ADDITION AND ELIMINATION REACTIONS

Among the various types of transformations used in organic synthesis, addition reactions are the cleanest since two components are combined into a single product with 100% atom efficiency [1518, 1519]. 

The asymmetric addition of small molecules such as hydrogen cyanide onto C=0 bonds or water and ammonia across C=C bonds is typically catalyzed by lyases. During such a reaction one or (depending on the substitution pattern of the substrate) two chiral centers are created from a prochiral substrate. 

The thermal elimination of alkyl halides to form nitriles is perhaps the best known reaction of imidoyl chlorides. J. v. Braun investigated this reaction in detail over a period of 40 years, and he developed a host of useful new synthetic procedures for the synthesis of compounds, which are otherwise more difficult to obtain. Unfortunately, most of his work has been written up in detailed form in Chemische Berichte and it requires some effort to retrieve this information. However, he wrote one review article in 1934 which is most informative. The elimination of alkyl halides on heating of imidoyl chlorides was recognized by Wallach in 1877 and v. Pechmann and Ley and Holzweissig reported examples of this reaction prior to the work of J. v. Braun. The elimination reaction, in its most general terms, can be described by the following sequences  

The overall reaction amounts to a stepwise conversion of a secondary amine via a primary amine to ammonia. Of course, both steps can be conducted simultaneously. For example, the reaction of N-benzoylpiperidine with phosphorus pentachloride at 120°C yields the imidoyl chloride CLXIII which is readily converted to the amide CLXIV. Hydrolysis of CLXIV yields s-chloropentylamine (CLXV) 

If the reaction is conducted at 200°C, double elimination with formation of benzonitrile and 1,5-dichloropentane (CLXVI) occurs ( ). 

The reaction can equally well be conducted with phosphorus penta-bromide, and 1,5-dibromopentane is obtained in good yield The dibromoalkanes, of course, can also be obtained from linear diamines and phosphorus pentabromide. For example, 1,6-dibromohexane (CLXVII) has been obtained upon reaction of N,N -dibenzoylhexamethylenediamine with phosphorus pentabromide ( ). 

The conversion of an alkyl halide into its next higher homolog can be accomplished by the following sequences ( )  

In this chapter we shall discuss the destruction and formation of carbon-carbon multiple bonds by addition and elimination reactions, respectively. The mechanism of aromatic substitution in which addition and elimination occur as separate steps will also be discussed. 

Carbon-carbon r bonds are relatively weak ( 65 kcal/mole-1). They are also, unless substituted by strong electron-withdrawing groups, electron-rich. For these reasons addition to them by electrophilic reagents usually occurs readily. The exact mechanism of addition depends on the reagent. 

The acid-catalyzed addition of water to a double bond is kinetically second-order, first-order each in olefin and in H30 +. 2 This fact is equally consistent with the concerted addition of a proton and a water molecule from the same H30+ and with initial attack of a proton to one side of the double bond followed by 

For example, the rate of hydration increases if the double bond hears electron-releasing snhsi-itiipnts. Schubert Lamm, and Keeffe have found that for a series of para-substituted styrenes (2), a linear correlation exists between 

Also in agreement with a cationic intermediate but not with concerted addition is the fact that Wagner-Meerwein rearrangements sometimes occur during hydration.6 

4 is characterized by AV values between -21 and —27 cm mol [79], i.e. significantly more negative than for the reaction with pyrrolidine mentioned above. The second order rate constant for the addition reactions exhibited an excellent correlation with the basicity of the selected aniline. The observed trend in the activation volumes could be correlated with an early or late transition state for the fast and slow addition reactions, respectively. 

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The stereochemistry of the most fundamental reaction types such as addition, substitution, and elimination are described by terms which specify the stereochemical relationship between the reactants and products. Addition and elimination reactions are classified as syn or anti, depending on whether the covalent bonds which are made or broken are on the same face or opposite faces of the plane of the double bond. 

When the addition and elimination reactions are mechanically reversible, they proceed by identical mechanistic paths but in opposite directions. In these circumstances, mechanistic conclusions about the addition reaction are applicable to the elimination reaction and vice versa. The principle of microscopic reversibility states that the mechanism (pathway) traversed in a reversible reaction is the same in the reverse as in the forward direction. Thus, if an addition-elimination system proceeds by a reversible mechanism, the intermediates and transition states involved in the addition process are the same as... 

Write a detailed mechanism for this condensation using only the molecules whose models are provided. Treat all proton transfers, nucleophilic additions, and elimination reactions as separate steps, and use curved arrows to show electron movement. Which of these steps do you think will be favorable Unfavorable Why ... 

Volume 8 Volume 9 Volume 10 Volume 12 Volume 13 Proton Transfer Addition and Elimination Reactions of Aliphatic Compounds Ester Formation and Hydrolysis and Related Reactions Electrophilic Substitution at a Saturated Carbon Atom Reactions of Aromatic Compounds Section 5. POLYMERISATION REACTIONS (3 volumes)... 

This chapter deals with (1) the transformation of the sulfone functionality into other functional groups by nucleophilic substitution reaction, and (2) the addition and elimination reaction of a,/i-unsaturated sulfones. Particular attention will be paid to recent uses of sulfones in organic syntheses1. 

Two approaches have been used in the synthesis of these types of compounds. Small boron-phosphorus ring compounds can serve as building blocks, and addition and elimination reactions with other main group elements can then extend the cage structure (see Schemes 23 and 24, Section 12.12.6.4.5). Alternatively, an unsaturated carbenoid fragment can be added to the bicyclic fragment as illustrated in Scheme 31 <1998IC490>. 

Cheletropic reactions include both addition and elimination reaction. The number of elimination reactions that have been studied in detail is not large but there is sufficient information to establish that orbital symmetry controls are operating. 

Addition to Double Bonds and Elimination Reactions. Both Lewis and Br nsted acidity of mineral surfaces can promote addition and elimination reactions (37). Equation 14 shows an example for an addition/elimination equilibrium catalyzed by Br nsted acidity ... 

In the present chapter, no explicit discussion or review of the acid-and/or base-catalyzed isomerization of olefins will be included. The discussion will be confined to isomerizations achieved with soluble transition metal complexes. However, it will be seen that addition and elimination reactions and allylic intermediates figure prominently in discussions of the mechanisms. 

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