Reactions of Ethoxysilanes with Silica: A Solid-State NMR Study

Behringer, Klaus D.; Blümel, Janet

doi:10.1021/ja00112a033

Cited by 137 publications

(127 citation statements)

References 31 publications

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“…[1±3,9±13] Special care has to be taken regarding the tempera- [62] Selected angles [8] [64] Selected angles [8] ture [17] and the solvent [16] used for the immobilization. It is important that the temperature of the immobilization be as low as possible, [17] the silica be as dry, [18] and the solvent be as unpolar [16] as possible. In the case of immobilization via Si(OEt) 3 groups, as soon as one covalent bond is formed, the linker cannot be detached from the surface any more.…”

Section: Crystal Structures Of the Nickel Complexesmentioning

confidence: 99%

“…[69] The reaction of alcohols with silica has already been described by Iler [70] and one of us. [18] However, regarding the robustness of covalent bonding to the support in the case of ligands with OH groups, such as 3 and 4, no systematic studies have been done yet to the best of our knowledge. Therefore, we undertook surface detachment studies, as we have previously done with ethoxysilane species.…”

Section: Crystal Structures Of the Nickel Complexesmentioning

confidence: 99%

“…[15] With silica, the different forms of leaching can be prevented efficiently by applying the right conditions concerning solvent [16] and temperature [17] during the immobilization step, and using rigorously dried silica. [16,18] As soon as there is one covalent Si-O-Si bond established, the linker is irreversibly bound. [15] Detachment of the metal fragment from the phosphine moieties [11,12] can be prevented by chelat-ing phosphines with ethoxysilane groups.…”

Section: Introductionmentioning

confidence: 99%

“…[19] As a complementary method, suspension NMR spectroscopy [15,20] allows the rapid check of a great number of routine samples, and also helps to detect merely adsorbed in contrast to covalently bound species. [15,18] The cyclotrimerization of alkynes represents one of the most useful methods for the selective synthesis of various substituted aromatic compounds. [21] Since many transition metal complexes are active catalysts for cyclotrimerizations, it is not surprising that the interest in both mechanistic as well as synthetic aspects of cyclotrimerizations has even been growing recently.…”

Section: Introductionmentioning

confidence: 99%

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New Silica‐Immobilized Nickel Catalysts for Cyclotrimerizations of Acetylenes

Reinhard¹,

Šoba²,

Röminger³

et al. 2003

Adv Synth Catal

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New dicarbonylnickel catalysts with a variety of mono-and bidentate phosphine ligands, such as Ph 2 P(CH 2 ) x PPh(CH 2 ) y Si(OEt) 3 (x 2, 3; y 2, 3), have been synthesized and fully characterized, three of them by X-ray analyses. All the catalysts have been immobilized on silica by reaction of the ethoxysilane or hydroxy groups of the phosphine linkers with the surface silanol groups. The clean immobilization was proved by 31 P solid-state NMR. It has been demonstrated that the C À O À Si bridge, in contrast to the strong Si À O À Si bond, is easily cleaved by organic solvents and water. With respect to the cyclotrimerization of phenylacetylene the optimal reaction conditions for all molecular and immobilized catalysts have been determined. Cyclooctane was the best solvent, and the optimal reaction temperature was 100 8C for reactions in homogeneous phase, and 155 8C for surface-bound catalysts. The bite angles and spacer lengths of the chelating ligands did not play a major role for the performance of the homogeneous or immobilized catalysts. The latter could be recycled for 12 times, with final substrate conversions still typically between 30 and 40%, and turnover numbers (TON) around 1500. With an excess of substrate in one run a TON of 4810 was obtained for an immobilized carbonylnickel complex with a dppp-type chelate phosphine (x 3, y 2). Reduction of the surface coverage generally resulted in catalysts with longer lifetimes, for example, reduction to one fourth gave a record TON of 7616 for this catalyst. The selectivities of the catalysts all followed one trend. The main products were the cyclotrimers 1,2,4-and 1,3,5-triphenylbenzene, besides dimers, linear trimers and tetramers as side-products. The fraction of the latter decreased at higher temperatures with homogeneous catalysts, and vanished entirely after a few cycles with the immobilized catalysts. On recycling, the initial ratio of unsymmetrical to symmetrical cyclotrimers of typically 5 : 1 gradually changed to about 1 : 1 for all catalysts.

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Section: Crystal Structures Of the Nickel Complexesmentioning

confidence: 99%

Section: Crystal Structures Of the Nickel Complexesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

New Silica‐Immobilized Nickel Catalysts for Cyclotrimerizations of Acetylenes

Reinhard¹,

Šoba²,

Röminger³

et al. 2003

Adv Synth Catal

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show abstract

“…A strained siloxane group generated by dehydroxylation has been shown to react with alkoxysilane groups, [12] and this reactivity opens up the possibility of supporting metallocene complexes with pendant alkoxy or silyloxy groups on dehydroxylated silica.…”

Section: Introductionmentioning

confidence: 99%