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Hydrogenation and Hydroamination Reactions Using Boron-Based Frustrated Lewis Pairs

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Hydrogenation and Hydroamination Reactions Using Boron-Based Frustrated Lewis Pairs Hydrogenation and Hydroamination Reactions Using Boron-Based Frustrated Lewis Pairs by Tayseer Mahdi A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Chemistry University of Toronto © Copyright by Tayseer Mahdi 2015 Hydrogenation and Hydroamination Reactions Using Boron-Based Frustrated Lewis Pairs Tayseer Mahdi Doctor of Philosophy Department of Chemistry University of Toronto 2015 Abstract New main group systems that provide avenues for small molecule activation have been illustrated using frustrated Lewis pairs (FLPs) – combinations of sterically encumbered Lewis acids and bases which cannot form adducts. The research presented herein expands the small molecule activation and transformation of FLPs using B(C 6F5)3. Combination of the aryl amine tBuNHPh and B(C 6F5)3 under H2 at room temperature leads to its heterolytic splitting forming the complex [tBuNH 2Ph][HB(C 6F5)3]. Exposing the salt to elevated temperatures is found to follow an alternative mechanism resulting in hydrogenation of the N- bound phenyl ring affording the isolable cyclohexylammonium salt [tBuNH 2Cy][HB(C 6F5)3]. This finding is extended to include a series of N-phenyl amines, in addition to mono- and di- substituted pyridines, quinolines and several other N-heterocycles. The reaction of B(C 6F5)3 and H 2 with other substrates, namely ketones and aldehydes are also investigated. Catalytic hydrogenation of the carbonyl functional group is achieved in an ethereal solvent to give alcohol products. In these cases, the borane and ether behave as a FLP to activate H2 and effect the reduction. Similar reductions are also achieved in toluene using B(C 6F5)3 in ii combination with cyclodextrins or molecular sieves. Reductive deoxygenation occurs in the particular case of aryl ketones. Finally, the Lewis acid B(C 6F5)3 is found to enable the intermolecular hydroamination of various terminal alkynes giving iminium alkynylborate complexes of the general formula 1 1 [RPhN=C(CH 3)R ][R C≡CB(C 6F5)3]. The three-component reaction can also be performed catalytically, generating enamine products which are amenable to subsequent hydrogenation reactions giving their corresponding amines. The chemistry is expanded to intramolecular systems forming N-heterocyclic compounds. Furthermore, a FLP route to stoichiometric hydrophosphination of alkynes is developed. iii Acknowledgments Graduate school is not a journey taken alone, rather it is one travelled with companions. I have a large group of wonderful people to thank for travelling by my side, continuously supporting me and putting a smile on my face. First and foremost, I would like to take this opportunity to express my sincere gratitude to my supervisor Prof. Doug Stephan. Thank you for your support, you were always positive and open to discussions. Aside from developing my knowledge in chemistry, you provided me with the opportunity to build relationships and grow professionally. I have also had the honour of having very helpful committee members over the past few years. Profs. Bob Morris and Datong Song, I would like to thank you for your guidance and feedback through the seminar series and committee meetings. Prof. Andrew Ashley, I truly appreciate the time you took to provide me with feedback for this thesis and attend my examination. Thank you to Prof. Erker at the University of Münster for accepting me to do an exchange in his research group. Of course the results in this thesis would not be publishable without the hard work of the staff at the University of Toronto. I would like to thank you all especially Darcy Burns, Dmitry Pichugin, Rose Balazs and Matthew Forbes. Also, I would like to thank Chris Caputo, Peter Mirtchev, Conor Prankevicius, Alex Pulis and Adam Ruddy for your time in editing this thesis. All of the past and present Stephan group members, thank you for the great times and of course for doing your lab jobs and keeping the lab functional. I definitely have to thank you Shanna for keeping us in check. I want to give a big shout out to all my Athletic Centre gym buddies, rock-climbing fellows, Chem Club soccer team champions and amazing Argon crossfitters. I cannot express how much I enjoyed every moment spent doing these outside-the-lab activities. A big I love you to my most amazing siblings Maithem, Christina, Jacob and Hoda. I do not have enough room here to express how much you guys mean to me, but through it all, we have stuck together and this is how we will continue until the end. To my future baby niece, you have put a smile on my face even while you are still inside the womb, I cannot wait to meet you. Finally, to the most supportive and kind-hearted person I have ever met, Renan, you have been there for me from the start of this journey until the end. Thank you all. iv Table of Contents Abstract ........................................................................................................................................... ii Acknowledgments .......................................................................................................................... iv Table of Contents ............................................................................................................................ v List of Figures ................................................................................................................................ xi List of Schemes ............................................................................................................................ xiv List of Tables ............................................................................................................................... xix List of Symbols and Abbreviations .............................................................................................. xxi Chapter 1 Introduction .................................................................................................................... 1 1.1 Science and Technology ..................................................................................................... 1 1.1.1 Boron: properties, production and uses .................................................................. 2 1.1.2 Boron chemistry ...................................................................................................... 3 1.2 Catalysis .............................................................................................................................. 4 1.3 Frustrated Lewis Pairs ......................................................................................................... 5 1.3.1 Early discovery ....................................................................................................... 5 1.3.2 Hydrogen activation and mechanism ...................................................................... 6 1.3.3 Substrate hydrogenation .......................................................................................... 9 1.3.4 Activation of other small molecules ..................................................................... 10 1.3.4.1 Unsaturated hydrocarbons ...................................................................... 10 1.3.4.2 Alkenes ................................................................................................... 11 1.3.4.3 Alkynes ................................................................................................... 11 1.3.4.4 1,1-Carboboration ................................................................................... 12 1.3.4.5 CO 2 and SO 2 ........................................................................................... 13 1.3.4.6 FLP activation of carbonyl bonds ........................................................... 14 1.3.4.7 Carbonyl hydrogenation ......................................................................... 15 v 1.3.4.8 Carbonyl hydrosilylation ........................................................................ 16 1.4 Scope of Thesis ................................................................................................................. 17 Chapter 2 Metal-Free Aromatic Hydrogenation of N-Phenyl Amines and N-Heterocyclic Compounds .............................................................................................................................. 19 2.1 Introduction ....................................................................................................................... 19 2.1.1 Hydrogenation ....................................................................................................... 19 2.1.2 Transfer hydrogenation ......................................................................................... 20 2.1.3 Main group catalysts ............................................................................................. 21 2.1.4 Hydrogenation of aromatic and heteroaromatic substrates ................................... 22 2.1.4.1 Transition metal catalysts ....................................................................... 22 2.1.4.2 Metal-free catalysts................................................................................. 23 2.1.5 Reactivity of FLPs with H 2 ................................................................................... 23 2.2 Results and Discussion ..................................................................................................... 24 2.2.1 H2 activation by amine/borane FLPs ...................................................................
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