DFT investigation of the catalytic hydromethylation of olefins by scandocenes. 2. Influence of the ansa ligand on propene and isobutene hydromethylation

Density functional theory (B3PW91) calculations have been carried out for the hydromethylation of propene and isobutene catalyzed by ansa-Me2Si(C5Me4)(2)ScCH, as modeled by H2Si(C5H4)(2)ScCH3. The calculations show that the hydromethylation of isobutene, but not propene, could be catalytic, and the difference may be attributed to a combination of electronic and steric factors. Several side reactions have also been considered. The catalytic cycle is completed only for cases in which both the olefin and methane can interact with the Lewis acidic metal center in separate steps. Olefins have greater coordination strengths than methane unless steric factors dominate. In the case of the ansa complex, which is more Lewis acidic and slightly more open than a nonansa complex, calculations show that the strong binding of propene blocks the catalytic cycle. This is not the case for the more bulky isobutene, which is a poorer ligand and thus allows competitive access of methane to the metal center.