extremly important to control again and again our insights into the ordering phenomena by experi­ ments under conditions as coherent as possible. We became more aware of the fact that chemistry - the science of "becoming" in complex systems - can not be understood by mechanistic details, i. e. THROUGHPUT-studies alone, because the mechanism is only true for the special system under inves­ tigation and does not offer a tool for the evolution of opposite order. We had to accept chemistry as a mediator between molecular physics and general epistemology. This quite unusual combination was directed by excel­ lent teachers and the realizations were made possible by enthusiastic, open­ minded coworkers (see references). The next target we will strive for on this journey will be to quantify the alternative principles, that means obtaining the order parameters of H. Haken (e. g. in asymmetric synthesis).