Cyclization Phenomena in the Sol−Gel Polymerization of α,ω-Bis(triethoxysilyl)alkanes and Incorporation of the Cyclic Structures into Network Silsesquioxane Polymers
摘要:
Intramolecular cyclizations during acid-catalyzed sol-gel polymerizations of alpha,omega-bis(triethoxysilyl)-alkanes substantially lengthen gel times for monomers with ethylene (1), propylene (2), and butylene (3) bridging groups. These cyclization reactions were found, using mass spectrometry and Si-29 NMR spectroscopy, to lead preferentially to monomeric and dimeric products based on six- and seven-membered disilsesquioxane rings. 1,2-Bis(triethoxysilyl)ethane (1) reacts under acidic conditions to give a bicyclic dimer (5) that is composed of two annelated seven-membered rings. Under the same conditions, 1,3-bis(triethoxysilyl)propane (2), 1,4-bis(triethoxysilyl)butane (3), and Z-1,4-bis(triethoxysilyl)but-2-ene (10) undergo an intramolecular condensation reaction to give the six- and seven-membered cyclic disilsesquioxanes 6, 7, and 11. Subsequently, these cyclic monomers slowly react to form the tricyclic dimers 8, 9, and 12. With NaOH as polymerization catalyst, these cyclic silsesquioxanes readily reacted to afford gels that were shown by CP MAS Si-29 NMR and infrared spectroscopies to retain some cyclic structures. Comparison of the porosity and microstructure of xerogels prepared from the cyclic monomers 6 and 7 with those of gels prepared directly from their acyclic precursors 2 and 3 indicates that the final pore structure of the xerogels is markedly dependent on the nature of the precursor. In addition, despite the fact that the monomeric cyclic disilsesquioxane species cannot be isolated from 1-3 under basic conditions due to their rapid rate of gelation, spectroscopic techniques also detected the presence of the cyclic structures in the resulting polymeric gels.
Regiodivergent hydrosilylation, hydrogenation, [2π + 2π]-cycloaddition and C–H borylation using counterion activated earth-abundant metal catalysis
作者:Riaz Agahi、Amy J. Challinor、Joanne Dunne、Jamie H. Docherty、Neil B. Carter、Stephen P. Thomas
DOI:10.1039/c8sc05391j
日期:——
The widespread adoption of earth-abundant metal catalysis lags behind that of the second- and third-row transitionmetals due to the often challenging practical requirements needed to generate the active low oxidation-state catalysts. Here we report the development of a single endogenous activation protocol across five reaction classes using both iron- and cobalt pre-catalysts. This simple catalytic
Manganese‐Catalyzed Hydrofunctionalization of Alkenes
作者:Jonathan R. Carney、Barry R. Dillon、Leonie Campbell、Stephen P. Thomas
DOI:10.1002/anie.201805483
日期:2018.8.13
The manganese‐catalyzed hydrosilylation and hydroboration of alkenes has been developed using a single manganese(II) precatalyst and reaction protocol. Both reactions proceed with excellent control of regioselectivity and in high yields across a variety of sterically and electronically differentiated substrates (25 examples). Alkoxide activation, using NaOtBu, was key to precatalyst activation and
锰催化烯烃的氢化硅烷化和氢硼化反应是使用单一的锰(II)预催化剂和反应方案开发的。两种反应均能很好地控制区域选择性,并能在多种空间和电子分化的底物上获得高收率(25个实例)。使用NaO t Bu进行的醇盐活化是催化剂前活化和反应活性的关键。对于这两种已开发的方法,催化剂的负载量都低至0.5 mol%,从而实现了跨各种官能团和以克为单位的催化作用。
Intramolecular Condensation Reactions of α,ω-Bis(triethoxysilyl)alkanes. Formation of Cyclic Disilsesquioxanes
作者:Douglas A. Loy、Joseph P. Carpenter、Sharon A. Myers、Roger A. Assink、James H. Small、John Greaves、Kenneth J. Shea
DOI:10.1021/ja961409k
日期:1996.1.1
alkylene-bridging groups had a pronounced effect on the competition between cyclization and polymerization of alpha},omega}-bis(triethoxysilyl)alkanes and on the formation of polymeric gels. While the intramolecularreaction clearly slows gelation, the cyclic disilsesquioxanes are still tetrafunctional monomers theoretically capable of forming polymeric gels. If the ring structures, which bear a striking resemblence
Pomerantseva, M. G.; Belyakova, Z. V.; Shchepinov, S. A., Journal of general chemistry of the USSR, 1984, vol. 54, # 2, p. 316 - 318
作者:Pomerantseva, M. G.、Belyakova, Z. V.、Shchepinov, S. A.、Efimova L. A.、Chernyshev, E. A.
DOI:——
日期:——
Sulfur-Modified SBA-15 Supported Amorphous Palladium with Superior Catalytic Performance for Aerobic Oxidation of Alcohols
作者:Kun Liu、Zhaoxiang Chen、Zhiqiang Hou、Yuanyuan Wang、Liyi Dai
DOI:10.1007/s10562-013-1184-2
日期:2014.5
A series of sulfur-modified SBA-15 supported amorphous palladium catalysts are prepared, and the effects of preparation parameters on the aerobic oxidation of benzyl alcohol are systematically investigated. The optimal catalyst exhibits remarkably enhanced catalytic activity (conversion 100 % and selectivity 99 %) and could be separated conveniently. The catalysts are characterized by X-ray diffraction patterns, BET, ICP, X-ray photoelectron spectroscopy, CO chemisorption and transmitting electron microscopy, and the results show that disulfur bond framework in the optimal catalyst act as both a stable linker and a good chelator for Pd species. Pd2+ is reduced to Pd-0 with the increasement of the carbon chain lengths between sulfur bonds, which is the cause of deactivation. The mechanism is that a base abstracts a proton from the coordinated alcohol to form a Pd alcoholate species that subsequently undergo beta-hydride elimination to give benzyl aldehyde, which is confirmed by the catalytic and characteristic results.