Kinetic studies on phenylphosphopolyperoxotungstates catalyzed epoxidation of olefins with hydrogen peroxide

✍ Scribed by Ahmad M. Al-Ajlouni; Özgül Sağlam; Tariq Diafla; Fritz E. Kühn

• Publisher: Elsevier Science
• Year: 2008
• Tongue: English
• Weight: 476 KB
• Volume: 287
• Category: Article
• ISSN: 1381-1169
• DOI: 10.1016/j.molcata.2008.03.005

✦ Synopsis

Kinetic studies on the catalytic activity of phenylphosphopolyperoxotungstate complexes

in the epoxidation of olefins with hydrogen peroxide as the oxidizing agent were carried out in CH 3 CN/H 2 O solvent mixtures. The nitro-substituted catalyst, B, was shown to be more active than A. However, the higher acidity of B enhances more epoxide ring-opening to yield 1,2-diol. Under biphasic conditions, the epoxide ring-opening is reduced, and in most cases the 1,2-diol products do not form. Under homogeneous conditions, the reaction is first-order with respect to the catalyst and the olefin, and between zero-and first-order with respect to the H 2 O 2 concentration. The rate of the reaction increases with the olefin nucleophilicity. The epoxidation rate constants of para and meta substituted styrenes are correlated with + according to the Hammett equation. The reaction constants ( ∼ -0.9) and the reaction stereoselectivity suggest a mechanism, which involves an external nucleophilic attack of the -system of the olefin on the electropositive oxygen of the W-peroxo group. A catalytic cycle involving a fast (pre-equilibrium) reaction between H 2 O 2 and the catalyst followed by the oxygen-transfer step (rate-determining) is proposed.