Control and Prediction of Eliminative Cyclisation Dinga Wonanke A thesis presented for the degree of Doctor of Philosophy in Chemistry School of Physical and Chemical Sciences University of Canterbury August, 2018 Abstract Eliminative cyclisation is a class of intramolecular reactions that provide a versatile synthetic route to functionalised and substituted polyaromatic and heteroaromatic compounds, which form the building blocks of molecular electronic devices such as organic photovoltaics and organic light-emitting diodes. These reactions may be either thermally or photochemically driven, and proceed via two steps: 1) planarisation to form a stable proto-aromatic intermediate, and 2) elimination of the groups or atoms adjacent to the forming bond to yield the final fully aromatic product. Although experimental studies have been performed on a wide range of starting materials to characterise reaction mechanisms and product distributions, there remains no predictive model for determining whether or not any given reactant will yield a product under specified reaction conditions. This thesis investigated the factors that control both the thermodynamics and kinetics of these reactions by explicitly mapping out reaction coordinate profiles i and analysing induced atomic forces upon photo-excitation for a diverse range of molecules. Results showed that the kinetics and thermodynamics of both planari- sation and elimination in thermal cyclisation are highly sensitive to the reaction conditions, which include the presence and position of the heteroatoms as well as the oxidation potential of the reactants. In photocyclisation, the planarisation step is a photo-activated excited-state process that is in contrast less sensitive to reaction condition; elimination is proposed to proceed in the ground state. The magnitude of the projected force along the bond vectors of the putative forming ring is shown to be a good predictor for the outcome of the photoplanarisation step. ii Dedication I dedicate this work to the Almighty God. Ps 127:1-3 iii Declaration I declare that the work presented in this thesis is mine except otherwise stated. This study was undertaken under the supervision of Deborah Crittenden in the School of Physical and Chemical Sciences, University of Canterbury. A part of this thesis has been incorporated into a previously published research paper: 1. Chapter 3: Wonanke A. D. Dinga, Crittenden Deborah L. (2018) Beyond the Woodward-Hoffman Rules: What Controls Reactivity in Eliminative Aromatic Ring-Forming Reactions?. Australian Journal of Chemistry 71, 249-256. iv Acknowledgements My most profound gratitude and thanks go to my supervisor, Deborah Critten- den, who has been an incredibly amazing, supporting, encouraging go-to person who makes even stressful moments still worth experiencing. Thanks so much for guiding and assisting me throughout this period. It has been a fantastic journey tapping your wealth of knowledge Special thanks go to my wonderful mother, who has been a continuous source of support and inspiration and who made it a duty to call me every morning and evening to ensure that I was keeping well. Mummy, you are special and one of the best gifts from God. I would like to thank all of the university lecturers and staff who are so friendly and approachable. Sincere thanks go especially to Sally Gaw, Sarah Masters, Bryce Williamson, Jan Wikaira and Nathan Alexander for all their continuous support throughout this period. v I would like to thank my lovely sisters, Dorothy, Akazong and Mbiatedem for being such wonderful human beings who make life so enjoyable and sweet. Big thanks to my one-and-only big brother Frank and my amazing mentor Terence A. Thank you all for making these years of studies enjoyable, regardless of the distance that separated us all. Many thanks as well to my office and group mates and especially to Ali, Nate and Mr Flanagan for their continual support and motivation. I would also like to thank my friends who have been so supportive during these last years. Vijay and Rachel, Biplang, Jess and Eion, Sam, Carissa, Richie, Chad, Nneka, Surin, Hans, Luis, Jake, Ryan, Lisa, Anna, Rob, Dave Jardin. Furthermore, I would like to thank my future wife. If you ever get to read this thesis, know that I was thinking of you. Finally, I would like to acknowledge the Marsden Fund Council from New Zealand Government funding, managed by the Royal Society, Te Ap¯arangi for sponsoring this research. vi Contents 1 Introduction 1 1.1 Optoelectronic technologies . .1 1.1.1 Organic semiconductors . .2 1.2 Polycyclic aromatic compounds . .4 1.3 Synthesis of PACs . .7 1.3.1 Eliminative cyclisation . .7 1.3.2 Problems with eliminative cyclization . 11 1.3.3 Computational modelling . 13 1.4 Objective of the present work . 18 1.4.1 Scope of thesis . 18 2 Computational Methods 34 2.1 Introduction . 35 2.2 Electronic structure methods . 35 2.2.1 Wave function methods: Hartree-Fock theory . 36 vii 2.2.2 Wave function methods: Post Hartree-Fock . 37 2.2.3 Density functional theory . 39 2.2.4 Excited-state wave function methods . 40 2.2.5 Density functional methods for excited states . 41 2.2.6 Basis sets . 41 2.2.7 Solvent model . 43 2.3 Characterising the potential energy surface . 45 2.3.1 Geometry optimisation . 46 2.3.2 Constrained geometry optimisation . 47 2.3.3 Freezing-string method . 48 2.4 Computational details . 48 3 Effect of Position and Nature of Heteroatom in Eliminative Cy- clisation 66 3.1 Introduction . 67 3.2 Methods . 72 3.3 Results and Discussion . 74 3.3.1 Thermal cyclisation . 74 3.3.2 Oxidative photocyclisation . 78 3.3.3 Eliminative photocyclisation . 81 3.4 Conclusions . 84 4 Substituent Effects on Substituted Benzylidene O-methyloximes 94 4.1 Substituents in organic reactions . 95 4.1.1 Inductive effect of substituents . 95 4.1.2 Resonance effect . 96 4.2 Substituent effects in cyclisation . 100 4.2.1 Basic steps in eliminative cyclisation . 100 viii 4.2.2 Substituent effects on thermal eliminative cyclisation . 101 4.2.3 Substituent effects on photochemical eliminative cyclisation . 102 4.3 Methods . 103 4.3.1 Data set . 103 4.3.2 Computational procedure . 106 4.4 Results and Discussion . 107 4.4.1 Unsubstituted benzylidene O-methyloximes . 107 4.4.2 Para-substituted benzylidene O-methyloximes . 112 4.4.3 Ortho-substituted benzylidene O-methyloximes . 114 4.4.4 Literature context . 117 4.4.5 Conclusion . 118 5 A New Photocyclisation Reactivity Predictor 128 5.1 Introduction . 129 5.2 Existing reactivity predictors for photocyclisation . 131 5.2.1 Localisation energy . 131 5.2.2 Free valence index . 132 5.2.3 Laarhoven rules for photocyclisation . 136 5.2.4 Variation in electronic overlap population . 138 5.2.5 Variation in bond order . 139 5.2.6 Summary . 141 5.2.7 Limitations of existing reactivity predictors . 142 5.3 Rationale for new reactivity predictor . 143 5.3.1 Induced atomic forces upon vertical excitation . 144 5.4 Methodology and computational details . 145 5.4.1 Force projection on bond vector . 145 5.4.2 Data set . 147 5.4.3 Procedure . 147 ix 5.5 Results and Discussion . 148 5.5.1 Predictive power . 164 5.5.2 Comparison with Laarhoven’s rules . 173 5.5.3 Practical application . 175 5.6 Summary . 176 6 Photocyclisation Reactivity Predictor: Generalised Case Studies and Testing 185 6.1 Introduction . 186 6.2 Dataset and Methodology . 187 6.3 Results and discussion . 187 6.3.1 Class one: Heteroatom-containing systems with a single cyclisation centre . 188 6.3.2 Class two: Heteroatom-containing molecules with multiple potential photocyclisation centres . 193 6.3.3 Case study 3: Systems that undergo non-oxidative elimination201 6.4 Summary . 204 7 Conclusion and Future Work 208 7.1 Conclusion . 209 7.2 Recommendation for synthetic chemists . 210 A Complete details of all species involved in reaction pathway and full details of thermochemical data 215 A.0.1 Detailed mechanism for Scholl reaction . 215 A.0.2 Detailed mechanism for Mallory reaction (oxidative pathway) 218 A.0.3 Detailed mechanism for Mallory reaction (eliminative path- way)............................... 219 x A.0.4 Geometries, energies and thermochemical data for reaction pathway minima, transition states and intermediates . 221 B Complete Thermochemical Data for benzylidene O-methyloximes228 C Simple Hückel molecular orbital theory calculation of bond order of trimethylene methane 232 xi CHAPTER 1 Introduction 1.1 Optoelectronic technologies The aim of every human civilisation from one generation to another is to devise new methods and means that can be used to meet the needs of humanity cost- effectively. The industrial revolution in the 19th century was a very significant human invention that brought a remarkable improvement in the living conditions and social wellbeing of humanity. [1–3] Impetus from this has led to an increased in the desire to develope new techniques and devices to accelerate production pro- cesses but with little or no human accountability for sustainability. [4,5] The direct consequence of this is the present day electronic revolution, which is the driving force of a vast majority of modern day technology. [6,7] Today, our world can be referred to as an electronic world. Electronic technol- 1 ogy has led to a remarkable improvement and growth in information technology, economic development, health, agriculture, energy, and national security. [8] For this reason, a great deal of scientific and technical research has focused on de- veloping electronically active materials and devices such as transistors, integrated circuits, lasers, optical fibres and solar cells. The primary raw materials responsible for the electronic properties of these devices are inorganic semiconductors such as silicon, germanium, and compounds obtained from combining group III and group V elements.