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Reagent Friday: OsO4 (Osmium Tetroxide)


In a blatant plug for the Reagent Guide, each Friday  I profile a different reagent that is commonly encountered in Org 1/ Org 2. 

Today’s reagent is among one of the best and most useful at what it does in all of organic chemistry. It’s blindingly good, in fact. So blinding, I don’t know if I’ve ever seen an example of it being used in an undergraduate teaching lab. It’s not a reagent for rookies: it’s genuinely dangerous, and best handled with extreme care.

Osmium tetroxide (OsO4) is a volatile liquid that is most useful for the synthesis of 1,2 diols from alkenes. (Side note: another name for 1,2 diols is vicinal diols, or vic-diols). The reaction is very mild, and usefully leads to the formation of syn diols. Another side note: this reaction doesn’t work with alkynes.

  The reaction works through a concerted process whereby two oxygens from the osmium interact with one face of the double bond. This results in a 5-membered ring (called an osmate ester) and generates the syn stereochemistry. The osmate ester is broken up into the 1,2-diol by use of a reducing agent such as potassium bisulfite, KHSO3. This is frequently omitted in textbooks, but is worth mentioning just in case.

By the way, dihydroxylation of alkenes can also be performed with cold, dilute potassium permanganate (KMnO4). One advantage of OsO4 is that it is much more compatible with other functional groups than KMnO4, which is kind of a ravenous beast.

Why is osmium “blindingly good”? One of the molecules required for vision is retinol:

What do you think would happen if the vapors from OsO4 reached your eyes? Everything would go dark, let me tell you.

The good news is that apparently the blindness wears off after several months.

P.S. You can read about the chemistry of OsO4 and more than 80 other reagents in undergraduate organic chemistry in the “Organic Chemistry Reagent Guide”, available here as a downloadable PDF.

P.P.S. One preparation in Woodward’s synthesis of cortisone (subscription req’d) calls for  Osmium tetroxide (68.48 g) . Damn!!!

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