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Sulfoxides of Thiophene and Dibenzothiophene: a Mechanistic Study of Photochemical Deoxygenation Mrinmoy Nag Iowa State University

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Sulfoxides of Thiophene and Dibenzothiophene: a Mechanistic Study of Photochemical Deoxygenation Mrinmoy Nag Iowa State University Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 2005 Sulfoxides of thiophene and dibenzothiophene: a mechanistic study of photochemical deoxygenation Mrinmoy Nag Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Organic Chemistry Commons Recommended Citation Nag, Mrinmoy, "Sulfoxides of thiophene and dibenzothiophene: a mechanistic study of photochemical deoxygenation " (2005). Retrospective Theses and Dissertations. 1843. https://lib.dr.iastate.edu/rtd/1843 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Sulfoxides of thiophene and dibenzothiophene: A mechanistic study of photochemical deoxygenation by Mrinmoy Nag A dissertation submitted to the graduate faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Major: Organic Chemistry Program of Study Committee William S. Jenks, Major Professor George A. Kraus Nicola L. Pohl Klaus Schmidt-Rohr Victor S.-Y. Lin Iowa State University Ames, Iowa 2005 UMI Number: 3184597 INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. UMI UMI Microform 3184597 Copyright 2005 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 ii Graduate College Iowa State University This is to certify that the doctoral dissertation of Mrinmoy Nag has met the dissertation requirements of Iowa State University Signature was redacted for privacy. Signature was redacted for privacy. For the Major Program iii To Gargi My wife iv TABLE OF CONTENTS ABSTRACT vii CHAPTER I. PHOTOCHEMISTRY AND PHOTOPHYSICS OF AROMATIC SULFOXIDES : A GENERAL REVIEW 1 1.1 Dissertation organization 1 1.2 Objectives 2 1.3 Fundamental properties of sulfoxide groups 2 1.4 A review of photochemistry of sulfoxides 5 1.4.1 a-Cleavage 5 1.4.1.1 a-Cleavage of dialkyl sulfoxides 6 1.4.1.2 a-Cleavage of benzylic sulfoxides 9 1.4.1.3 a-Cleavage of alkyl aryl and diaryl sulfoxides 1 * 1.4.1.4 SO extrusions * ^ 1.5 Hydrogen abstraction * ^ 1.6 Photochemical deoxygenation of aromatic sulfoxides ^ 1.6.1 Disproportionate mechanism 1 ^ 1.6.2 Dimer mechanism 1 & 1.6.3 Sulfmyl mechanism 20 1.6.4 Hydrogen abstraction 21 1.7 Bimolecular photoreduction of aromatic sulfoxides 23 1.8 Stereomutation of sulfoxides 26 V 1.9 Photophysics of aromatic sulfoxides 29 References 32 CHAPTER II. PHOTOCHEMISTRY AND PHOTOPHYSICS OF HALOGEN-SUBSTITUTED DIBENZOTHIOPHENE OXIDES Abstract 36 2.1 Introduction 37 2.2 Results 39 2.3 Discussion 50 2.4 Conclusion 55 2.5 Experimental Section 56 Acknowledgment 63 References 63 CHAPTER III. PHOTOCHEMISTRY OF SUBSTITUTED DIBENZOTHIOPHENE OXIDES: THE EFFECT OF TRAPPING GROUPS Abstract 68 3.1 Introduction 69 3.2 Results 72 3.3 Discussion 81 3.4 Conclusion 87 vi Experimental Section 88 Acknowledgement 97 References 97 CHAPTER IV. PHOTOCHEMISTRY OF THIOPHENE-S-OXIDE DERIVATIVES Abstract 102 4.1 Introduction 102 4.1.1 Detection of the adduct of 0(3P) with acetonitrile 103 4.1.2 Synthetic challenge 104 4.1.3 Structure of thiophene oxide 108 4.1.4 Characteristic of the excited state of thiophene oxides 110 4.1.5 Photochemistry of thiophene-^-oxides 110 4.2 Results 111 4.3 Discussion 120 4.4 Summary 123 4.5 Experimental Section 124 References 130 CHAPTER V. GENERAL CONCLUSIONS 134 ACKNOWLEDGEMENTS 138 vii Abstract Dibenzothiophene oxide (DBTO) produces dibenzothiophene (DBT) as the only detectable photoproduct. Although the chemical yield of deoxygenation is very high, the quantum yield is relatively low. Indirect evidence also indicates formation of 0(3P) as the other photoproduct. The quantum yield of deoxygenation increases in the presence of oxygen trapping solvents. From these results, a mechanism for the deoxygenation was proposed which indicates a unimolecular S-0 bond scission and a S-0 bond stretch coupled to intersystem crossing. Heavy atom substituted DBTOs show high efficiency of phosphorescence but a moderate increase in the deoxygenation quantum yield. However, the order of increase in both phosphorescence and deoxygenation quantum yields are in accord with the heavy atom effect. 4-Iodo DBTO also undergoes deiodination via a secondary photolysis. Alkenyl and sulfenyl groups were appended to the 4-position of DBTO to check whether they increase the efficiency of the deoxygenation. Photolysis of these substituted DBTOs showed increase in the quantum yield and for substituted DBTOs detectable amount of intramolecular trapped products were also formed. The quantum yield of deoxygenation was less solvent dependent for this series of DBTOs. However, in cyclohexene the quantum yield was similar to that observed for DBTO, and no intramolecular trapped products were found. This could be due to the fact that cyclohexene is a better trapping agent than the appended alkenyl and sulfenyl groups, and due to its presence in a large excess compared to the trapping group concentration. However, detection of intramolecular trapped products suggests that solvent effects observed in previous studies of DBTO derive at least mainly from the reactivity between the oxidizing species that is released - presumably 0(3P) - and the solvent, rather than from other macroscopic solvent parameters. Unlike DBTO, mechanistic studies for the deoxygenation of thiophene oxide (TO) are little known. The S-O bond breaking energy in TO is about 15 kcal/mol less than that of DBTO, whereas its first excited singlet state has almost the same energy as DBTO. Stable substituted TO derivatives were prepared hoping that they would produce 0(3P) with high efficiency. Results obtained for 2,5-bis(trimethylsilyl) TO are promising. The quantum yield of deoxygenation in benzene was about 14 times higher than for DBTO. However, only about half of the disappearance was accounted for by the appearance of the corresponding thiophene. Other thiophene oxides, which were photolyzed either produced furan or other unknown products. It is possible that there is an analogy between the deoxygenation mechanism of DBTO and TO but it is too early to speculate that. At least, it is fair to say that the photochemistry of TO derivatives is more complicated and more substituent dependent than that of DBTO derivatives. 1 CHAPTER 1 PHOTOCHEMISTRY AND PHOTOPHYSICS OF AROMATIC SULFOXIDES: A GENERAL REVIEW 1.1 Dissertation organization This dissertation contains five chapters. Chapter 1 is a general review of the photochemistry of dibenzothiophene (DBTOs) and thiophene oxides (TOs). This chapter is composed of mainly the literature reviews available for the photochemical processes of the previously mentioned compounds. Chapter 2 and Chapter 3 are based on two independent published papers. Chapter 4 is the basis of another publication. Chapter 2 discusses the photochemistry and photophysics of heavy atom substituted DBTOs. In Chapter 3, the photochemical deoxygenation of DBTOs with O atom accepting substituents is discussed. In Chapter 4, the photochemistry of various substituted stable thiophene oxides is discussed. Chapter 5 contains general conclusions and a summary of previous chapters. While the majority of the work described here was carried out by the author, there were contributions from another group member. Some of the experiments discussed in Chapter 4 were performed by Melanie Heying. 2 1.2 Objectives The overall goal of this dissertation is to develop a better understanding of the photochemical processes of dibenzothiophene and thiophene oxides. The contents of the next two chapters are related and discuss the photochemical deoxygenation process of the substituted aromatic sulfoxide, DBTO. The fourth chapter is about another aromatic sulfoxide, thiophene oxide, whose photochemistry is relatively unexplored compared to the DBTO derivatives. 1.3 Fundamental properties of sulfoxide groups Superficially, the sulfoxide or sulfinyl functional group has many similarities to carbonyl functional groups. Both of the functional groups have polar bonds where oxygen is the more electronegative atom. Both the functional groups show a-cleavage as one of their main photochemical reactions, and both groups tend to stabilize a-carbanions. Despite these similarities, there is a fundamental difference between them. In carbonyl groups, the carbon atom is sp2 hybridized, whereas in sulfoxides the sulfur atom is approximately sp3 hybridized. Hence, unlike the carbonyl groups no true Jt-bond can be found in the sulfoxide groups. At the same time, the sulfoxide bond cannot be represented by a simple single bond. Because of the complexity of the sulfoxide bond there is no universally accepted
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