National iibrafy Bibliothèque nationale 1+1 of Canada du Canada Acquisitions and Acquisitions et Bibliographic Services services bibliographiques 395 Wellington Street 395. rue Wellington Ottawa ON KlA ON4 Ottawa ON K1A ON4 Canada Canada The author has granted a non- L'auteur a accordé une licence non exclusive licence allowing the exclusive permettant à la National Library of Canada to Bibliothèque nationale du Canada de reproduce, loan, distriiute or sell reproduire, prêter, distribuer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic formats. la forme de microfiche/nlm, de reproduction sur papier ou sur format électronique. The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts from it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. A Theoretical Study of Stereoselectivity in the Diels-Alder Reaction James Dimitrios Xidos BSc. (Chemistry and Applied Mathematics) A thesis submitted to the School of Graduate Studies in partial fulfilment of the requirements for the degree of Doctor of Philosophy Department of Chemistry Mernorial University of Newfoundland St. John's Newfoundland Abstract The control of stereochemistry in chernical reactions is a primary concem for synthetic organic chemists. The Diels-Alder reaction ofien provides good yields of a single stereoisomer. However, the reasons for this stereoselectivity are not always obvious. The Diels-Alder reaction of 5-substituted- l,3-cyclopentadienes has been the focus of considerable debate. 1$3-Cyclopentadienes substituted at the Csposition with OR, NRR', and F yielded 100% syn addition products, while substitution with CH,, SR and CI yielded mixtures of the products of syn and anti addition, and substitution with Si(CH3),, Br, SePh and 1 yielded primarily anti addition products. Explanations in the literature for the preferential syn addition of some dienes usuaily invoived eiectronic phenornena. The degree of facial selectivity for the Diels-Alder reactions of 5-substituted-1,3- cyclopentadienes with a variety of dienophiles can be predicted correctly at the ab inifio HF/6-3 1 G(d) level. It has been determined that the energy required to deform the diene into its syn transition state geometry is the prirnary factor controlling facial selectivity in these reactions. This energy is related to the amount of angular change about C, of the diene in the syn transition state. Facial selectivity roughly correlates with empincal measures of the size of the C, substituent. The size of the bond between Cs and its substituent (Cs-X bond) has been defined to be the second moment, evaiuated at the centroid of charge of the Cs-X bond, of the locaiized molecular orbital which describes the Cs-X bond. This measure of size correlates with facial seiectivity. The Cs-O,C,-N and Cs-F bonds are predicted to be smaller than the Cs-H bond. 11 A substituent factor has been defined by dividing the value of size by the distance between C, and the centroid of charge. The substituent factor correlates excellently with facial selectivity. Thus. facial selectivity cm be fully accounted for based on steric arguments. An additional electrostatic repulsion has been shown to exist for syn addition of 1,3- cyclopentadienes substituted with halogen atoms, C=Nor C =CH. with 1 J.4-uiazoline-3.5- dione. Protonation of the C,-substituent is predicted to stabilize the anti addition transition state. resulting in more anri addition product. Deprotonation of the C,-substituent is predicted to stabilize the syn addition transition state. and to destabilize the anti addition transition state. thus increasing the proportion of syn addition product. The reaction of 3-substituted- 1,2-cyclopropenes with 1,3-butadiene is predicted to yield, in most cases. mainly the product of endo-anti addition, with a lesser amount of the product of exo-anri addition. It is suggested that endo-syn and exo-syn additions are disfavoured due to steric hindrance between the diene and the C,-substituent of the dienophile. The preference for endo-anti addition is primarily due to a favourable interaction between the methylene hydrogen of 3-substituted-1.2-cyclopropene and 1,3-butadiene. This interaction becomes less favourable as the electronegativity of the C,-substituent increases. The substituent factor correlates well with both steric hindrance and electronegativity. It is transferable to other systems, and correlates with geometric trends. Acknowledgments I would like to extend my most sincere appreciation to my supervisors, Dr. D. Jean Burnell and Dr. Raymond A. Poirier, for their instruction, guidance, discussions and support during the course of my research work, and for their faith in my capabilities. 1 would like to acknowledge Dr. Cory C. Pye for his contrîbutions to this research. for his helpful instruction and discussions, and for his fnendship. 1 would dso like to acknowiedge the complementary expenmental study performed by Dr. Lori C. Burry, Mr. Johnathon E. Letoumeau and Mr. Mark A. Wellman. 1 would like to express my appreciation to Dr. C. Robert Lucas for allowing me to use his office during the writing of this thesis. I extend special thanks to Dr. Graham J. Bodwell for his comments and suggestions concenùng this thesis. I would like to express my gratitude to Mr. Randy Dodge. Mr. Paul Fardy, Mr. Alan Goulding. and Mr. Gilbert Wong of the Department of Computing and Communications for their assistance in the computational aspects of this study. 1 wouid like to acknowledge Ms. Diane Burke and Ms. Tamrny Gosse who assisted me with most of the calculations and who tabulated much of the data for the study presented in Section 4 of this thesis. The submission of this thesis ends ten years of study at Mernorial University of Neu-foundland. 1 would like to extend my most sincere appreciation to al1 of the faculty, staff. and students who enlightened my life during my tenure. Special thanks are extended to Dr. Peter Golding. I would like to extend my wannest thanks to my wife Lisa and to my three children. Matthew. Meghan and Nicholas, for their love, patience and understanding. 1 dedicate this thesis to them. Financial support fiorn Memonal University, Dr. D. Jean Burnell and Dr. Raymond A. Poirier is gratefully appreciated. Table of Contents .. Abstract .............................................................. ri Acknowledgments ..................................... ,... ......... iv Table of Contents ....................................................... vi List of Tables ......................................................... viii List of Figures ........................................................ xiv List of Abbreviations and Symbols ........................................ xx 1 . Stereoselectivity in the Diels-Alder Reaction ................................ 1 1.1 Introduction ...................................................1 1-2 Synlanfi Stereoselectivity .........................................5 1.2.1 The Facial Selectivity Hypotheses ......................... 11 1 -3 Endolexo Stereoselectivity ....................................... 19 3 . Facial Selectivity in the Diets-Alder Reactions of 5~Substituted~l.3~Cyclopentadienes........................... -27 2.1 A Systematic Experimental and Theoretical Study .................... 27 2.2 The ab iniiio Study ............................................-31 2.2.1 Facial Selectivity Mode1 ................................. 32 2.2.2 Computational Methods ................................. 35 2.3 Activation Barriers and Energies of Reaction ........................38 2.4 The Search for an Electronic Effect ............................... -49 2-5Geometry .................................................... 70 2.6 Partitioning Activation Energy ................................... 81 2.7 Size and Steric Hindrance ...................................... 100 2.8 What is Measuring? ....................................... 122 3 . Facial Selectivity of Protonated and Deprotonated 5-Substituted- 1, 3-Cyclopentadienes ........................... 126 3.1 Computationai Method ........................................ 126 3 -2 Activation Energy and its Components ............................ 128 3 -3 Geometry and Electronic Structure ............................... 137 3 -4 Steric Hindrance .............................................. 146 4 . Stereoselectivity in the Diels-Alder Reaction of 1,3-Butadiene and 3-Substituted Cyclopropene ................................................... 150 4.1 Computational Method ........................................ 150 4.2 Activation Energy and its Components ............................ 153 4.3 The DestabiIization of the endo-syn and exo-syn Transition States ....... 166 4.4 Endo-anti versus exo-anii Stereoselectivity ......................... 171 4.5 Reactivity of 3-Substituted-1.2-Cyclopropenes ...................... 176 5 . Conclusions and Future Work .......................................... 191 References ........................................................... 196 Appendix ............................................................ 202 vii List of Tables Experimental facial selectivities for the Diels-Alder reaction of CpX with NPM. PTAD and tetracyanoethene . -28 Experimental facial selectivities for the