Nucleophilic Attack at Ring Nitrogen

Nucleophilic Attack at Ring Atoms 5.16.3.5.1 Oxygen, nitrogen and sulfur nucleophiles... 

The thermolysis and photolysis of mesoionic phenyl-1,3,2-oxathiazolones were extensively investigated during the period 1980-1990 <1996CHEC-II(4)433>. Electrophilic and nucleophilic attack at ring atoms of 1,3,2-dithiazoles (nitrogen, carbon, and sulfur) occurs quite rarely and little attention was paid to it in CHEC-II(1996). A large part of the section was devoted to the transformation of 1,3,2-dithiazolium radicals to corresponding cations and vice versa. 

Nucleophilic Attack at Hydrogen Attached to Ring Carbon or Ring Nitrogen... 

As depicted in Scheme 11, ylides 39 derived from 4-methyl-[l,2,3]triazolo[l,5- ]pyridine react with Michael acceptors, which, upon nucleophilic attack at C3 and ring opening, lead to nucleophilic displacement of nitrogen. The intermediate diradical led to a mixture of compounds, including alkenes and a cyclobutane derivative when methyl acrylate was used, and the indolizine 40 with methyl propiolate as the electrophile <1998T9785>. Heating 4-methyl triazolopyridine with benzenesulfonyl chloride in acetone also confirmed decomposition via a radical pathway. 

Carrying out the same reaction with N-labeled potassium amide/liquid ammonia, the aminodechlorination leads to 100% incorporation of the label in the ring of 95. This exclusive enrichment of the nitrogen of the ring in 95 means that the replacement of the 2-chloro substituent starts by an exclusive nucleophilic attack at C-4 and formation of the C-4 adduct 93. Ring opening to 2-iminobenzoyl-3-cyanamino-5,6-diphenylpyrazine (94) and subsequent recyclization yield 95. 

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). 

Reactivity of Fully Conjugated Rings. 5.1 Thermal and Photochemical Reactions. 5.2 Electrophilic Attack at Nitrogen. 5.3 Electrophilic Attack at Carbon. 5.4 Nucleophilic Attack at Carbon... 

The —S—C—C=N— chelate ring is greatly weakened or does not tend to form for steric reasons and the electron-withdrawing effect of the metal ion coordinated to the nitrogen atom facilitates nucleophilic attack at the imine carbon atom. Similar observations have been made with bis (2-pyridinal) ethylenedi-imine... 

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