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January 7, 2013
Oxidizing Alcohols - With Water
This looks like an interesting reaction; let's see what gets made of it. David Milstein's group at the Weizmann Institute in Israel report a new catalytic system to oxidize alcohols to carboxylic acids, with water as the oxygen donor (as shown from labeling experiments). Hydrogen gas bubbles out of the mixture. The catalyst is a ruthenium complex, and although the reaction is not especially fast (18 hour timescale), the turnover numbers seem to be good (0.2% catalyst loading). Interestingly, oxygen actually seems to hurt the catalyst; the system runs better under argon. One possible drawback is that the ruthenium catalyst can serve as a hydrogenation catalyst - alkenes are reduced, what with all the hydrogen around.
Getting rid of (most of) the metals and the high-valent reagents will be worth the trouble industrially, as will getting rid of the need for pressurized oxygen. As it is now, many carboxylic acid compounds are produced on scale via either alkenes (hydroformylation and then oxidation of the aldehydes with a catalyst, or carbonylation), from alkanes via nonselective oxidation in air, or from alcohols via carbonylation.
We're still a long way from ditching the current processes, but if this reaction is robust enough, it could open up some new industrial feedstock routes. (One that I wonder about is replacing the current route to adipic acid, used in Nylon production. It's currently made through a rather foul nitric acid process - if there's enough hexanediol in the world. (Not sure if that's feasible, though - it looks like most of the hexanediol is made instead by reducing adipic acid! Makes you wonder if there's a potential biological route, as there is for butanediol). Edit - fixed this part, due to dropped some carbons between my brain and the keyboard this morning). Someone may also find a nice use for the hydrogen that's given off, and get some sort of two-for-one process. At the very least, this is a reminder of just how much more metal-catalyzed chemistry there is to be discovered. . .
Update: one of the paper's authors has dropped by the comments section, with interesting further details. . .
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