Catalysis of Organic..

18Supported Re Catalysts for Olefin MetathesisThis material (167 mg) was treated with 1 mL of a CDCl 3 solution of SnMe 4(0.17 mM) at room temperature. Analysis of the supernatant by 1 H NMRspectroscopy revealed a singlet at 2.63 ppm for MeReO 3 , in addition to a singlet at0.081 ppm with 117 Sn and 119 Sn satellites, assigned to unreacted SnMe 4 . Thisexperiment demonstrates that Me 4 Sn generates MeReO 3 from grafted perrhenatesites, as predicted by Scheme 2.Reaction of perrhenate/silica-alumina with SnMe 4 . Silica-alumina is neither anadmixture of silica and alumina, nor a poorly ordered aluminosilicate, but a solidsolution that possesses both strong Lewis acidity and strong Brønsted acidity withoutbridging hydroxyls. Therefore, our computational model for silica-alumina consistsof a silsesquioxane monosilanol cube in which one corner has been replaced byaluminum, to reproduce the Lewis acidity of the support. This substitution alsoenhances the acidity of the adjacent terminal silanol, creating Brønsted acidity.Perrhenate grafted to the aluminosilsesquioxane cube is shown in Scheme 3. Incontrast to the simple C 3v symmetry of the perrhenate/silsesquioxane model (Scheme2), the optimized structure obtained for silica-alumina adopts a lower symmetry inorder to accommodate a Lewis acid-base interaction between an oxo ligand of thegrafted perrhenate fragment and the Al center.SnSnMe 4COSiReAl≡SiORe(O) 2 OAl- 43 kJ/molSnSiAlReCScheme 3 Reaction of perrhenate on aluminosilsesquioxane cube (model for silicaalumina)with SnMe 4 is predicted to form grafted MeReO 3.Transmetalation of the perrhenate/aluminosilsesquioxane cube model withSnMe 4 is considerably more exothermic than for the perrhenate/silsesquioxane cubemodel. A similar grafted trimethyltin fragment is formed, as is MeReO 3 ; however,the latter is not liberated. It remains bound to the aluminosilsesquioxane cube via theLewis acid-base interaction with the Al center. The optimized structure also containsa Lewis acid-base interaction between Re and an adjacent framework oxygen