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741.
Lukas Tendera Moritz Helm Dr. Mirjam J. Krahfuss Dr. Maximilian W. Kuntze-Fechner Prof. Dr. Udo Radius 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(71):17849-17861
A case study on the effect of the employment of two different NHC ligands in complexes [Ni(NHC)2] (NHC=iPr2ImMe 1Me , Mes2Im 2 ) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni2(iPr2ImMe)4(μ-(η2 : η2)-COD)] B / [Ni(iPr2ImMe)2(η4-COD)] B’ or [Ni(Mes2Im)2] 2 , respectively, with alkynes afforded complexes [Ni(NHC)2(η2-alkyne)] (NHC=iPr2ImMe: alkyne=MeC≡CMe 3 , H7C3C≡CC3H7 4 , PhC≡CPh 5 , MeOOCC≡CCOOMe 6 , Me3SiC≡CSiMe3 7 , PhC≡CMe 8 , HC≡CC3H7 9 , HC≡CPh 10 , HC≡C(p-Tol) 11 , HC≡C(4-tBu-C6H4) 12 , HC≡CCOOMe 13 ; NHC=Mes2Im: alkyne=MeC≡CMe 14 , MeOOCC≡CCOOMe 15 , PhC≡CMe 16 , HC≡C(4-tBu-C6H4) 17 , HC≡CCOOMe 18 ). Unusual rearrangement products 11 a and 12 a were identified for the complexes of the terminal alkynes HC≡C(p-Tol) and HC≡C(4-tBu-C6H4), 11 and 12 , which were formed by addition of a C−H bond of one of the NHC N-iPr methyl groups to the C≡C triple bond of the coordinated alkyne. Complex 2 catalyzes the cyclotrimerization of 2-butyne, 4-octyne, diphenylacetylene, dimethyl acetylendicarboxylate, 1-pentyne, phenylacetylene and methyl propiolate at ambient conditions, whereas 1Me is not a good catalyst. The reaction of 2 with 2-butyne was monitored in some detail, which led to a mechanistic proposal for the cyclotrimerization at [Ni(NHC)2]. DFT calculations reveal that the differences between 1M e and 2 for alkyne cyclotrimerization lie in the energy profile of the initiation steps, which is very shallow for 2 , and each step is associated with only a moderate energy change. The higher stability of 3 compared to 14 is attributed to a better electron transfer from the NHC to the metal to the alkyne ligand for the N-alkyl substituted NHC, to enhanced Ni-alkyne backbonding due to a smaller CNHC−Ni−CNHC bite angle, and to less steric repulsion of the smaller NHC iPr2ImMe. 相似文献
742.
743.
744.
Kathrin Kunowa Sebastian Schmidt-Lehr Werner Pauer Hans-Ulrich Moritz Christian Schwede 《Macromolecular Symposia》2007,259(1):32-41
Summary: In this article particular attention is paid to processes of mixing fluids with different viscosities relevant for polymerization where the reaction is fast and mixing is the limiting factor. Apart from this, mixing fluids with different viscosities is still one of the challenging tasks in industrial chemistry. Therefore, the characterization of mixing elements is another important topic. Two different multiple chemical reactions, based on the principle of competitive-parallel reactions, were used and compared to investigate (micro)mixing efficiency in polymerization reactors. The well-known Villermaux-Dushman reaction and the third Bourne reaction were applied. The observed product distribution represents the quantity of segregation of the fluid which gives in turn information about the dependency on certain parameters like type and speed of stirrer, dosing period, feed position, and the viscosity of the fluid. The results from semibatch and continuous stirred tank reactors and two different stirrers, Rushton and INTERMIG® impeller, are discussed. 相似文献
745.
Sabine Beuermann Michael Buback Moritz Gadermann Michael Jürgens Fabian Günzler 《Macromolecular Symposia》2004,206(1):229-240
Free-radical terpolymerizations of styrene, methyl methacrylate, and glycidyl methacrylate were carried out in a tubular reactor in the presence of 20 wt.% CO2 at temperatures between 120 and 180°C and pressures of 300 and 350 bar. The number average molecular weights, MN, were mostly between 2000 and 3000 g·mol−1 and polydispersity indices around 2. In part of the experiments molecular weights were controlled by n-dodecyl mercaptan serving as the chain-transfer agent. PREDICI modeling indicates that the targeted molecular weights of MN∼2500 g·mol−1 and polydispersities around 2 may also be reached by using an initiator cocktail, a mixture of two initiators with significantly different decomposition rate coefficients. The predictions are confirmed experimentally. 相似文献