Zirconium triflates, preparation

Although in the recent years the stereochemical control of aldol condensations has reached a level of efficiency which allows enantioselective syntheses of very complex compounds containing many asymmetric centres, the situation is still far from what one would consider "ideal". In the first place, the requirement of a substituent at the a-position of the enolate in order to achieve good stereoselection is a limitation which, however, can be overcome by using temporary bulky groups (such as alkylthio ethers, for instance). On the other hand, the ( )-enolates, which are necessary for the preparation of 2,3-anti aldols, are not so easily prepared as the (Z)-enolates and furthermore, they do not show selectivities as good as in the case of the (Z)-enolates. Finally, although elements other than boron -such as zirconium [30] and titanium [31]- have been also used succesfully much work remains to be done in the area of catalysis. In this context, the work of Mukaiyama and Kobayashi [32a,b,c] on asymmetric aldol reactions of silyl enol ethers with aldehydes promoted by tributyltin fluoride and a chiral diamine coordinated to tin(II) triflate... 

The aim of this contribution is to present data on the preparation of catalysts containing as embedding species a large family of eolloids such as colloids of ruthenium, platinum, or palladium-gold alloys and triflate derivatives such as lanthanum and silver triflate or tert-butyldimethylsilyltrifluoromethanesulfonate (BMSTM). Silica, zirconia and tantalum oxides were used as carrier. All these preparations considered the polymeric sol-gel route using as starting materials silicon, zirconium or tantalum alcoxides. 

Hafnium and zirconium mononitrate tris(triflate), [M(H20)x(N03)](0Tf)35 were prepared from their tetrachlorides in analogous fashion to the lanthanide salts. Much to our delight these (deliquescent) salts displayed A, nitrate stretching frequencies in their IR spectra at 1651 and 1650 cm1 respectively. Armed with this pleasing information and with a specific programme aim of nitrating o-nitrotoluene (ONT) to dinitrotoluenes (DNTs) with these catalyst types, (catalytic quantities of ytterbium(III) triflate were essentially ineffective for this transformation) hafnium(IV) and zirconium(IV) triflate were prepared as their hydrated salts via metathesis of their tetrachlorides with silver triflate in water. 

The behavior of zirconium is somehow close to that of titanium. However, the preparation of zirconium inflates does not require the additional presence of organic ligands to afford the stability against moisture. gem-Disubstituted alkenes were directly and readily etherified with alcohols using Zr(OTf)4 as recyclable Lewis acid catalysts (Equation (8.17)) [38]. Primary or secondary alkene analogues were inert under these conditions, enabling excellent chemose-lectivity. It is worth no notice that other triflates, as lanthanide triflates, for example, are inactive as catalysts for such reactions. 

Metal alkoxides of silicon, titanium, zirconium, and aluminum have been used to prepare optically transparent composite films with the etheric phosphazene polymer poly[bis(2-(2-methoxyethoxy)ethoxy)-phosphazene] (MEEP). Dynamic mechanical and stress-strain techniques are used to evaluate and compare the mechanical properties of the different composites. Despite strong interactions with the inorganic oxide network, the glass transition temperature (Tg) of MEEP does not shift to higher temperature upon addition of the inorganics, but the polymer is reinforced by the oxides. The addition of potassium triflate to the composites resulted in materials with both excellent mechanical properties and increased conductivity. 

---