Interaction of Water, Alkyl Hydroperoxide, and Allylic Alcohol with a Single-Site Homogeneous Ti−Si Epoxidation Catalyst:  A Spectroscopic and Computational Study

Abstract

Tetrakis(trimethylsiloxy)titanium (TTMST, Ti(OSiMe3)4) possesses an isolated Ti center and is a highly active homogeneous catalyst in epoxidation of various olefins. The structure of TTMST resembles that of the active sites in some heterogeneous Ti-Si epoxidation catalysts, especially silylated titania-silica mixed oxides. Water cleaves the Ti-O-Si bond and deactivates the catalyst. An alkyl hydroperoxide, TBHP (tert-butyl hydroperoxide), does not cleave the Ti-O-Si bond, but interacts via weak hydrogen-bonding as supported by NMR, DOSY, IR, and computational studies. ATR-IR spectroscopy combined with computational investigations shows that more than one, that is, up to four, TBHP can undergo hydrogen-bonding with TTMST, leading to the activation of the O-O bond of TBHP. The greater the number of TBHP molecules that form hydrogen bonds to TTMST, the more electrophilic the O-O bond becomes, and the more active the complex is for epoxidation. An allylic alcohol, 2-cyclohexen-1-ol, does not interact strongly with TTMST, but the interaction is prominent when it interacts with the TTMST-TBHP complex. On the basis of the experimental and theoretical findings, a hydrogen-bond-assisted epoxidation mechanism of TTMST is suggested.

Catalytic consequences of open and closed grafted Al(III)-calix[4]arene complexes for hydride and oxo transfer reactions

“…The comparable activation energies when using either open or closed catalyst active sites reinforce the direct mechanism of Lewis acid-catalyzed epoxidation, which is supported by the results of experiments using heterogeneous catalysts 6 and 9 above. Such a result is also consistent with the mechanism of ethylene epoxidation catalysis that is observed when using tert-butylhydroperoxide as oxidant and Ti(OSi(CH 3 ) 3 ) 4 as soluble catalyst by Urakawa and coworkers (29), as well as the original DFT studies by Vayssilov and van Santen on TS-1-catalyzed epoxidation of ethylene, which found that the barrier for oxo transfer via deprotonation of hydroperoxide is higher by 1.7 kcal/mol compared with the mechanism with nondissociatively bound hydroperoxide (28).…”

“…This mechanism, as well as related recently proposed mechanisms involving organic hydroperoxide (29), are directly at odds with one proposed mechanism based on experimental evidence (30-32) (and supported with DFT modeling) (31,32), which involves H−O-bond cleavage during hydrogen peroxide chemisorption and the open site (SI Appendix, Fig. 1S entry 1a) as a relevant intermediate.…”

“…2S for summary of calculation results and calculation data on cases with and without an extra equivalent of organic hydroperoxide). For both the open and closed structures, the extra equivalent of hydroperoxide lowers the transition-state energy via hydrogen-bond-donating interactions to the hydroperoxide involved in oxygen transfer, as described previously (29), and this additional transition-state energy lowering corresponds to ∼0.6-1.0 kcal/mol. We note that the hydrogen bond donor within this context can in principle be either organic hydroperoxide, alcohol byproduct of organic hydroperoxide that is formed following oxygen transfer, or silanol (30)(31)(32)(33).…”

Facile preparation of silica-supported Ti catalysts effective for the epoxidation of cyclooctene using Ti-bridged silsesquioxanes

“…Note that previous experimental and theoretical studies have revealed that Ti(IV) species with more electron-deficient characters could be more active for alkene epoxidation because of the very facile electrophilic oxygen transfer from the Ti(IV)-OOH species to alkenes. 38 As discussed above, the addition of various silicas was found to significantly increase the yield of the epoxide by the epoxidation of cyclooctene in the presence of a catalytic amount of Ti-POSSs. At a higher substrate-to-catalyst ratio, 1a with Cabosil or JRC-SiO-8 showed more than five times higher catalytic activity than that of 1a only.…”

Recent Developments in Epoxide Preparation

“…The Ti complexes prepared with tBu 2 Si 2 O(OH) 4 were supported on silica to produce active heterogeneous epoxidation catalysts [21]. Recently, Urukawa et al [22] demonstrated that the structure of tetrakis(trimethylsiloxy) titanium (TTMST, Ti(OSiMe 3 ) 4 ), possessing an isolated Ti center, resembles that of the active sites in some heterogeneous Ti-Si epoxidation catalysts, especially silylated titania-silica mixed oxides.…”