Vapor Phase Photochemistry of Cyanopyridines and Pyridine
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VAPOR PHASE PHOTOCHEMISTRY OF CYANOPYRIDINES AND PYRIDINE. DEUTERIUM LABELING STUDIES by Somchoke Laohhasurayotin A Dissertation Submitted to the Faculty of the WORCESTER POLYTECHNIC INSTITUTE in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in Chemistry by ____________________________________________ April, 2005 APPROVED: Examining Committees ________________________________ __________________________________ Professor James W. Pavlik, Advisor Professor W. Grant McGimpsey __________________________________ Professor Venkat R. Thalladi __________________________________ Professor Richard P. Johnson University of New Hampshire ABSTRACT The vapor phase photochemistry of the three isomeric cyanopyridines and the three methylpyridines was studied by irradiating their vapors at 254 nm. It was found that direct irradiation of any one cyanopyridine isomer resulted in the formation of the other two isomers of cyanopyridine or methylpyridines respectively. The reactivity of each isomer was found to be different. This was suggested to be based on the stability of their azaprefulvene intermediates formed during interconversion. The phototransposition of these molecules was suggested to result from 2,6-bonding, nitrogen migration around the five sides of cyclopentenyl ring followed by rearomatization. This mechanism was found to be consistent with the results of deuterium labeling studies of cyanopyridines These result suggest that cyanopyridines undergo phototransposition via the intermediacy of azaprefulvenes instead of Dewar-pyridine and azaprismane. Thus, photochemical studies showed that the six trideuteriopyridine isomers constitute two separate photochemical triads. Each triad consists of three isomers that are photointerconverting upon irradiation at 254 nm in the vapor phase. Similary, it was found that the three isomeric tetradeuteriopyridine isomers also constitute a photochemical triad and are interconverting upon irradiation at 254 nm in the vapor phase. These phototranspositions are best explained by the cyclization, nitrogen migration, and rearomatization mechanism. These results are in contrast to the long-held belief that pyridine is photostable in the vapor phase. Instead, unlabeled pyridine undergoes a hidden phototransposition leading back to itself. ACKNOWLEDGEMENT I would like to express my gratitude to my advisor, Professor James W. Pavlik whose help, guidance, and encouragement helped me all the time during my Ph.D. research and writing of this dissertation. He showed me how to use time and effort efficiently, which is very important for my professional career in the future. I am also grateful to Professor W. Grant McGimsey, Professsor Venkat R. Thalladi, and Professor Richard P. Johnson for the interest they have shown in this dissertation by accepting to be my Ph.D examination committee. I would like to thank Dr. Naod Kebede, friend and colleague, who gave me bunches of his knowledge and advice. Special thanks to Professor Robert E. Connors for letting me use his vacuum rack for my photochemistry work. All my thanks go also to my colleagues, Chuchawin Changtong, for his help in NMR and other instruments, as well as Vuthichai Ervithayasuporn, Jessica Martinez, Tharinee Vongnakorn for their chemistry techniques and the nice work atmosphere. Nantanit Wanichacheva, Hubert Nienaber and Ernesto Soto should not be forgotten because of several times of refreshment and research discussions. I also thanks to all people in Department of Chemistry and Biochemistry, WPI, for their helps and friendliness. I would like to thank my father, mother, and my brother for their understanding and support during these years of study. Last thank go to Supara Tokunalai, for her endless support through my Ph.D study. ii TABLE OF CONTENTS ABSTRACT........................................................................................................................i ACKNOWLEDGEMENT................................................................................................ ii TABLE OF CONTENTS ................................................................................................iii LIST OF FIGURES........................................................................................................ xv LIST OF TABLES........................................................................................................ xxv CHAPTER I Introduction........................................................................................... 1 CHAPTER II Literature review................................................................................... 4 2.1. Photochemistry of pyridine and its derivatives....................................................... 4 2.1.1. Spectroscopic properties of pyridine and methylpyridines................................. 24 2.2. Photochemistry of cyanopyridines ......................................................................... 32 2.2.1. Spectroscopic properties of cyanopyridines ....................................................... 32 2.3. The triplet state of 4-cyanopyridine ....................................................................... 39 CHAPTER III Statement of purpose........................................................................ 40 CHAPTER IV Results and discussion ...................................................................... 43 4.1. Photochemistry of cyanopyridines in the vapor phase......................................... 43 4.1.1. Irradiation of 2-cyanopyridine (36) at 254 nm ................................................... 43 4.1.2. Irradiation of 3-cyanopyridine (37) at 254 nm ................................................... 48 4.1.3. Irradiation of 4-cyanopyridine (38) at 254 nm ................................................... 53 4.1.4. Irradiation of 2-cyanopyridine (36) at ≥ 290 nm ................................................ 58 iii 4.1.5. Irradiation of 3-cyanopyridine (37) at ≥ 290 nm ................................................ 59 4.1.6. Irradiation of 4-cyanopyridine (38) at ≥ 290 nm ................................................ 61 4.1.7. Mechanistic discussion for cyanopyridines ........................................................ 64 4.2. Photochemistry of methylpyridines in vapor phase ............................................. 75 4.2.1. Irradiation of 2-methylpyridine (6)..................................................................... 75 4.2.2. Irradiation of 3-methylpyridine (7)..................................................................... 81 4.2.3. Irradiation of 4-methylpyridine (8)..................................................................... 87 4.2.4. Mechanistic Discussion of methylpyridines ....................................................... 94 4.3. Deuterium labeling studies.................................................................................... 105 4.3.1. Cyanopyridines ................................................................................................. 105 4.3.2. Proton-Deuterium exchange reaction with base catalyst.................................. 105 4.3.3. Synthesis of 4-cyanopyridine-2,6-d2 (38-2,6-d2).............................................. 109 4.3.4. Synthesis of 3-cyanopyridine-2,6-d2 (37-2,6-d2)............................................. 114 4.3.5. Synthesis of 2-cyanopyridine-4,6-d2 (36-4,6-d2).............................................. 118 4.3.6. Pyridines ........................................................................................................... 127 4.3.7. Synthesis of dideuterio-2,6-pyridine (1-2,6-d2)................................................ 129 4.3.8. Synthesis of 3,4,5-trideuteriopyridine (1-3,4,5-d3)........................................... 133 4.3.9. Synthesis of 2,4,6-trideuteriopyridine (1-2,4,6-d3)........................................... 136 4.3.10. Synthesis of 2,3,5,6-tetradeuteriopyridine (1-2,3,5,6-d4) ............................... 142 4.3.11. Synthesis of 2,3,4,6-tetradeuteriopyridine (1-2,3,4,6-d4) ............................... 149 4.3.12. Synthesis of 2,3,4,5-tetradeuteriopyridine (1-2,3,4,5-d4) ............................... 155 4.3.13 Synthesis of 2,3,4-trideuteriopyridine (1-2,3,4-d3), 2,4,5-trideuteriopyridine 160 (1-2,4,5-d3), and 2,3,6-trideuteriopyridine (1-2,3,6-d3).............................................. 160 4.3.14. Synthesis of 2,3,5-trideuteriopyridine (1-2,3,5-d3)......................................... 170 4.4. Photochemistry of deuterated cyanopyridines.................................................... 174 4.4.1. Irradiation of 2-Cyanopyridine-4,6-d2 .............................................................. 174 4.4.2. Irradiation of 3-cyanopyridine-2,6-d2 ............................................................... 180 4.5.3. Irradiation of 4-cyanopyridine-2,6-d2 ............................................................... 186 iv 4.5. Photochemistry of deuterated pyridines.............................................................. 191 4.5.1. Irradiation of 2,6-dideuteriopyridine ................................................................ 191 4.5.2. Mechanistic discussion of deuterated pyridines ............................................... 196 4.5.3. 2,6-Dideuteriopyridine...................................................................................... 198 4.6. Photochemistry of trideuteriopyridines............................................................... 203 4.6.1. Irradiation of 3,4,5-trideuteriopyridine