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Transition Metal-Catalyzed Reduction of Carbonyl Compounds Transition metal-catalyzed reduction of carbonyl compounds Fe, Ru and Rh complexes as powerful hydride mediators Elina Buitrago ©Elina Buitrago, Stockholm 2012 ISBN 978-91-7447-506-7 Printed in Sweden by US-AB, Stockholm 2012 Distributor: Department of Organic Chemistry, Stockholm University ii Abstract A detailed mechanistic investigation of the previously reported ruthenium pseudo-dipeptide-catalyzed asymmetric transfer hydrogenation (ATH) of aromatic ketones was performed. It was found that the addition of alkali metals has a large influence on both the reaction rate and the selectivity, and that the rate of the reaction was substantially increased when THF was used as a co-solvent. A novel bimetallic mechanism for the ruthenium pseudo- dipeptide-catalyzed asymmetric reduction of prochiral ketones was pro- posed. There is a demand for a larger substrate scope in the ATH reaction, and heteroaromatic ketones are traditionally more challenging substrates. Nor- mally a catalyst is developed for one benchmark substrate, and a substrate screen is carried out with the best performing catalyst. There is a high prob- ability that for different substrates, another catalyst could outperform the one used. To circumvent this issue, a multiple screen was executed, employing a variety of ligands from different families within our group’s ligand library, and different heteroaromatic ketones to fine-tune and to find the optimum catalyst depending on the substrate. The acquired information was used in the formal total syntheses of (R)-fluoxetine and (S)-duloxetine, where the key reduction step was performed with high enantioselectivities and high yield, in each case. Furthermore, a new iron-N-heterocyclic carbene (NHC)-catalyzed hydros- ilylation (HS) protocol was developed. An active catalyst was formed in situ from readily available imidazolium salts together with an iron source, and the inexpensive and benign polymethylhydrosiloxane (PMHS) was used as hydride donor. A set of sterically less demanding, potentially bidentate NHC precursors was prepared. The effect proved to be remarkable, and an unprec- edented activity was observed when combining them with iron. The same system was also explored in the reduction of amides to amines with satisfac- tory results. iii iv List of publications This thesis is based on the following papers, which will be referred to by Roman numerals. Reprints were produced with the kind permission of the publisher. I. Mechanistic Investigations into the Asymmetric Transfer Hydrogenation of Ketones Catalyzed by Pseudo-dipeptide Ruthenium Complexes Wettergren, J.; Buitrago, E.; Ryberg, P.; Adolfsson, H. Chem. Eur. J. 2009, 15, 5709 – 5718 II. High throughput screening of a catalyst library for asymmet- ric transfer hydrogenation of heteroaromatic ketones. For- mal syntheses of (R)-fluoxetine and (S)-duloxetine. Buitrago, E.; Lundberg, H.; Andersson, H. G.; Ryberg, P.; Adolfsson, H Manuscript III. Selective hydrosilylation of ketones by in situ generated iron NHC complexes Buitrago, E.; Zani, L.; Adolfsson, H. Appl. Organomet. Chem. 2011, 25, 748 – 752 IV. Efficient and Selective Hydrosilylation of Carbonyls Cata- lyzed by Iron Acetate and N-Hydroxyethylimidazolium Salts Buitrago, E.; Tinnis, F.; Adolfsson, H. Adv. Synth. Catal. 2012, 354, 217 – 222 Appendix B Direct hydrosilylation of tertiary amides to amines in an efficient iron-NHC-catalyzed procedure Buitrago, E.; Volkov, A.; Adolfsson, H. v vi Contents Abstract ..................................................................................................................... iii List of publications ..................................................................................................... v Abbreviations ............................................................................................................. ix 1. Introduction ............................................................................................................. 1 1.1 Reductions ............................................................................................................................ 2 1.2 Transition metal-catalyzed transfer hydrogenation ............................................................... 3 1.2.1 The transfer hydrogenation reaction ............................................................................. 3 1.2.2 Mechanistic aspects ...................................................................................................... 4 1.2.3 The asymmetric transfer hydrogenation reaction ......................................................... 5 1.3 Hydrosilylation of carbonyl compounds ............................................................................... 6 1.3.1 The hydrosilylation reaction ......................................................................................... 6 1.3.2 Transition metal-catalyzed asymmetric hydrosilylation ............................................... 7 2. Mechanistic studies on the asymmetric transfer hydrogenation of ketones catalyzed by pseudo-dipeptide ruthenium complexes (Paper I) .............................. 11 2.1 Results and discussion ......................................................................................................... 13 2.1.1 Kinetic investigations ................................................................................................. 13 2.1.2 Overall kinetic analysis .............................................................................................. 15 2.1.3 Kinetic isotope effects ................................................................................................ 17 2.1.4 Different ruthenium precursors .................................................................................. 18 2.1.5 Mechanistic models .................................................................................................... 19 2.1.6 Substrate scope ........................................................................................................... 21 2.2 Conclusions ......................................................................................................................... 22 3. Selective reduction of heteroaromatic ketones: A combinatorial approach (Paper II) .............................................................................................................................. 23 3.1 Results and discussion ......................................................................................................... 24 3.1.1 Reaction parameters ................................................................................................... 24 3.1.2 Evaluation of the multidimensional screening ............................................................ 25 3.1.3 Formal syntheses of biologically active compounds .................................................. 29 3.3 Conclusions ......................................................................................................................... 31 vii 4. A simple and selective iron-NHC-catalyzed hydrosilylation of ketones (Paper III) .................................................................................................................................. 33 4.1 Results and discussion ......................................................................................................... 35 4.1.1 Optimization of the catalytic system .......................................................................... 35 1.2 Substrate scope .............................................................................................................. 37 4.2 Conclusions ......................................................................................................................... 38 5. Iron-catalyzed hydrosilylation of carbonyl compounds with hydroxyethyl NHC ligands (paper IV) ..................................................................................................... 39 5.1 Results and discussion ......................................................................................................... 39 5.1.1 Synthesis of the ligands .............................................................................................. 39 5.1.2 Optimization of the catalytic system .......................................................................... 40 5.1.3 Substrate scope ........................................................................................................... 41 5.2 Conclusions ......................................................................................................................... 43 6. Efficient iron-NHC-catalyzed reduction of tertiary amides to amines (Appendix B) .............................................................................................................................. 45 6.1 Results and discussion ......................................................................................................... 46 6.2 Conclusions ......................................................................................................................... 49 Concluding remarks .................................................................................................. 51 Acknowledgements ................................................................................................... 53 Appendix A ............................................................................................................... 55 Appendix B ..............................................................................................................
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