Preparation Methods
Stable carbenes are very reactive molecules and so it is important to consider the reaction conditions carefully when attempting to prepare these molecules. Stable carbenes are strongly basic (the pKa value of the conjugate acid of an imidazol-2-ylidene was measured at ca. 24) and react with oxygen. Clearly these reactions must be performed under a dry, inert atmosphere, avoiding protic solvents or compounds of even moderate acidity. Furthermore, one must also consider the relative stability of the starting materials. Whilst imidazolium salts are stable to nucleophilic addition, other non-aromatic salts are not (i.e. formamidinium salts).
Consequently in these cases, strong unhindered nucleophiles must be avoided whether they are generated in situ or are present as an impurity in other reagents (e.g. LiOH in BuLi).
Several approaches have been developed in order to prepare stable carbenes, these are outlined below.
Read more about this topic: Persistent Carbene
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