I have been travelling extensively, which is the reason for not being able to write any posts. Now the summerly pace is settling in and I am back in Toronto. Not too long ago, my graduate student Sean Liew brought up an interesting JACS manuscript to my attention. I asked Sean to sum it up and here is his take:
“When we consider the fundamental properties of water, hydride formation to furnish H– and HO+ is not the first thing that comes to mind. Prof. Benjamin Stokes at UC Merced just published a creative strategy for transfer hydrogenation of alkenes using water in the presence of palladium and tetrahydroxydiboron. The authors propose a mechanism in which the oxidative addition product of palladium and tetrahydroxydiboron allows for the recruitment of water molecules to the Lewis acidic boron ligands. This zwitterionic Pd-B-O-H complex can be likened to that of the very well known Pd-C-C-H moiety that is notorious for β-hydride elimination chemistry. The reaction appears to proceed through generation of boric acid and the active palladium hydride species that recruits the alkene or alkyne for migratory insertion. Repeat the water recruitment process with the other boron centre followed by reductive elimination provides the hydrogenated product. So why is this so intriguing? H-O+-B–-Pd species is just being deprotonated, but by way of the unique reactivity of this short-lived intermediate, water’s proton is turned into a hydride through an entirely “inorganic” β-hydride elimination process.”
must be the most expensive hydride source ever. Surely formate, borohydride, or even hydrogen must be hundred times cheaper
I agree with you, but the conceptual part is interesting.
it could be useful for preparative deuterations if you don’t want to buy a deuterium tank.
Another underused hydrogenation source is NaH2PO2 (Pd-C, water, EtOAc)
This is a good point. Could definitely be used for that.