Alkoxy Hydrosilanes As Surrogates of Gaseous Silanes for Hydrosilylation of Alkenes

Me2SiH2, MeSiH3, and SiH4 are gaseous and flammable silanes that are inconvenient to use in chemical reactions. Catalytic amounts of a nickel pincer complex and NaO'Bu are reported to allow the synthesis of alkyl hydrosilanes from alkenes and alkoxy hydrosilanes, leading to the replacement of Me2SiH2, MeSiH3, and SiH4 by Me-2(MeO)SiH, Me(EtO)(2)SiH, and (MeO)(3)SiH in hydrosilylation reactions of alkenes. The scope and mechanism of the reactions are also described.

Nickel-catalyzed Hydrosilylation of Alkenes and Transition-Metal-Free Intermolecular α-C-H Amination of Ethers

Ivan Buslov

Transition-metal-catalyzed hydrosilylation of olefins is one of the most important methods for the preparation of organosilicon compounds, which have broad applications in both synthetic and material chemistry. For decades, precious metals, principally platinum-based complexes, have been utilized as catalysts for olefin hydrosilylation. However, due to the high and volatile cost and low abundance of Pt, the development of active and selective catalysts based on Earth-abundant metals is highly desirable. In the first chapter, a short overview of state-of-the-art Pt catalysts and recent advances in development of Fe, Co and Ni-catalysts for olefin hydrosilylation is given. The applications, mechanisms and remaining challenges of transition-metal-catalyzed hydrosilylation are discussed. In the chapter 2, chemoselective anti-Markovnikov hydrosilylation of functionalized alkenes using well-defined bis(amino)amide nickel pincer complexes is described. The catalysts exhibit both high turnover frequencies and turnover numbers. Alkenes containing amino, ester, amido, ketone, and formyl groups are selectively hydrosilylated with Ph2SiH2. Chemoselective hydrosilylation of carbon-carbon double bond in the presence of formyl group using a base metal catalyst is reported for the first time. A modification of reaction conditions allows tandem isomerization-hydrosilylation reactions of internal alkenes to give terminal alkyl silanes. Hydrosilylation of alkenes with tertiary silanes is more attractive from practical point of view. The screening of various nickel alkoxide complexes with reduced steric bulk led us to discovery of an efficient heterogeneous catalyst for alkene hydrosilylation with commercially relevant tertiary silanes (chapter 3). The catalyst exhibits high activity in anti-Markovnikov hydrosilylation of unactivated terminal alkenes and isomerizing hydrosilylation of internal alkenes. The catalyst can be applied to synthesize a single terminal alkyl silane from a mixture of internal and terminal alkene isomers. Furthermore, the same catalyst can be used to remotely functionalize an internal alkene derived from a fatty acid. Chapter 4 describes a catalytic system composed of a nickel amido(bisoxazoline) complex and NaOtBu for an unexpected synthetic transformation, leading to the synthesis of functionalized alkyl hydrosilanes from readily available alkenes and alkoxysilanes. This method provides a convenient and safe alternative to hydrosilylation using flammmable and potentially dangerous Me2SiH2, MeSiH3 and SiH4. The reaction mechanism was also described. Efficient and atom-economical creation of C-N bond is one of the major tasks in synthetic organic chemistry. Direct C-H amination has emerged as an attractive method for the construction of new C-N bonds. In the chapter 5 a new transition-metal free method for the intermolecular amination of the alpha-C-H bonds of ethers is described. Using a hypervalent iodine reagent as oxidant the amination of cyclic and acyclic alkyl ethers with a wide range of amides, imides, and amines was achieved. The utility of this method was demonstrated in the synthesis of Tegafur and its analogues. This transition-metal-free method provides a rapid access to a large number of nitrogen-containing organic molecules that may serve as useful synthetic intermediates or biologically active agents.

EPFL2016

Compact reactor for continuous multi-phase reactions

Martin Grasemann

The objective of this thesis is the development of a compact reactor for continuous three-phase reactions in single-pass configurations. The design is based on a bubble column reactor staged with layers of catalytic material. To make the reactor suitable for reactions with high adiabatic temperature rise such as highly exothermic hydrogenation reactions under solvent free conditions, work focuses on two different aspects of integrated reactor and catalyst design: Preparation and characterization of structured catalyst stages optimized for high intrinsic activity as well as for intense gas-liquid redistribution and mass transfer on each reactor stage Development of heat transfer equipment integrated in the staged bubble column for efficient, quasi continuous in-line removal of process heat. Chapter 3 is dedicated to the preparation and characterization of a structured catalyst for the use in a staged bubble column reactor. Sintered Metal Filters (SMF), a material consisting of metal fibers with a diameter of ~ 20 µm forming thin and highly porous sheets is chosen as support material. The fibers are coated with a 5wt% layer of ZnO grains, leaving the open structure of the SMF material intact. Monodispersed Pd nanoparticles as active phase are deposited on the ZnO surface layer with a total metal loading of 0.2wt%. The resulting Pd/ZnO/SMF catalyst shows high activity in the model hydrogenation reaction of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-butene-2-ol (MBE). Semi-batch autoclave hydrogenations show rates of up to 30 mol/molPds with target product (MBE) yields of up to 96.5%. Activity and selectivity of the Pd/ZnO/SMF catalyst are strongly influenced by Strong Metal Support Interaction (SMSI) between the ZnO support and the deposited Pd nanoparticles. XPS and XRD analysis show the formation of a PdZn alloy already under ambient conditions, decreasing the catalyst activity but, on the other hand, preventing strong catalyst deactivation. The formation of this alloy phase was observed under ambient conditions as well as low temperature reaction conditions, but can also be achieved by high temperature treatment of the catalyst in hydrogen atmosphere. The intrinsic kinetics of the MBY hydrogenation was studied in semi-batch experiments. First order towards hydrogen was found and a Langmuir-Hinshelwood kinetic model was developed, describing the experimental data reasonably well. Chapter 4 describes a compact heat exchange (HEX) element integrated in the SBCR. The cross flow heat exchanger design is based on vertical micro-slits of 0.3mm width, a breadth between 12mm and 40mm and 6mm length as reaction side channel geometry, ensuring a minimum of added reaction volume and small heat transfer distances. The single and two-phase heat transfer performance of the HEX element is studied considering the influence of gas and liquid superficial velocities as well as liquid properties. With one SMF layer placed at the HEX element entrance redispersing gas and liquid phase, heat transfer performance was observed to increase linearly with gas and liquid throughput. The maximum heat transfer performance per reaction volume was determined as ~ 0.5 MW/m3K, obtained for moderate gas and liquid superficial velocities. The hydrodynamics of a bubble column staged only with ZnO/SMF sheets show an narrow residence time distribution (RTD) and negligible backmixing through the catalyst layers. The RTD can be described accurately by a "tanks-in-series" model. The gas-liquid pressure drop through the SMF material is mainly governed by capillary effects and the gas velocity. A semi-empirical model is developed, combining both friction and surface tension effects and predicting the pressure drop through the SMF material with an accuracy of ±10%. In Chapter 5, the novel SBCR based on Pd/ZnO/SMF catalytic stages and the integrated micro-HEX elements are tested in the hydrogenation model reaction of MBY to MBE. The reaction in a loop setup comprising 5 HEX/catalyst stages is shown to be strongly influenced by external mass transfer, resulting in an observed catalyst effectiveness below 65%. Intense gas-liquid mass transfer in the SMF structure leads to high volumetric mass transfer coefficient in the staged bubble column of 1s-1 - 4s-1 and an observed reactor productivity of up to ~ 12000 kg/m3h. This signifies an increase of around two orders of magnitude compared to standard industrial semi-batch equipment. High catalyst densities in the reactor lead to high reactor productivity, but have a detrimental effect on product selectivity. The drop in product yield amounts up to 6% for high catalyst density, but can be decreased to only 0.5% if a 5mm distance ring is inserted between HEX stages to decrease the catalyst loading in the reactor.

EPFL2009

Nickamine and Analogous Nickel Pincer Catalysts for Cross-Coupling of Alkyl Halides and Hydrosilylation of Alkenes

Xile Hu, Renyi Shi, Zhikun Zhang

CONSPECTUS: Ligand development plays an essential role in the advance of homogeneous catalysis. Tridentate, meridionally coordinating ligands, commonly termed pincer ligands, have been established as a privileged class of ligands in catalysis because they confer high stability while maintaining electronic tenability to the resulting metal complexes. Pincer ligands containing "soft" donors such as phosphines are typically used for late transition-metal ions, which are considered "soft" acids. Driven by our interest to develop base-metal catalysis and in view of the "hard" character of base-metal ions, our group explored a pincer ligand containing only "hard" nitrogen donors. A prototypical nickel complex of this ligand, "Nickamine", turned out to be an efficient catalyst in a wide range of organic reactions. Because of its propensity to mediate single-electron redox chemistry, Nickamine is particularly suited to catalyze cross-coupling of nonactivated alkyl halides through radical pathways. These coupling partners have been challenging substrates for traditional, palladium-based catalysts because of difficult oxidative addition and nonproductive beta-H elimination. The high activity of Nickamine for cross-coupling leads to high chemoselectivity and functional group tolerance, even when reactive Grignard reagents are employed as nucleophiles. The scope of the catalysis is broad and encompasses spa(3)-spa(3), sp(3)-sp(2), and sp(3)-sp cross-coupling. The defined nature of Nickamine facilitated the mechanistic study of cross-coupling reactions. Experiments involving radical-probe substrates, presumed intermediates and dormant species, kinetics, and density functional theory computations revealed a bimetallic oxidative addition pathway. In this pathway, two Ni centers each provide one electron to support the two-electron activation of an alkyl halide substrate. The success of Nickamine motivated our systematic structure-activity studies aiming at improved activity in certain reactions through ligand modification. Indeed, better catalysts have been developed for cross-coupling of secondary alkyl halides as well as direct alkynylation of alkyl halides. The improvement is attributed to a more accessible Ni center in the new catalysts than in Nickamine. Surprisingly, the improvement could be obtained simply by replacing a dimethyl amino group in Nickamine with a pyrrolidino group. During the study of the catalytic cycle of Nickamine in cross-coupling reactions, we synthesized the corresponding Ni-H species. Consequently, we explored the catalytic application of Nickamine in Ni-H mediated reactions, such as hydrosilylation. To our delight, Nickamine is a chemoselective catalyst for hydrosilylation of alkenes while tolerating a reactive C=O group. An analogous Ni pincer complex was found to catalyze unusual hydrosilylation reactions using alkoxy hydrosilanes as surrogates of gaseous silanes.

AMER CHEMICAL SOC2019