Vanadium-catalyzed Epoxidations

The phenomenon that early transition metals in combination with alkyl hydroperoxides could participate in olefin epoxidation was discovered in the early 1970s [30, 31]. While m-CPBA was known to oxidize more reactive isolated olefins, it was discovered that allylic alcohols were oxidized to the corresponding epoxides at the same rate or even faster than a simple double bond when Vv or MoVI catalysts were employed in the reaction [Eq. (2)] [30]. 

While the titanium-catalyzed epoxidation reaction was flourishing, the pioneer version of the reaction remained undeveloped and it was not until 1999 that new 

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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 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 . 

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,... 

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. 

Peracid epoxidation C strongly favored diastereomeric transition state D unfavored diastereomeric transition state vanadium-catalyzed epoxidation E favored diastereomeric transition state. 

Although these reactions obviously involve intramolecular oxygen transfer within a titanium(IV) tartrate-ally lie alcohol-alkyl hydroperoxide complex, analogous to the vanadium-catalyzed epoxidations discussed above, the exact nature of the catalytic species and the mechanism of enantioselection remain controversial [39, 40],... 

Similar results were observed with vanadium catalyzed epoxidation of cyclohexene (1) and limonene (2) with TBHP and PHP (Table 2). Epoxidations with TBHP gave reasonable conversions and... 

Table 2. Vanadium catalyzed epoxidation with TBHP selectivities while...

Oxidations. Ketones are cleaved to afford carboxylic acids in moderate yields by ReiOv-t-BuOOH in acetic acid at 100°. Vanadium-catalyzed epoxidation of unsaturated alcohols in liquid carbon dioxide is feasible.- A resin-bound cobalt-phosphine complex is also effective as a catalyst for oxidation. ... 

Ziegler, F.E., and B.B. Jaynes Rearrangement of a Hindered Allylic Alcohol During Vanadium-Catalyzed Epoxidation. A Short Synthesis of Uvidin C. Tetrahedron Letters, 26, 5875 (1985). 

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