Addition-elimination reaction, multiple

The reaction between lithiated diethyl propenylphosphonate and ethyl propenoate leads to carbon-carbon bond formation involving either C(i) or C(3) of the phosphonate ester. Thus, ethyl but-2-enoate or but-3-en-2-one yielded 481 and 482, respectively, and the esters 483 were obtained from coumarin, all by simple addition, but addition-elimination and multiple addition processes were also described. 

Summary Carbonyl compounds readily undergo multiple addition-elimination reactions ... 

In Part 2 of this book, we shall be directly concerned with organic reactions and their mechanisms. The reactions have been classified into 10 chapters, based primarily on reaction type substitutions, additions to multiple bonds, eliminations, rearrangements, and oxidation-reduction reactions. Five chapters are devoted to substitutions these are classified on the basis of mechanism as well as substrate. Chapters 10 and 13 include nucleophilic substitutions at aliphatic and aromatic substrates, respectively, Chapters 12 and 11 deal with electrophilic substitutions at aliphatic and aromatic substrates, respectively. All free-radical substitutions are discussed in Chapter 14. Additions to multiple bonds are classified not according to mechanism, but according to the type of multiple bond. Additions to carbon-carbon multiple bonds are dealt with in Chapter 15 additions to other multiple bonds in Chapter 16. One chapter is devoted to each of the three remaining reaction types Chapter 17, eliminations Chapter 18, rearrangements Chapter 19, oxidation-reduction reactions. This last chapter covers only those oxidation-reduction reactions that could not be conveniently treated in any of the other categories (except for oxidative eliminations). 

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. 

The high element effects for the reactions of amines with (11) and (12) suggest multiplicity of mechanistic routes. The second-order kinetics and the very slow exchange of cis-bromo-(ll-a-D) in isopropanol fit addition-elimination (Ghersetti et al., 1965). In methanol, exch/ nub values for reaction of cyclohexylamine with cis and iraws-chloro-(ll) and cis- and Jrans-bromo-(ll) are 13, 11, 20 and 23, respectively, and 0-9, 0-6, 1 4 and 0-8 for the corresponding reactions of di-n-butylamine. While (rans-bromo-(ll) and cis-chloro-(ll) showed normal kinetics in methanol and in ethanol, the rate constants with cis-bromo- (11) and (12) decreased with time, but steady second-order behaviour could be achieved by addition of the perchlorate of the amine used. While this fits an amine-promoted elimination-addition, where the ammonium salt formed shifts the equilibrium to the left (equation 15), the slow... 

This elimination is reminiscent of the last step in the aqueous palladium chloride oxidation mentioned above and this reaction also may involve multiple hydride addition-elimination steps. Minor amounts of the normal products and Markovnikov products are also generally found in these reactions. Cupric chloride can be used as a reoxidant although the yields are generally lower than with an all acetate, non-catalytic reaction. 

Formation of dimer products seems to be due to increased electrophilicity of the multiple bond of the fluoroalkeiie. Thus, the initial stage of the reaction of triphenylphosphane with perfluoro(l-methylcyclopentenc) is addition to the multiple bond. The intermediate (7/8) attacks a second molecule of cycloalkene, and difluorotriphenyl-/ -phosphanc is eliminated producing perfluoro[l-methyl-2-(2-melhylcyclopent-l-enyl)cyclopentenc] (9) in 75% yield. 

Addition reactions Another type of organic reaction appears to be an elimination reaction in reverse. An addition reaction results when other atoms bond to each of two atoms bonded by double or triple covalent bonds. Addition reactions typically involve double-bonded carbon atoms in alkenes or triple-bonded carbon atoms in alkynes. Addition reactions occur because double and triple bonds have a rich concentration of electrons. Therefore, molecules and ions that attract electrons tend to form bonds that use some of the electrons from the multiple bonds. The most... 

We have now seen general acid- and general base-catalyzed processes for polarized multiple bond additions, eliminations, and enolizations. There is good reason to expect that all reaction types, additions, eliminations, substitutions, and rearrangements have general acid- or general base-catalyzed routes under the correct conditions. 

The most common representatives of the L-C=Y class of electron sinks are the carboxyl derivatives with Y equal to oxygen. In basic media there is only one pathway the addition-elimination path, path Ad y + Ep (see Section 4.5.1). The leaving group should be a more stable anion than the nucleophile, or the reaction will reverse at the tetrahedral intermediate. A follow-up reaction of a second addition to the polarized multiple bond occasionally occurs. With lone pair sources a second addition is rare because the nucleophile is usually a relatively stable species the second tetrahedral intermediate tends to kick it back out (see Section 9.2). 

Figure A.4 shows the usefulness of the reaction cube as a data structure. Additions to carbonyls often occur between different charge types, and frequently three-dimensional energy surfaces are used to clarify the various equilibria. We have seen two faces of this cube before as individual energy surfaces. The bottom faee of the cube is Figure 7.16, polarized multiple bond addition/elimination mechanisms in basic media. The back face of the cube is Figure 7.17, polarized multiple bond addition/elimination mechanisms in acidic media.

While there are unlimited possible organic reactions, general patterns do exist. Reactions are often presented with a step-by-step reaction mechanism that demonstrates each bit of the reaction in detail. Some organic reactions fit into multiple categories. For example, some substitution reactions follow an addition-elimination pathway. Movies demonstrating a number of organic reactions can be found here http //www.chem.ox.ac.uk/vrchemistrv/nor/reactions.asp... 

Some properties of diethyl allylphosphonate have been investigated, in particular the behaviour of its anion (500) towards < /t-unsaturated ketones and carboxylic esters. Although in. some cases the conjugate addition of the anion has been observed, e.g. in the formation of (501) and (502), in most ca.ses the reaction leads to carbocyclic products. y-Nucleophilicity, with addition-elimination, has been observed in reactions with e.g. ( )-4-methoxybut-3-en-2-one, when the final product is (503), following expulsion of the methoxy group. A third mode of reaction consists of multiple addition ... 

Many carbene complexes undergo nucleophilic attack at the carbene carbon. This chemistry is presented in more detail in Chapter 13 on metal-ligand multiple bonds. In brief, cationic carbene complexes tend to imdergo simple addition processes to generate neutral products (Equation 11.8), whereas neutral complexes can react by either addition (Equation 11.9) or by a sequence of addition and elimination reactions (Equation 11.10). Fischer has shown that the aminolysis of alkoxycarbene complexes occurs by initial attack at the carbene carbon by a mechanism similar to that for the aminolysis of organic esters. Although less studied than reactions of carbenes with nucleophiles, reactions of carbyne complexes with nucleophiles are also known, and these reactions generate carbene complexes. ... 

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