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Multiple Substituent Effects on the Rate Constants for N(2)

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Multiple Substituent Effects on the Rate Constants for N(2) Multiple Substituent Effects on the Rate Constants for N(2)-N(3) Restricted Rotation of cis-1,3-Diphenyltriazenes by Herlina Lim A thesis presented to the University of Waterloo in fulfillment of the thesis requirement for the degree of Master of Science in Chemistry Waterloo, Ontario, Canada, 2006 © Herlina Lim 2006 AUTHOR'S DECLARATION FOR ELECTRONIC SUBMISSION OF A THESIS I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis, including any required final revisions, as accepted by my examiners. I understand that my thesis may be made electronically available to the public. ii Abstract Triazenes represent an interesting class of organic photochromic materials based on cis-trans isomerization around a nitrogen-containing double bond, and are of potential use in optical memory devices. One structural property of triazenes related to cis-trans isomerization is restricted rotation around the N(2)-N(3) bond. The purpose of the study presented in this thesis is to investigate the influence of aryl substitution on the N(2)-N(3) rotational barrier of meta and/or para substituted cis-1,3-diphenyltriazenes. Rate constants for restricted rotation in cis-triazenes were measured by means of a laser-flash photolysis system, via trans-to-cis photoisomerization of corresponding trans-triazenes (which were synthesized either by classical or non-classical diazonium coupling reactions). A quantitative structure-reactivity correlation analysis of rate constants for restricted rotation of cis-triazenes was carried out in terms of an extended Hammett equation. Resulting Hammett reaction constants for restricted rotation are –1.88 ± 0.08 and 0.70 ± 0.08 for N(1) and N(3), respectively, indicating that restricted rotation of cis-triazenes is more sensitive to the electronic character of the aryl group at N(1) than to that at N(3). Interestingly, the observed rate constants for restricted rotation in cyano-containing substrates are found to be pH-dependent. The latter is attributed to a change in the ionization state of the substrate, which is supported by solvent isotope effects. Furthermore, the pH dependence observed for unsymmetrical cyano-containing substrates indicates that upon deprotonation of the amino group, the negative charge concentrates either on the 4-CN substituent attached to the phenyl ring at N(3) or on the triazeno moiety. Thus, the primary cis-anionic 4-cyano-containing forms are not the same ions and do not equilibrate rapidly, since they undergo cis-to-trans isomerization. Finally, an extended Hammett equation was also employed in the analysis of pKa values of trans-triazenes. The results show that there is a direct mesomeric interaction between the charge at N(3) in the triazeno moiety with 4-CN and 4-NO2 groups in the systems studied. Consequently, the Yukawa-Tsuno modification of the Hammett equation is introduced to correlate pKa values and Hammett substituent constants. iii Acknowledgements I would like to greatly thank my supervisor, Professor Mónica Barra. Her support, guidance and friendship over the past two years have made my research study a rewarding experience. Special thanks are given to my committee members, Professors Gary Dmitrienko and Xiangdong Fang for their valuable advice. Thanks are also due to Mrs. Jan Venne for her great help in acquiring NMR spectra and to Mrs. Cathy Van Esch for her help with administrative issues. I would also like to acknowledge all the undergraduate students who worked and who are working in our laboratory: Atefeh Asadi, Marianne Heppleston, Kelvin Lau, and Ryan Tabone. Special thanks are given to all my friends, particularly Minh Nguyen and Jarrod Johnson for their support. Last but not least, I would like to express my appreciation to my parents for their unconditional love and support that allowed me to pursue a higher education level. iv This thesis is dedicated to my dearest and beloved parents, brother and sister. v Table of Contents Abstract ................................................................................................................................ iii Acknowledgements............................................................................................................... iv Table of Contents ................................................................................................................. vi List of Tables........................................................................................................................ ix List of Figures....................................................................................................................... xi List of Schemes................................................................................................................... xiii List of Charts.......................................................................................................................xiv List of Abbreviations, Acronyms, and Symbols..................................................................xv Chapter 1. Introduction.........................................................................................................1 1.1 Properties of Triazenes .............................................................................................3 1.1.1 Stability of Triazenes ........................................................................................3 1.1.2 Restricted Rotation............................................................................................8 1.1.3 Tautomerization..............................................................................................13 1.1.4 Cis-Trans Isomerization..................................................................................15 1.2 Structure-Reactivity Correlation using the Hammett Equation ................................19 1.3 Research Objectives................................................................................................23 References..........................................................................................................................25 Chapter 2. Synthesis and Acid Dissociation Equilibrium Constants of Substituted 1,3- Diphenyltriazenes.................................................................................................................29 2.1 Synthesis of Substituted 1,3-Diphenyltriazenes.......................................................29 2.2 Determination of Acid Dissociation Equilibrium Constants of Substituted 1,3- Diphenyltriazenes...............................................................................................................33 vi 2.2.1 Results and Discussion....................................................................................33 2.2.2 Conclusions ....................................................................................................45 References..........................................................................................................................47 Chapter 3. Restricted Rotation around the N(2)-N(3) Bond of Cis-1,3-Diphenyltriazenes. ..............................................................................................................................................49 3.1 Background ............................................................................................................49 3.2 Results and Discussion ...........................................................................................50 3.3 Conclusions ............................................................................................................71 References..........................................................................................................................73 Chapter 4. Experimental......................................................................................................75 4.1 Synthetic Methods ..................................................................................................75 4.1.1 Materials and Instruments ...............................................................................75 4.1.2 Synthesis of Symmetrical Triazenes................................................................76 4.1.2.1 1,3-Bis(3,4-dimethylphenyl)triazene (9a)....................................................76 4.1.2.2 1,3-Bis(4-ethynylphenyl)triazene (9b).........................................................77 4.1.2.3 1,3-Bis[3-(trifluoromethyl)phenyl]triazene (9c)...........................................78 4.1.2.4 1,3-Bis(3-cyanophenyl)triazene (9d) ...........................................................78 4.1.2.5 1,3-Bis(3-nitrophenyl)triazene (9e) .............................................................79 4.1.2.6 1,3-Bis(3,5-dichlorophenyl)triazene (9f) .....................................................80 4.1.2.7 1,3-Bis(3,4,5-trichlorophenyl)triazene (9g)..................................................80 4.1.2.8 Attempted Synthesis of 1,3-Bis(3-methylphenyl)triazene (9h) by the Classical Diazonium Method ......................................................................................81 vii 4.1.2.9 Attempted Synthesis of 1,3-Bis(3-methylphenyl)triazene (9h) by SHNC Method ....................................................................................................................82 4.1.2.10 Attempted Synthesis of 1,3-Bis(3-methylphenyl)triazene (9i)..................83 4.1.3 Synthesis of Unsymmetrical Triazenes............................................................84 4.1.3.1
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