Eliminations of a-Substituents

The addition of a nucleophile to an aromatic ring, followed by elimination of a substituent, results in nucleophilic substitution. The major energetic requirement for this mechanism is formation of the addition intermediate. The addition step is greatly facilitated by strongly electron-attracting substituents, and nitroaromatics are the best reactants for nucleophilic aromatic substitution. Other EWGs such as cyano, acetyl, and trifluoromethyl also enhance reactivity. 

The addition of a nucleophile to an aromatic ring, followed by elimination of a substituent, results in nucleophilic substitution. The major energetic requirement for this... 

It should be noted that for N-substituted dihydropyridines and pyrimidines, an alternative aromatization is observed. It includes the elimination of a substituent from the nitrogen atom [246, 247, 288, 289] (Schemes 3.88, 3.89). Heteroaromatized derivatives of pyrimidine, in addition, can be obtained as a... 

During dioxygenation, elimination of a substituent may occur simultaneously. Illustrative examples of simultaneous elimination and dioxyygen-ation during degradation are discussed in the appropriate sections in Chapter 6, but may usefully be summarized here. They are represented formally by the reactions given in Figure 4.48. 

Yet another reaction for the ketimine illustrated in Fig. 14 is the elimination of a substituent (labeled Y in this drawing) with formation of a double bond. The product of this elimination sometimes decomposes, with loss of nitrogen as ammonia (NH3), but in other cases a molecule... 

The general mechanism for electrophilic substitution suggests that groups other than hydrogen could be displaced, provided the electrophile attacked at the substituted carbon. Substitution at a site already having a substituent is called ipso substitution and has been observed in a number of circumstances. The ease of removal of a substituent depends on its ability to accommodate a positive charge. This fector determines whether the newly attached electrophile or the substituent is eliminated from the [Pg.588]

The rate of iodine formation depends on the degree of A"-substitu-tion. Compounds which are unsubstituted on both the iV-atoms (35) and those wdth a single A -substituent (43) liberate instantly the calculated quantity of iodine in the cold. However, the 1,2-disubstituted diaziridines (44) need brief heating with the acid iodine solution they then give 95-100% of the calculated iodine. " This effect of substitution is so well defined that it can be used for a proof of constitution. The diaziridino-triazolidincs (37) prepared from aldehydes, ammonia, and chloramine give complete iodine liberation only on heating. Thus the structure 57 which is isomeric with 37 can be eliminated. ... 

An interesting free radical carbon-carbon bond formation with concomitant elimination of a /5-thio substituent was achieved during the course of Boger s impressive synthesis of CC-1065.26-27 In the event, treatment of aryl bromide 70 (see Scheme 13) with tri-n-... 

Several studies have been undertaken in an attempt to understand factors that affect the susceptibility of a complex to reductive elimination. These include investigations on Me-Pd-CNC pincer complexes [39] studies on the influence of the geometry of the complexes [40] the impact of A-substituents [41] and a study on the influence of chelating spectator ligands in complexes of the type [PdMe(NHC) (P-P)]BF [42]. 

The synthesis of new heterocyclic derivatives under conservation of a preformed cyclic structure is not only of particular importance for the synthesis of ionic 1,3,2-diazaphosphole or NHP derivatives but has also been widely apphed to prepare neutral species with reactive functional substituents. The reactions in question can be formally classified as 1,2-addition or elimination reactions involving mutual interconversion between 1,3,2-diazaphospholes and NHP, and substitution processes. We will look at the latter in a rather general way and include, beside genuine group replacement processes, transformations that involve merely abstraction of a substituent and allow one to access cationic or anionic heterocycle derivatives from neutral precursors. 

A reaction involving the loss or elimination of two substituents attached to a common atom (usually carbon), a-Elimination is often observed in the synthesis of certain carbenes and nitrenes. See also Carbene... 

The reaction of 39 with methyl 4,6-0-benzylidene-2-deoxy-2-iodo-a-D-altropyranoside (48) gave a complex mixture of products, from which methyl 4,6-0-benzylidene-2,3-dideoxy-a-D-eryf/iro-hex-2-eno-pyranoside (50) could be isolated. The formation of 50 was ex-plained83(b) by attack by chloride ion on the iodine atom in intermediate 49, followed by elimination of the substituent at C-3. Compound 50 itself reacts with reagent 39, and, therefore, prolonged reaction times led to extensive decomposition. 

Benzyl-2,4,6-triphenyl-4//-thiopyran (2451) reacts with perchloric acid with loss of the 4-benzyl group to afford perchlorate 404.236 A similar elimination of the substituents was found for 4-benzyl derivatives 253.3<8 The reported conversion of 4,4-dichloro-4//-thiopyran (336b) to perchlorate 408, involving the loss of a 4-chlorine,325 is not an oxidation process. Similar transformations are also reported.349... 

Path A Nucleophilic attack at a hydrogen atom of a substituent with subsequent elimination (discussed under the relevant substituent in Section 3.2.3.1). 

---