OsO4 (Osmium Tetroxide) for Dihydroxylation of Alkenes
1. Osmium Tetroxide, OsO4 And The Dihydroxylation of Alkenes
Osmium tetroxide (OsO4) is a very useful reagent for the dihydroxylation of alkenes [Note 1] . In this reaction,For example, the reaction of cyclohexene with OsO4 gives exclusively cis-cyclohexan-1,2-diol, with none of the trans diol formed.Since we are breaking a C-C bond and forming two C-O bonds, this is an example of an oxidation reaction (See article: Oxidation and Reduction in Organic Chemistry)Two alkenes that differ only in their configuration (e.g. the stereoisomers cis- and trans- pent-2-ene) will result in products that are themselves stereoisomers.This fits the definition of a stereospecific reaction, as per IUPAC. (See article - Stereoselective and Stereospecific Reactions)Chiral molecules with exactly opposite (R,S) designations are enantiomers. Chiral molecules that share the configuration at at least one chiral center and differ at the configuration of another chiral center will be diastereomers. For more, see article: Types of Isomers)Note that in each case the two new C-OH bonds form on the same face of the alkene.
2. The Mechanism for Dihydroxylation of Alkenes With OsO4
The mechanism of alkene dihydroxylation is a concerted cycloaddition reaction where the C-C pi bond combines with two Os=O bonds to give a five-membered ring structure known as an osmate ester. (Note that in the osmate ester the Os is in the +6 oxidation state as opposed to the +8 oxidation state found in OsO4)This concerted mechanism nicely accounts for the cis stereochemistry observed in the dihydroxlyation of cyclohexene.Osmate esters are fairly stable products and can be isolated. [Note 2] However, since we are generally much more interested in the diol, a reagent such as potassium bisulfite (KHSO3) or sodium bisulfite (NaHSO3) is commonly used to break the Os-O bonds and liberate the diol.Just a heads-up - in introductory courses, this second reagent may or may not be included. It purpose is just to get rid of the osmium.(It is much more common nowadays to use catalytic OsO4 and a stoichiometric amount of an oxidant such as N-methylmorpholine N-oxide (NMO) or H2O2 to regenerate OsO4 from the Os(VI) species. In these cases, KHSO3 is not needed. See section below.)As a fairly electron-poor reagent, reactions with OsO4 increase in rate as the alkene becomes more electron-rich.For practical purposes, this means thatIt’s possible to selectively dihydroxylate an electron-rich alkene in the presence of other alkenes. For a few examples, see Note 3 below.
3. Predicting The Stereochemistry Of Dihydroxylation Products
cis- and trans- alkenes can be each be prepared from alkynes, depending on the reagent(s) used for reduction.Why is this important right now?Well, since cis- and trans- alkenes give dihydroxylation products that are stereoisomers of each other, dihydroxylation reactions provide great fodder for exam questions that challenge your understanding of stereochemistry.See if you can answer this classic quiz question:For a refresher on solving these kinds of stereochemistry problems, see article - Enantiomers, Diastereomers or the Same?Just as important as determining the stereochemistry of products is being able to work backwards from the products of dihydroxylation to the starting alkenes.This is more challenging with linear (as opposed to cyclic) products, since it will require that you successfully perform a bond rotation.See if you can work backwards from this diol to the starting alkene:Here is a similar example:
4. OsO4 versus KMnO4 As A Reagent for Dihydroxylation
A reagent similar to OsO4 that is also capable of performing dihydroxylation is potassium permanganate, KMnO4.Treatment of alkenes with cold, alkaline KMnO4 will also result in vicinal diols.Like the reaction with OsO4, this also proceeds through a cyclic, concerted transition state that results in a cyclic metal species (this time called a “manganate ester”).The key difference here is that unless the manganate ester will react further to give the products of oxidative cleavage unless hydroxide ion HO(-) is present to hydrolyze the Mn-O bonds and liberate the vicinal diol. This is not generally a problem with OsO4.This is also why the temperature is kept low for KMnO4 oxidations.Yields with KMnO4 tend to be lower and KMnO4 is also much less tolerant of sensitive functional groups like alcohols and aldehydes. Dihydroxylations with KMnO4 are often used with a phase transfer catalyst.
5. Catalytic OsO4 Using Stoichiometric Oxidant
It’s one thing to write a reaction down on a sheet of paper that uses a stoichiometric amount of osmium tetroxide.It’s another thing entirely to carry it out in the lab.For one thing, OsO4 is expensive - $332/g last time I checked, slightly cheaper if you b...
6. Reactions of Vicinal Diols - Cleavage with NaIO4
vicinal diols can undergo oxidative cleavage with various reagents to break a C-C bond and form two new C-O (pi) bonds.The most commonly used reagents for these purposes are sodium periodate (NaIO4) and lead tetraacetate Pb(OAc)4, although earlier we also touched on the fact that this is a prominent side reaction when performing dihydroxylations with KMnO4.Sequentially treating a double bond with OsO4 to give a diol followed by oxidative cleavage with NaIO4 or Pb(OAc)4 gives the functional equivalent of ozonolysis (reductive workup).(Later in Org 2, you will learn that diols will react with aldehydes and ketones to form acetals - See article: Hydrates, Hemiacetals, and Acetals)
7. Summary
Let’s summarize the key points we’ve covered about OsO4.
Notes
Note 1. OsO4 is prepared through burning metallic osmium in an atmosphere of pure oxygen. From Brauer’s Handbook of Preparative Inorganic Chemistry (Academic Press, 1963, New York). (page 1603)“Pure OsO4 is best prepared by a dry method. Osmium powder is heated in a boat placed in a glass or quartz tube through which a stream of dry oxygen is passed. The metal burns t...
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(Advanced) References and Further Reading
For examples of reactions employing OsO4, see:‘,’OsO4 (Osmium Tetroxide) for Dihydroxylation of Alkenes
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