Nucleophilic catalysts

This process is usually excluded from reaction mechanisms.

There are many compounds which act as both good nucleophiles and good leaving groups. These can produce an intermediate which makes the molecule more reactive with a weaker nucleophile. These are called nucleophillic catalysts. Below is an example with pyridine helping an ROH attack a C=O:

Pyridine is useful above because it acts as a solvent, a weak base, and a catalyst.

Notice pyridine will end up protonated instead of the chloride ion. So at least two equivalents of pyridine are required.