Preparation of a Periodically Ordered Mesoporous Organosilica Material Using Chiral Building Blocks

Ordered mesoporous materials (OMMs), of which MCM41 is perhaps the most prominent example, are characterized by well-defined pore systems and large surface areas on the order of 700–1000 m g. This makes OMMs very interesting for numerous applications, such as in chromatographic separation or as supports for catalytically active compounds. Conventional pure silica materials are somewhat limited in their range of functional properties. Therefore, the preparation of high-surface-area (porous) organic–inorganic hybrid materials has received a lot of attention. The modification of mesoporous silica materials with organic groups can be achieved in three different ways. Because of the reactive Si– OH groups on the pore walls of OMMs, it is possible to use silane derivatives X3–ySi–Ry (X = OR′ or halogen; R is an organic group) to carry out a post-synthetic grafting process. Alternatively, the silane derivatives can be used directly during the synthesis of the OMMs. However, a major drawback of these two techniques is that, depending on the steric requirements and the degree of hydrophobicity of the silanes, only a fraction of the inorganic matrix can be organically modified. Typically, not more than 25 % (R′O)3Si–R can be used in a co-condensation process. The rest of the material network is formed with Si(OR′)4 as the source, yielding purely inorganic SiO2. The recent development of the so-called periodically ordered mesoporous organosilica materials (PMOs) seems to be a solution to the above-mentioned problems. Bis-alkoxysilanes with a bridging organic group (R′O)3Si–R–Si(OR′)3 are used as precursors for the formation of mesoporous RSi2O3 materials. The “interface contact” of the bridging organic groups is maximized for PMO materials that contain “undiluted” O1.5Si–R–SiO1.5 motifs. [7] The advantage of using the bis-alkoxysilanes is that the addition of Si(OR′)4 and its cocondensation is not mandatory for obtaining highly ordered materials. A further advantage of this interesting and new class of materials is their enhanced mechanical stability; also, the use of molecular building blocks leads to the homogeneous distribution and accessibility of the organic groups. PMOs can contain more than one group bound to each Si atom, and the assembly of larger PMO building blocks like dendrimers has also been reported. Although the number of bis-alkoxysilanes used as precursors for PMOs is steadily increasing, the field is still quite unexplored. Therefore, a PMO material containing O1.5Si–R–SiO1.5, where R possesses a chiral group, represents a very tempting target. To the best of our knowledge, there has been only one previous report of a comparable system. The previous work described the synthesis of a mesoporous material by the cocondensation route involving a bis-alkoxysilane with a chiral vanadyl salen complex as the bridging organic ligand along with tetraethoxysilane (TEOS). The maximum amount of the chiral bis-alkoxysilane used was 15 %, and the new sol–gel precursor contains a potentially labile thioether group. A cocondensation process was probably necessary in order to incorporate the large and hydrophobic salen complex into the mesoporous framework. In order to overcome the increased hydrophobicity, the authors used ethanol as a co-solvent. The key to obtaining a PMO material from a single precursor is therefore the synthesis of a bis-alkoxysilane with a chiral bridging organic group that is as small as possible, while also being potentially hydrophilic. Fairly well-ordered PMO materials have been prepared using (R′O)3Si–CH2CH2–Si(OR′)3 as a precursor. Thus, molecules (R′O)3Si–CHXCH2– Si(OR′)3 represent an attractive group of chiral PMO precursors, where X can be any group except hydrogen. In this paper, we report the synthesis of a PMO precursor of this type and use it to prepare an ordered mesoporous material. Co-condensation with an additional tetraalkoxysilane is not necessary in order to obtain a well-ordered material. We have used the synthetic approach shown in Scheme 1 to prepare the desired compound. First, bis(1,2-trimethoxysilyl)ethene is prepared via a metathesis reaction, as previously reported in the literature. The enantioselective hydroboration of carbon–carbon double bonds is a well-established technique. We have adapted this method by using rhodium(I) catalysts in combination with (R)-(+)-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (R-BINAP) as a chiral ligand (see Experimental). The product 2 is obtained as a colorless liquid. The compound is purified under high vacuum (p = 10 mbar; 1 mbar =100 Pa) at a temperature of 150 °C using bulb-to-bulb distillation. However, due the sensitive character of the compound, the final yield of the pure product is only 35 %. NMR investigations in-