Alcohols vanadium-catalyzed epoxidation

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,... 

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],... 

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). 

Mihelich ED, Daniels K, Eickhoff DJ (1981) Vanadium-catalyzed Epoxidations Highly Stereoselective Epoxidations of Acyclic Homoallylic Alcohols Predicted by a Detailed Transition-state Model. J Am Chem Soc 103 7690... 

In contrast to the results obtained with simple olefins, olefins containing alcohol functionality were epoxidized much more rapidly in the presence of vanadium complexes than with molybdenum [409,410]. The efficiency of the vanadium catalyzed epoxidation of allyl alcohol has been rationalized on the basis of an intermediate complex having a geometry which places the electron-deficient oxygen of the hydroperoxide in the vicinity of the double bond, equation (265). 

Thus, vanadium complexes preferentially epoxidize small amounts of allylic oxygen species formed in situ to give epoxy alcohols whereas molybdenum complexes catalyze epoxidation of the excess of unreacted olefin to give epoxides. The mechanism of vanadium catalyzed epoxidation of allylic alcohols has been discussed in an earlier section. 

DR Pesiri, DK Morita, T Walker, W Tumas. Vanadium-catalyzed epoxidations of oleflnic alcohols in liquid carbon dioxide. Organometallics 18 4916-4924, 1999. 

The development of Sharpless model was the result of a systematic investigation of the epoxidation reaction of a wide range of acyclic allylic alcohols [65], Two illustrative examples are shown below in Schemes 9.2 and 9.3. In the vanadium-catalyzed epoxidation of olefin 30, transition state 31 is believed to be favored. It includes an acute 0-C-C=C dihedral angle of 50° while at the same time, the dominant interactions are minimized (Scheme 9.2). In the epoxidation of 33 with a peracid, structure 34 incorporates an obtuse 0-C-C=C angle and concomitantly minimizes severe non-bonded A, j interactions (Scheme 9.3). 

Van Tamelen showcased the ability of homoallylic alcohols to bias the stereochemical course of epoxidations in investigations of the biosynthesis of sterol precursors (Equation 11) [68]. The polyunsaturated substrate 39 readily participated in a vanadium-catalyzed epoxidation of the 1,1-disubsti-tuted olefin with complete regio- and stereoselectivity. Monoepoxide 40 was isolated as the only detectable product in quantitative yield. 

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

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... 

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