Nucleophilic substitution reactions summary

A large number of nucleophilic substitution reactions involving interconversions of pyridopyrimidines have been reported, the majority of which involve substituents in the pyrimidine ring. This subject has been reviewed previously in an earlier volume in this series which dealt with the theoretical aspects of nucleophilic re-activiti in azines, and so only a summary of the nucelophilic displacements of the substituent groups will be given here. In general, nucleophilic substitutions occur most readily at the 4-position of pyrido-... 

Summary of the Nucleophilic Substitution Reactions of Carboxylic Acids and Their Derivatives... 

Imidazole reactivity was fully covered in CHEC-I, and only a brief summary is included here. The neutral molecule is Jt-excessive, being subject to electrophilic attack at N-3, less readily at C-4(5), and seldom at C-2. In benzimidazole electrophiles preferentially attack N-3 and the benzene ring carbons. Nucleophilic substitution reactions usually require some form of electron withdrawal elsewhere in the system, with displacements of groups at C-2 often favored. Imidazolium species are naturally more susceptible to nucleophilic attack, and they only undergo electrophilic substitutions with difficulty (e.g. nitration, sulfonation). The corresponding imidazole anions, when they can form, are highly reactive towards electrophiles. 

The stereochemistry of nucleophilic substitution reactions has been examined for substrates ranging in complexity from primary alkyl to triarylmethyl. A summary of representative examples is presented in Table 5.12. Chiral 1-butanol-l-rf and its derivatives have small, but measurable, optical rotations and provide useful substrates for the important case of substitution in primary systems. Entry 1 in Table 5.12 illustrates the stereospecific inversion observed in 1-butyl-1-rf... 

Summary This is an example of anchimeric assistance (also called neighbouring group participation) in nucleophilic substitution reactions... 

Summary of the Mechanisms of the Nucleophilic Substitution Reactions in Chlorocyclophosphazenes... 

Transition metals are widely used as catalysts for nucleophilic substitution reactions and there has been summary of their use in amination reactions. Pd(dba)2-ligand A is reported to be an efficient catalyst for the amination of a variety of aryl chlorides. Poly[(j9-cyclopentadienylmethylstyrene)rhodium] + grafted on to a polymeric sublayer has been used as an effective catalyst for alkoxydefluorination reactions of fluoroarenes. The activity is similar to that of the homogeneous catalyst and may be conserved for repeated applications. 

Table 9.4 Summary of the Reactivity of Alkyl Halides in Nucleophilic Substitution Reactions...

In Summary Pyridine undergoes slow electrophilic aromatic substitution preferentially at C3. Nucleophilic substitution reactions occur more readily to expel hydride or another leaving group from either C2 or C4. 

In the above examples, the nucleophilic role of the metal complex only comes after the formation of a suitable complex as a consequence of the electron-withdrawing effect of the metal. Perhaps the most impressive series of examples of nucleophilic behaviour of complexes is demonstrated by the p-diketone metal complexes. Such complexes undergo many reactions typical of the electrophilic substitution reactions of aromatic compounds. As a result of the lability of these complexes towards acids, care is required when selecting reaction conditions. Despite this restriction, a wide variety of reactions has been shown to occur with numerous p-diketone complexes, especially of chromium(III), cobalt(III) and rhodium(III), but also in certain cases with complexes of beryllium(II), copper(II), iron(III), aluminum(III) and europium(III). Most work has been carried out by Collman and his coworkers and the results have been reviewed.4-29 A brief summary of results is relevant here and the essential reaction is shown in equation (13). It has been clearly demonstrated that reaction does not involve any dissociation, by bromination of the chromium(III) complex in the presence of radioactive acetylacetone. Furthermore, reactions of optically active... 

Mechanism 6-1 Allylic Bromination 228 Summary Methods for Preparing Alkyl Halides 229 6-7 Reactions of Alkyl Halides Substitution and Elimination 231 6-8 Second-Order Nucleophilic Substitution The Sn2 Reaction 232 Key Mechanism 6-2 The S j2 Reaction 233 6-9 Generality of the SN2 Reaction 234... 

Summary Nucleophilic substitution and p elimination reactions of alcohols... 

Summary Phosfdiazene bases represent a new class of highly active non-ionic catalysts that rapidly polymerize cyclosiloxanes with equilibrium attained in very short reaction times at very low catalyst levels. To date, phosphazene base catalysts have been considered an academic curiosity because of the complicated and hazardous synthetic protocol used to prepare them. A facile synthetic process has been developed, which yields ionic phosphazene bases in three steps with an overall yield of qrproximately 75%. This is achieved through nucleophilic substitution of ionic phosphonitrilic chloride oligomers with secondary amines, followed by anion exchange. These ionic phosphazenes were found to exhibit similar reactivity in the ring-opening polymerization of cyclosiloxanes to that of the non-ionic phosphazene base. 

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