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From Ruthenium to Iron for the Catalytic Reduction of Ketones: Catalysis and Mechanistic Insights

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From Ruthenium to Iron for the Catalytic Reduction of Ketones: Catalysis and Mechanistic Insights From Ruthenium to Iron for the Catalytic Reduction of Ketones: Catalysis and Mechanistic Insights by Alexandre Mikhailine A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Chemistry Department University of Toronto © Copyright by Alexandre Mikhailine 2012 ii From Ruthenium to Iron for the Catalytic Reduction of Ketones: Catalysis and Mechanistic Insights Alexandre Mikhailine Doctor of Philosophy Chemistry Department University of Toronto 2012 Abstract A range of air- and moisture-stable phosphonium salts was prepared. Compounds were isolated in high yield and fully characterized. The properties of these compounds and the nature of their formation were explored. The phosphonium salts react with base to give phosphino- aldehydes which are important building blocks in the synthesis of PNNP ligands. The condensation reaction between phosphino-aldehydes and a diamine usually employed for the preparation of PNNP ligands was not applicable to the phosphino-aldehydes derived from these phosphonium salts as a result of the high reactivity of the nucleophilic phosphorus causing uncontrollable side-reaction. In order to resolve this problem, a template reaction with iron(II) Lewis acid was used to suppress the reactivity of the phosphorus via coordination. The reaction was successful and gave rise to bis -tridentate complexes with PNN ligands ([Fe(Ph 2PCH 2CH=N---NH 2)2][BPh 4]2, where N---NH 2 depends on diamine used) as the kinetic product and to desired tetradentate complexes with PNNP ligands ( trans -[Fe(Ph 2PCH 2CH=N--- N=CHCH 2PPh 2)(CH 3CN) 2][BPh 4]2, where N---N depends on diamine used) as a thermodynamic product of the reaction. The reaction appeared to be very general; complexes iii with various diamines incorporated in the ligand backbone were prepared in high yield and fully characterized. Mono-carbonylation reaction of the complexes containing tetradentate PNNP ligands resulted in the formation of the precatalysts with a general formula ( trans -[Fe(Ph 2PCH 2CH=N-- -1 -N=CHCH 2PPh 2)(CO)(Br)][BPh 4]. These precatalysts give active (TOF up to 28000 h ) and enantioselective (up to 95 % ee) catalytic systems for the ATH of ketones when activated with base in a solution of 2-propanol as the reducing agent. On the basis of a kinetic study and other evidence, we propose a mechanism of activation and operation of the catalytic system involving the precatalyst trans - [Fe(CO)(Br)(Ph 2CH 2CH=N-(( S,S)-C(Ph)H-C(Ph)H)-N=CHCH 2PPh 2)][BPh 4] and acetophenone as a model substrate. We determined that the activation of the precatalyst to the active species involves the stereoselective reduction of one imine group of the ligand, since when the active species are quenched with acid, the complex trans -[Fe(CO)(Cl)(Ph 2CH 2CH- (H)N-(( S,S)-C(Ph)H-C(Ph)H)-N=CHCH 2PPh 2)][BPh 4] containing amine and imine functionalities in the backbone is produced. iv Acknowledgements I would like to say a special thank you to my supervisor, Professor Bob Morris, who guided me through my graduate studies, was always helpful in all possible aspects of the research and significantly influenced my personal and professional development. His kindness and patience made my stay in Morris group a pleasant and fruitful experience that I will never forget. I would like also to thank past and present members of the Morris group for their involvement and dedication to the research projects that we were involved in as well as friendly and professional environment that they created inside and outside of the laboratory. I would like to acknowledge the staff personal in the Department of Chemistry at the University of Toronto for their help hard and dedicated work, in particular, Dr. Alan Lough in the X-ray crystallography and Dr. Tim Burrow in the NMR laboratories. At the end I want to say thank you to my family and friend who supported me throughout my studies. v Table of Contents Abstract ......................................................................................................................................... ii Acknowledgements ...................................................................................................................... iv Table of Contents .......................................................................................................................... v List of Figures .............................................................................................................................. xi List of Schemes ........................................................................................................................... xv List of Tables ............................................................................................................................ xvii List of Abbreviations ................................................................................................................. xix Chapter 1: Introduction ................................................................................................................. 1 1.1 The importance of chiral alcohols .................................................................................. 1 1.2 Synthesis of Chiral alcohols: Stoichiometric Reactions ..................................................... 5 1.3 Synthesis of Chiral alcohols: Catalytic Reactions .............................................................. 6 1.4 Transition metal catalyzed asymmetric reduction of ketones ............................................. 7 1.4.1 Dihydrogen complexes ................................................................................................ 7 1.4.2 Formation of metal hydrides: oxidative addition ......................................................... 9 1.4.3 Formation of metal hydrides: deprotonation of dihydrogen complexes .................... 10 1.4.4 Catalytic reduction of ketones ................................................................................... 11 1.5 Transfer hydrogenation of ketones ................................................................................... 15 1.6 Iron as an alternative metal for the catalytic asymmetric transfer hydrogenation of ketones. ................................................................................................................................... 18 1.7 Thesis outline .................................................................................................................... 20 1.8 References ......................................................................................................................... 24 Chapter 2: Cyclic Phosphonium Salts as Versatile Ligand Precursors: Synthesis and Characterization .......................................................................................................................... 29 2.1 Abstract ............................................................................................................................. 29 vi 2.2 Introduction ....................................................................................................................... 29 2.3 Results and Discussion...................................................................................................... 33 2.3.1 One carbon spacer between the phosphorus and reactive carbon centre ................... 33 2.3.2 Two carbon spacer between the phosphorus and reactive carbon centre .................. 37 2.3.3 Three carbon spacer between the phosphorus and reactive carbon centre ................ 39 2.5 Conclusions ....................................................................................................................... 40 2.4 Experimental ..................................................................................................................... 41 2.4.1 General comments ..................................................................................................... 41 2.4.2 General procedure for preparation of the dimers 2.3a and 2.3b ................................ 42 2.4.3 Dimer 2.3a ................................................................................................................. 42 2.4.4 Dimer 2.3b ................................................................................................................. 43 2.4.5 Procedure for preparation of the dimer 2.3d .............................................................. 43 2.4.6 Major diastereomer of 2.3d ........................................................................................ 44 2.4.7 Minor diastereomer of 2.3d ....................................................................................... 44 2.4.8 Procedure for preparation of the tetramer 2.5 ............................................................ 45 2.4.9 Procedure for preparation of the monomer 2.7 .......................................................... 46 2.4.10 Structure determination of 2.3a, 2.3d and 2.7 by X-ray diffraction ........................ 47 2.6 References ......................................................................................................................... 49 Chapter 3: Template Syntheses of Iron(II) Complexes Containing Chiral PNNP and PNN Ligands ....................................................................................................................................... 51 3.1 Abstract ............................................................................................................................. 51 3.2 Introduction ......................................................................................................................
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