SYNTHESIS AND PHOTOCHEMICAL TRANSFORMATIONS OF A FEW TETHERED BARRELENES Thesis submitted to the Cochin University of Science and Technology In partial fulfilment of the requirements for the degree of Doctor of Philosophy In Chemistry In the Faculty of Science By Eason M. Mathew Under the supervision of Dr. P. A. Unnikrishnan DEPARTMENT OF APPLIED CHEMISTRY COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY KOCHI – 682 022, Kerala, INDIA July 2013 DECLARATION I hereby declare that the work presented in the thesis entitled “Synthesis and Photochemical Transformations of a few Tethered Barrelenes” is the result of genuine research carried out by me under the supervision of Dr. P. A. Unnikrishnan, Assistant Professor of Organic Chemistry, Department of Applied Chemistry, Cochin University of Science and Technology, Kochi‐22, and the same has not been submitted elsewhere for the award of any other degree. Kochi-22 Eason M. Mathew July 1, 2013 DEPARTMENT OF APPLIED CHEMISTRY COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY COCHIN – 682 022, KERALA, INDIA Dr. P. A. Unnikrishnan Assistant Professor Ph:- 0484-2575804 email: [email protected] CERTIFICATE This is to certify that the thesis entitled “Synthesis and Photochemical Transformations of a few Tethered Barrelenes” is a genuine record of research work carried out by Mr. Eason M. Mathew, under my supervision, in partial fulfilment of the requirements for the degree of Doctor of Philosophy of Cochin University of Science and Technology, and further that no part thereof has been presented before for the award of any other degree. Kochi-22 P. A. Unnikrishnan July 1, 2013 (Supervising Guide) Acknowledgements I would like to take this opportunity to acknowledge and extend my sincere thanks for those who help me to make this Ph.D. thesis possible. First and foremost, I express my sincere gratitude and obligation to my mentor Dr. P. A. Unnikrishnan, Assistant Professor of Organic Chemistry, Department of Applied Chemistry, Cochin University of Science and Technology, for giving me an opportunity to be in his research group, for his inspiration, valuable suggestions and support throughout my research work. I express my sincere thanks to Dr. S. Prathapan, Associate Professor, Department of Applied Chemistry, Cochin University of Science and Technology for his valuable suggestions and fruitful discussions with patience, throughout the research, being my doctoral committee member. I wish to thank Dr. N. Manoj, Head, Department of Applied Chemistry, CUSAT and former Department Heads, Dr. K. Sreekumar and Dr. K. Girishkumar for providing the facilities of the department for carrying out my research work. I express my heartfelt gratitude to Dr. M. R. Prathapachandra Kurup for helping me in the interpretation of the SCXRD data. I would like to extend my sincere thanks to all the teaching and non-teaching staff of the Department of Applied Chemistry, for their help and support. My thanks are also due to, Dr. M. Sithambaresan, Faculty, Eastern University, Srilanka, for his valuable assistance in the elucidation of the SCXRD data when I needed it most. vii Staff, Administrative block, CUSAT, for technical assistance. Dr. Shibu, M. E and Mr. Saji, T. K. SAIF, CUSAT, for, SXRD and NMR analyses whenever I wanted. Mr. Vishnu, P., Alpha Chemicals and Diagnostics for supplying chemicals in time. Former members of organic group Dr. Jean J. V., Dr. John P. R., Dr. Gisha, G., Dr. Ambily, M. J., Dr. Saino, H. V. and Mr. Jayakumar, K. My colleagues Mr. Jomon, P. J., Mr. Rakesh, N., Mr. Senju, D., Ms. Sandhya, R., Ms. Seena, S, Ms. Pravitha, N. P., Ms. Sajitha, T. S., Ms. Reshma, G., Ms. Minu, J. Ms. Manjula, K., Ms. Cisy, A., Ms. Suma, C. S., Ms. Saumya, T. S., Ms. Remya, L., Ms. Kala, K., Ms. Nithya, C., Ms. Ligi, M. L. Mr. Shan, D., Mr. Tomson, D., Ms. Parvathy and Ms. Vineetha for making a homely atmosphere in the lab. Friends in the DAC and other departments of CUSAT for their corporation. SAIF, CUSAT for analytical and spectral data. Council of Scientific and Industrial Research (CSIR) for research fellowship. Very special thanks to my family members especially Appa, Amma and Kochumon for their endless support and love. Above all, I thank God Almighty for His blessings. Eason M. Mathew viii PREFACE Dibenzobarrelenes undergo facile photorearrangement to the corresponding dibenzosemibullvalenenes under triplet sensitized irradiation through di- -methane rearrangement whereas on direct irradiation dibenzocyclooctatetraenes are formed. Number and variety of organic molecules obtained by di- -methane rearrangement is incredible and makes it particularly synthetically useful. Such photoproducts are not readily available by alternate routes. Besides, di- -methane rearrangement of appropriate dibenzobarrelene derivatives finds application in the alignment of nematic liquid crystals. It is observable from the literature that generalization is possible for the observed regioselectivity of di- -methane rearrangements of numerous acyclic, cyclic and bicyclic 1,4-dienes. The reaction regioselectivity of such compounds has been predicted from mechanistic point of view. However, there are conflicting reports on the effect of electronic and steric factors on the regioselectivity observed with barrelene-semibullvalene photorearrangement. We have designed few barrelene molecules in such a way that the structural features of these compounds will enable them to undergo intriguing triplet state mediated di- -methane rearrangement. Our strategy involved the preparation of dibenzobarrelenes appended with a fused ring systems, thereby restricting the rotational freedom of the bridgehead substituent. We describe these systems as ‘tethered barrelenes’. These tethered barrelenes enabled us to examine the effect of orientation and the nature of the bridgehead-substituents in controlling ix the regioselectivity of di-π-methane rearrangement in a more systematic fashion. In this background, the thesis entitled “SYNTHESIS AND PHOTOCHEMICAL TRANSFORMATIONS OF A FEW TETHERED BARRELENES” reveals our attempts to explore the factors controlling the regioselectivity of di-π-methane rearrangement displayed by dibenzobarrelenes. Moreover, we have observed interesting dark reactions of suitable substituted tethered dibenzosemibullvalenes in a few cases. The thesis is divided into five chapters. The first chapter briefly introduces regioselectivity of di-π-methane rearrangement, some photochemical transformations of dibenzobarrelenes and an application of di-π-methane rearrangement. The definition of the research problem is given at the end of this chapter. The second chapter deals with the synthesis of the barrelenes through intramolecular Diels-Alder reactions of anthracene derivatives appended with suitable acetylenic substituents. Photochemical transformations of tethered ethers and tethered esters are given in the third and fourth chapters respectively. The fifth chapter describes di-π-methane rearrangement of tethered sulfides and one tethered sulfone. Major outcomes are given at the end of the thesis. The structural formulae, schemes, tables and figures are numbered chapter-wise as each chapter of the thesis is as an independent unit. All new compounds are fully characterized on the basis of their spectral and analytical data including the single crystal X-ray analysis in many cases. A comprehensive list of references is given at the end of each chapter. x List of Abbreviations br : broad C : centigrade DCM : dichloromethane DEPT : distortionless enhancement by polarisation transfer DMF : dimethylformamide d : doublet FT IR : fourier transform infrared g : gram h : hour GC-MS : gas chromatography-mass spectrometry HCl : hydrochloric acid Hz : hertz IMDA : intramolecular Diels-Alder m : multiplet Me : methyl mg : milligram min : minute mL : millilitre mp : melting point nm : nanometre NMR : nuclear magnetic Resonance ORTEP : oak ridge thermal ellipsoid plot program KBr : potassium bromide Ph : phenyl KOH : potassium hydroxide NaOH : sodium hydroxide s : singlet Sens. : sensitized SN : substitution t : triplet THF : tetrahydrofuran TLC : thin layer chromatography XRD : X-ray diffraction xi CONTENTS Page No. CHAPTER 1 PHOTOISOMERIZATIONS AND PHOTOCHEMICAL REARRANGEMENTS – A SHORT INTRODUCTION 1.1 Abstract 1 1.2 Introduction 1 1.3 Classification of Photochemical Rearrangements 3 1.4 The di- -methane rearrangement 9 1.4.1 Mechanistic Aspects 10 1.4.2 Reaction Multiplicity 12 1.4.3 Regioselectivity 26 1.5 Isomerization of Dibenzobarrelenes 31 1.6 Application of the di- -methane rearrangement 40 1.7 Definition of the research problem 42 1.8 Objectives 44 1.9 References 46 CHAPTER 2 SYNTHESIS OF A FEW TETHERED ESTERS, TETHERED SULFIDES, TETHERED SULFONES AND TETHERED ETHERS 2.1 Abstract 53 2.2 Introduction 53 2.3 Results and Discussion 56 2.3.1 Synthesis of 9-substituted anthracenes 57 2.3.2 Formylation of 9-substituted anthracenes 58 2.3.3 Synthesis of tethered esters 59 xiii 2.3.4 Synthesis of tethered sulfides 61 2.3.5 Synthesis of tethered sulfone 64 2.3.6 Synthesis of tethered ethers 65 2.3.7 Conclusion 67 2.4 Experimental Section 67 2.5 Reference 87 CHAPTER 3 PHOTOCHEMICAL TRANSFORMATIONS OF THE TETHERED ETHERS 3.1 Abstract 91 3.2 Introduction 91 3.3 Results and Discussion 97 3.3.1 Direct Irradiation 98 3.3.2 Sensitised Irradiation 99 3.3.3 Conclusion 102 3.4 Experimental Section 103 3.5 References 109 CHAPTER 4 PHOTOCHEMICAL TRANSFORMATIONS OF THE TETHERED ESTERS 4.1 Abstract 111 4.2 Introduction 111 4.3 Results and Discussion 114