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Catalytic Amide Reductions Under Hydrosilylation Conditions

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Catalytic Amide Reductions Under Hydrosilylation Conditions Catalytic Amide Reductions under Hydrosilylation Conditions Alexey Volkov © Alexey Volkov, Stockholm 2016 Cover picture: Photo of the tube by Tove Slagbrand Edited by Alexey Volkov ISBN: 978-91-7649-406-6 Printed in Sweden by Holmbergs, Malmö 2016 Distributor: Department of Organic Chemistry, Stockholm University ii “Somewhere, something incredible is waiting to be known.” ― Carl Sagan iii iv Abstract This thesis covers the development of catalytic methodologies for the mild and chemoselective reductions of amides. The first part of the thesis describes the use of a Fe(II)/NHC catalyst for the deoxygenation of aromatic tertiary amides to corresponding amines. The protocol is characterized by low catalyst loading, mild reaction conditions and the use of air and mois‐ ture stable polymethylhydrosilaxane (PMHS) as the hydride source. The second part concerns the development of a protocol for the room temperature deoxygenation of a wide range of tertiary amides to amines using catalytic amounts of Et2Zn and LiCl together with PMHS. The system displayed high levels of chemoselectivity tolerating various reducible groups such as nitro, nitrile, and olefin functionalities, and was shown to be applicable for the reduction of aromatic, heteroaromatic and aliphatic ter‐ tiary amides. The attempts to expand the scope of the Fe‐based protocol to accom‐ modate benzylic tertiary amides led to the development of a transition metal‐free catalytic system based on KOtBu for the formation of enamines. The final products constitute an important class of precursors for a wide range of valuable compounds in organic chemistry. Moreover, avoiding the use of transition metals in the protocol allowed the desired products to be obtained without the hazardous metal contaminants. The last chapter of the thesis describes the Mo(CO)6‐catalyzed hydrosi‐ lylation of amides. The Mo‐based catalyst was proven to mediate the deox‐ ygenation of α,β‐unsaturated tertiary and secondary amides to the corre‐ sponding allylamines without reduction of the olefinic bonds. Further de‐ velopment of the catalytic system revealed an unprecedented chemoselec‐ tivity in the hydrosilylation of aromatic and certain aliphatic tertiary amides in the presence of a variety of reducible groups along with aldehydes and imines that were tolerated for the first time. Moreover, it was possible to control the reaction outcome by variation of the reaction temperature to obtain either amines or aldehydes as the major products. The synthetic utility of the developed Mo(CO)6‐catalyzed protocols was further demon‐ strated in the synthesis of the pharmaceuticals Naftifine and Donepezil. v Populärvetenskaplig sammanfattning på svenska De flesta kemiska processer inom industrin samt flertalet viktiga transformationer inom biokemi är katalyserade. Katalytiska system tillåter reaktioner att ske under milda förhållanden med minskad mängd genererat avfall, i motsats till reaktioner baserade på stökiometriska reagens där betydligt fler biprodukter bildas. De ovan nämnda fördelarna med katalys resulterar i mer kostnadseffektiva, milda och miljövänliga processer. Reduktion av amider är en transformation som kan leda till en rad viktiga föreningar som aminer, iminer, enaminer, aldehyder och alkoholer. Amidbindningen, även känd som peptidbindning, förekommer naturligt i proteiner och enzymer och är mycket stabil i jämförelse med andra karbonyl föreningar som karboxylsyror, estrar, ketoner och aldehyder. De flesta publicerade katalytiska system för amidreduktion är i överlag baserade på dyra ädelmetaller som iridium, rodium och platinum. Ett delmål med denna avhandling var att utveckla katalytiska system baserade på rikligt förekommande och ogiftiga metaller som järn och zink. Resultatet av dessa studier presenteras i kapitel två och tre. Att helt undvika användandet av metaller och istället applicera en organisk molekyl som katalysator är ett annat sätt att närma sig en miljövänlig process. En effektiv organokatalytisk metod för reduktion av amider till enaminer utvecklades och behandlas i kapitel fyra. Eftersom amider generellt är minst reaktiva bland karbonylföreningarna är det en svår uppgift att selektivt reducera denna i närvaro av andra funktionella grupper. Kemoselektiva reduktioner av amider är viktigt då det kan möjliggöra nya syntesvägar samt förkorta kemiska processer. I den sista delen i avhandlingen beskrivs två protokoll där reduktion av amider har utförts med en mycket hög kemoselektivitet. Den syntetiska användbarheten av de sistnämda protokollen demonstrerades i nya syntesvägar för två läkemedel, Naftifin och Donepezil. vi List of publications This thesis is based on the following papers, which will be referred to by their Roman numerals I‐V. Reprints were made with the kind permission from the publishers. The contribution by the author to each publication is clarified in Appendix A. I. Direct Hydrosilylation of Tertiary Amides to Amines by an In Situ Formed Iron/N‐Heterocyclic Carbene Catalyst Alexey Volkov, Elina Buitrago,* Hans Adolfsson* European Journal of Organic Chemistry, 2013, 11, 2066‐2070 II. Mild and Selective Et2Zn‐Catalyzed Reduction of Tertiary Amides under Hydrosilylation Conditions Oleksandr O. Kovalenko, Alexey Volkov, Hans Adolfsson* Organic Letters, 2015, 17, 446‐449 III. Catalytic Reductive Dehydration of Tertiary Amides to Enamines under Hydrosilylation Conditions Alexey Volkov, Fredrik Tinnis, Hans Adolfsson* Organic Letters, 2014, 16, 680‐683 IV. Mo(CO)6 Catalyzed Chemoselective Hydrosilylation of α,β‐ Unsaturated Amides for the Formation of Allylamines Alexey Volkov, Fredrik Tinnis,* Tove Slagbrand, Ida Pershagen, Hans Adolfsson* Chemical Communications, 2014, 50, 14508‐14511 V. Chemoselective Reduction of Tertiary Amides under Thermal Control for the Formation of Aldehydes or Amines Fredrik Tinnis,* Alexey Volkov, Tove Slagbrand, Hans Adolfsson* Angewandte Chemie International Edition, 2016, 55, 4562‐4566 vii Papers not included as part of this thesis: Mild Deoxygenation of Aromatic Ketones and Aldehydes over Pd/C Using Polymethylhydrosiloxane as the Reducing Agent Alexey Volkov, Karl P. J. Gustafson, Cheuk‐Wai Tai, Oscar Verho,* Jan‐E. Bäckvall,* Hans Adolfsson* Angewandte Chemie International Edition, 2015, 54, 5122‐5126 viii Contents Abstract ................................................................................................................................ v Populärvetenskaplig sammanfattning på svenska ........................................ vi List of publications........................................................................................................ vii Contents ............................................................................................................................. ix Abbreviations ................................................................................................................... xi 1 Introduction ..................................................................................... 1 1.1 Catalysis ................................................................................................................... 2 1.2 Reduction of amides ........................................................................................... 3 1.2.1 Non‐catalytic reduction protocols .......................................................................... 4 1.2.2 Catalytic hydrogenation protocols ......................................................................... 6 Heterogeneous catalysts for hydrogenation of amides .................................................... 7 Homogeneous catalysis in hydrogenation of amides ....................................................... 8 1.2.3 Catalytic hydrosilylation of amides ..................................................................... 10 Hydrosilylation of amides proceeding via C‐O bond scission .......................................... 13 a) Precious metal‐catalyzed deoxygenation ................................................................13 b) First row and non‐precious transition metal‐catalyzed protocols ..............................16 c) Transition metal‐free systems ................................................................................19 Hydrosilylation of amides to aldehydes and precursors thereof ..................................... 21 1.3 Objectives of the thesis ................................................................................... 22 2 Direct Hydrosilylation of Tertiary Amides by an In Situ Formed Iron/N‐Heterocyclic Carbene Catalyst (Paper I) ................................. 23 2.1 Introduction ........................................................................................................ 23 2.2 Results and discussion ..................................................................................... 24 2.3 Conclusions ......................................................................................................... 28 3 Mild and Selective Et2Zn‐Catalyzed Reduction of Tertiary Amides under Hydrosilylation Conditions (Paper II) ....................................... 29 3.1 Introduction ........................................................................................................ 29 ix 3.2 Results and discussion ..................................................................................... 29 3.3 Conclusions ......................................................................................................... 32 4 Catalytic Reductive Dehydration of Tertiary Amides
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