Optically pure diethyl tartrate

Enantioselective epoxidation of allylic alcohols using t-butyl peroxide, titanium tetra-wo-propoxide, and optically pure diethyl tartrate. 

Catalytic Reactions. Certain catalytic reactions show a considerable departure from the linear relationship between the ee of a chiral source and the extent of the asymmetric induction (Scheme 42) 68). The Sharpless epoxidation of geraniol using Ti(IV) tetraisopropoxide modified by enantiomerically pure diethyl tartrate (DET) (Ti DET =1 1) gives the epoxide in 94% ee. Kagan and Agami first found that, by using the tartrate auxiliary in 50% ee, the optical yield varied to 70% ee. This optical yield is considerably higher than the expected value, 47 % ee... 

The precatalyst, however, is a chiral rather than a Mo complex. It is generated by the in situ treatment of titanium isopropoxide with optically pure diethyl or diisopropyl tartrate. As L-tartaric acid is a natural product, the optically pure ligand is easily made. As shown by reaction 8.5.3.1, at the optimum Ti tartarate ratio (1 1.2), complex 8.32 is the predominant species in solution. This gives the catalytic system of highest activity and enantioselectivity. 

This chemical bond between the metal and the hydroxyl group of ahyl alcohol has an important effect on stereoselectivity. Asymmetric epoxidation is weU-known. The most stereoselective catalyst is Ti(OR) which is one of the early transition metal compounds and has no 0x0 group (28). Epoxidation of isopropylvinylcarbinol [4798-45-2] (1-isopropylaHyl alcohol) using a combined chiral catalyst of Ti(OR)4 and L-(+)-diethyl tartrate and (CH2)3COOH as the oxidant, stops at 50% conversion, and the erythro threo ratio of the product is 97 3. The reason for the reaction stopping at 50% conversion is that only one enantiomer can react and the unreacted enantiomer is recovered in optically pure form (28). 

The methyl ester of acid 76 was adsorbed onto NaY zeolite, in which optically pure inductors like ephedrine and diethyl tartrate had been preloaded. In this instance, however, the results of photoreaction were disappointing [84]. Diethyl tartrate proved to be the best chiral inductor (12% ee) all the rest gave ee values of 5% or less. Photoisomerization of the methyl ester of the diphenylben-zoyl acid 78 using zeolites modified with chiral indicators also gave low ee values [85]. Norephedrine was found to be the best chiral indicator, resulting in 20% ee. 

---