Hydroboration Studies of Olefins with Heterocyclic Hydroborating Reagents with Various Transition Metal Complexes: Attemped Diastereoselective Synthesis of Homoallylic Alcohols via Hydroboration. Thesis submitted in fulfilment of the requirements for the degree Master of Science By Mbongeni M. Shungube School of Chemistry and Physics College of Agriculture, Engineering and Science University of KwaZulu-Natal Pietermaritzburg December 2016 Declaration The experimental work described in this dissertation was carried out in the School of Chemistry, College of Agriculture, Engineering and Science, University of KwaZulu-Natal, Pietermaritzburg, under the supervision of Professor Ross S. Robinson. I hereby declare that these studies represent original work by the author and have not otherwise been submitted in any form for any degree or diploma to any tertiary institution. Where the use of published information from other authors has been made and it is duly acknowledged in the text. Signed …………………………………………… Mbongeni M. Shungube (Candidate) I hereby certify that this statement is correct. Signed …………………………………. Professor Ross S. Robinson (Supervisor) i Abstract Hydroboration is one of the key routes to afford organoborane esters which are versatile intermediate in organic synthesis. Investigations of different hydroborating reagents and catalysts toward hydroboration present alternative to achieve organoborane ester with different stability, selectivity, and yield. Hence, a part of this study was focused on the preparation of heterocyclic hydroborating reagents with a single site available for hydroboration. Four reagents were prepared which included the well-known catecholborane (23) and pinacolborane (24) and the hardly studied 2,3-dihydro-1,3,2-benzodiazaborole (31) and 2,3-dihydro-1,3,2-benzoxazaborole (34). A study to utilize hydroboration to prepare allylic boronic esters using the four hydroborating reagent above was also undertaken. These esters were to be further employed for diastereoselective synthesis of homoallyl alcohols. Unfortunately, study failed after attempts to preparation and extraction these esters such as (Z)-4,4,5,5-tetramethyl-2-(3-methylpent-2-en-1-yl)-1,3,2-dioxaborolane (104) were unsuccessful. The 11B−NMR spectroscopy results indicated the formation of a new signal of boron compound which was assumed as the corresponding esters, however, was never isolated for further evidence. The four hydroboration reagents mentioned above were further investigated for hydroboration with different transition metal catalysts namely Wilkinson’s catalyst (58), dimethyltitanocene ipr 2,6-bis-[1-(2,6-diisopropylphenylimino)ethyl]pyridine iron(ІІ) chloride (( BIP)FeCl2) (79) and Mes 2,6-bis-[1-(2,4,6,-trimethylphenylimino)ethyl]pyridine iron(ІІ) chloride (( BIP)FeCl2) (80). ipr These study indicated the better hydroboration efficiency of the ( BIP)FeCl2 (79) and Mes ( BIP)FeCl2 (80) for the hydroboration with 2,3-dihydro-1,3,2-benzodiazaborole (31) and 2,3-dihydro-1,3,2-benzoxazaborole (34) compared to the Wilkinson’s catalyst (58). Poor reactivity of dimethyltitanoce toward hydroboration was also observed. Furthermore, within this study two boronic esters 2-octyl-1,3,2-benzoxazaboralane (53) and 2-(2-phynylpropyl)-1,3,2-benzoxazaborolane (116) were isolated for the first time. ii Acknowledgements I would like to thank my supervisor Professor Ross S. Robinson for providing me with the opportunity to carry out this research and his continuous professional guidance and encouragements throughout this project. I would also like thank Dr. Sphamandla S. Sithebe for all his imports and advice. I would also like thank my fellow research group members Lesley Dralle and Tim Underwood I also extend my gratitude to the technical staff as well as the postgraduate students particularly Wonder W. Mpapane, Nontokozo Duma, Thobeka Hlengwa for their encouragement and support. Special thanks to the following support staff: Mr. Craig Grimmer for his support and most efficient running of the NMR spectrometers, Mrs. Caryl Janse van Rensburg for running mass spectrometer, I would like to express my gratitude to Mr. Feyzil Shaik, Mr. Shawn Ball, Mr. Paul Forder and Mr. Clerence Mortlock and to all my colleagues in the Warren lab. I wish to express my deepest gratitude to my family sister Lindiwe P. Shungube, my brother Thembikosi M. Shungube, my nephew Thando B. Tshabalala, my mother Lizzy N. Mahlalela, my father Elphas M. Shungube, my aunt Grace Shungube, Phumzile Shungube, Sfiso J. Mhlaba, Nelsiwe Jwankie and my friends Wonder W. Mpapane, Menzi F. Mahlalela and Patrice S. Shongwe for all the love, supportive, motivations and patient I have received from all. I also gratefully acknowledge the financial support from NRF and my supervisor Professor Ross S. Robinson iii Table of Contents Declaration ....................................................................................................................................... i Abstract ........................................................................................................................................... ii Acknowledgements ........................................................................................................................ iii Table of Contents ........................................................................................................................... iv List of Figures ................................................................................................................................ ix List of Schemes ............................................................................................................................. xii List of Tables ................................................................................................................................ xvi Abbreviations .............................................................................................................................. xvii 1. Introduction ............................................................................................................................. 1 1.1. Organoboranes and their importance: Direct focus on boronic esters .......................... 1 1.2. Discovery of Hydroboration ......................................................................................... 2 1.3. Stoichiometry and Regioselectivity .............................................................................. 4 1.4. Mechanism of Hydroboration ....................................................................................... 6 1.5. Hydroborating Reagents ............................................................................................. 10 1.5.1. Borane and its Adducts ............................................................................................. 10 1.5.2. Alkylborane Hydroborating Reagents ...................................................................... 12 1.5.2.1. Thexylborane (9) .............................................................................................. 12 1.5.2.2. Bis(3-methyl-2-butyl)borane (7) ...................................................................... 13 1.5.2.3. Dicyclohexylborane (18). ................................................................................. 14 1.5.2.4. 9-borabicyclo[3.3.1]nonane (11). ..................................................................... 14 1.5.2.5. Diisopinocampheylborane (21). ....................................................................... 15 1.5.3. Heterocyclic Hydroborating Reagents. .................................................................... 17 1.5.3.1. Oxygen based heterocyclic hydroborating reagents ........................................ 19 1.5.3.2. Sulphur based heterocyclic hydroborating reagents. ....................................... 24 iv 1.5.3.3. Nitrogen based heterocyclic hydroborating reagents ....................................... 26 1.5.3.4. Mixed Based Heterocyclicboranes ................................................................... 28 1.6. Metal-catalyzed hydroboration ................................................................................... 31 1.6.1. Wilkinson’s catalysts (58) ........................................................................................ 31 1.6.2. Other rhodium(I) Catalysts employed for Hydroboration ........................................ 38 1.6.3. Iridium-Catalyzed Hydroboration ............................................................................ 40 1.6.4. Titanium catalyzed hydroboration ............................................................................ 42 1.6.5. Iron-catalyzed hydroboration ................................................................................... 45 2. Research rational ................................................................................................................... 52 2.1. Previous research with the research group. ................................................................. 52 2.2. Project Aims and Objectives ....................................................................................... 54 3. Results and Discussions ........................................................................................................ 56 3.1. Preparation of Hydroborating reagents. ...................................................................... 56 3.1.1. Synthesis of catecholborane (23) ............................................................................