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

Abstract

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 X0.15), 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 x0.5), 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.

The design of organic catalysis for epoxidation by hydrogen peroxide

“…Since this reaction almost certainly involves nucleophilic attack at the carbonyl group of the carboxylic acid, equilibration is much slower for acetic and other simple carboxylic acids [20], although it could be faster for strong acids like CF 3 CO 2 H. The slow rates for this equilibration rule out its incorporation into a viable catalytic cycle. Most discussions of the promotion of hydrogen peroxide reactions by acids concentrate on the formation of new peroxidic species [21], rather than the proton transfer process proposed here, although there have been several recent reports of electrophilic activation of hydrogen peroxide by perfluorinated alcohols [22–24] and phenol [25]. In the absence of strong acids, it is likely that this activation involves clusters of strongly hydrogen-bonded solvent molecules, as suggested by Berkessel for catalysis by HFIP (1,1,1,3,3,3-hexafluoro-2-propanol) [24].…”

“…Most discussions of the promotion of hydrogen peroxide reactions by acids concentrate on the formation of new peroxidic species [21], rather than the proton transfer process proposed here, although there have been several recent reports of electrophilic activation of hydrogen peroxide by perfluorinated alcohols [22–24] and phenol [25]. In the absence of strong acids, it is likely that this activation involves clusters of strongly hydrogen-bonded solvent molecules, as suggested by Berkessel for catalysis by HFIP (1,1,1,3,3,3-hexafluoro-2-propanol) [24]. …”

Kinetic Solvent Effects in Organic Reactions

“…These studies together show for some reaction families, hydrogen bonding with solvent is more important than polarity to explain kinetic solvent effect. However, treating the solvent explicitly is occasionally necessary to deduce these effects, as was the case of HFIP vs. methanol 24,75,76 . kcal/mol with two water molecules (Figure 12c), indicating that the transition state is more stabilized by the solvent than in the stepwise mechanism.…”

“…The investigations together illustrate that continuum solvation models can be sufficient to explain trends in solvation kinetics, and also that reaction rate can vary with solvent polarity alone. The epoxidation of olefins ( Figure 10) by hydrogen peroxide was studied both experimentally and with DFT by Berksessel and co-workers 24,75,76 . For these reactions, using fluorinated alcohols such as hexafluoroisopropanol (HFIP) accelerates the rate in relation to 1,4-dioxane 75,76 .…”

“…The epoxidation of olefins ( Figure 10) by hydrogen peroxide was studied both experimentally and with DFT by Berksessel and co-workers 24,75,76 . For these reactions, using fluorinated alcohols such as hexafluoroisopropanol (HFIP) accelerates the rate in relation to 1,4-dioxane 75,76 . From 0-4 molecules of HFIP were studied with the reactants quantum mechanically in the gas-phase, and then the whole system was treated with PCM 24 .…”