Oxovanadium alkoxides

Priebsch, W. and D. Rehder. 1990. Oxovanadium alkoxides Structure, reactivity, and 51-V NMR characteristics. Crystal and molecular structures of VO(OCH2CH2Cl)3 and V0C12(THF)2H20. Inorg. Chem. 29 3013-3019. 

It was claimed that la can be epoxidized with cumene hydroperoxide in the presence of catalytic amounts of [VO(acac)2] to give 98% l,2-epoxy-3-methyl-3-butanol, 3a (at 70% conversion). This epoxide could be separated in 99% purity from the reaction mixture by distillation [22]. Due to the chelating effect of the acetylacetonate ligand, it may be better to use oxovanadium alkoxide complexes. The V 0(0R) moiety has been known for many years [23-25], but only recently has its chemistry attracted significant attention. For example, it was found useful in catalytic systems for the oxidative cleavage of C-C bonds of ketones [26] and... 

Scheme 39 Surface chemistry of molecular oxovanadium alkoxides and chlorides.

Priebsch W., Rehder D. Oxovanadium alkoxides structure, reactivity and vanadium-51 NMR characteristics. Crystal and molecular structures of tris(2-chloroethoxo)oxovanadium and aquadichlorooxobis(tetrahydrofuran) vanadium. Inorg. Chem. 1990 29 3013 Razuvaev GA., Drobotmko V.V. Synthesis and properties of alkoxy derivatives of vanadium R2V(0R )2, J. Organometal. Chem. 1977 131 43 Reuter H., Kremser M. Investigations into tin(IV) alkoxides 1. Crystal and molecular structure of tin(IV) isopropoxide-isopropanol solvate, Sn(0 Pr)4 PrOH. Z. Anorg. AUg. Chem. 1991 598 259... 

Addition of alkyllithium to cyclobutanones and transmetallation with VO(OEt)Cl2 is considered to give a similar alkoxide intermediates, which are converted to either the y-chloroketones 239 or the olefinic ketone 240 depending on the substituent of cyclobutanones. Deprotonation of the cationic species, formed by further oxidation of the radical intermediate, leads to 240. The oxovanadium compound also induces tandem nucleophilic addition of silyl enol ethers and oxidative ring-opening transformation to produce 6-chloro-l,3-diketones and 2-tetrahydrofurylidene ketones. (Scheme 95)... 

Fortunately, the equilibrium of the active catalyst (HPS)VO(OH) and another species (HPS)VO(OR) permits a direct probe of the active species. The addition of ethanol or methanol significantly retards the rate of tribromide formation when the catalyst is derived from (HPS)VO(OEt)(EtOH), although the rate is unchanged (within experimental error) when the catalyst is derived from sodium or ammonium vanadate. This disparity is readily rationalized by considering the coordination environment in the two catalysts. In (HPS)VO(OH), only one site is available to bind peroxide, the equatorial site occupied by hydroxide. The presence of ROH in solution results in a competition between peroxide and alkoxide for the sole available site. The equilibrium between (HPS)VO(OR) and (HPS)VO(02) favors the peroxo species, but addition of 0.13 M methanol (20-fold excess over [H202]) results in a greater than 2-fold decrease in rate. By contrast, oxovanadium(V) has several available coordination sites, as well as the ability to bind two peroxide moieties. The addition of 0.5 M methanol does not cause any change in the rate of tribromide formation. 

One subset of Schiff base complexes are the hydrazone complexes which have a R—C=N—N functional group instead of the R C=N—C functionality. These compounds have been prepared as models for bromoperoxidase389 and other biological systems.390 In the solid state, the hydrazones have coordination patterns similar to those of the Schiff bases and the majority of the complexes are oxovanadium(V) hexadentate or pentacoordinate complexes with tridentate 02N donor sets (see Table 5). While most structures contain alkoxide donors,390-403 complexes have been reported with diols and catechols,129,404,405 hydroxamic acids,288,391 hydroxy quinolinate,406, 7 and benzoylhydrazine.408 In addition, dioxo,375,409-412 oxo-bridged... 

Structures of oxovanadium tris(alkoxides) (triesters of H3VO4 ). In solution, there are concentration-dependent equilibria between monomers and dimers. In the solid state, monomers (bulky R substituents), dimers (R = cyclopentyl) or polymers exist (R = Me).l l... 

Table 3.2 Isotropic and anisotropic NMR parameters for oxovanadium(V) alkoxides.

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