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Paraquat is 4,4'dipyridinium chloride. There are two common ways to synthesise it starting with pyridine.
In one reaction that can be done on a large scale in water or liquid ammonia, methyl-pyridinium chloride is reacted with a base and catalytic amounts of sodium cyanide in the solvent to give a dimeric intermediate that oxidises easily into paraquat.
What is the chemical mechanism of the dimerisation reaction?
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.
I remember PET (photoinduced electron transfer) works by the Mariano group in which N-dimethylallyl pyridinium perchlorate underwent intramolecular radical cyclizations upon uv irradiation and in fact, I've seen N-methyl chinolinium salts undergo dimerization under these conditions.
Imagine that the N-methylpyridinium cation is irradiated at $\lambda$ = 300 nm. The resulting $S_1$ state oxidizes cyanide to a cyanogen radical.
The N-methylpyridinium cation is thereby reduced to a pyridyl radical. Radical dimerization furnishes a 4-4'-dihydrobiaryl (see drawing).
Aromatization to paraquat requires the loss of two hydrogen atoms and two electrons; the latter are being used to regenerate cyanide.
The mechanism is a bit speculative but consistent and reflects the catalytic use of cyanide.
