Help Us Solve a Mechanistic Mystery: Benzimidazole Formation using Sodium Metabisulfite

If you're a synthetic chemist, you may have seen seen (or run) this reaction before:

It's the oxidative conversion of aromatic diamines and aromatic aldehydes into benzimidazoles. One common way of performing this reaction is in the presence of sodium metabisulfite. The question is: what's the role of the sodium metabisulfite and what's the oxidant?

Over at Chempedia Lab, this question about the role of sodium metabisulfite in benzimidazole formation stands as an excellent example of why the service was created. Five answers have been posted, three of which are starting to get lengthy discussions associated with them. Four Chempedia Lab users have been involved so far in coming up with an answer. New papers have been brought into the discussion.

Unfortunately, we have no answer as of yet. What we're looking for is a study that clearly shows the specific role being played by sodium metabisulfite in this reaction. Know the answer? Enlighten us and claim your bounty!

If you're tempted to post your answer or idea as a comment here, I encourage you to instead to create a Chempedia Lab account and join in the discussion there.

You can read more about Chempedia Lab in articles posted here. I'll also be giving a talk on Chempedia Lab (#16027) at the Fall ACS Meeting.