Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1998 Photochemical and computational study of sulfoxides, sulfenic esters, and sulfinyl radicals Daniel D. Gregory Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Organic Chemistry Commons Recommended Citation Gregory, Daniel D., "Photochemical and computational study of sulfoxides, sulfenic esters, and sulfinyl radicals " (1998). Retrospective Theses and Dissertations. 11926. https://lib.dr.iastate.edu/rtd/11926 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]. 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This microform edition is protected against unauthorized copying under Title 17, United States Code. UMI 300 North Zeeb Road Ann Arbor, MI 48103 11 Graduate College Iowa State University This is to certify that the Doctoral dissertation of Daniel D. Gregory has met the dissertation requirements of Iowa State University Signature was redacted for privacy. Signature was redacted for privacy. For the Major Program Signature was redacted for privacy. For the 0»radi](alfe College Ill To Mary My wife and best friend. iv TABLE OF CONTENTS Page ABSTRACT viii CHAPTER I; PHOTOCHEMISTRY OF SULFOXIDES: A GENERAL REVIEW I 1.1 Dissertation organization 1 1.2 The sulfoxide functional group 1 L3 The a-cleavage reaction 5 L3.1 a-Cleavage of dialkyl systems 6 L3.2 a-Cleavage of benzylic sulfoxides 8 1.3.3 a-Cleavage of alkyl aryl and diaryl sulfoxides 8 1.4 SO extrusion reactions of sulfoxides 12 1.5 Hydrogen abstraction 14 1.6 Stereomutation 16 References 18 CHAPTER II: PHOTO-DEOXYGENATION OF DIBENZOTHIOPHENE SULFOXIDE: EVIDENCE OF A UNIMOLECULAR S-0 HOMOLYSIS 22 Abstract 22 2.1 Introduction 23 2.2 Dimer mechanism 25 2.2.1 Evidence against the dimer mechanism 28 2.3 Sulfmyl mechanism 31 V 2.3.1 Evidence against the sulfinyl mechanism 32 2.4 Hydrogen abstraction 33 2.4.1 Evidence against hydrogen abstraction mechanism 33 2.5 Dibenzothiophene sulfoxide 35 2.6 Direct cleavage of the S-0 bond 37 2.7 Chemical reactivity of 0(^P) 38 2.7.1 Photolysis of NiO 39 2.7.2 Photolysis of pyridine N-oxides 41 2.8 Energetics of S-O homolysis 44 2.9 Electronic state of dibenzothiophene sulfoxide 45 2.10 Observed oxidations 46 2.10.1 Oxidations by the triplet oxygen atom 47 2.11 Origin of the oxygen atom 49 2.12 Epoxidation of alkenes 50 2.13 Oxidation of allyl benzene 53 2.14 Competitive oxidations with dueterated solvents 55 2.15 Direct detection of 0(^P) 59 2.16 Synthesis of 2,5-di-phenylthiophene sulfoxide 62 2.17 Photolysis of 2,5-di-phenylthiophene sulfoxide 65 2.18 Synthesis of 2,5-di-/err-butylthiophene sulfoxide 67 2.19 Photolysis of 2,5-di-fe;rr-butyUhiophene sulfoxide 68 2.20 Excited state of thiophene sulfoxide 74 vi 2.21 Conclusions 76 2.22 Experimental 78 References 91 CHAPTER III: THERMOCHEMISTRY OF SULFENIC ESTERS (RSOR'): NOT JUST A PRETTY PEROXIDE 97 Abstract 97 3.1 Introduction 98 3.2 General plan of attack and computational methods 101 3.3 Model sulfenic acid and ester systems 105 3.4 Results 107 3.5 Discussion 109 3.5.1 Comparison of the weakening of the O-H, O-C, S-H and S-C bonds by peroxides, disulfides and sulfenic esters. 113 3.5.2 Comparison of 0-0, S-S, and S-O bond enthalpies 118 3.6 Isomerization of sulfenic esters to sulfoxides 119 3.7 Comparison of less expensive computational methods with G2 120 3.8 Photochemistry of sulfenic esters 124 3.9 Conclusions 127 References 128 CHAPTER IV: COMPUTATIONAL STUDY OF SULFINYL RADICAL CHEMISTRY 132 4.1 Introduction 132 4.2 Experimental structure of sulfinyl radicals 139 4.3 Computational characterization of sulfinyl radicals 142 vii 4.4 Excited states of sulfinyl radicals 151 4.5 Self coupling reaction of sulfinyl radicals 158 4.6 Computational study of the sulfinyl radical coupling reaction 165 4.7 Reaction of sulfenyl radicals with oxygen 175 4.8 Computational study of the reaction of sulfinyl radicals with oxygen 182 References 187 CHAPTER V: GENERAL CONCLUSIONS 193 APPENDIX A: COMPUTATIONAL METHODS 198 APPENDIX B; CARTESIAN COORDINATES OF MOLECULES 213 ACKNOWLEDGMENTS 224 VUl ABSTRACT Despite a fairly long history of research, sulfoxide photochemistry has not been extensively developed. This is particularly true in terms of structural effects on the observed sulfoxide photochemistry. Because of this, our group has embarked on an extensive study to gain a better understanding of sulfoxide photochemistry and the effects of structure on this photochemistry. The intention of this research is to obtain knowledge of sulfoxide photochemistry that can be used as a predictive tool in the photolysis of other sulfoxide systems. Sulfoxide photochemistry can be divided into four general reaction types. These include a-cleavage, hydrogen abstraction, stereomutaiion, and deoxygenation. All of these reactions will be discussed to varying extents in this dissertation with an emphasis on the deoxygenation reaction. Photolysis of dibenzothiophene sulfoxide results in the formation of dibenzothiophene and oxidized solvent. Though quantum yields are low, chemical yields of the sulfide are quite high. Yields of the oxidized solvents can also be high. Typical products are phenol from benzene, cyclohexanol, and cyclohexene from cyclohexane and 2- cyclohexenol and epoxycyclohexane from cyclohexene. A number of experiments designed to elucidate the mechanism of the hydroxylation were carried out, including measurements of quantum yields as a function of concentration, solvent, quenchers, and excitation wavelength. These data are inconsistent with a mechanism involving a sulfoxide dimer, which also does not properly account for the solvent oxidations. It is suggested that the active oxidizing agent may be atomic oxygen 0(^P) or a closely related noncovalent complex, based on the nature ix of the oxidation chemistry, comparison to known rate constants for 0('P) reactivity, and the quantum yield data. A computational study on the thermochemistry of several simple sulfenic acids (RSOH) and esters (RSOR') is reported. The enthalpies of R-S, S-0, and O-R' homolytic cleavage are calculated at the G2 level of theory and compared to related peroxides and disulfldes. Less expensive B3LYP calculations were unsatisfactory. When R and R' are both alkyl, the 0-C bond is expected to be the weakest in the molecule; for CH3SOCH3, C-S, S-O, and O-C bond dissociation enthalpies of 67, 64, and 49 kcal/mol are predicted by G2. Compared to peroxides, sulfenic esters are predicted to have weaker O-C bonds and S-O bonds that are stronger than the analogous O-O bonds. The C-S bonds of sulfenic esters are predicted to be somewhat stronger than those of disulfides. A rationalization is given for the observation that radical stabilization is greater for RSO* than ROO*, RSS*, or ROS*. The sulfinyl radical has been proposed to be an important sulfur containing molecule in environmental, combustion, and certain biological processes. A computational investigation of sulfinyl radicals is reported. The ground and excited electronic states are developed using ab initio and density functional methods. The self coupling reaction of sulfinyl radicals is investigated using the G2 method. The meso isomer of the a-disulfoxide was found to be more stable than the eclipsed dl isomer by 4.2 kcal/mol at the G2(MP2) method. Of the three possible coupling pathways, the two paths which forms the OS-sulfenyl sulfinate and the a-disulfoxide are both calculated to be very close in energy. The rearrangement of the methyl OS-suIfenyl sulfinate to the thiosulfonate via S-S hemolysis was calculated to be endothermic by 44 kcal/mol, which may suggest a concerted rearrangement.