Venturello anion

A particularly interesting system for the epoxidation of propylene to propylene oxide, working under pseudo-heterogeneous conditions, was reported by Zuwei and coworkers [61]. The catalyst, which was based on the Venturello anion combined with long-chained alkylpyridinium cations, showed unique solubility properties. I11 the presence of hydrogen peroxide the catalyst was fully soluble in the solvent, a 4 3 mixture of toluene and tributyl phosphate, but when no more oxidant was left, the tungsten catalyst precipitated and could simply be removed from the... 

NMR spectroscopy and electrospray mass spectrometry.376 Other research in this area has focused mainly on the catalytic activities of the Venturello anion, [P04 W0(02)2 2]3, 377 and complexes that are structurally related to this. These and other peroxo-containing heteropolyanions that have been reported recently are summarized in Table 26. 

The first examples of an electrostatie attachment of POMs on solid supports are dated to the mid 1980. Baba et al. reported immobilization of heteropolyacids on the anion-exchange resin Amberlist-15 [68]. Later on, this type of the support was successfully used by Jacob s group to immobilize the Venturello complex P04[W0(02)2]4, well known as highly selective homogeneous catalyst for H202-based epoxidation of alkenes... 

Hydrogen peroxide may be efficiently employed for the epoxidation of unfunctionalized aikenes in the presence of polyoxo complexes of tungsten and molybdenum . Venturello and coworkers first developed a phosphotungstate catalyst , with particular emphasis on the tetrakis(diperoxotungsto)phosphate anion P04[W(0)(02)2]4 , 37 , that in conjunction with H2O2 and a lipophilic tetra-alkylammonium salt Q+X as phase transfer... 

The tetra-n-hexylammonium (THA) salt of [P04 W0(02)2 4]3- was isolated and characterized crystallographically by Venturello and coworkers [77]. The anion consisted of the P043 and two [W202(02)4] species. As mentioned above, [P04 W0(02)2 4]3 has been postulated to be a catalytically active species for the H3[P i204 )]/H202 system developed by Ishii and coworkers [35-44] because P04[W0(02)2 4]3 exhibited a very similar catalytic activity and selectivity to those of the Ishii system [77, 78]. 

POMs can he immobilized onto anion-exchange resins and surface-modified metal oxides with quaternary ammonium cation- or amino-functional groups via anion-exchange. Jacobs and coworkers tethered Venturello s catalyst [P04(W0(02)2)4]3-on a commercially available nitrate-form resin with alkylammonium cations and have carried out the epoxidation of allylic alcohols and terpenes with this supported catalyst [166, 167]. The regio- and diastereoselectivity of the parent homogeneous catalysts were preserved in the supported catalyst. For bulky alkenes, the reactivity of the POM catalyst was superior to that of Ti- 3 zeolite with a large pore size. The catalytic activity of the recycled catalyst was maintained completely after several cycles. 

Venturello and co-workers reported that in the presence of aqueous H2O2 a phosphotungstate of the Keggin type gives phosphotungstate anions possessing four W atoms, each one displaying 2 peroxo moieties. 

C. Venturello, R. D Aloisio, J. C. J. Bart, M. Ricci, A new peroxotungsten heteropoly anion with special oxidizing properties synthesis and structure of tetrahexylammonium tetra(diperoxotungsto)phosphate(3-), J. Mol. Catal. 32,107-110(1985). 

The group of Ishi described a similar system to that of Venturello consisting of H3[PWi204o] in combination with H2O2 and a PTR [147,148]. However, it has been demonstrated that the two catalyst systems involve the oxoperoxo anion [PO4 WO(O2)2 4] [149,150] derived from polyoxometallates. 

Tungsten-based epoxidation systems with H2O2 have attracted much attention because of their high reactivity compared with molybdenum analogues and inherently poor activity for the decomposition of H2O2. The tetranuclear perox-otungstate 1 was isolated and characterized crystallographically by Venturello and coworkers [26,27]. The anion 1 consists of the FO4 anion and two... 

Venturello, C., D Aloisio, R., Bart, J., et al. (1985). A New Peroxotungsten Heteropoly Anion with Special Oxidizing Properties Synthesis and Structure of Tetrahexylammoium Tetra(diperoxotungsto)phosphate (3-), J. Mol. Catal, 32, pp. 107-110. 

P-Elimination-coupled to in situ metalation opens an entry also to halogen-free organometallic intermediates. For example, the I-ethoxy-1,3-butadiene emerging from a 1,4-elimination of ethanol from the 2-butenal diethyl acetal reacts rapidly in an ensuing step with 5ec-butyllithium and potassium tert-butoxide to give 1-ethoxy-1,3-butadienyl-potassium/lithium (242), a synthetic equivalent of the 2-butenoyl ("crotonoyl") anion (Scheme 1-187). Numerous applications of such sequences to organic preparations have been reported by Paolo Venturello et... 

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