Electrophilic substitution at nitrogen

Odum and Aaronson found that p-cyanophenyl azide gave a substituted hydrazine (32) (70%) on direct photolysis in dimethylamine. On carrying out the photolysis in the presence of a triplet sensitizer (9 xanthenone) the yield of hydrazine dropped from 70% to 6% and the yield of amine rose to 70%, indicating that the hydrazine arose by attack of the singlet nitrene on dimethylamine  

In a further study, Odum and Wolf established that ring expansion could compete with hydrazine formation depending upon the wavelength of light used. This behevior was rationalized in terms of excess of excitation energy producing a hot nitrene  

Thermolysis of j 7-substituted aryl azides in a series of suitably substituted anilines yields mixed azo compounds as illustrated  

Several other examples of azo compound formation from reaction of aryl azides with anilines are known and support for a hydrazo intermediate (34) comes from the work of Huisgen and von Fraunberg. They showed that 34 could be converted to 35 in a separate experiment 

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Hexamethyleneimine and its benzo derivatives are typical secondary amines and undergo electrophilic substitution at nitrogen, e.g. alkylation and acylation, under standard conditions. Most form stable hydrochlorides and simple derivatives such as picrates (B-67MI51600). Table 4 contains pjRTa values of some azepines. 

Electrophilic substitution at nitrogen can occur in either ring, and the preference may depend on the nature of the substituents. Electrophilic substitution at carbon proceeds readily in the presence of electron-releasing substituents. 

Electrophilic substitution at nitrogen can occur in either ring depending on the nature of the substituents present. Electrophilic substitution at carbon proceeds readily in the presence of electron-releasing substituents. A wide variety of electrophiles have been studied. Mesoionic systems are readily substituted and cationic structures may become very reactive when conditions are chosen so as to promote intermediate formation of a pseudo-or anhydro-base. Sulfoxides are formed in the peracid oxidation of fused dihydrothiazoles. 

Electrophilic substitution at ring nitrogen atoms has been limited to protonation and iV-alkylation of the anion derived from a pyrido-pyrimidinone.i - Thus, the sodium salt of pyrido-[2,3-d]pyrimidine-2,4-(l//,3ir)-dione and dimethylsulfate yield the 1,3-dimethy] derivative (176). 

The reactions in this chapter are arranged in order of leaving group hydrogen, metals, halogen, and carbon. Electrophilic substitutions at a nitrogen atom are treated last. 

Electrophilic addition reactions. See also Addition reactions with butylenes, 4 405-408 of maleic anhydride, 75 490 with methacrylic acid/derivatives, 76 236-237 of propylene, 20 774 Electrophilic aromatic substitution, benzene, 3 599-601 Electrophilic attack, at nitrogen and carbon, 27 98... 

Reactions at ring atoms consist mainly of electrophilic attack at nitrogen and cycloadditions. Examples of the reaction of 2-substituted 1,3,4-oxadiazoles with bifunctional compounds at both ring nitrogen and at the substituent, leading to cyclic systems, are included in Section 4.06.5.2 irrespective of where the initial point of attack took place. A few examples of nucleophilic attack at unsubstituted carbon are described, the more common nucleophilic attack at substituted carbon being included under reactions of the appropriate substituent (Sections 4.06.7.1-7.5). 

Like other 7r-excessive heterocycles9 (e.g., azoles), the main reactions of azapentalenes are electrophilic substitutions at electron-rich centers (nitrogen or carbon atoms) in the molecule. 

The relatively low electron density at carbon, coupled with the possibility of protonation at nitrogen, makes electrophilic substitution at carbon difficult. A further problem is acid-catalyzed ring cleavage, particularly with alkyloxadiazoles. No examples of nitration or sulfonation of the oxadiazole ring have been reported and attempted brominations were unsuccessful. A low yield of 2-(2-furoyl)-5-phenyl-l,3,4-oxadiazole is obtained when 2-phenyl-l,3,4-oxadiazole is treated with 2-furoyl chloride in the presence of triethylamine (77LA159). 

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