Kinetic studies of olefin epoxidation with hydrogen peroxide in 1,1,1,3,3,3-hexafluoro-2-propanol reveal a crucial catalytic role for solvent clusters

The epoxidation of cyclooctene and 1-octene with hydrogen peroxide was studied kinetically in HFIP (1,1,1,3,3,3-hexafluoro-2-propanol)/1,4-dioxane mixtures. In the case of cyclooctene, no additional catalyst was applied, whereas the epoxidation of 1-octene was run in the presence of phenylarsonic acid as catalyst. For both reaction types, two kinetic regimes can be distinguished: at low HFIP concentration (n(HFIP)/n(total) less than or equal to 0.15), both the catalyzed and the uncatalyzed reaction show first order dependence on the HFIP concentration. This result is interpreted by the HFIP "charge template" for the uncatalyzed epoxidation (as postulated by Shaik and Neumann) and by the formation of an arsonic acid mono-HFIP ester for the catalyzed reaction. At high HFIP concentration (n(HFIP)/n(total) greater than or equal to 0.5), both reaction types are up to ca. 5 orders of magnitude faster and show ca. 12(th), order dependence on the HFIP concentration. We propose that the dramatic accelerations observed at high HFIP concentrations are brought about by HFIP clusters. Based on literature data (Fioroni, Roccatano, Hong, Suhm), it is concluded that the epoxidation reactions studied here take place in a HFIP coordination sphere comprising ca. 12 HFIP molecules, and that this coordination sphere effects the enormous increases in epoxidation rates. This mechanistic model bears resemblance to enzymatic reactions taking place in and being catalyzed by a protein matrix.