[phil81]

I was wondering what the mechanism of hydrogenolytic debenzylation is. The usual conditions are: H₂, Pd/C, solvent (e.g., EtOH) and sometimes addition of AcOH. This cleaves benzyl ethers and amines. I cannot think of anything that makes sense.

[nj_bartel]

I don't know the answer, but I'd say looking into the reactivity of the ether/amine and the Pd/C catalyst would be a solid starting point.  The H₂ reactivity with Pd/C is easily found.  AFAIK, the actual arene is essentially inert in those conditions.  Maybe following addition of the H₂ to the Pd/C (and assuming the ether/amine attach to the Pd/C by donating one of it's (the ether/amine's) lone pairs to the catalyst in a mechanism similar to the H₂), the complexed hydrogen could insert into the complexed ether/amine forming a good leaving group.  The benzylcation could then somehow get a hydrogen.  Maybe with H₂ serving as a nucleophile.

[azmanam]

It's surprisingly hard to find anything on the internet or in my protective groups book... but here's my take (and I don't have anything to back this up):

Pd/H₂ is a reducing system.  It pumps an electron into the aryl ring to give a radical anion.  Anion collapses to give an exocyclic double-bond and generates O^- and a benzyl radical.  Benzyl radical and hydroxide pick up protons from solvent.

Although my protective group book has separate sections for reductive cleavage (hydrogenolysis, H₂/Pd) and reductive cleavage (single electron, Li/NH₃), so maybe the two are different mechanisms?

something like this, maybe...

edit: mechanism probably wrong... For the mechanism to be correct, it should be lithium donating an electron (as in the Li/NH₃ debenzylation)

[azmanam]

Addendum...

I think the mechanism I proposed is for single electron cleavage, NOT H2/Pd cleavage.

Perhaps it's an insertion into the ether bond followed by reductive elimination?

[phil81]

Thank you both.

I think azmanam's mechanism does kind of make sense. In this way, H2 would actually be required to regenerate Pd(0), which would otherwise be consumed...
Some additional information from Wuts/Greene. The relative speed for the hydrogenation of benzyl ethers in varying solvents: THF > AcOH > octanol > hexanol > propanol > ethanol > hexane > methanol > toluene. No explanation is given.