Epoxide vanadium-catalyzed

Vanadium-catalyzed asymmetric epoxidation has recently been re-examined with a newly designed chiral hydroxamic acid (3).43-45 The hydroxamic acid (3) forms a 1 1 complex with vanadium ions and induces high enantioselectivity (Scheme 6). 

The vanadium-catalyzed epoxidation of hindered homoallylic alcohols has been described by Prieto and coworkers [339]. Reaction times for the epoxidation in a series of cis- and trans-2-methyl-alkenols were significantly reduced from 6-10 days to... 

In a recent paper, Zhang and Yamamoto have described a modified BHA ligand (235d) that is suitable for highly enantioselective vanadium-catalyzed epoxidation of homoallylic alcohols (Scheme 102). Both tram- and cA-substituted epoxides were achieved with nearly complete enantioselectivities and good yields. 

SCHEME 60. Optically active hydroxamic acid ligands for the vanadium-catalyzed asymmetric epoxidation of allylic alcohols,... 

SCHEME 61. Vanadium-catalyzed epoxidation of allylic alcohols using different hydroperoxides... 

Table 17) with two substituents in position C3 the oxygen transfer by the chiral hydroperoxides occurred from the same enantioface of the double bond, while epoxidation of the (ii)-phenyl-substituted substrates 142c,g,i resulted in the formation of the opposite epoxide enantiomer in excess. In 2000 Hamann and coworkers reported a new saturated protected carbohydrate hydroperoxide 69b , which showed high asymmetric induction in the vanadium-catalyzed epoxidation reaction of 3-methyl-2-buten-l-ol. The ee of 90% obtained was a milestone in the field of stereoselective oxygen transfer with optically active hydroperoxides. Unfortunately, the tertiary allylic alcohol 2-methyl-3-buten-2-ol was epoxidized with low enantioselectivity (ee 18%) with the same catalytic system . 

TABLE 18. Enantiomeric excesses obtained in the vanadium-catalyzed asymmetric epoxidation of aUyhc alcohols using enantiomeric pure ligand 140a and racemic as well as enantiomericaUy pure hydroperoxide 16a... 

The retarding effect of alcohols on the rate of epoxidation manifests itself in the observed autoretardation by the alcohol coproduct.428,434 446,447 The extent of autoretardation is related to the ratio of the equilibrium constants for the formation of catalyst-hydroperoxide and catalyst-alcohol complexes. This ratio will vary with the metal. In metal-catalyzed epoxidations with fe/T-butyl hydroperoxide, autoretardation by tert-butyl alcohol increased in the order W < Mo < Ti < V the rates of Mo- and W-catalyzed epoxidations were only slightly affected. Severe autoretardation by the alcohol coproduct was also observed in vanadium-catalyzed epoxidations.428 434 446 447 The formation of strong catalyst-alcohol complexes explains the better catalytic properties of vanadium compared to molybdenum for the epoxidation of allylic alcohols.429 430 452 On the other hand, molybdenum-catalyzed epoxidations of simple olefins proceed approximately 102 times faster than those catalyzed by vanadium.434 447 Thus, the facile vanadium-catalyzed epoxidation of allyl alcohol with tert-butyl hydroperoxide may involve transfer of an oxygen from coordinated hydroperoxide to the double bond of allyl alcohol which is coordinated to the same metal atom,430 namely,... 

It has recently been reported495 that the complex CsH5V(CO)4 (CSHS = cy-clopentadienyl) is an efficient catalyst for the stereoselective oxidation of cyclohexene to ris-l,2-epoxycyclohexane-3-ol in good yield (65% at 10% conversion). This high stereoselectivity is reminiscent of the highly selective vanadium-catalyzed epoxidations of allylic alcohols with alkyl hydroperoxides discussed earlier. The mechanism of reaction,... 

I to 5. 0 mol per mol of hydroperoxide. The presence of sodium naphtheoate, by prevenling side reaction, helps to reduce the excess propylene required (from lO/l to 2/1 in moles). In the Shell technology, epoxidation is catalyzed by metallic oxides (molybdenum, vanadium, titanium, etc.) supported on sih cau The liighiy exothe c reaction takes place around 100 to 130 at 3.5.10 Pa absolute. Hydroperoxide conver> sion is very hi (> 97 per cent). Propylene oxide molar selectivity exceeds 70 per cent and that of the styrene precursors 93 per cent As for propylene, its once-through conversion is about 15 per cent, for a oxide molar selectivity greater than 90 per cent, and the main by-products are dimers and heavier hydrocarbons. 

The epoxides obtained from the chlorohydrins are the opposite diastereomers to those formed by vanadium-catalyzed epoxidations (9, M-82). 

Vanadium-catalyzed epoxidation of the diene (156) having hydroxy allylic to one double bond and homoallylic to the other double bond does not furnish exclusively (157 equation 54). ° The epoxidation of (156) with dibutyltin oxyperoxide is regioselective, fUmishing exclusively the regioisomer (157) as a 9S S mixture of erythro and threo diastereoisomers."" Epoxidations of the diol (159) and its epimer (160) take place selectively from the a-face. On the basis of this observation and further studies it has been concluded that in (159) and (160) the epoxidation is directed by the homoallylic, but not the allylic,... 

Epoxidations of triisopropylsilyl (TIS) ethers (170) and (171) with W05-HMPA in dichloroethane take place stereoselectively to furnish n-epoxides these epoxidations, along with the vanadium-catalyzed epoxidations of (152) and (153) (described in Section 3.1.3.3), make available a group of all the four possible diastereoisomeric epoxidies having four consecutive chiral centers in an acyclic carbon framework,... 

In the respective vanadium-catalyzed epoxidation, a substantial discriminating steric interaction in either of the two diastereomeric transition states is also absent. 

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