One might believe that sodium or potassium hydride would be the ideal base for deprotonating these precursor salts. The hydride should react irreversibly with the loss of hydrogen to give the desired carbene, with the inorganic by-products and excess hydride being removed by filtration. In practice this reaction is often too slow, requiring the addition of DMSO or t-BuOH. These reagents generate soluble catalysts, which increase the rate of reaction of this heterogeneous system, via the generation of tert-butoxide or dimsyl anion. However, these catalysts have proved ineffective for the preparation of non-imidazolium adducts as they tend to act as nucleophiles towards the precursor salts and in so doing are destroyed. The presence of hydroxide ions as an impurity in the metal hydride could also destroy non-aromatic salts.
Deprotonation with sodium or potassium hydride in a mixture of liquid ammonia/THF at −40 °C has been reported for imidazole-based carbenes. Arduengo and coworkers managed to prepare a dihydroimidazol-2-ylidene using NaH. However, this method has not been applied to the preparation of diaminocarbenes. In some cases, potassium tert-butoxide can be employed without the addition of a metal hydride.