Isolation of Enantiomers via Diastereomer Crystallisation Parathy Anandamanoharan 12 March 2010 A thesis submitted for the degree of Doctor of Philosophy of the University of London Department of Chemical Engineering University College London Torrington Place London WC1E 7JE ABSTRACT Enantiomer separation remains an important technique for obtaining optically active materials. Even though the enantiomers have identical physical properties, the difference in their biological activities make it important to separate them, in order to use single enantiomer products in the pharmaceutical and fine chemical industries. In this project, the separations of three pairs of diastereomer salts ( Fig1 ) by crystallisation are studied, as examples of the ‘classical’ resolution of enantiomers via conversion to diastereomers. The lattice energies of these diastereomer compounds are calculated computationally (based on realistic potentials for the dominant electrostatic interactions and ab initio conformational energies). Then the experimental data are compared with the theoretical data to study the efficiency of the resolving agent. CH 3 R +- NH 3 OOC Phenylethylammonium R= CH 3 2-phenylpropanoate R= C2H5 2-phenylbutanoate R= OH mandelate Fig1 Three pairs of diastereomer salts studied All three fractional crystallisations occurred relatively slowly, and appeared to be thermodynamically controlled. Separabilities by crystallisation have been compared with measured phase equilibrium data for the three systems studied. All crystallisations appear to be consistent with ternary phase diagrams. In the case of R = CH 3, where the salt-solvent ternaries exhibited eutonic behaviour, the direction of isomeric enrichment changed abruptly on passing through the eutonic composition. In another example, R = OH, the ternaries indicated near-ideal solubility behaviour of the salt mixtures, and the separation by crystallisation again corresponded. i Further, new polymorphic structures and generally better structure predictions have been obtained through out this study. In the case of R = CH 3, an improved structure of the p-salt has been determined. In the case of R = C 2H5, new polymorphic forms of the n-salts, II and III, have been both discovered and predicted. This work also demonstrates that chemically related organic molecules can exhibit different patterns of the relative energies of the theoretical low energy crystal structures, along with differences in the experimental polymorphic behaviour. This joint experimental and computational investigation provides a stringent test of the reliability of lattice modelling to explain the origins of chiral resolution via diastereomer formation. All the experimental and computational works investigated in this thesis are published (see APPENDIX 1 ). ii ACKNOWLEDGEMENTS I would like to thank my supervisors, Professor Alan Jones and Professor Peter Cains for the continuous guidance, time, advice, support and encouragement throughout my project. It has been a privilege to work with them, as they have taught me to resolve complex problems throughout my project and all I have learned from them will be invaluable tools in my future career. I would like to say special thanks to Dr Panagiotis (Panos) Karamertzanis, Professor Sally (Sarah) Price and all the other helpful staffs from the Chemistry Department, at UCL, for guidance on using the molecular modelling tools and scientific advices throughout my studies and also for performing some of the characterisation studies. A special thanks to Ghulam Warsi and other technical staffs for providing help with the materials necessary to carry out this project and for their patience. Finally, I would also like to thank the EPSRC organisation for funding my research through a departmental research studentship. iii TABLE OF CONTENTS Abstract i Acknowledgements iii Table of contents iv List of Tables ix List of figures xi 1 INTRODUCTION 1 2 SURVEY OF LITERATURE AND METHODS 9 2-1 Stereochemistry and Crystallisation 9 2-1-1 Stereochemistry 9 2-1-2 Enantiomeric molecules or enantiomers 11 2-1-3 Diastereomeric molecules or diastereomers 13 2-1-4 Crystallisation 14 2-1-5 Racemic crystallisation 15 2-1-6 Diastereomeric crystallisation 15 2-1-7 Separation of racemates via classical resolution 18 2-1-8 Separation of racemates via resolution by direct and preferential crystallisations 18 2-1-9 Separation of racemates via kinetic resolution 20 2-1-10 Resolution of racemates by diastereomeric salt formation 21 2-2 Crystallography and polymorphs 22 2-2-1 Crystallography 22 2-2-1-1 Crystal structures 23 2-2-1-2 Packing and symmetry 27 2-2-1-3 Space group nomenclatures 29 2-2-1-4 Forces responsible for crystal packing – Hydrogen bonding 30 2-2-1-5 A given substance can crystallise in different ways 34 2-2-1-6 How crystals form? 34 2-2-1-7 Nucleation 35 2-2-1-8 Crystal growth 38 iv 2-2-1-9 Survey of crystallisation methods 38 2-2-2 Polymorphism 40 2-2-2-1 Definitions 40 2-2-2-2 Effect of additives 43 2-2-2-3 Examples 44 2-2-2-4 Polymorphic compounds in the Cambridge Structural Database 45 2-2-2-5 Powder Diffraction File 46 2-2-2-6 What polymorphism can do in resolution? 46 2-2-2-7 How to choose polymorphs in pharmaceutical applications? 47 2-3 Phase Equilibria 48 2-3-1 Phase Equilibria 48 2-3-1-1 The Gibbs phase rule 48 2-3-1-2 Triangular phase diagrams 50 2-3-1-3 The lever rule 51 2-3-1-4 Solubility diagrams of diastereomer salt mixtures 52 2-3-2 Solubility Measurements 57 2-4 Analysis and characterisation methods and techniques 60 2-4-1 Chromatographic methods-HPLC 60 2-4-2 Diffraction methods 64 2-4-2-1 X-ray powder diffraction methods 65 2-4-2-2 Single-crystal X-ray diffraction methods 65 2-4-3 Calorimetric methods 66 2-5 Previous work in this field 67 2-6 Molecular modelling 70 2-6-1 Introduction 70 2-6-2 The assumptions which may allow lattice energy calculations to be used to predict diastereomeric resolution 71 2-6-3 Lattice energy 74 2-6-3-1 Modelling the lattice energy of simple ionic solids 74 2-6-3-2 Lattice energy for molecular salts 76 2-6-3-3 Molecular flexibility 77 2-6-3-4 Programs used for calculations in this thesis 77 v 2-7 Summary 79 3 EXPERIMENTAL AND MODELLING METHODS 80 3-1 Experimental methods 80 3-1-1 Solubility measurements 81 3-1-2 Ternary equilibrium measurements 84 3-1-3 Separability measurements 87 3-1-4 HPLC measurements 88 3-1-5 Preparation of RRA2 single-crystal diastereomer salt samples for structure determination 90 3-1-6 Further experimental work to determine different RRA2 polymorphs 91 3-1-6-1 By changing acid:base ratio 91 3-1-6-2 By replacing ethanol with other solvents 92 3-1-6-3 By changing the pH of ethanol 92 3-1-7 Slurry experiments 93 3-1-7-1 Choice of solvent 93 3-1-7-2 Slurry experiment 94 3-1-8 Characterisation of crystallisation samples 94 3-1-9 Experimental relative stabilities 95 3-2 Modelling methods 96 3-2-1 Experimental lattice energy minimisation calculation – ExptMinExpt 96 3-2-2 Ion conformational energy or Intramolecular energy - ∆EIntra 98 3-2-3 Theoretical lattice energy minimisation calculation accounting for conformational contributions to lattice energy – ExptMinCOpt 100 3-3 Summary 101 4 EXPERIMENTAL RESULTS AND DISCUSSION 102 4-1 Experimental crystal structures and their structural characteristics 102 4-1-1 Experimental crystal structures 102 4-1-2 Experimental relative stability 109 4-2 Solubility measurements 111 vi 4-2-1 Solubility curve of (R)-1-phenylethylammonium- (R,S)-2-phenylpropanoate A1 112 4-2-2 Solubility curve of (R)-1-phenylethylammonium- (R,S)-2-phenylbutyrate A2 113 4-2-3 Solubility curve of (R)-1-phenylethylammonium- (R,S)-mandelate A3 114 4-3 Phase equilibria 115 4-3-1 Phase equilibria of system (R)-1-phenylethylammonium-(R,S)-2- phenylpropanoate A1 115 4-3-2 Phase equilibria of system (R)-1-phenylethylammonium-(R,S)-2- phenylbutyrate A2 118 4-3-3 Phase equilibria of system (R)-1-phenylethylammonium-(R,S)- mandelate A3 120 4-4 Separability Measurements 121 4-4-1 Separability measurements of (R)-1-phenylethylammonium- (R,S)-2-phenylpropanoate A1 122 4-4-2 Separability measurements of (R)-1-phenylethylammonium- (R,S)-2-phenylbutyrate A2 124 4-4-3 Separability measurements of (R)-1-phenylethylammonium- (R,S)-mandelate A3 126 4-5 Further study on polymorph determination 127 4-5-1 Extra work results to determine different (R)-1-phenylethylammonium- (R)-2-phenylbutanoate (RRA2) polymorphs 127 4-5-2 Slurry experiment results 128 4-6 Discussion on the experimental results 129 4-7 Summary 133 5 MODELLING RESULTS AND DISCUSSION 134 5-1 Experimental lattice energy minimisation calculations (ExpMinExpt) 134 5-2 The crystal energy to predict the relative thermodynamic stability 136 5-3 Discussion on the modelling results 139 5-4 Summary 143 6 CONCLUSION 144 vii 7 FUTURE WORK 147 LIST OF PUBLICATIONS 150 REFERENCES 151 APPENDIX 1: PUBLISHED JOURNALS 161 APPENDIX 2: THE CRYSTALLOGRAPHIC INFORMATION FILES (CIF) FOR THE DETERMINED AND REDETERMINED STRUCTURES 186 APPENDIX 3: PACKING DIAGRAMS 190 APPENDIX 4: THERMAL MEASUREMENTS FOR A2 192 APPENDIX 5: THERMAL MEASUREMENTS FOR A3 194 APPENDIX 6: DATA CORRESPONDING TO ENTHALPY ∆H AND ENTROPY ∆S CALCULATIONS 196 APPENDIX 7: EQUILIBRIUM OF A1 AT 30 AND 50°C 198 APPENDIX 8: EQUILIBRIUM OF A2 AT 30 AND 50°C 204 APPENDIX 9: EQUILIBRIUM OF A3 AT 30 AND 50°C 208 APPENDIX 10: SEPARABILITY CALCULATION SHEET OF A1 AT DIFFERENT TEMPERATURES 211 APPENDIX 11: SEPARABILITY CALCULATION SHEET OF A2 AT DIFFERENT TEMPERATURES 215 APPENDIX 12: SEPARABILITY CALCULATION SHEET OF A3 30°C 218 viii