Vanadium-catalyzed

Figure 6.6 Chiral ligands employed in the vanadium-catalyzed AE of allylic alcohols.

Great interest in vanadium-catalyzed AE has been shown in recent years, and many promising results continue to appear [35]. Some of the most successful complexes to date are based on the ligands shown in Figure 6.6. A comparison of their catalytic activities is presented in Table 6.4. 

Keywords hefera-Diels-Alder reaction, cooperativity of chirality, vanadium-catalyzed... 

The mechanism by which the enantioselective oxidation occurs is generally similar to that for the vanadium-catalyzed oxidations. The allylic alcohol serves to coordinate the substrate to titanium. The tartrate esters are also coordinated at titanium, creating a chiral environment. The active catalyst is believed to be a dimeric species, and the mechanism involves rapid exchange of the allylic alcohol and /-butylhydroperoxide at the titanium ion. 

The 1,5- and 1,6-dialdehydes 22 and 24 undergo the annulative pinacol coupling to give the cyclic vzc-diols 23 and 25, respectively (Scheme 13) [29]. The vanadium-catalyzed intramolecular coupling reaction of 1,5-diketone 26 also proceeds with excellent selectivity (Scheme 14) although the intermolecular coupling of ketones such as acetophenone results in low diastereoselectivity under these conditions [21]. 

Cyclodimerization is observed in the vanadium-catalyzed reaction of the arylidene malononitriles 39 in the presence of MesSiCl, giving 40 diastereos-electively as shown in Scheme 23 [59]. 

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... 

Recently, Trost et al. reported the vanadium-catalyzed addition reaction of 2,3-allenols [180], Here the oxygen in 401 served as an intramolecular nucleophile to attack the center carbon atom of allene to form a vanadium enolate 402. Aldol condensation of 402 with an aldehyde afforded (2-hydroxy)alkyl vinylic ketones 403. 

Scheme 1.25 Vanadium-catalyzed racemization of allylic alcohol.

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... 

SCHEME 142. Vanadium-catalyzed oxidation of aromatic compounds using H2O2... 

Vanadium-catalyzed hydrocarbon oxidation with peroxides can be carried out also by supporting the catalyst with the appropriate ligand on polymers " , on sUica " or encapsulating it in zeolites ". Similar activity has been obtained with vanadium-containing... 

Vanadium-catalyzed hydroxylation of benzene and cyclohexane has also been obtained with in situ generation of hydrogen peroxide from H2/O2 in the presence of palladium. A similar process has been settled for methane oxygenation to methyl trifluoroacetate and formic acid. Monoperoxovanadate, as well as copper hydroperoxides, have been indicated as the active species for the activation of the C—H bond of methane. 

BUTYLPHENYL)METHYLENE]IMINE] AS A LIGAND FOR VANADIUM-CATALYZED ASYMMETRIC OXIDATION OF SULHDES. 293... 

BENZYL-4,6-O-ISOPROPYLIDENE-a-D-GLUCOPYRANOSIDE, 2-DEOXY-2-[[(2-HYDROXY-3,5-di-lerl-BUTYLPHENYL)METHYLENE]IMINE] AS A LIGAND FOR VANADIUM-CATALYZED ASYMMETRIC OXIDATION OF SULFIDES... 

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