Titanium complexes Sharpless Ti tartrate asymmetric epoxidation catalyst

Titanium complexes (Sharpless Ti tartrate asymmetric epoxidation catalyst) 

Tartaric acid is the least expensive chiral starting material with twofold symmetry available from natural sources. The Sharpless Ti tartrate asymmetric epoxidation catalyst consists of titanium(IV) tetraisopropoxide (Ti(OiPr)4) in combination with a chiral tartrate diester to induce asymmetry in the reaction of allylic alcohols. It is used with an alkyl hydroperoxide such as TBHP in the presence of 3A or 4A molecular sieves (to remove water) for the epoxidation of allylic alcohols [75]. It was the first effective asymmetric epoxidation catalyst reported. 

The mechanism of epoxidation has been studied in depth [489,490]. The mixing of 1 equiv. of dialkyl tartrate with 1 equiv. of titanium tetraisopropoxide produces 2 equiv. of alcohol in accordance with the following reaction  

Molecular weight determinations by a technique related to vapor-phase osmometry indicated that the Ti(tartrate) is dimeric [490], so n = 2. The kinetic rate expression is as follows [489]  

The rate is first order with respect to allyl alcohol, Ti(tartrate), and the hydroperoxide oxidizing agent, and is inhibited by alcohol. The rate expression is consistent with the reaction sequence (Fig. 1.17). The Ti(tartrate) complex is formed by removal of two alkoxide ligands, and then the remaining two alkoxide ligands are displaced by TBHP and the allyl alcohol. The order of displacement is immaterial so fhe loaded complex can be reached by either pathway shown. 

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