Oxo-titanium species

Mikami and co-workers16-19 have done extensive work for developing catalysts for the asymmetric carbonyl-ene reaction. Excellent enantioselectivites are accessible with the binol-titanium catalyst 17 (Equation (10)) for the condensation of 2-methyl butadiene (R1 = vinyl) and glyoxalates (binol = l,T-binaphthalene-2,2 -diol).16 The products were further manipulated toward the total synthesis of (i )-(-)-ipsdienol. The oxo-titanium species 18 also provides excellent enantioselectivity in the coupling of a-methyl styrene with methyl glyoxalate.17 Reasonable yields and good enantioselectivites are also obtained when the catalyst 19 is formed in situ from titanium isopropoxide and the binol and biphenol derivatives.18... 

Although the identification of tetrahedrally coordinated, tetra- and tripodal Ti4+ ions on the surface of titanosilicates, as the likely active sites in reactions that require Lewis acidity, seems convincing, the structure and role of the sites active in catalytic oxidation, presumably oxo-titanium species, formed by the interaction of H202 (or H2 + 02) with these surface Ti ions, are not clear. In recent years, this problem has been investigated by FTIR (133), Raman (39,40), XANES (46-48), electronic (54-57), and EPR (51-54) spectroscopies. This is one of the areas in which major progress has been made since the reviews of Notari (33) and Vayssilov (34). Zecchina et al. (153) recently summarized some of the salient features of this progress. 

The rate also decreases with an increase in the chain length of the alkene molecule (hex-l-ene > oct-1-ene > dodec-l-ene). Although the latter phenomenon is attributed mainly to diffusion constraints for longer molecules in the MFI pores, the former (enhanced reactivity of terminal alkenes) is interesting, especially because the reactivity in epoxidations by organometallic complexes in solution is usually determined by the electron density at the double bond, which increases with alkyl substitution. On this basis, hex-3-ene and hex-2-ene would be expected to be more reactive than the terminal alkene hex-l-ene. The reverse sequence shown in Table XIV is a consequence of the steric hindrance in the neighborhood of the double bond, which hinders adsorption on the electrophilic oxo-titanium species on the surface. This observation highlights the fact that in reactions catalyzed by solids, adsorption constraints are superimposed on the inherent reactivity features of the chemical reaction as well as the diffiisional constraints. 

Appendix B. Characteristics of the Oxo-Titanium Species Generated on TS-1 on Contact with Aqueous H202... 

D. Srinivas, P. Manikandan, S. C. Laha, R. Kumar, and P Ratnasamy, Reactive oxo-titanium species in titanosilicate molecular sieves EPR investigations and structure—Activity correlations, J. Catal. 217, 160-171 (2003). 

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