NMR-Untersuchungen und Konfigurationsanalyse an Cyclopentasilanderivaten

Hengge, E.; Kovar, D.; Söllradl, H.

doi:10.1007/bf00906674

Cited by 14 publications

(6 citation statements)

References 4 publications

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“…On the other hand, the narrow chemical shift window (5.0−5.2 ppm) of the cyclics in Figure a indicates a much more limited number of stereocenters and ring sizes. In fact, it has been suggested earlier that resonances in this region are due to pentaphenylcyclopentasilane isomers. , The 1 H NMR spectra of the precipitated polymers show the presence of smaller amounts of species resonating between 5.0 and 6.0 ppm than is evident in Figure a. It may therefore be concluded that the precipitation procedure removes most, but not all, of the cyclic material.…”

Section: Resultsmentioning

confidence: 69%

Stereostructures of Linear and Cyclic Polyphenylsilanes Produced by Dehydrocoupling in the Presence of Group 4 Metallocene Catalysts

et al. 1999

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Phenylsilane was polymerized with a number of group 4 metallocene precatalysts, including (S,S)-[1,2-bis(η 5 -tetrahydroindenyl)ethane]titanium binaphtholate (1)/2BuLi, Cp 2 HfCl 2 (2)/ 2BuLi/B(C 6 F 5 ) 3 , and CpCp*ZrCl 2 (3)/2BuLi/B(C 6 F 5 ) 3 . With 1, polymerization proceeds extremely slowly and it is possible to observe the stepwise oligomerization to Si 2 , Si 3 , Si 4 , and Si 5 species. One-and two-dimensional 1 H and 29 Si NMR experiments permit the assignment of peaks to the stereoisomers of the Si 4 and Si 5 compounds. For the linear oligomers PhSiH 2 SiHPh(SiHPh) n SiHPhSiH 2 Ph (n g 0), the 29 Si NMR resonances group cleanly into the regions δ (ppm) 58-59 (PhSiH 2 ), 59-63 (SiHPhSiHPhSiHPh), and 64-65 (PhSiH 2 SiHPhSiHPh). The chiral catalyst 1 has little influence on the sterochemical outcome of the coupling reaction. Higher molecular weight polymers synthesized with 2 and 3 or obtained by fractionation of lower molecular weight polymers all have nearly identical 29 Si NMR spectra. It is proposed that this spectrum is a reflection of the pentad composition of the polymers and that any deviations from randomness are a result of intrachain interactions at the growing chain end and are unrelated to the structure of the catalyst.

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Section: Resultsmentioning

confidence: 69%

Stereostructures of Linear and Cyclic Polyphenylsilanes Produced by Dehydrocoupling in the Presence of Group 4 Metallocene Catalysts

et al. 1999

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“…Other than the 29 Si resonance for Ph 2 SiH 2 , all of the signals present in this region correspond to Ph 2 SiH– groups that most likely formed from redistribution of a terminal −PhSiH 2 unit. The broad range of overlapping resonances from δ = −30 to −32 likely corresponds to products containing more than one −PhSiH– unit in a cyclosilane that consists of mostly Ph 2 Si– units (e.g., Ph 8 Si 5 H 2 ). , …”

Section: Resultsmentioning

confidence: 96%

“…The broad range of overlapping resonances from δ = −30 to −32 likely corresponds to products containing more than one −PhSiH− unit in a cyclosilane that consists of mostly Ph 2 Si− units (e.g., Ph 8 Si 5 H 2 ). 35,36 The second region of silane products in the DEPT 29 Si NMR spectra is in the range δ = −55 to −70 and corresponds to proton resonances in the range δ 4.4−4.8 as determined by 1 H− 29 Si HSQC NMR spectroscopy. The signals from δ = −56 to −60 are due to terminal PhSiH 2 − groups.…”

Section: ■ Introductionmentioning

confidence: 99%

Iridium Pincer Catalysts for Silane Dehydrocoupling: Ligand Effects on Selectivity and Activity

Mucha

Waterman

2015

Organometallics

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Catalytic reactions of bisphosphinite pincerligated iridium compounds p-X R (POCOP)IrHCl (POCOP) [2,6-(R 2 PO) 2 C 6 H 3 , R = i Pr, X = H (1); R = t Bu, X = COOMe (2); = H (3); = NMe 2 (4)] with primary and secondary silanes have been performed. Complex 1 is primarily a silane redistribution precatalyst, but dehydrocoupling catalysis is observed for sterically demanding silane substrates or with aggressive removal of H 2 . The bulkier compounds (2−4) are silane dehydrocoupling precatalysts that also undergo competitive redistribution with less hindered substrates. Products generated from reactions utilizing 2−4 include low molecular weight oligosilanes with varying degrees of redistribution present or disilanes when employing more sterically demanding silane substrates. Selectivity for redistribution versus dehydrocoupling depends on the steric and electronic environment of the metal but can also be affected by reaction conditions. ■ INTRODUCTIONPolysilanes are not primarily prepared by metal catalysis, despite how essential metal catalysis is for the preparation of specialty polyolefins. 1 This is a surprising dichotomy given the premium on monodisperse, linear polysilanes of high molecular weight to promote optimal σ-electron delocalization. 2,3 Reasons for the lack of industrial adoption of metal-catalyzed silane dehydrocoupling depend on the general category of catalyst employed. For example, well-studied and highly active d 0 metal catalysts tend to suffer from competitive backbiting and formation of cyclosilanes, which can limit molecular weight and increase polydispersity. 4 Despite the fact that the first report on silane dehydrocoupling used Wilkinson's catalyst, 5 initial observations of low dehydrocoupling activity with late metals stunted their development relative to d 0 metal catalysts. 6 Additionally, the use of late metal catalysts in dehydrocoupling is potentially problematic due to two metal-catalyzed side reactions, the oxidation of Si−Si bonds 7 and redistribution. 8 Thus, the potential advances that metal catalysis can offer to polysilane preparation cannot be realized until the factors that impact metal catalysis are better understood. Efforts to minimize these side reactions have included promoting hydrogen loss, 9 employing designer substrates, 10,11 inhibiting silyl coordination, 12 and rigorous exclusion of water and oxygen.While conditions for promoting linear polysilanes over cyclosilanes are known for d 0 metallocene complexes, general conditions to promote dehydrocoupling over redistribution are not known. 4 However, late metal catalysts using nickel, 13−16 rhodium, 17,18 and platinum 19 have shown dehydrocoupling activity and selectivity comparable to that of the group 4 metallocenes, although these methods of polymerization still generally suffer from lower than desired molecular weights and high polydispersity.Iridium pincer complexes bearing the POCOP ligand (POCOP = 2,6-( t Bu 2 PO) 2 C 6 H 3 − ) are thermally robust molecules that have shown high activity as dehydrogenation ca...

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“…The reaction with HX/AlX 3 allows only the preparation of (SiX 2 ) m (m = 4-6, X = Cl, Br, I) [46][47][48] or (PhSiI) 5 [49]; ortherwise the reaction products are mixtures of partially halogenated products. In contrast, dearylation reactions with triflic acid proceed more selectively.…”

Section: Methodsmentioning

confidence: 99%

Silyl triflates: valuable materials for the synthesis of organosilicon oligomers and polymers

Uhlig

1997

Polym. Adv. Technol.

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NMR-Untersuchungen und Konfigurationsanalyse an Cyclopentasilanderivaten

Cited by 14 publications

References 4 publications

Stereostructures of Linear and Cyclic Polyphenylsilanes Produced by Dehydrocoupling in the Presence of Group 4 Metallocene Catalysts

Stereostructures of Linear and Cyclic Polyphenylsilanes Produced by Dehydrocoupling in the Presence of Group 4 Metallocene Catalysts

Iridium Pincer Catalysts for Silane Dehydrocoupling: Ligand Effects on Selectivity and Activity

Silyl triflates: valuable materials for the synthesis of organosilicon oligomers and polymers

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