Addition elimination reactions

Scheme 5 Transformations of 1,3,2-diazaphospholenes by elimination/addition reactions (X=C1, Br)...

This special feature arises from the combination of the transition metal behavior such as the coordination of a carbon-carbon multiple bond, oxidative addition, reductive elimination, P-hydride elimination, addition reactions and the behavior of classical c-carbanion towards electrophiles. 

Going over the basics and mechanisms of nucleophilic substitution reactions Mastering mechanisms of elimination/addition reactions Determining synthesis strategies for aromatic systems... 

An elimination/addition reaction is another distinct type of reaction mechanism that occurs in aromatic systems. In these mechanisms, the elimination involves the loss of an HX molecule. While this may seem like a dehydrohalo-genation as seen in Organic Chemistry 1, it really is a different reaction. The HX loss leads to the formation of a benzyne intermediate (see Figure 8-2). The mechanism ends with addition to the bond formed by the loss of HX. 

An Sj Ar mechanism is an addition/elimination, not an elimination/addition reaction. 

The Dow Process utilizes an elimination/addition reaction to convert chlorobenzene to phenol. The proposed mechanism for this reaction is shown in Figure 8-3. The high-temperature reaction begins with chlorobenzene and aqueous sodium hydroxide. Note that this mechanism starts with the hydroxide attacking as a base, beginning dehydrohalogenation to form benzyne. The second hydroxide ion attacks as a nucleophile to form a carbanion intermediate, which behaves as a base in the last step to yield the final product. 

Two other important modes of substitution require mention here. They are the SNAr and elimination-addition reactions. Actually, it is sometimes difficult to distinguish between true aromatic nucleophilic substitutions and addition-elimination processes. The second group involves pyridyne intermediates (Scheme 53). Both of these reaction types are discussed fully under substituent reactions (Chapter 2.06). 

Condensation of sodium phenoxide with 2,2,2-trifluoroethyl iodide gives a product of direct substitution in a low yield, several other ethers are formed by elimination-addition reactions [7] Use of mesylate as a leaving group and hex amethyl phosphoramide (HMPA) as a solvent increases the yield of the substitution [5] Even chlorine can be replaced when the condensation is performed with potassium fluonde and acetic acid at a high temperature [9] (equations 6-8)... 

Rearrangement is associated with the fact that hydroboration is reversible at elevated temperatures. This makes possible a sequence of elimination-addition reactions in which boron becomes attached to different carbons and ultimately leads to the most stable product that has boron bonded to the carbon at the end of the chain ... 

Elimination-addition reactions of aryl halides with alkali-metal amides are discussed in Section 14-6C high-temperature copper-catalyzed amination, also effective, usually does not lead to rearrangement. 

Benzyl chloride reacts easily with methyl acrylate in the presence of tri-n-butylamine and palladium acetate (1 mol %) as catalyst.51 The product is a mixture of (E)-methyl 4-phenyl-3-butenoate (67%) and (E)-methyl 4-phenyl-2-butenoate (9%), arising from elimination-addition reactions of the palladium hydride group which largely isomerize the initial elimination product. 

A number of other groups have investigated the clustering reactions of small cation silicon species with silanes and other small molecules. The ion-molecule reactions occurring between SiHv+ (x = 0-3) cations and neutral ammonia, as well as the reactions between NHt+ cations and SiH4 were studied by FIMS113,114. The main channel for the reaction between SiHv+ ions and NH3 was found to correspond to the elimination-addition reaction, well-known for silanes (equation 20), which formally corresponds to the transfer of a nitrene-unit (NH)113. 

The elimination/addition reaction already proceeded at room temperature when the dichloropropionic acid had been linked as an ester to the support, but required heating when an amide linkage had been chosen. When amines with low nucleophilicity were used, such as aniline or a-amino acid esters, higher reaction temperatures were also beneficial. Occasional by-products for this reaction sequence were acrylic acid derivatives or the corresponding hydrogenated products (2-thiopropionic acid derivatives). These by-products were usually formed when a very small excess of amine was used in the elimination/addition step. Both the thiols and the amines used in this reaction sequence could be polyfunctional, as illustrated by the examples sketched in Fig. 3. 

SILENE-TYPE SPECIES IN ELIMINATION-ADDITION REACTIONS... 

Electrophilic attack may also lead to an elimination-addition reaction involving the silylonium intermediate [Eq. (73)]. Route 2 involving single... 

Cyclic ketone with exo cyclic methylenes can be prepared in just the same way and used in situ. Morpholine is often used as a convenient secondary amine for the Mannich reaction and the resulting amino-ketones can be methylated and undergo elimination-addition reactions with stabilized enoUtes such as that derived from ethyl acetoacetate. This starting material wias prepared from natural menthone and the mixture of diastereoisomers produced is unimportant because the product is to be used in a Robinson annelation (see below). 

This reaction occurs by an elimination/addition reaction that involves a benzyne intermediate. 

An efficient procedure for the nucleophilic displacement of the A, A -dimethylamino group of 1-triisopropylsilyl-gramines via the fluoride ion-induced elimination-addition reaction has been devised. 1-Triisopropylsilylgramine methiodide 1371 reacted smoothly with a variety of nucleophiles in the presence of tetrabutylammonium fluoride (TBAF) to give 3-substituted indoles 1372 (Scheme 262) <1995TL5929>. 

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