title: Interaction of water, alkyl hydroperoxide, and allylic alcohol with a single-site homogeneous Ti-Si epoxidation catalyst: A spectroscopic and computational study.
abstract: Tetrakis(trimethylsiloxy)titanium (TTMST, Ti(OSiMe3)4) possesses an isolated Ti center and is a highly active homogeneous catalyst in epoxidation of various olefins. The structure of TTMST resembles that of the active sites in some heterogeneous Ti-Si epoxidation catalysts, especially silylated titania-silica mixed oxides. Water cleaves the Ti-O-Si bond and deactivates the catalyst. An alkyl hydroperoxide, TBHP (tert-butyl hydroperoxide), does not cleave the Ti-O-Si bond, but interacts via weak hydrogen-bonding as supported by NMR, DOSY, IR, and computational studies. ATR-IR spectroscopy combined with computational investigations shows that more than one, that is, up to four, TBHP can undergo hydrogen-bonding with TTMST, leading to the activation of the O-O bond of TBHP. The greater the number of TBHP molecules that form hydrogen bonds to TTMST, the more electrophilic the O-O bond becomes, and the more active the complex is for epoxidation. An allylic alcohol, 2-cyclohexen-1-ol, does not interact strongly with TTMST, but the interaction is prominent when it interacts with the TTMST-TBHP complex. On the basis of the experimental and theoretical findings, a hydrogen-bond-assisted epoxidation mechanism of TTMST is suggested.
authors.affiliation: Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, ETH-Hönggerberg, CH-8093 Zürich, Switzerland.
authors.forename: Atsushi
authors.initials: A
authors.lastname: Urakawa
authors.forename: Thomas
authors.initials: T
authors.lastname: Bürgi
authors.forename: Peter
authors.initials: P
authors.lastname: Skrabal
authors.forename: Felix
authors.initials: F
authors.lastname: Bangerter
authors.forename: Alfons
authors.initials: A
authors.lastname: Baiker