The weedkiller paraquat can be produced from methyl-pyridinium chloride using a cyanide catalyst: what is the mechanism?
While the mechanism is not 100% certain (or wasn't when I worked on it some time ago), the current best guess is that it is similar to the well known Benzoin condensation:



This is based on the following intermediate:



Applied to the paraquat conditions this might go like this:



The neutral quinone-like intermediate crystallises out of solution and can be separated and then oxidised (it is very oxygen sensitive) to the desired paraquat product.

The details are partially guesswork, though, as it isn't the easiest reaction to investigate mechanistically.