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Durham E-Theses Durham E-Theses The photochemistry of highly uorinated pyridines Middleton, Roderick How to cite: Middleton, Roderick (1977) The photochemistry of highly uorinated pyridines, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/8331/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk UNIVERSITY OF DURHAM A THESIS entitled THE PHOTOCHEMISTRY OF HIGHLY FLUORINATED PYRIDINES Submitted by RODERICK MIDDLETON, B.Sc.(DUNELM) (Van Mlldert College) The copyright of this thesis rests with the author. No quotation from it should be published without his prior written consent and information derived from it should be acknowledged. A candidate for the degree of Doctor of Philosophy . 1977 To My Mother MEMORANDUM This work was carried out at the University of Durham between October 1973 and October 1976. It is the original work of the author unless indicated by reference and has no been submitted for any other degree. Part of this work has been the subject of the followin publications. R. D. Chambers, R. Middleton and (in part) R. P. Corbally, J. C. S. Chec. Comms.. 1975, 731. R. D. Chambers and R. Middleton, J. C. S. Chetxi. Comms.. 1977, 15*+ R. D. Chambers and R. Middleton, J. C. S. Perkln IT 1977, 1500 ACKNOWLEDGEMENTS I would like to express my gratitude to Professor R. D. Chambers for his help and encouragement and to Dr. R. S. Matthews for his assistance in interpretation of n.m.r. spectra. I would also like to thank the many technical and laboratory staff for their assistance and Mrs. M. Towers for typing this thesis. Thanks are also due to the S.R.C. for the provision of a maintenance grant. SUMMARY A series of highly fluorinated pyridines were irradiated using ultra violet radiation, in an attempt to detect re• arrangements in these systems. The pyridines used were penta-, tetra-, tri- and di- substituted by perfluoroalkyl groups. Perfluoro-^-ethyl-2,6-di-isopropyl-3,5-dimethylpyridine, upon prolonged irradiation, in solution, at both 253»7nm. and 300nm., isomerised to an inseparable mixture containing two remarkably stable azaprismanes, which were subsequently identi• fied from their pyrolysis products. The mechanism for formation of this azaprismane mixture was shown to involve the rearrange• ment of a 2-azabicyclo[2.2.0]hexadiene intermediate to a 1- azabicyclo[2.2.0]hexadiene intermediate. l Irradiation of perfluoro- +-ethyl-2,6-di-isopropyl-3)5- dimethylpyridine, at both 253«7nm. and 300nm., for a short time, resulted in formation of a stable l-azabicyclo[2.2.0]- hexa-2,5-diene derivative. Irradiation of perfluoro-2,lf,6-tri-isopropyl-3»5-dimethyl- pyridine, at 253«7nm., gave a single symmetrical azaprismane, the identity of which was confirmed from the pyrolysis product. The penta-alkyl pyridines, obtained from the pyrolysis of azaprismanes, showed interesting features in their n.m.r. spectra which enabled their identification. A transference technique vas used for the irradiation of perfluorotetra- and tri-alkyl pyridines. Perfluoro-2,6-di-isopropyl-3,5-dimethylpyridine isomerised, on irradiation, to a moderately stable 2-azabicyclo- [2.2.0]hexadiene derivative. The thermal rearrangement of this derivative was studied using differential scanning calorimetry. Irradiation of perf luoro-2,l+,6-tri-isopropylpyridine gave the first stable derivative of 2-azabicyclo[2.2.0]hexa-2,5- diene. A l-azabicyclo[2.2.0]hexadiene derivative was also obtained in low yield. Irradiation of perf luoro-25-tri-isopropylpyridine gave a mixture containing a 2-azabicyclo[2.2.0]hexadiene derivative (55#) and a 1-azabicyclo[2.2.0]hexadiene derivative {h%). The valence isomers obtained were shown to rearrange upon further irradiation. The thermal stability of valence isomers is discussed and the half-lives of some isomers, obtained in this work, measured. Valence isomers were not detected from irradiation of either perfluoro-2,5-di-isopropylpyridine or pentafluoropyridine Attempts were made to react valence isomers with NaOMe, bromine and furan. Irradiations were also carried out on a few fluorinated azacyclohexadienes. Both isomerisation and fragmentation reactions were observed. CONTENTS Page CHAPTER 1 ISOMERISATION IN SIX MEMBERED AROMATIC RINGS 1.1 Introduction 1 1.2 The Photolysis of Benzene 1 1.3 Synthesis of Valence Isomers of Benzene *+ i) Bicyclo[2.2.0]hexa-2,5-diene (Dewar Benzene) h ii) Tricyclo[3-l-0.02,6]hex-3-ene (Benzvalene) *f iii) Tetracyclo[2.2.0.02'6.o3>5]hexane (Prismane) 6 l.^f Theoretical Treatment of Rearrangement of Benzene to its Valence Isomers 7 1.5 Theoretical Treatment of Rearrangements in Aromatic Rings i) Introduction 12 ii) Rearrangements proceeding via Benzvalene Intermediates 12 iii) Rearrangements Proceeding via Prismane Intermediates lh 1.6 Theoretical Rearrangements and Interconversions of Valence Isomers i) Introduction 15 ii) Interconversion of Dewar Benzene and Prismane 15 iii) Rearrangement of Benzvalenes 16 iv) Rearrangement of Benzvalene to Fulvene v) Isomerisation of Benzvalene to Dewar Benzene vi) Isomerisation of Prismane to Benzvalene Photolysis of Labelled Benzenes i) Introduction ii) Labelling using Deuterium iii) Photochemical Rearrangement of Di-substituted Benzenes A. Di-t-butyl benzenes B. Xylenes iv) Photochemical Rearrangement of Tri-t-butyl benzenes v) Photochemical Rearrangement of Tetra-substituted Benzenes A. Tetra-t-butyl benzenes B. Tetrakis(trimethylsilyl) benzenes vi) Valence Isomers of Hexamethyl- benzene Rearrangement of Heterocyclic Systems i) Pyridines ii) The Photolysis of Pyridinium Ions iii) Photolysis of Pyrones and Pyrylium Cations iv) Photoisomerisation of Pyraglne FluorlnsiLed ByeteuiB i) Fluorinnted Benzenes ii) Valence Isomers from Perfluoro alkyl Benzenes Page iii) Rearrangement in Fluorinated Benzenes iv) Pyridines 51 v) Rearrangement of Fluorinated Pyridazines 5*+ vi) Photoisomerisation of Fluorinated Pyrazines 56 1.10 Bicyclopropenyls 57 1.11 Ring Transpositions: a Suggested Notation 63 1.12 2,3-Diazabicyclo[2.2.0]hexa-2,5-diene 65 CHAPTER 2 THE SYNTHESIS OF POLYFLUOROALKYLPYRIDINES 2.1 Introduction 66 2.2 Pentafluoropyridine 66 2.3 Perfluoropolyisopropylpyridines 66 2»h Reactions of Pentefluoropyridine and P.T.F.E. 69 2.5 Reactions Between Perfluoro-3,5- dimethylpyridlne and Hexafluoro- propene, in the Presence of Fluoride Ion 71 2.6 Nucleophilic Substitution in Perfluoro-3-methylpyridine by Heptafluoro-isopropyl Anion 73 2.7 Reaction Between Perfluoro-2,6-di- isopropyl-3,5-dimethylpyridine and Tetrafluoroethylene, in the Presence of Fluoride Ion 73 2.8 Reaction Between Perfluoro-3,5- dimethylpyridine and Pentafluoro- ethyl Anion 71+ £&££ CHAPTER 3 THE PHOTOCHEMISTRY OF SOKE PERFLUORO- ALKYL PYRIDINES 3.1 Introduction 76 3.2 The Photochemistry of Perfluoropenta- alkylpyridines i) A. Irradiation of Perfluoro-^- ethyl-2,6-di-isopropyl- 3,5-dimethylpyridine 77 B. Pyrolysis of Azaprismane Mixture 80 C. n.m.r. Spectra of Penta- alkylpyridines - Chemical Shifts D. Pyrolysis of Azaprismane Mixtures of Varying Compositions 87 E. Deduction of Azaprismane Structures 88 F. Rearomatisation of Azaprismanes (159) and (160) 88 (1) Rearomatisation vis Benz• valene Intermediates 90 (2) Rearomatisation via Para- bonded Intermediates 92 G. Mechanism for Formation of (160) from (i5J) 95 H. Mechanism for Formation of (159) from (153) (1) A One Stage Mechanism 96 (2) Mechanisms Which Involve Rearrangement of Para- bonded Isomers 97 I. Irradiation of Perfluoro-^- ethyl-2,6-di-isopropyl-3,5- dimethylpyridine (153). at 253-7 n.m. in a Transference System 103 ii) A. Irradiation of Perfluoro-2,- i+.6-tri-isopropyl-3,5- dimethylpyridine ( IM-8). at 253.7 n.m. B. Pyrolysis of Perfluorp-2,*+»D- tri-isopropyl-3,5-dimethyl-l- azatetracyclo[2.2.0.02>°.03>5 hexane (179) iii) Irradiation of Perfluoro-2- ethyl-3,6-di-isopropyl-l+,5- dimethylpyridine (156). at 253.7 n.m. iv) Irradiation of Perfluoro-2-ethyl 5,6-di-isopropyl-3,lf-dimethyl- pyridine (15_2), at 253-7 n.m. The Photochemistry of Perfluorotetra- alkylpyridines i) A. Introduction B. Irradiation in a Transference System ii) Irradiation of Perfluoro-2,6-di- isopropyl-3,5-dimethylpyridine (JLk2)i at 253.7 n.m. A. In Solution B. In a Transference System iii) Irradiation of Perf luoro-2 6- tri-isopropyl-3-methylpyridine (151)• at 253-7 n.m. The Photochemistry of Perfluorotri- alkylpyridines i) Irradiation of Perf luoro-2,>+,6- tri-isopropylpyridine (H8) A. Static Irradiations B. In a Transference System ii) Irradiation of Perfluoro-2.5- tri-isopropylpyridine (1^.7)T at 253.7 n.m. Under Transference iii) Irradiation of Perfluoro-2- isopropyl-3)5-dimethylpyridine (150). at 253.7 n.m. Under Transference Pass iv) Irradiation of Perfluoro-'f,6- di-isopropyl-3-methylpyridine (152), at 253.7 n.m. Under Transference 12o 3.5 Irradiation of a Perfluorodi-elkyl- pyridine. Perfluoro-2,5-di-iso- propylpyridine (132) 127 3.6 Irradiation of Pentafluoropyridine (H6) 127 3.7 Irradiation of Perfluorotetra-kis- isopropylpyrazine (207) 129 3.8 Conclusions 129 CHAPTER h THE THERMAL STABILITY AND PROPERTIES OF VALENCE ISOMERS OF PYRIDINES *+.l Introduction 132 h.2 D.S.C. as Applied to Valence Isomer Rearrangements i) Introduction 132 ii) Activation Energy from D.S.C. 133 iii) Valence Isomer Rearrangements Studied by D.S.C.
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