Ti-bridged silsesquioxanes as precursors of silica-supported titanium oxide catalysts for the epoxidation of cyclooctene

Silica-supported titanium oxide catalysts were prepared by the controlled calcination of Ti-bridged polyhedral oligomeric silsesquioxanes (POSS) bearing four Si–O–Ti bonds, Ti[(c-C5H9)7Si7O11(OSiMe2R)]2 (1a; R = vinyl, 1b; R = methyl) and Ti[(c-C5H9)8Si8O13)]2 (2), supported on various silicas, and these catalysts showed excellent activities for the epoxidation of cyclooctene by tBuOOH as an oxidant to selectively give cyclooctene oxide in high yields. On the other hand, catalysts prepared using a corner-capped-type Ti-containing POSS, CpTi[(c-C5H9)7Si7O12] (3, Cp = cyclopentadienyl), or tetraisopropoxytitanium (4) showed lower activities. The catalysts prepared from Ti-bridged POSS can be easily separated from the reaction mixture and reused without a significant loss of activity. These catalysts were also applicable for the epoxidation of limonene. A spectroscopic study revealed that calcination of Ti-bridged POSS supported on silica gave isolated tetrahedral Ti(IV) species, which would be responsible for the high activity, while calcination of silica-supported 3 or 4 gave aggregated surface titanium oxide species.