Oxidation reactions, transition-metal Sharpless titanium

Hie first of Sharpless s reactions is an oxidation of alkenes by asymmetric epoxidation. You met vanadium as a transition-metal catalyst for epoxidation with r-butyl hydroperoxide in Chapter 33, and this new reaction makes use of titanium, as titanium tetraisopropoxide, Ti(OiPr)4, to do the same thing. Sharpless surmised that, by adding a chiral ligand to the titanium catalyst, he might be able to make the reaction asymmetric. The ligand that works best is diethyl tartrate, and the reaction shown below is just one of many that demonstrate that this is a remarkably good reaction. 

Allylic alcohols are converted to epoxides by oxidation with tert-hutyl hydroperoxide in the presence of certain transition metals. The most significant aspect of this reaction— called the Sharpless epoxidation— is its high enantioselectivity when carried out using a combination of tert-hutyl hydroperoxide, titanium(IV) isopropoxide, and diethyl tartrate. 

The types of reactions that can be catalyzed by transition metal complexes are now very numerous and are very widely used in synthesis. We have already met a number of them—osmium in catalysis of dihydroxylation reactions, titanium in Sharpless epoxidation, various metals in hydrogenation reactions of alkenes, and the Ziegler-Natta process for polymerization. In this section, we will just highlight a few types that have been popular—an oxidation, some hydrogenations, and some coupling reactions. Although outline reaction mechanisms will be given, this is for interest only—they are beyond the scope of this text, and many are more complicated than is shown here. 

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