The 2001 Nobel Prize in Chemistry was awarded to three organic...

Question

The 2001 Nobel Prize in Chemistry was awarded to three organic chemists who have developed methods for catalytic asymmetric syntheses. An asymmetric (or enantioselective) synthesis is one that converts an achiral starting material into mostly one enantiomer of a chiral product. K. Barry Sharpless (The Scripps Research Institute) developed an asymmetric epoxidation of allylic alcohols that gives excellent chemical yields and greater than 90% enantiomeric excess.

The Sharpless epoxidation uses tert-butyl hydroperoxide, titanium(IV) isopropoxide, and a dialkyl tartrate ester as the reagents. The following epoxidation of geraniol is typical.

(a) Which of these reagents is most likely to be the actual oxidizing agent? That is, which reagent is reduced in the reaction? What is the likely function of the other reagents?

(b) When achiral reagents react to give a chiral product, that product is normally formed as a racemic mixture of enantiomers. How can the Sharpless epoxidation give just one nearly pure enantiomer of the product?

(c) Draw the other enantiomer of the product. What reagents would you use if you wanted to epoxidize geraniol to give this other enantiomer?

Accepted Answer

Hey everyone. Let's do this problem. And says in an an anti a selective or asymmetric synthesis and a Cairo starting material is converted into mostly one an anti more of a Cairo product for example in the asymmetric a pox sedation developed by K. Barry Sharpless, A lilic alcohols are converted to a pox sides with more than 90% in anti american excess. A typical Sharpless iP oxidation uses TERT beautiful hydro peroxide titanium for isopropyl oxide and a dye alcohol. Tar trade Esther as the regents. As is shown in the approximation of a gerund dial derivative below one identify the oxidizing agent in this reaction. What is the likely function of the other re agents? We'll start with just part one and then move on to parts two and three after. So part one identify the oxidizing agent in this reaction. So we know that an oxidizing agent is going to donate an oxygen atom to another compound. So in this reaction the TERT beautiful hydro peroxide donates an oxygen to oxidize the geranium derivative to the corresponding up oxide. So TERT beautiful hydro peroxide is the oxidizing agent. Also it asks what is the likely function of the other re agents? Well, di ethyl tar trait is the source of asymmetry in this reaction because it is the only Cairo molecule in the reaction system. So without this di ethyl titrate the reaction does not give a high and anti America high and anti american excess. Right? Remember the problem said about 90% E and anti american excess. So this carbon molecule will help direct the react in a certain way to produce that specific and anti american excess. So this di ethyl Tartarus is the source of asymmetry and uh selectivity in our reaction. And for part three we have are not part three. The third re agent titanium for isopropyl oxide is going to sort of form a dime er structure with the other re agents specifically di ethyl tar trait and it's going to hold all the reagents together so that they can react together. Alright, so that is part one, Part two says. How can the reaction give just one mostly pure and anti tumor of the product? Well we said that the asymmetry relates to die ethel L. Tar trait. Right. So the answer for part two is the key to an anti a selective a pock sedation is di ethyl titrate. It forms a Cairo structure when it complexes with titanium which directs the spatial orientation of TERT bugle, hydro peroxide and geranium oil derivative when they approach each other, leading to only one in an tumor of product. So if we have day ethel L. Tar trait, maybe it prefers one side of the re agent or the geranium derivative and the other an antimatter prefers the other side. So we can use those different orientations, those different Ananta murders of di ethyl tar trait to direct our reaction. How we wanted to go. So with that being said, Part three says give the structure of the other end of the product. What re agents will you use to a poxy dies, the geranium derivative to produce this other Ananta mur Well, like we just mentioned, if we use di ethyl L tar trait to give our wedge dash combo seen in this reaction, Maybe if we use di ethyl D. Tar trait, it will produce the other nan tumor. Right? So let's draw what that will look like. Everything is going to stay the same except for the wedge and dash orientation. So, originally we had the dash up oxide in the wedge metal and the wedge alcohol group out here. Or I should say methanol group out there. So now we're going to have a wedge a Parkside in dash carbon groups. So a dashed metal and dashed methanol in the regions here, like we said, the asymmetry is depending on di ethyl tar trade. So that one will change to di ethyl Ditto or trait and the other regents will stay the same. Oops, we have the church, beautiful hydro peroxide and the titanium for this Oprah Parkside. All right. And all of this makes choice D the correct answer for this problem. That's it for this one. I hope this video is helpful. And thanks for watching

Key Concepts

Asymmetric Synthesis

Oxidizing Agents

Enantiomeric Excess

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