IMHO performing the reaction without any added water doesn't work nearly as well, the water is necessary. One of the driving forces in this reaction is the formation of the insoluble Al oxides/hydroxides, and yields achieved by me and described in the literature prove that a little water will not affect the reaction at all. On the other hand I never used MeOH as the solvent, and MeOH is just as acidic as H2O, so Al(OMe)3 might be just as favorable as the hydroxides, but I'm still sceptical. I have stored amalgamated sheet Al for a day under EtOH and there was almost no reaction. When water was added the whole mother gradually heated up to the boiling point, boiling off the EtOH. This makes me believe that the oxide/hydroxide formation is a major driving force in this reaction.
Pyro: wrong wrong wrong!
The reaction taking place is an electron transfer from the Hg to the imine, and the hydrogen (actually the protons) necessary to form the end product are then supplied by the solvent.
H2 gas produced in this reaction is completely worthless. It will not contribute anything to this reaction, and when done properly there is no almost no H2 produced until the very end, when nearly all imine is reduced and the only possible reaction left for the Al/Hg is reaction with the solvent. The reason for this is the overvoltage(?), the difficulty of H2 production on Hg surfaces, the same effect that allows sodium amalgam to be formed in NaCl electrolysis, even though this reaction is electrochemically very unlikely to happen when you look at the standard potentials of the reactions taking place.
Another important observation: when doing the Al/Hg with thick (=low surface area), well-amalgamated sheet Al there is almost nothing happening until you start adding the ketone. The imine is formed, and as soon as there is something to be reduced around the reduction takes place. You stop adding ketone and the reaction dies down. Once all ketone has reacted the reaction cools down and pretty much comes to an end, leaving the leftover unreacted Al/Hg behind. This means you will need less Al in such a reaction compared to the one using foil, and you will end up with much less sludge to deal with.
