Functional group exchanges

There are many factors that contribute to the greenness of a chemical reaction, and there is no clear preferred route, although the two systems that replace C-H directly are, on paper at least, cleaner than those that require functional group exchange. These reactions are summarised in Table 4. 

Another transformation hinging on the use of stoichiometric amounts of peroxide is an exchange reaction that replaces the xanthate group with a bromide. This synthetically valuable functional group exchange is illus-... 

Amides and ureas The sulfonamides undergo functional group exchange on reaction with thiolacetic acid, hydroxamic acids, and R2NC(=S)SH, in the presence of cesium carbonate. Reaction of the sulfonamides with dithioacids affords thioamides. 

Acetattzation. Diethyl acetals are formed from carbonyl compounds by functional group exchange with triethyl orthoformate (EtOH also present) under the influence of DDD." Benzylic activation. Hydride abstraction by DDQ from benzyl ethers, where the benzylic position is also activated by a nuclear substitutent (e.g., methoxy group), prepares such compounds to be attacked by nucleophiles. The reaction constitutes an important step in a synthesis of deoxyfrenolicin. ... 

Functional group exchanges. TiCla serves as a catalyst in the conversion of THP ethers, silyl ethers, and propargyl esters to the corresponding esters and ethers. A direct synthesis of a-(benzotriazol-l-yl)alkyl ethers involves treatment of dialkyl ethers with 1-chlorobenzotriazole andTiCl4. ... 

Functional group exchange. A synthesis of cyanoguanidines from O-substi-tuted -cyanoisoureas is promoted by MesAl. 

Figure 1.1. Functional group exchange reaction wheel. A visual reminder of the interconvertihility of functional groups.

C-C-Br => C=C ). In the actual synthesis, the required reaction is the reverse of this retroreaction.. It is also possible to write a functional group exchange based on the alkene as C=C C—C-Br. 

The introduction to organic reactions will be continued in the functional group exchange reactions presented in Chapter 2. That chapter will review the reactions introduced in a typical first organic chemistry course. Some of the concepts will be expanded and updated to include more synthetically useful reactions. Oxidation and reduction will be presented in separate chapters to focus attention on the vast number of reactions that fall under these categories. 

ACIDS, BASES AND FUNCTIONAL GROUP EXCHANGE REACTIONS... 

Chapter 2, Acids, Bases, Functional Group Exchanges... 

A.i. The Sn2 Reaction. In reactions where the substrate (see 61) has a leaving group (X) connected to a sp3 carbon, displacement by a nucleophile leads to a product in which X has been replaced by the nucleophile to give 63. This process can be described as a bimolecular (it follows second order rate equation) substitution that involves a nucleophile. The mechanistic descriptor is, therefore, Sn2. Reactions labeled as Sn2 may be observed in several different functional group exchange reactions and the substrate and the reagent can be either simple or complex. All Sn2 reactions can be represented by the simple reaction sequence 61 63,... 

The most synthetically useful nucleophilic substitution reaction is the bimolecular Sn2 reaction, shown in the section 2.7.A. A unimolecular substitution (ionization followed by substitution) is less useful in synthesis in a general sense, but is often the best specific reaction to effect a particular functional group exchange. In Chapter 12 we will see many examples where cationic reactions are very useful. Unimolecular substitution also occurs as a side reaction in aqueous media and can influence both the yield, stereochemistry, and regiochemistry of the final product. 

---