Manganese, Iron and Cobalt Catalyzed Reductive Hydrogenation and Cross-Coupling Reactions Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften Dr. rer. nat. an der Fakultät für Chemie und Pharmazie der Universität Regensburg vorgelegt von Efrain Reyes-Rodriguez Regensburg 2018 ii iii Für meine Mutter iv v The experimental part of this work was carried out between January 2015 and March 2018 at the University of Regensburg, Institute of Organic Chemistry under the su- pervision of Prof. Dr. Axel Jacobi von Wangelin. The thesis was submitted on: 19.12.2018 Date of the defense: 25.01.2019 Board of examiners: Prof. Dr. Rainer Müller (chairman) Prof. Dr. Axel Jacobi von Wangelin (1st referee) Jun.-Prof. Dr. Ivana Fleischer (2nd referee) Prof. Dr. Frank-Michael Matysik (examiner) vi Contents 1 Introduction 1 1.1 Environmental Aspects of Chemical Transformations . .2 1.2 Current State of 3d Transition Metal Catalysis . .3 1.3 Scope of the Thesis . .8 1.4 References . 12 2 Recyclable Cobalt(0) Nanoparticle Catalysts for Hydrogenations 15 2.1 Introduction . 16 2.2 Results and Discussion . 17 2.3 Conclusion . 24 2.4 Experimental Section . 25 2.4.1 General information . 25 2.4.2 General Procedures . 26 2.4.3 Synthesis of Starting Materials . 28 2.4.4 Hydrogenation Reactions . 30 2.4.4.1 Catalyst and Substrate Screening . 30 2.4.4.2 Isolated Hydrogenation Reaction Products . 38 2.4.5 ICP-OES Measurement . 60 2.4.6 ICP-MS Measurement . 61 2.4.7 Functional Group Tolerance Tests . 62 2.4.8 Comparison of Different Co-Np Preparations . 63 2.4.9 Recycling Experiments . 64 2.4.10 Particle Analyses . 64 2.5 References . 67 vii viii Contents 3 A Manganese Nanosheet: New Cluster Topology and Catalysis 73 3.1 Introduction . 74 3.2 Cluster Synthesis and Characterization . 75 3.3 Hydrogenation Results . 79 3.4 Conclusion . 83 3.5 Experimental Section . 84 3.5.1 General Information . 84 3.5.2 General Procedures . 85 3.5.3 Synthesis and Characterization of the Manganese Nanocluster [Mn6(m3-H)4(m2-H)2{m2-N(SiMe3)2}4{N(SiMe3)2}2] (2) . 86 3.5.4 Radical clock reaction . 88 3.5.5 Cyclotrimerization Reactions of Phenyl Acetylene . 89 3.5.6 Reaction of 2 with 4-Me-Pyridine . 90 3.5.7 Hydroboration of Pyridine . 91 3.5.8 X-Ray Structure . 95 3.5.9 Magnetic Measurements . 96 3.5.10 Poisoning Studies . 97 3.5.11 Synthesis of Starting Materials . 98 3.5.12 Hydrogenation products . 103 3.6 References . 113 4 Alkene Metalates as Hydrogenation Catalysts 119 4.1 Introduction . 120 4.2 Results and Discussion . 122 4.2.1 Precatalyst Syntheses . 122 4.2.2 Catalytic Hydrogenations . 126 4.2.3 Mechanistic Studies . 130 4.2.4 Methodology Extensions . 138 4.3 Conclusions . 143 4.4 Experimental Section . 144 4.4.1 General Information . 144 4.4.2 General Procedures . 146 4.4.3 1H-NMR Spectra of the New Complexes 5−7 . 148 4.4.4 Photographic Images of Monitoring Experiments . 150 4.4.5 Negative-Ion Mode ESI Spectra . 151 4.4.6 X-Ray Crystallography . 155 4.5 References . 157 Contents ix 5 Iron-Catalyzed Cross-Coupling of Secondary Alkyl Chlorides 163 5.1 Introduction . 164 5.2 Results and Discussion . 165 5.3 Conclusion . 179 5.4 Experimental Section . 180 5.4.1 General Information . 180 5.4.2 General Procedures . 181 5.4.3 Synthesized Starting Materials . 182 5.4.4 Ligand Synthesis . 187 5.4.5 Synthesis of b-Ketiminato Iron Complexes . 188 5.4.6 Optimization Experiments . 190 5.4.6.1 Catalyst Loading Experiments . 190 5.4.6.2 Optimization Experiments Using Electron Withdraw- ing Aryl Grignard Reagents . 190 5.4.6.3 Effect of Fluorinated Substrates as Additive . 191 5.4.6.4 Amine and Amide Ligand Screening . 192 5.4.6.5 Further Investigation into Ligand Activity . 196 5.4.6.6 Use of Well-Defined Iron(II) Complexes . 197 5.4.7 Mechanistical Investigations . 199 5.4.7.1 Using 4-Chlorophenylmagnesium Bromide . 199 5.4.7.2 Catalyst Poisoning Experiments Under Ligand-Free Con- ditions . 200 5.4.7.3 Radical Clock Experiments . 200 5.4.7.4 Competition Reactions Between LMeFeAr and Ar’MgBr 201 5.4.8 Isolated Coupling Products . 202 5.4.9 Cyclic Voltammetry . 217 5.4.10 UV/Vis Experiments . 218 5.4.11 NMR Reaction Analysis . 219 5.4.12 X-Ray Crystallography . 221 5.4.13 Selected NMR-Spectra . 223 5.5 References . 226 x Contents 6 Transition Metal-Free Reductive Silylation of (Het)Aryl Bromides 233 6.1 Introduction . 234 6.2 Results and Discussion . 235 6.3 Conclusion . 240 6.4 Experimental Section . 241 6.4.1 General Information . 241 6.4.2 General Procedures . 242 6.4.3 Reaction Optimization . 244 6.4.4 Synthesis of Starting Materials . 246 6.4.5 Isolated Coupling Products . 250 6.5 References . 274 7 Appendix 279 7.1 List of Figures . 279 7.2 List of Schemes . 283 7.3 List of Tables . 285 7.4 Acknowledgements . 287 Chapter 1 Introduction Reductive hydrogenation and cross- coupling utilizing cheap and readily available base metal catalysts H X R' X R' R H2 R Fe H ArMgBr R Cl Co Mn R Ar R' R' ArX transition metal free R3Si-X Mg R3Si-Ar One-pot reductive silylation of aryl bromides and chlorides mediated by microwave irradiation Abstract: The need for sustainable chemical transformations has been steadily in- creasing in recent years due to the rising prices of noble metals and new ecological policies to reduce atmospheric pollution and hazardous waste. This introductory chapter strives to describe the basic principles necessary to attain “green” and sus- tainable chemistry in our modern industrial age as well as going into more detail on the importance of base metal catalysis. Lastly, an overview of the subsequent chapters is given. 1 2 Introduction 1.1 Environmental Aspects of Chemical Transformations The biosphere of our planet has enabled the proliferation of aerobic organisms and the accumulation of oxidized matter on the earth’s surface (water, CO2, oxides, car- bohydrates). On the other hand, reduced chemical compounds are valuable sources of energy found only in deeper layers or isolated reservoirs (hydrogen, methane, petroleum, natural gas, coal, solid metals). Many oxidation events proceed spon- taneously or are highly thermodynamically favored, while reduction reactions often require an external driving force through the supply of energy or high-energy re- agents. The scarcity of high-energy resources makes the reductive transformation of available oxidized raw materials (biomass, water, atmosphere) into energy sources (H2, methanol) and intermediates (NH3, synthesis gas, platform chemicals) one of the greatest challenges of modern industrial societies.[1] The emerging field of Green Chemistry aims to overcome these challenges in an environmentally sustainable way. Its main goals are summarized by the Twelve Prin- ciples of Green Chemistry which were postulated by Paul Anastas and John Warner in 1998.[2] In general, Green Chemistry strives to minimize the use of hazardous chemicals and waste production as well as maximizing the efficiency of chemical reactions while relying mostly on renewable feedstocks. A lot of different methods have been developed to reduce energy consumption (e.g. by using microwave irradi- ation instead of conventional heating methods),[3] waste generation (e.g. recycling of catalyst material[4] or the use of flow reactors[5]) and the use of hazardous chemicals (e.g. substitution of solvents for water[6] or ionic liquids[7]). The use of catalysts represent a cornerstone in the abovementioned principles of Green Chemistry and is generally seen as a very important aspect for the economical generation of chemical compounds in industry. Catalytic processes are a vital tool in the development of new synthetic routes for the functionalization of molecules. It is estimated that 75% of the existing industrial chemical transformations and 90% of newly developed processes include the use of catalysts.[8] The established cata- lyst systems are based predominantly on second- and third-row transition metals, i.e. ruthenium, rhodium, palladium and iridium[9] as well as on nickel and cop- per.[10] Despite their high efficiency and broad application, the development of novel synthetic routes and catalytic systems is in full swing. This is not least owed to the increasing prices of noble metals, the toxicity of nickel and the new ecological policies to reduce atmospheric pollution and to eliminate hazardous waste. In this regard, first-row transition metals, like Mn, Co and Fe constitute a viable alternative to the existing catalyst metals. The selected examples in Figure 1.1 show the advantages of first-row transition metals in comparison to the most-used noble metals - they are abundant, very cheap and pose small threat to the environment and to human health. A direct comparison shows a difference of multiple orders of magnitude between the selected 3d transition Chapter 1.2: Current State of 3d Transition Metal Catalysis 3 Figure 1.1: Comparison of abundance, price, and greenhouse potential for highly- used catalyst metals. A logarithmic.