Interaction of Water, Alkyl Hydroperoxide, and Allylic Alcohol with a Single-Site Homogeneous Ti−Si Epoxidation Catalyst: A Spectroscopic and Computational Study†
journal contributionposted on 17.02.2005, 00:00 by Atsushi Urakawa, Thomas Bürgi, Peter Skrabal, Felix Bangerter, Alfons Baiker
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.