Vanadium complexes oxygen ligands

Reactions of these ligands have not been studied in aqueous solution. However, their complexes are readily synthesized and are stable but reactive towards heteroligands [41,42], The reported structures all show the vanadium coordinated in monomeric units after the fashion depicted in Scheme 4.9. The multidentate thiolato complexes with tri- or tetradentate functionality are sufficient to satisfy the coordination requirements of the vanadium nucleus. Structurally, the compounds are not much different from analogous complexes formed with oxygen ligands (Section 4.4.2). 

Asymmetric sulfoxidation with chiral vanadium complexes is much older than Ti and A1 because this metal provides more robust catalysts which are not deactivated by the presence of water. The first contribution to the field was made by Bolm and co-workers in the second half of 1990s. These authors developed chiral catalysts formed in situ by the reaction of VO(acac)2 with chiral enantiopure Schiff ligands bearing one stereocenter based on a t-leucinol scaffold." The maximum ee achieved was 85% using low catalyst loadings (<1% mol) and without any precautions to avoid moisture or oxygen, which was unusual at that time as hydroperoxides... 

Hu and coworker developed the silica, titania, ceria, and y-alumina supported vanadium catalyst, which shown unique reactivity for the direct amination of toluene with hydroxylamine hydrochloride, over 50 % total yield of toluidines was obtained on the y-alumina supported vanadium catalyst operated at optimal conditions [87]. They also investigated a sodium metavanadate catalyzed one-step amination of benzene to aniline with hydroxylamine [88], The reaction became more efficient in the presence of oxygen. A free-radical mechanism was proposed based on the results of EPR, NMR, and UV-Vis characterization. Moreover, the catalytic activity of a series of vanadium complexes with N,0- or 0,0-ligands for the liquid-phase direct amination of benzene with hydroxylamine to aniline has been investigated [89]. [VO(OAc>2] was proved to be the most active catalyst for the amination because of its relatively greater electrophilicity and smaller steric hindrance of ligand. 

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