Regioselective Silylations of C-2 Hydroxyl Groups of Cyclodextrins Dependent on Reaction Temperature

Silylations of the C-2 hydroxyl group of cyclodextrins were carried out using t-butyldimethylsilyl imidazole in the presence of 4A molecular sieves in N,N-dimethylformamide. A unique aspect of this silylation method is the temperature dependence of the regioselectivity; silylation at 0 °C regioselectively favored the C-6 position to afford mono-6-O-t-butyldimehylsilyl-cyclodextrins, whereas silylation at 140 °C exhibited high regioselectivity on the C-2 hydroxyl group.     

Synthesis and structures of tris(pentafluorophenyl)silylamines

Tris(pentafluorophenyl)silylamines were synthesized by silylation of amines and imines with (C₆F₅)₃SiCl or (C₆F₅)₃SiOTf in the presence of triethylamine. The crystal structures of the (C₆F₅)₃SiN(H)CH₂Ph and (C₆F₅)₃SiN(CH=CMe₂)CH₂Ph compounds were studied by X-ray diffraction. The crystal packings were analyzed by quantum chemical calculations in terms of the density functional theory (PBE exchange-correlation functional). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1345–1352, July, 2007.     

Synthesis and transformations of 1,3,5-triazabicyclo[3.1.0]hexanes

The conditions for the condensation of 1,2-nonsubstituted diaziridines with CH₂O and NH₃ (or AlkNH₂) to the corresponding 1,3,5-triazabicyclo[3.1.0]hexanes have been found and 3-phenylsulfonyl, 3-trimethylsilyl, 3-nitroso, and 3-nitro derivatives of the latter have been obtained.For the previous communication, see Ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2091–2095, December, 1993.     

Rhodium‐Catalyzed Enantioselective Silylation of Cyclopropyl C−H Bonds

Hydrosilyl ethers, generated in situ by the dehydrogenative silylation of cyclopropylmethanols with diethylsilane, undergo asymmetric, intramolecular silylation of cyclopropyl C?H bonds in high yields and with high enantiomeric excesses in the presence of a rhodium catalyst derived from a rhodium precursor and the bisphosphine (S)‐DTBM‐SEGPHOS. The resulting enantioenriched oxasilolanes are suitable substrates for the Tamao–Fleming oxidation to form cyclopropanols with conservation of the ee value from the C?H silylation. Preliminary mechanistic data suggest that C?H cleavage is likely to be the turnover‐limiting and enantioselectivity‐determining step.     

Aryl(triethyl)silanes for Biaryl and Teraryl Synthesis by Copper(II)‐Catalyzed Cross‐Coupling Reaction

Aryl(triethyl)silanes are found to undergo cross‐coupling with iodoarenes in the presence of catalytic amounts of CuBr₂ and Ph‐Davephos, as well as cesium fluoride as a stoichiometric base. Because the silicon reagents are readily accessible through catalytic C?H silylation of aromatic substrates, the net transformation allows coupling of aromatic hydrocarbons with iodoarenes via triethylsilylation.     

Introduction of PEG-SANM nanocomposite as a new and highly efficient reagent for the promotion of the silylation of alcohols and phenols and deprotection of the silyl ethers

Poly (ethylene glycol)-sulfonated sodium montmorillonite (PEG-SANM) nanocomposite was prepared by a simple method and characterized using XRD, TGA, SEM, TEM, and FT-IR techniques. After preparation and characterization, this reagent was used as a highly efficient and reusable solid acid catalyst for the chemoselective silylation of alcohols and phenols and deprotection of the obtained silyl ethers. The method offers several advantages including high to excellent yields of the products, short reaction times, easy preparation of the catalyst and easy work-up procedure. In addition, the catalyst can be recycled and reused at least for five times without significant decrease in the catalytic activity.     

硅烷化改性对纳米ZSM-5甲醇制汽油催化剂性能的影响

对比了水热处理后微米ZSM-5和纳米ZSM-5分子筛的物化性质和催化甲醇制汽油(MTG)的反应性能,发现采用纳米ZSM-5分子筛催化剂能得到较高的汽油收率和较长的寿命,但汽油中均四甲苯含量较高.对纳米ZSM-5分子筛进行硅烷化处理,利用低温N_2吸附-脱附、X射线衍射(XRD)、氨气程序升温脱附(NH_3-TPD)对改性前后的样品进行表征.在温度380℃、压力2.0 MPa、空速3.0 h~(-1)的反应条件下进行MTG反应,对硅烷化改性后的催化剂进行评价.结果表明,负载SiO_2后催化剂的强酸中心降低,比表面积和孔容降低.纳米ZSM-5分子筛合适的SiO_2负载量为2%,硅改性后用于MTG反应,催化剂的寿命和汽油收率分别由改性前的144 h和33.6%显著增加到180 h和34.4%.当SiO_2负载量继续增加时,催化剂寿命和汽油收率逐渐降低.另外,随SiO_2负载量的增加,其催化MTG所得汽油产品中的异构烷烃和芳烃含量降低,烯烃和正构烷烃含量增加,均四甲苯含量显著降低,改善了油品质量.     

Surface silylation and pore structure development of silica aerogel composites from colloid and TEOS-based precursor

Flexible aerogel-fiber composites were prepared by silylation and ambient drying of colloidal silica and tetraethylorthosilicate (TEOS)-based sol. After immersing glass fiber matrices into silica sol with colloid-based, colloid/TEOS-based, and TEOS-based silica sol, it was surface-modified in a trimethylchlorosilane/n-hexane solution and heat-treated at 230 °C in ambient atmosphere. Surface silylation of silica aerogel synthesized from colloid and TEOS-based silica sols showed different behaviors. For colloid silica gel, it was comprised of small sized mesopores because colloid-based silica gel has dense networks through great degrees of hydrolysis and condensation. On the contrary, TEOS-based aerogel was consisted of relatively large-sized pores because of comparatively lesser degree of hydrolysis and condensation. Through this study, we can know that the pore structures of silica aerogel could be controlled by choosing colloid or TEOS-based precursor and surface silylation reaction.