Francis A. Carey

Carbonium ion-silane hydride transfer reactions IV. Structure and reactivity at silicon

Abstract Rates of hydride transfer from substituted silanes to the tris(2,6-dimethoxyphenyl)methyl cation were measured spectrophotometrically in acetic acid. For a series of triarylsilanes the value of p was −1.84 and for aryldimethylsilanes p was −1.01. For both series a better correlation was obtained for σ than for σ + . The primary kinetic-isotope effect k H / k D was measured for transfer from triphenylsilane and triphenyldeuteriosilane to the tris(2,6-dimethoxyphenyl)methyl cation, the 9- p -anisylxanthyl cation and the phenyl- p -tolyldeuteriomethyl cation and found to be between 1.51 and 1.89 for the three carbonium ions. The results indicate that the transition state for hydride transfer from silicon involves a four center array (trigonal bipyramid at silicon) in which nucleophilic participation by solvent aids in displacement of hydride from silicon.

Synthesis of Macromolecules

Much of the effort of organic chemists since about 1930, especially in industrial research laboratories, has been directed toward the synthesis of polymeric materials, and many such substances have come to play a prominent role in industry and commerce. In this chapter, some of the reactions that are utilized to create useful polymers will be illustrated. It will be seen from the discussion that the basic mechanisms of polymerization reactions are the same as those encountered in the reactions of small organic molecules, and that the special features of polymerizations are the result of the high molecular weights of the molecules involved.

REGIOSELECTIVITY OF METALATION OF 1,3-DITHIOLANES AND 1,3-DITHIOLANE 1-OXIDES

Abstract Deprotonation of 1,3-dithiolane 1-oxide (3) and trans-2-phenyl-1,3-dithiolane 1-oxide (5) leads to cleavage of the derived anions. Cleavage reactions do not occur with trans-hexahydro-1,3-benzodithiole (7), its 1-oxide, or its 1,1-dioxide all of which can be metalated and alkylated quantitatively at C-2. The preparation and reactions of the sulfoxides derived from 7 are not highly stereoselective.

Chemical Bonding and Structure

Organic chemistry is a broad field which intersects with such diverse areas as biology, medicine and pharmacology, polymer technology, agriculture, and petroleum engineering. At the core of organic chemistry are fundamental concepts of molecular structure and reactivity of carbon-containing compounds. The purpose of this text is to cover the central core of organic chemistry. This knowledge can be used within organic chemistry or applied to other fields, such as those named above, which require significant contributions from organic chemistry. One organizational approach to organic chemistry divides it into three main areas—structure, dynamics, and synthesis. Structure includes the description of bonding in organic molecules and the methods for determining, analyzing, and predicting molecular structure. Dynamics refers to study of the physical properties and chemical transformations of molecules. Synthesis includes those activities which are directed toward finding methods which convert existing substances into different compounds. These three areas are all interrelated, but synthesis is built on knowledge of both structure and reactions (chemical dynamics), while understanding dynamic processes ultimately rests on detailed knowledge about molecular structure. A firm grounding in the principles of structure and chemical bonding is therefore an essential starting point for fuller appreciation of dynamics and synthesis. In this first chapter, we will discuss the ideas that have proven most useful to organic chemists for describing and correlating facts, concepts, and theories about the structure of organic molecules.

Alkylation of Nucleophilic Carbon Intermediates

Carbon-carbon bond formation is the basis for the construction of the molecular framework of organic molecules by synthesis. One of the fundamental processes for carbon-carbon bond formation is a reaction between a nucleophilic carbon and an electrophilic one. The focus in this chapter is on enolate ions, imine anions, and enamines, which are the most useful kinds of carbon nucleophiles, and on their reactions with alkylating agents. Mechanistically, these are usually SN2 reactions in which the carbon nucleophile displaces a halide or other leaving group. Successful carbon-carbon bond formation requires that the SN2 alkylation be the dominant reaction. The crucial factors which must be considered include (1) the conditions for generation of the carbon nucleophile; (2) the effect of the reaction conditions on the structure and reactivity of the nucleophile; (3) the regio- and stereoselectivity of the alkylation reaction; and (4) the role of solvents, counterions, and other components of the reaction media that can influence the rate of competing reactions.

Chemical Bonding and Molecular Structure

One currently held view about organic chemistry that seems to be receiving broad acceptance is that for organizational purposes, it can best be divided into three main areas. These areas are structure, dynamics, and synthesis, and they are interdependent.1 In order to appreciate fully the latter two areas, both of which will be developed at length in subsequent chapters, a firm grounding in the principles of structure and chemical bonding is essential.

Organometallic Compounds of Group I and II Metals

The use of organometallic reagents in organic synthesis had its beginning around 1900 with the work of Victor Grignard, who discovered that alkyl and aryl halides reacted with magnesium metal to give homogeneous solutions. The “Grignard reagents” proved to be reactive carbon nucleophiles and have remained very useful synthetic reagents since that time. Organolithium reagents came into synthetic use somewhat later. In the last 25 years, the synthetic utility of reactions involving metal ions and organometallic compounds has expanded enormously. Certain of the transition metals, such as copper, palladium, and nickel, have gained important places in synthetic methodology. In addition to providing reagents for organic synthesis, the systematic study of the reactions of organic compounds with metal ions and complexes has created a large number of organometallic compounds, many having unique structures and reactivity. In this chapter, we will discuss the Grignard reagents and organolithium compounds. In Chapter 8, the role of transition metals in organic synthesis will be given attention.

Aromatic Substitution Reactions

This chapter is concerned with reactions that introduce or interchange substituent groups on aromatic rings. The most important group of such reactions are the electrophilic aromatic substitutions, but there are also significant reactions that take place by nucleophilic substitution mechanisms, and still others that involve radical mechanisms. Examples of synthetically important reactions from each group will be discussed. Electrophilic aromatic substitution has also been studied in great detail from the point of view of reaction mechanism and structure-reactivity relationships; these mechanistic studies received considerable attention in Part A, Chapter 9. In this chapter, the synthetic aspects of electrophilic aromatic substitutions will be emphasized.

Free-Radical Reactions

A free-radical reaction is a chemical process in which molecules having unpaired electrons are involved. The radical species could be a starting compound or a product, but in organic chemistry, the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates or transition states in which all electrons remain paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur.

Reactions Involving Carbocations, Carbenes, and Radicals as Reactive Intermediates

Trivalent carbocations, carbanions, and radicals are the most fundamental classes of reactive intermediates. Discussion of carbanion intermediates began in Chapter 1 and has continued in several other chapters. The focus in this chapter will be on electron-deficient reactive intermediates. Carbocations are the most fundamental example, but carbenes and nitrenes also play a significant role in synthetic reactions. Each of these intermediates has a carbon or nitrogen atom with six valence electrons, and they are therefore electron-deficient and electrophilic in character. Because of their electron deficiency, carbocations and carbenes have the potential for skeletal rearrangements. We will also discuss the use of carbon radicals to form carbon-carbon bonds. Radicals, too, are electron-deficient but react through homolytic bond-breaking and bond-forming reactions. A common feature of all of these intermediates is that they are of high energy, relative to structures with filled bonding orbitals. Their lifetimes are usually very short. Reaction conditions designed to lead to synthetically useful outcomes must take this high reactivity into account.