Many free mesoionic carbenes are synthesized from their protonated salt form by deprotonation using strong potassium bases, such as potassium bis(trimethylsilyl)amide (KHMDS) or potassium tert-butoxide (KOt-Bu).  Potassium bases are used because they do not form stable carbene-alkali metal adducts.
Imidazolin-4-ylidenes (MIC) would form rather than imidazolin-2-ylidenes (NHC) due to blocking the C2 position. The C2 carbenes are thermodynamically more stable than their C4 counterparts due to resonance and inductive carbon-nitrogen interactions. Also, calculations show that the C4 hydrogen is less acidic than the C2 hydrogen of imidazole. This data suggests that the C2 position should be activated preferentially to the C4 position unless the C2 position is blocked. Aryl and bulky alkyl groups (such as isopropyl) are good at blocking the C2 position from being activated.