Transition asymmetric epoxidation

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). 

Transition metal-catalyzed epoxidations, by peracids or peroxides, are complex and diverse in their reaction mechanisms (Section 5.05.4.2.2) (77MI50300). However, most advantageous conversions are possible using metal complexes. The use of t-butyl hydroperoxide with titanium tetraisopropoxide in the presence of tartrates gave asymmetric epoxides of 90-95% optical purity (80JA5974). 

Chiral epoxides and their corresponding vicinal diols are very important intermediates in asymmetric synthesis [163]. Chiral nonracemic epoxides can be obtained through asymmetric epoxidation using either chemical catalysts [164] or enzymes [165-167]. Biocatalytic epoxidations require sophisticated techniques and have thus far found limited application. An alternative approach is the asymmetric hydrolysis of racemic or meso-epoxides using transition-metal catalysts [168] or biocatalysts [169-174]. Epoxide hydrolases (EHs) (EC 3.3.2.3) catalyze the conversion of epoxides to their corresponding vicinal diols. EHs are cofactor-independent enzymes that are almost ubiquitous in nature. They are usually employed as whole cells or crude... 

The development of transition metal mediated asymmetric epoxidation started from the dioxomolybdcnum-/V-cthylcphcdrinc complex,4 progressed to a peroxomolybdenum complex,5 then vanadium complexes substituted with various hydroxamic acid ligands,6 and the most successful procedure may now prove to be the tetroisopropoxyltitanium-tartrate-mediated asymmetric epoxidation of allylic alcohols. 

Fig. 11 The competing transition states for the epoxidation with ketone 55 Table 4 Asymmetric epoxidation with ketone 55...

The Julia - Colonna asymmetric epoxidation of electron-deficient unsaturated ketones to the corresponding epoxides with high yields and high ee is well known. This technique produces chiral chemical entities from the clean oxidant, hydrogen peroxide, without the use of a toxic or water sensitive transition metal additive. 

An alternate mechanism invoking an ion-pair transition-state assembly has been proposed to account for the enantioselectivity of the asymmetric epoxidation process [137]. In this proposal, two additional alcohol species are required in the transition-state complex. This... 

In 1967, Henbest et al. reported the first asymmetric epoxidation with an optically active peracid as the chiral oxidant [3], Since then, many optically active peracids have been used for this purpose but enantioselectivity remains low (<20% ee). This is probably because the substrate and the asymmetric center in the peracid are distant from each other in the transition state of the epoxidation as shown in Figure 6B.1 [4],... 

Figure 4.11 Plausible transition state for the asymmetric epoxidation.

Jacobsen, E. N. Transition Metal-catalyzed Oxidations Asymmetric Epoxidation. In Comprehensive Organometallic Chemistry IT, Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds. Elsevier Oxford, 1995 Vol. 12, pp 1097-1136, and references cited therein. 

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. 

The explanation for these experimental results, i.e. the lack of label transfer, is that the tetrahedral species (A) resulting from the addition of HSOf to the carbonyl group is capable of epoxidation. Ring closure of (A) is likely to be the rate-determining step in dioxirane formation. This work is important from a synthetic viewpoint, since it is crucial in the development of chiral ketones for the catalytic asymmetric epoxidation and the design of probes of transition state stereoselectivities that the nature of the oxidizing species is understood. 

This is mainly due to the fact that by means of chiral ligands it is comparatively facile to transfer absolute stereochemical information to a cat-alytically active metal center. However, the success of some of these reactions (e.g. the Sharpless asymmetric epoxidation or the Noyori hydrogenation) must not hide the fact that the number of powerful transition metal-catalyzed C-C coupling reactions, which proceed reliably with high enantioselectivity, is still rather small. 

The allylic alcohol binds to the remaining axial coordination site where stereochemical and stereoelec-tronic effects dictate the conformation shown in Figure 5. The structural model of catalyst, oxidant and substrate shown in Figure 5 illustrates a detailed version of the formalized rule presented in Figure 1. Ideally, all the observed stereochemistry of epoxy alcohol and kinetic resolution products can be rationalized according to the conq>atibility of their binding with the stereochemistry and stereoelectronic requirements imposed by this site. A transition state model for the asymmetric epoxidation complex has been calculated by a frontier orbital preach and is consistent with the formulation portrayed in Figure... 

This class of catalysts covers chemocatalysts that do not contain a transition metal. The class has been known for many years, but it is relatively recently that the term organocatalyst has been used (209). A wide variety of transformations can be performed, which is currently an area of intense research (209-218). Table 5 (220-252) summarizes some key transformations in which organocatalysis can be useful. Reactions range from the asymmetric epoxidation of alkenes, which need not be conjugated to another functional group, to aldol reactions and... 

The hrst of Sharpless s reactions is an oxidation of alkenes by asymmetric epoxidation. You met vanadium as a transition-metal catalyst for epoxidation with f-butyl hydroperoxide in Chapter 33,... 

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