The enantioselective cyanosilylation of aldehydes on a chiral VO(Salen) complex encapsulated in SBA-16

Yang, Hengquan; Zhang, Lei; Wang, Peng; Yang, Qihua; Li, Can

doi:10.1039/b815997a

Cited by 81 publications

(46 citation statements)

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“…[12] FDU-12 is a mesoporous silica material with a cavity diameter of 18.6 nm and a pore-entrance size less than 4.0 nm. [13] These dimensions make FDU-12 an ideal nanoreactor to host the molecular catalyst (see Figures S1 and S2 in the Supporting Information).…”

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Hydration of Epoxides on [Co^III(salen)] Encapsulated in Silica‐Based Nanoreactors

Bai

Wang

et al. 2012

Angewandte Chemie

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Monoethylene glycol (MEG) and 1,2-propylene glycol (PG) are important raw materials for the manufacture of polyester resins, antifreezes, cosmetics, medicines, and other products. [1] The total global demand for MEG has been estimated to be over 19 million metric tons per year.[2] The production of MEG in industry predominantly involves the liquid-phase hydration of ethylene oxide (EO), whereby a large excess of water (20-25 mol of water/mol of EO) is required for the high conversion of EO and high selectivity for MEG. The concentration of MEG in the final aqueous solution is only approximately 10 wt %, and huge energy is consumed for the distillation of the product from the aqueous solution. As a result, epoxide hydration is one of the most cost-and energy-intensive processes in the chemical industry.Possible catalytic hydration processes have been extensively investigated for the environmentally friendly production of MEG at a low energy cost. Various types of catalysts, including liquid and solid acids or bases, have been explored, such as sulfuric acid, [3] the salts of some acids, [4] cyclic amines, [5] cation-and anion-exchange resins, [6] quaternary phosphonium halides, [7] polymeric organosilane ammonium salts, [8] macrocyclic chelating compounds, [9] and supported metal oxides.[10] However, a high H 2 O/EO molar ratio (i.e. > 10) is still required for high MEG selectivity. The MEG selectivity is very low at low H 2 O/EO molar ratios owing to the formation of diethylene glycol (DEG) and triethylene glycol (TEG) by the self-condensation of MEG, which is readily catalyzed by acid and base catalysts. Moreover, the inherent corrosion and environmental problems associated with the liquid acids/bases limit their application in industry. The development of an efficient and environmentally benign process for the hydration of epoxides with an H 2 O/epoxide molar ratio approaching the stoichiometric value of the chemical reaction is still a huge challenge.In this study, we developed a solid catalyst that is different from the conventional liquid/solid acid or base catalysts for the hydration of epoxides. We constructed the solid catalyst by encapsulating the molecular catalyst [Co III (salen)], with a salen ligand derived from 3,5-di-tert-butylsalicyclaldehyde and trans-1,2-diaminocyclohexane, [11] in the nanocage of the mesoporous silica FDU-12. This catalyst exhibits high activity and selectivity in the hydration of epoxides under mild reaction conditions. Furthermore, the H 2 O/epoxide molar ratio can be decreased to as low as 2:1 while maintaining the conversion of EO above 98 % and the selectivity for MEG above 98 %. This novel catalytic approach has great potential for the green and energy-saving hydration of epoxides, as well as many other conventional chemical reaction processes in industry.The solid catalyst (denoted as FDU-12-[Co III

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Hydration of Epoxides on [Co^III(salen)] Encapsulated in Silica‐Based Nanoreactors

Bai

Wang

et al. 2012

Angewandte Chemie

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“…For this purpose, the prepared material in the rst step was reacted with triethoxyoctylsilane to modify the pore entrance size of the SBA-16. Earlier work by Li et al showed that silylation with this reagent resulted in reducing the pore entrance rather than pore blocking [50].…”

Section: Catalytic Studiesmentioning

confidence: 99%

“…The excess water was slowly vaporized and the solid dried in vacuum oven overnight at 100 C. Based on the method in the literature [50], the modi cation and adjusting the pore entrance size of SBA-16 were performed by silylation reaction. The obtained solid was dispersed in 1.5 ml dry toluene, followed by the addition of 1.25 ml anhydrous pyridine and 5 mmol triethoxyoctylsilane, and then re uxed for 24 h under nitrogen atmosphere.…”

Section: Preparation Of the Catalystmentioning

confidence: 99%

“…Recently, SBA-16 mesoporous material has attracted much attention as support of heterogeneous catalysts due to its uniform and large pores, high surface area, high thermal stability, and tunable pore size [48][49][50][51][52]. It has a three-dimensional cubic mesoporous structure and, due to its large nanocages, it is a promising support for the encapsulation of POMs.…”

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confidence: 99%

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Encapsulation of Keggin-type phosphotungstic acid into the mesopores of SBA-16 as a reusable heterogeneous catalyst for the epoxidation of olefins

Masteri‐Farahani

Modarres

2017

Scientia Iranica

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Abstract. A heterogeneous catalyst for the epoxidation of ole ns was prepared by encapsulating Keggin-type phosphotungstic acid (H3PW12O40) into the mesopores of SBA-16. After the encapsulation, the pore entrance size of SBA-16 was reduced through a silylation method to encompass the catalyst in the mesopores and allow easy di usion of the reactants and products during the catalytic process. The prepared catalyst was characterized by FT-IR and Inductively Coupled Plasma-Optical Emission Spectroscopies(ICP-OES), X-Ray Di raction (XRD), and Transmission Electron Microscopy (TEM). The analysis results revealed that the mesoporous nature of SBA-16 was conserved after encapsulation of the catalyst following the silylation step. The catalytic activity of the prepared material was assessed in the epoxidation of ole ns with H2O2. The heterogeneous catalyst was recovered and reused up to ve cycles without considerable decrease in activity.

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“…Cr(Salen) was encapsulated in the nanocages of FDU-C using propyltrimethoxysilane (C3) as the silylation reagent in the presence of pyridine according to the strategy we reported previously (Scheme 1). [17][18][19][20][21] The textural properties of FDU-12, FDU-C and the synthesized solid catalyst (0.76 wt% Cr) were detected by the N 2 sorption experiment. As exhibited in Fig.…”

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Promoted activity of Cr(Salen) in a nanoreactor for kinetic resolution of terminal epoxides

Bai

Peng

et al. 2012

Chem. Sci.

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Cr(Salen) catalyzed kinetic resolution of terminal epoxides via asymmetric ring opening (ARO) with TMSN 3 is an important approach for the synthesis of enantiopure 1,2-amino alcohols, however, the high catalyst usage amount (1-2 mol%) impedes its practical applications. An efficient solid nanoreactor was constructed by encapsulation of Cr(Salen) and pyridine in the nanocages of mesoporous silica. This solid nanoreactor exhibits high activity (TOF: 1325 h À1) and high enantioselectivity (91% ee) for the kinetic resolution of 1,2-epoxyhexane via ARO with TMSN 3 at a catalyst concentration as low as 0.002 mol%, whereas the homogeneous counterpart affords almost no conversion of epoxide under similar reaction conditions. The high activity of the solid nanoreactor is mainly attributed to the greatly intensified cooperative activation in the nanocages as evidenced by the sharply increased TOF in parallel with Cr(Salen) concentration in each nanocage. The increased nucleophilicity of Cr(Salen) after coordination to pyridine could also promote the catalytic activity. The solid nanoreactor can be easily separated and recycled. We demonstrated the possibility for designing an efficient solid nanoreactor for asymmetric catalysis by taking the advantages of the cooperative activation.The synthesis of enantiopure 1,2-amino alcohols through the kinetic resolution of terminal epoxides via asymmetric ring opening (ARO) with TMSN 3 on Cr(Salen) was first reported by Jacobsen and co-workers.1,2 However, high catalyst concentration (1-2 mol%) is needed to achieve desirable activity and enantioselectivity due to the bimolecular reaction pathway and the low activity of Cr(Salen).3 For enhancing the activity, dimeric Cr(Salen) catalysts were prepared and they can catalyze the ARO of epoxide at concentrations an order of magnitude below the lower limit of reactivity of their monomer counterparts due to the enhanced cooperative activation effect. 4Recycling the homogeneous catalysts is quite important regarding the green, atomic efficient, economic and sustainable production of chemicals for the practical applications. Thus far, several heterogenization strategies have been described for Cr(Salen) catalysts for ARO reactions.5-7 However, the reduced activity and enantioselectivity of Cr(Salen) covalently bonded to solid supports through ligand tethering was observed because of the impossible concerted participation of two Cr(Salen) complexes required for the bimetallic activation pathway. 8Cr(Salen) complexes were also immobilized on solid support through a weak interaction, such as coordinative bond or electrostatic adherence.9 During the reaction, equilibrium is reached and part of the Cr(Salen) complexes migrates to the solution for promoting the reaction in the homogeneous phase. The weak interaction between the metal salen complex and supports is the driving force to rebind the complex to the solid through a mechanism that has been termed as boomerang catalyst. Although the asymmetric induction in this case can be as high as the h...

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The enantioselective cyanosilylation of aldehydes on a chiral VO(Salen) complex encapsulated in SBA-16

Cited by 81 publications

References 38 publications

Hydration of Epoxides on [Co^III(salen)] Encapsulated in Silica‐Based Nanoreactors

Hydration of Epoxides on [Co^III(salen)] Encapsulated in Silica‐Based Nanoreactors

Encapsulation of Keggin-type phosphotungstic acid into the mesopores of SBA-16 as a reusable heterogeneous catalyst for the epoxidation of olefins

Promoted activity of Cr(Salen) in a nanoreactor for kinetic resolution of terminal epoxides

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The enantioselective cyanosilylation of aldehydes on a chiral VO(Salen) complex encapsulated in SBA-16

Cited by 81 publications

References 38 publications

Hydration of Epoxides on [CoIII(salen)] Encapsulated in Silica‐Based Nanoreactors

Hydration of Epoxides on [CoIII(salen)] Encapsulated in Silica‐Based Nanoreactors

Encapsulation of Keggin-type phosphotungstic acid into the mesopores of SBA-16 as a reusable heterogeneous catalyst for the epoxidation of olefins

Promoted activity of Cr(Salen) in a nanoreactor for kinetic resolution of terminal epoxides

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Product

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Hydration of Epoxides on [Co^III(salen)] Encapsulated in Silica‐Based Nanoreactors

Hydration of Epoxides on [Co^III(salen)] Encapsulated in Silica‐Based Nanoreactors