Vanadyl salen complexes

According to a different approach to tailoring a catalyst for the desired ionic character, the catalyst was modified by the attachment of an imidazolium ion. This approach was demonstrated for a vanadyl salen complex, VO(salen) IL (Scheme 14) (187). 

Scheme 9.20 Synthesis of a chiral vanadyl salen complex with an ionic tag...

Vanadyl salen complexes epoxidized cyclohexene presumably through intermediate hydroperoxides formed by radical chain autoxidation <1997105927, 1998TL5923>. Reactions using vanadium complexes have been carried out in liquid carbon dioxide <19990M4916>. 

Vanadyl salen complexes can be used to catalyze cyanosilylation of aldehydes. As a consequence, onium salt supported analogs of these active species have been prepared and compared to other method of immobilization. Unsurprisingly, the OS supported version was much more active (TOF18 Ir1) than the solid supported version but afforded the product in lower enantiomeric excesses than the non-supported salen complex, most certainly due to the detrimental presence of chloride anion [101, 102] (Table 5) ... 

Murphy DM, Fallis lA, Landon J et al (2009) Enantioselective binding of structural epoxide isomers by a chiral vanadyl salen complex a pulsed EPR, CW-ENDOR and DFT investigation. Phys Chem Chem Phys 11 6757-6769... 

The chiral vanadyl salen complex was anchored on mesoporous materials by a covalent grafting method. These heterogenized complex catalysts were evaluated as asymmetric catalysts for the asymmetric oxidation of sulfides to sulfoxides [92]. 

Vanadyl salen provides a model of mixed heteroatom metal coordination characteristic of Ni and VO in the maltenes and asphaltenes. Approximately 50-90% of the metals in petroleum are not contained in the free porphyrin fraction. Yen (1975, 1978) has postulated that these metals exist in a variety of environments such as highly aromatic bound porphyrins, complexed to tetradentates of mixed N, S, and O ligands, or defect sites in large aromatic sheets. Analytical work by Fish et al. (1984) has indicated the presence of metals complexed to salen-type ligands in petroleum. 

Allylic alcohols, for example geraniol, 2-methylallyl alcohol, 3,3-dimethylallyl alcohol, 3-buten-2-ol, l-octen-3-ol, and l-hexen-3-ol, are epoxidized with tert-butyl hydroperoxide in the presence of a vanadyl salen oxo-transfer catalyst in supercritical CO2. The metal catalyst was prepared in a simple two-step, Schiff base-type reaction to form the salen ligand, followed by complexation to the vanadyl group. The use of non-toxic supercritical CO2 in the presence of the new epoxidation vanadium catalyst led to yields and diastereoselectivities that were comparable to those resulting from the use of environmentally hazardous solvents such as CH2CI2 [59]. 

Styryl-fnnctionalized vanadyl(IV) salen complexes (nonchiral and chiral versions) were covalently anchored onto mercapto-modified ACs and SWCNTs... 

The vanadium(IV) complex of salen in zeolite was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butyl hydroperoxide as oxidant.88 Well-characterized vanadyl bis-bipyridine complexes encapsulated in Y zeolite were used as oxidation catalysts.101 Ligation of manganese ions in zeolites with 1,4,7-triazacyclononanes gives rise to a binu-clear complex stabilized by the zeolites but allows oxidation with excellent selectivity (Scheme 7.4). 

Oxo-vanadium derivatives include numerous examples of Schiff base complexes, the importance of which is increasing in that they can act as insulin-enhancing agents.3 For instance, the vanadyl(IV) derivatives of TV.TV -ethylenebisCsalicylideneimine), [VO(salen)], A TV -disalicylidene-o-phenylenediamine, [VO(saloph)], and iV,Ar -ethylenebis(acetylaceto-natoimine), [VO(acen)], are shown in Scheme 1. 

Schiff s base complexes of vanadium were encapsulated in the supercages of zeolite NaY and their catalytic activities were tested in the epoxidation of several alkenes and allylic alcohols with /er/-butylhydroperoxide The complexes investigated were VO(HPS) (vanadyl-N-(2-hydroxyphenyl)salicylideneimine) and VO(salen) (vanadyl-N,N (bis)salicylidene-imine). Particular attention was devoted to the question of leaching of vanadium during reaction. 

Some square pyramidal derivatives of thio-vanadyl, (V=S) +, have also been prepared from the corresponding vanadyl complexes deep magenta [VS(salen)] and [VS(acen)] [H2acen = V,V -ethylenebis(acetylacetonylideneimine)] by... 

Styryl-functionalized vanadyl(lV) salen covalently anchored onto mercapto-modifled ACs and SWCNTs showed high catalytic activity in the cyanosilylation of benzaldehyde with substrate conversion of 83 and 93%, respectively [91]. The SWCNTs were shown to be more suitable support for the VO complex relative to the high-surface-area AC, because the latter support exhibited some adventitious activity. The asymmetric version of the reaction was also performed using the chiral vanadyl complex the SWCNTs also behaved as a better support, as the %ee was 66%, whereas for AC the %ee was 48% [91]. The VO(IV) complex immobilized onto SWCNTs was also tested in the catalytic cyanosilylation of hexanal and 4-fluorobenzaldehyde with high substrate conversions 97 and 96%, respectively. 

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