Catalyst Design for Alkene Epoxidation by Molecular Analogues of Heterogeneous Titanium-Silicalite Catalysts

The
epoxidation of allylic alcohols with H2O2 catalyzed
by the hybrid [α-B-SbW9O33(tBuSiO)3Ti­(OiPr)]3– (1) anion as a molecular model of heterogeneous
Ti-silicalite TS-1 catalyst was analyzed by means of DFT to determine
the main factors that control the catalytic process and, finally,
to improve the value of the available catalysts. Our calculations
revealed that unlike other alkenes, allylic alcohols can bind the
Ti center after activation of the precatalyst via hydrolysis to give
the corresponding Ti-alcoholate, which is the catalyst resting state.
Next, the dissociative addition of hydrogen peroxide to Ti causes
the cleavage of a Ti–OSi junction to form a Ti­(η2-OOH) moiety. The partial detachment of the Ti from the catalyst
structure yields an intermediate with a flexible Ti center from which
the Ti-OOH group can transfer an electrophilic oxygen to the alkene
substrate in an inner-sphere fashion. The rate-determining
process, which involves the heterolytic activation of H2O2 over the Ti­(IV) and the electrophilic O-transfer, accounts
for an overall free-energy barrier of 23.0 kcal mol–1 for 2-methyl-2-buten-1-ol, in line with the experimental value of
22.3. Conversely, the outer-sphere O-transferalso accessible
to nonfunctionalized alkenesoccurs through a more strained
transition state that lays above in energy (by ∼4 kcal mol–1), giving a clue to explain the low yields reported
experimentally for nonfunctionalized olefins. We also found that reducing
the bulkiness of the substituents in the silanol
functions of the catalyst has a positive influence on the catalytic
activity, decreasing the overall free-energy barriers for the outer-sphere
path. With this knowledge, we developed other catalytic species with
tailored steric properties based on [SbW9O33(RSiOH)3]3– structure
(R =iPr
and nPr), which were synthesized, characterized,
and successfully applied to the catalytic epoxidation of unfunctionalized
alkenes. Present results clearly show that the detailed knowledge
of the reaction mechanisms, even for complex processes, is possible
nowadays and that the acquired information allows designing catalysts
with desired activities.