Stoichiometry and Kinetics of Gas Phase Cyclohexene Epoxidation by a Silica-Supported tert- Butylperoxidititanium Complex

Peroxo and alkylperoxo metal complexes are presumed intermediates in the homogeneous and heterogeneous catalytic epoxidations of olefins. Both solid and soluble versions containing high valent, d° transition metal ions activate peroxides heterolytically, thereby facilitating oxygen atom transfer to electron-rich substrates (1). However, mechanistic studies have been undertaken largely with the more readily characterized soluble catalysts (2-4). For heterogeneous catalysts, the fraction of metal sites which participate in selective oxidation is generally unknown. Although measurements and comparisons of overall activity and selectivity are common (5), it is not generally possible to distinguish between mechanisms or measure elementary rate constants for individual sites. Titanium alkoxides are effective homogeneous catalysts for the epoxidation of substituted olefins. Their propensity for association to multinuclear species is also well-established (6). The active form of a homogeneous, enantioselective titanium-tartrate catalyst was demonstrated to be dinuclear in titanium (7). In contrast, heterogeneous catalysts consisting of titanium embedded in an aluminosilicate framework contain mostly isolated titanium sites (8), although the assertion that such sites are uniquely responsible for catalyst activity is based on