Catalytic epoxidation by heteropolyoxoperoxo complexes: From novel precursors or catalysts to a mechanistic approach

In the course of systematic investigations, both the 'H2WO4/H2O2-H2O/H3PO4' and 'H-3[PW12O40] . yH(2)O/H2O- H2O/H3PO4' systems are found to show several P-31 and W-183 NMR signals which can be assigned to new phosphatoox-operoxotungstate species, [PWxOy](z-) (x=1-4). It is clear that in the oxidations, the Keggin heteropolyanion is only a precursor to the true catalyst. The addition of Q(+)Cl(-) (an appropriate onium salt) to these systems leads to the isolation of pure crystalline salts: Q(3)[PO4{WO(O-2)(2)}(4)], 1, and Q(2)[HPO4{WO(O-2)(2)}(2)], 2. The solid state structure of [(n-C4H9)(4)N](2)[HPO4{WO(O-2)(2)}(2)] is known. Synthesis and structural investigation of complexes with other assembling anions were also reported. Some of these complexes are: Q(3)[PO4{MoO(O-2)(2)}(4)], 3, Q(2)[SO4{MoO(O-2)(2)}(2)], 4, Q(3)[AsO4{WO(O-2)(2)}(4)], 5, Q(2)[HAsO4{WO(O-2)(2)}(2)], 6, Q(2)[CH3AsO3{WO(O-2)(2)}(2)] . H2O, 7, 2Q(3)[(HSO4)(SO4){W3O6(O-2)(3)}] . 7H(2)O, 8, Q(2)[W4O6(O-2)(6)(CH3OH)(2)(CH3O)(2)], 9, etc. Some of these polyanions (1 to 7) have one or two neutral [M(2)O(2)(mu-O-2)(2)(O-2)(2)] moieties with one bridging and one non-bridging peroxo group on each W (or Mo) center. Infrared and/or Raman and and W-183 NMR spectra show that the structures of the anions are conserved in organic solvents at room temperature. There is also clear evidence for the formation of [(PO4){Mo4-xWxO20}](3-) in the reaction of [PMo4O24](3-) and [PW4O24](3-); novel mixed compounds can be isolated. The salts with the [M(2)O(2)(mu-O-2)(2)(O-2)(2)] moiety, in particular with M = W, are among the most efficient for the transfer of 'active oxygen' to olefinic substrates. (R)-(+)-limonene is stoichiometrically epoxidized: only half the peroxide oxygen is transferred to the olefinic substrate at room temperature. The results show an effect of the assembling Ligand in stoichiometric systems and in phase transfer catalytic systems. Extended Huckel molecular orbital calculations have been used to support an active oxygen-to-olefin transfer mechanism. The best mechanistic model involves direct interaction of one olefinic carbon atom with an oxygen atom of the peroxo groups without binding of the alkene to the transition metal center.