Nucleophilic attack at hydrogen Deprotonation

Deprotonaton at the ring carbon(s) and following reactions are treated in Section 2.01.2.5.2. 

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Nucleophilic Attack at Hydrogen Attached to Heterocyclic Carbon Atoms (Deprotonation)... 

As mentioned above and to be described in later reactivity sections, the ring chemistry is dominated by the heteroatom. As a result, for CN-3 systems, only one example of electrophilic and none of nucleophilic attack at carbon are known. No examples of nucleophilic attack at hydrogen attached to carbon (i.e., deprotonation) are known. 

Base-catalyzed hydrogen exchange occurs at the 3- and 5-positions of 1,2-dimethyl-pyrazolium salts. 2-Unsubstituted 1,3-dithiolylium salts are easily deprotonated by nucleophilic attack of hydrogen. The intermediate carbene easily undergoes dimerization. Hydrogen exchange can also occur (Scheme 23) (80AHC(27)15l). 

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