Asymmetric hydroperoxidation

The first practical method for asymmetric epoxidation of primary and secondary allylic alcohols was developed by K.B. Sharpless in 1980 (T. Katsuki, 1980 K.B. Sharpless, 1983 A, B, 1986 see also D. Hoppe, 1982). Tartaric esters, e.g., DET and DIPT" ( = diethyl and diisopropyl ( + )- or (— )-tartrates), are applied as chiral auxiliaries, titanium tetrakis(2-pro-panolate) as a catalyst and tert-butyl hydroperoxide (= TBHP, Bu OOH) as the oxidant. If the reaction mixture is kept absolutely dry, catalytic amounts of the dialkyl tartrate-titanium(IV) complex are suflicient, which largely facilitates work-up procedures (Y. Gao, 1987). Depending on the tartrate enantiomer used, either one of the 2,3-epoxy alcohols may be obtained with high enantioselectivity. The titanium probably binds to the diol grouping of one tartrate molecule and to the hydroxy groups of the bulky hydroperoxide and of the allylic alcohol... 

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). 

The Sharpless-Katsuki asymmetric epoxidation reaction (most commonly referred by the discovering scientists as the AE reaction) is an efficient and highly selective method for the preparation of a wide variety of chiral epoxy alcohols. The AE reaction is comprised of four key components the substrate allylic alcohol, the titanium isopropoxide precatalyst, the chiral ligand diethyl tartrate, and the terminal oxidant tert-butyl hydroperoxide. The reaction protocol is straightforward and does not require any special handling techniques. The only requirement is that the reacting olefin contains an allylic alcohol. 

The asymmetric epoxidation of an allylic alcohol 1 to yield a 2,3-epoxy alcohol 2 with high enantiomeric excess, has been developed by Sharpless and Katsuki. This enantioselective reaction is carried out in the presence of tetraisopropoxyti-tanium and an enantiomerically pure dialkyl tartrate—e.g. (-1-)- or (-)-diethyl tartrate (DET)—using tcrt-butyl hydroperoxide as the oxidizing agent. 

In light of the previous discussions, it would be instructive to compare the behavior of enantiomerically pure allylic alcohol 12 in epoxidation reactions without and with the asymmetric titanium-tartrate catalyst (see Scheme 2). When 12 is exposed to the combined action of titanium tetraisopropoxide and tert-butyl hydroperoxide in the absence of the enantiomerically pure tartrate ligand, a 2.3 1 mixture of a- and /(-epoxy alcohol diastereoisomers is produced in favor of a-13. This ratio reflects the inherent diasteieo-facial preference of 12 (substrate-control) for a-attack. In a different experiment, it was found that SAE of achiral allylic alcohol 15 with the (+)-diethyl tartrate [(+)-DET] ligand produces a 99 1 mixture of /(- and a-epoxy alcohol enantiomers in favor of / -16 (98% ee). 

Based on detailed kinetic investigations, a tentative mechanism for this asymmetric oxidation was proposed (Scheme 2) according to which optically active sulphoxides may be formed by two pathways external attack on the sulphur atom by the chiral titanium hydroperoxide (path A) or coordination of sulphur to titanium prior to the oxidation step (path B). Although paths A and B could not be distinguished experimentally, the temperature effect was tentatively ascribed to a change of the mechanism, path A being predominant above — 20 °C and path B becoming competitive at lower temperatures (or vice versa). 

The modified Sharpless reagent was also successfully applied288 for the asymmetric oxidation of a series of 1,3-dithiolanes 248 to their S-monooxides 249 (equation 134). It was observed that the optical induction on sulphur (e.e. from 68 to 83%) is not significantly affected by the substituents R1 and R2. Asymmetric oxidation of a few aryl methyl sulphides by organic hydroperoxides in the presence of a catalytic amount of the optically active Schiff base-oxovanadium(IV) complexes gave the corresponding sulphoxides with e.e. lower than 40%289. 

The enantioselective epoxidation method developed by Sharpless and co-workers is an important asymmetric transformation known today. This method involves the epoxidation of allylic alcohols with fcrt-butyl hydroperoxide and titanium (sopropoxide in the presence of optically active pure tartarate esters, see Eqn. (25). 

The epoxidation of allylic alcohols can also be effected by /-butyl hydroperoxide and titanium tetraisopropoxide. When enantiomerically pure tartrate ligands are included, the reaction is highly enantioselective. This reaction is called the Sharpless asymmetric epoxidation.55 Either the (+) or (—) tartrate ester can be used, so either enantiomer of the desired product can be obtained. 

The scope of metal-mediated asymmetric epoxidation of allylic alcohols was remarkably enhanced by a new titanium system introduced by Katsuki and Sharpless epoxidation of allylic alcohols using a titanium(IV) isopropoxide, dialkyl tartrate (DAT), and TBHP (TBHP = tert-butyl-hydroperoxide) proceeds with high enantioselectivity and good chemical yield, regardless of... 

Another interesting asymmetric epoxidation technique using metal catalysis involves the vanadium complexes of A-hydroxy-[2.2]paracyclophane-4-carboxylic amides (e.g., 19), which serve as catalysts for the epoxidation of allylic alcohols with f-butyl hydroperoxide as... 

The idea of double asymmetric induction is also applicable to asymmetric epoxidation (see Chapter 1 for double asymmetric induction). In the case of asymmetric epoxidation involving double asymmetric induction, the enantiose-lectivity depends on whether the configurations of the substrate and the chiral ligand are matched or mismatched. For example, treating 7 with titanium tet-raisopropoxide and t-butyl hydroperoxide without (+)- or ( )-diethyl tartrate yields a mixture of epoxy alcohols 8 and 9 in a ratio of 2.3 1 (Scheme 4 3). In a... 

In 1980 a useful level of asymmetric induction in the epoxidation of some alkenes was reported by Katsuki and Sharpless121. The combination of titanium (IV) alkoxide, an enantiomerically pure tartrate ester and tert-butyl hydroperoxide was used to epoxidize a wide variety of allylic alcohols in good yield and enantiomeric excess (usually >90%). This reaction is now one of the most widely applied reactions in asymmetric synthesis131. 

Asymmetric epoxidation of ( -benzylideneacetophenone using THE La-(R)-BINOL-Ph3PO/CUMENE hydroperoxide system... 

Table 5.1 Catalytic asymmetric epoxidation of allylic alcohols using a combination of titanium wopropoxide. enantiomerically pure tartrate ester ((+)-DET or (+)-DIPT) and rerr-butyl hydroperoxide (yield and enantiomeric excess, according to the relevant publication). ...

Asymmetric trishomoallylic epoxidation.1 The 6,7-double bond of 1 is epox-idized stereoselectively to afford 2 with trityl hydroperoxide catalyzed by (-)-diethyl tartrate and Ti(0-i-Pr)4 in the presence of molecular sieves. (-Butyl hydroperoxide is not useful in this case. The product was used for an enantioselective... 

Anomeric hydroperoxides are readily prepared by treatment of 2-deoxy sugars with H202 in the presence of acid (Fig. 59). They are used as reagents for enantioselective epoxidation of a,(Tunsaturated olefins (e.g. chalcone) in the presence of sodium hydroxide, the epoxidations showed exceptionally high asymmetric induction.76... 

Successful asymmetric epoxidation of 2-cyclohexen-l-one was achieved using /-butyl hydroperoxide, toluene, solid sodium hydroxide, and benzylquininium chloride. Cyclohexenone epoxide obtained in this manner has an e.e. of 20% ([ ] = -39°). 

Although it was also Henbest who reported as early as 1965 the first asymmetric epoxidation by using a chiral peracid, without doubt, one of the methods of enantioselective synthesis most frequently used in the past few years has been the "asymmetric epoxidation" reported in 1980 by K.B. Sharpless [3] which meets almost all the requirements for being an "ideal" reaction. That is to say, complete stereofacial selectivities are achieved under catalytic conditions and working at the multigram scale. The method, which is summarised in Fig. 10.1, involves the titanium (IV)-catalysed epoxidation of allylic alcohols in the presence of tartaric esters as chiral ligands. The reagents for this asyimnetric epoxidation of primary allylic alcohols are L-(+)- or D-(-)-diethyl (DET) or diisopropyl (DIPT) tartrate,27 titanium tetraisopropoxide and water free solutions of fert-butyl hydroperoxide. The natural and unnatural diethyl tartrates, as well as titanium tetraisopropoxide are commercially available, and the required water-free solution of tert-bnty hydroperoxide is easily prepared from the commercially available isooctane solutions. 

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