Self-assembled monolayers (SAMs) are an excellent tool to change surface properties of materials. The method has attracted a lot of interest and has developed rapidly in the past years. Nanotechnology has developed rapidly in recent decades,and it is widely used in several areas. SAMs are often used to prepare nanoparticles [1, 2, 3],because most nanoparticles need to be functionalized and have their biological toxicity reduced. Glass plates are also good supports for functional molecules. Fluorescent sensors were assembled on glass,which were used for the detection of nitroaromatic compounds in the vapor phase [4, 5, 6]. Fluorescent proteins were immobilized on glass surfaces,and they were very important for potential applications in bionanotechnology . Some metal catalysts were supported on glass surfaces, which were used to catalyze oxidation reactions [8, 9].
Chiral compounds are very important in our daily life,especially in medicinal chemistry,because different configurations have different drug activities. As such,enantioselective synthesis has attracted increasing interest from chemists during the past few decades. There are three main approaches to asymmetric synthesis,which are chiral pool synthesis,chiral auxiliaries,and asymmetric catalysis. Asymmetric transfer hydrogenation (ATH) is one of the most important asymmetric catalytic reactions,and it is normally promoted by transition metal catalysts [10, 11, 12, 13, 14, 15, 16, 17]. Noyori et al. found that chiral TsDPEN exhibited excellent catalytic activity and enantioselectivity in the asymmetric transfer hydrogenation of aromatic ketones ,and they have made outstanding contributions in this area [19, 20]. Compared with homogeneous catalysts,heterogeneous immobilization molecular catalysts at the surfaces of materials have several outstanding advantages, including facile product purification,easy catalyst recovery from reaction mixtures and reusability. Normally,the materials used for immobilizing molecular catalysts are mesoporous silica [21, 22], magnetic materials ,zeolites ,organic polymers ,or high surface area carbon . However,to the best of our knowledge there is no chiral catalyst that is immobilized on a glass surface.
What we are interested in is material supported catalysts in an effort to achieve highly active and recyclable heterogeneous catalysts [27, 28, 29, 30, 31]. Herein,the ligand of TsDPEN (substitutive phenylsulfonyl-1,2-diphenylethylenediamine) with ethynyl group was synthesized,and the ligand was conjuncted with a silica source via click chemistry. Then,the homogeneous catalyst was prepared through stirring the solution of the ligand and [Cp*RhCl2]2 in dichloromethane at room temperature. At last,the obtained catalyst was immobilized at the surface of glass plates to form the heterogeneous catalyst,which showed good catalytic efficiency and enantioselectivity.
Microscope glass slides were used for monolayer preparation. The substrates were oxidized with piranha solution for 15 min (concentrated H2SO4 and 33% aqueous H2O2 in 3:1 ratio; caution: piranha should be handled carefully) and rinsed with MilliQ water (MQ). After drying with nitrogen stream,the substrates were used immediately to provide a freshly hydroxyl terminated surface on which to form the silanized monolayer.
As shown in Scheme 1,processed glass were immersed in the solution of silanizing homogeneous catalyst in CH2Cl2 until the solvent evaporated completely. To remove the unreacted homogeneous catalyst,the prepared glass were treated with ultrasound in CH2Cl2 and EtOH. The glass-supported heterogeneous catalyst (GH-catalyst) was obtained after the glass dried.
A typical procedure was as follows : GH-catalyst, HCOONa (20 mg,0.3 mmol),ketones (2.0 mmol),and 2.0 mL water were added in a 10 mL round bottom flask. The mixture was allowed to react at 40 8C for 4 h. During the reaction,it was monitored constantly by TLC. The conversion and ee value could be determined by chiral GC using a Supelco b-Dex 120 chiral column (30 m × 0.25 mm (i.d.),0.25 mm film) or HPLC analysis with UV-vis detector using Daicel OJ-H chiral column (1 0.46 cm × 25 cm).
The click reaction was used for the synthesis of the TsDPEN ligand with the trimethoxysilane group,which combinedwith Rh to form the homogeneous catalyst (see Supporting information). The heterogeneous catalyst was synthesized by grafting the homogeneous catalyst onto the oxidized glass with excess hydroxy groups. According to the XPS data,the O/Si ratio of normal glass was 1.53,and that of oxidized glass was 2.11. That indicated that excess hydroxy groups were obtained after the glass was oxidized. The content of sulfur increased from 0 to 0.61%. This was because a few sulfonic groups were produced when the glass was oxidized by sulfuric acid. The glass we used were normal slides,so therewas plentiful carbon according to the XPS data. After grafting the homogeneous catalyst onto the slides, the XPS N1s and Rh3d signals indicated that the catalyst was immobilized successfully. As shown in Fig. 1,the binding energy of Rh3d was 308.8 eV,which was nearly the same with Cp*RhTsDPEN (309.3 eV).
The resulting AFM images are shown in Fig. 2. The surface roughness exhibited by the root-mean-square (RMS) height value for glass was 2.60 nm. After grafting,the RMS value for the catalyst 6 decreased to 2.19 nm. This decrease in roughness can be explained by the grafted homogeneous catalyst decreasing the altitude difference.
The catalytic activity of GH-catalyst was evaluated following successful preparation of heterogeneous catalyst. Table 1 summarizes catalytic performances for asymmetric transfer hydrogenation of aromatic ketones in aqueous medium. GH-catalyst delivered high conversions for most of the aromatic ketones and in most cases the enantioselectivities were good,with ee values reaching up to 93%. Comparing entries 2,7,and 8 with 6 and 10,it was obvious that aromatic ketones with electron-drawing groups were generally more active than those with electron-donating groups. That may be because the electron-donating groups give lower LUMO values,which accelerates the catalytic reaction. Another advantage of GH-catalyst was the active centers were at the surface of the slides,which can show good catalytic activity for the huge size substrate (entry 12). In addition,the recyclability of GH-catalyst was also investigated with acetophenone as a substrate in the same conditions. GH-catalyst was recovered very easily and could be reused directly. However,the conversion at the third cycle was 25%,and decreased rapidly from 99% (the second cycle). That may because the active center was lost from the glass.
In conclusion,we have synthesized trimethoxysilane-substitute Cp*RhTsDPEN via click reaction that can be directly grafted onto oxidized glass slides to prepare heterogeneous catalysts. Using XPS and AFM,we have demonstrated the heterogeneous catalyst was prepared successfully. The catalyst displayed good catalytic activity and enantioselectivity in the asymmetric transfer hydrogenation of aromatic ketones in aqueous medium,and the substrates with electron-donating groups had more enantioselectivity than those with electron-drawing groups.
We are grateful to the Shanghai Sciences and Technologies Development Fund (Nos. 13ZR1458700 and 12nm0500500),the Shanghai Municipal Education Commission (Nos. 14YZ074, 12ZZ135),Shanghai Normal University (Nos. DXL122, SK201329) for financial support.
Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2014.01.007.
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