A New, Efficient Route to Titanium-Silsesquioxane Epoxidation Catalysts Developed by Using High-Speed Experimentation Techniques

Abstract

Titanium centers dispersed on, and supported by, various forms of silica are catalysts with remarkable properties for partial oxidations. [1] The character of the active site varies with the silica support used. Although it has been established that in the best (on the basis of activity per gram of titanium) heterogeneous catalysts the Ti centers are all four-coordinate, [2±4] it is still unclear whether, for optimum performance, the active catalytic species requires the specific configuration of four siloxy groups, or if being partially hydrolyzed to, say, one hydroxy and three siloxy ligands is equally good. Chemical modeling studies by various groups [5±7] have shown that Ti catalysts with fewer than four siloxy groups should also be active as catalysts. These considerations indicate that there is still some residual uncertainty regarding the active site, despite the many experimental and computational studies carried out to resolve it. [8,9] Indeed, several mechanisms, or variations of one underlying mechanism, might operate depending on the precise configuration of the Ti site. Hence, different types of Ti centers might exhibit the desired characteristics.For these catalysts, which are difficult to characterize, organic ± inorganic hybrid compounds have proved to be useful models. Especially silsesquioxanes [10±12] (RSiO 1.5 ) a -(H 2 O) 0.5b (R is an organic group; a ! 1, b ! 0) have been a focus of attention as model compounds for silica. Of interest for catalysis are those silsesquioxanes that contain SiÀOH groups, that is partially condensed structures to which a catalytically active compound can be complexed. [13,14] The incompletely condensed silsesquioxane trisilanol 1 (a 7, b 3) has recently been reported as a precursor for the soluble, titanium model compound 2 (Scheme 1), which results in an active catalyst for the epoxidation of alkenes. [5±7] Silsesquioxane 1 can be obtained by the slow hydrolytic condensation of the corresponding monosilane RSiX 3 [Eq. (1) and (2); R organic group; X Cl, OMe, OEt; a b 2 n; n 1,2,3...;Scheme 1. Complexation of titanium to the incompletely condensed silsesquioxane R 7 Si 7 O 12 H 3 (R cyclohexyl, cyclopentyl), modeling the grafting of titanium on silica.b 3 a]. [15] Besides compound 1, the reaction may produce other silsesquioxane structures, some of which may also contain silanol groups that could react with titanium species to produce catalytic materials. The preparation of compound 1 is time-consuming and requires a number of purification steps. [15] Here we describe the application of high-speed experimentation techniques [16] to optimize the preparation of silsesquioxanes as precursors for Ti catalysts active in the epoxidation of alkenes with peroxides. The aim is to identify a faster and cheaper way to synthesize silsesquioxane precursors for Ti catalysts. Since silsesquioxanes other than 1 may also show relevant catalytic activity after reaction with titanium, the synthesis of the silsesquioxane precursors was optimized not as a function of the yield i...

Synthesis and Application of Easily Recyclable Thiomorpholines for Use in Sulfur Ylide Mediated Asymmetric Epoxidation of Aldehydes

“…The high‐speed experimentation screening: Silsesquioxane precursors for titanium catalysts were synthesised by the hydrolytic condensation of six different trichlorosilanes (R = cyclohexyl, cyclopentyl, phenyl, methyl, ethyl, and tert ‐butyl) in three highly polar solvents (dimethyl sulfoxide (DMSO), water and formamide) by means of high‐speed experimentation techniques. Previous work showed that the R group on the organosilane and the solvent in which the hydrolytic condensation is carried out are the most relevant parameters influencing the synthesis of silsesquioxanes 8. 11 The parameter space studied was defined by the full‐factorial combination of the six trichlorosilanes and the three solvents.…”

“…This parameter space was screened as a function of the activity of the catalysts obtained after coordination of [Ti(OBu) 4 ] to the silsesquioxane structures in the epoxidation of 1‐octene with tert ‐butyl hydroperoxide (TBHP) 12. In Figure 1, the relative activities of the titanium silsesquioxanes are shown together with those of the titanium silsesquioxanes obtained from silsesquioxanes synthesised in acetonitrile, which gave the best results among the solvents tested in previous work 8. 9 The values are normalised to the activity of the most active titanium silsesquioxane found so far, specifically that obtained by reacting [Ti(OBu) 4 ] with cyclopentyl silsesquioxane a 7 b 3 [( c ‐C 5 H 9 ) 7 Si 7 O 12 TiOC 4 H 9 ] 12.…”

“…Therefore, a combinatorial chemistry and high‐speed experimentation7 approach, which allows the fast screening of broad parameter spaces, is particularly suitable to study and optimise the synthesis of these compounds. Recently, high‐speed experimentation (HSE) techniques have been successfully applied to the optimisation of silsesquioxane precursors for titanium catalysts active in the epoxidation of alkenes 8. 9 This work resulted in a new and efficient way of synthesising silsesquioxane precursors and generated knowledge about the effect of the various parameters involved in the synthesis.…”

“…The experimental approach is similar to that used in previous studies 8. 9 The synthesis of silsesquioxane precursors is optimised as a function of the epoxidation activity of the catalysts obtained after titanium complexation.…”

Organic–Inorganic Hybrid Saponites Obtained by Intercalation of Titano‐Silsesquioxane

“…This leads to the formation of metal‐containing silsesquioxanes (M‐POSS), which are relevant as building blocks for multifunctional composite materials or as model systems for heterogeneous catalysts 4–7. M‐POSS compounds, for example, have been dispersed on different silicas and polymeric matrices for the preparation of active catalysts in epoxidation,8–9 polymerization,10–11 and oxidative dehydrogenation of olefins 12–15…”