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Electrochemical Thiocyanation of Organic Compounds by Anna Gitkis

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Electrochemical Thiocyanation of Organic Compounds by Anna Gitkis Electrochemical Thiocyanation of Organic Compounds Thesis submitted in partial fulfillment of the requirements for the degree of “DOCTOR OF PHILOSOPHY” by Anna Gitkis Submitted to the Senate of Ben-Gurion University of the Negev September 2010 Beer Sheva Electrochemical Thiocyanation of Organic Compounds Thesis submitted in partial fulfillment of the requirements for the degree of “DOCTOR OF PHILOSOPHY” by Anna Gitkis Submitted to the Senate of Ben-Gurion University of the Negev Approved by the advisor Prof. James Y. Becker: _______________ Date: __________________ Approved by the Dean of the Kreitman School of Advanced Graduate Studies: _______________________ Date: ___________________ September 2010 Beer Sheva This work was carried out under the supervision of Professor James Y. Becker In the Department of Chemistry Faculty of Natural Sciences Acknowledgments I wood like to thank my supervisor, Prof. James Y. Becker, for providing me with consistent stimulus and inspiration to proceed and for his valuable support and assistance in all aspect of my scientific work. I want to especially mention my colleagues, Ph.D. students Natalie Geinik, Alex Shtelman, and Efrat Korin; new members of our laboratory Libi Brakha, Anna Rakovchic, and Tatyana Golub; and technical assistant Mrs. Ethel Solomon, who helped me in solving scientific and technical problems and, most importantly, ensured an unforgettable atmosphere of friendship and joy. I am thankful to my family for supporting and encouraging me on the way towards the Ph.D. degree, for their patience and understanding when this thesis was being written. I am sincerely grateful to my mother Ekaterina who since childhood has instilled in me a desire to study. I am honored to receive my Ph.D. degree. This work is dedicated to my amazing children, Daniel and Liron, whose love in them I am carrying in my heart. Table of Contents Abstract………………………………………………………………………………………... i 1. Introduction……………………………………………………………………………….. 1 1.1. Electroorganic synthesis……………………………………………………………... 1 1.1.1. Advantages of electrochemical synthesis……………………………………….. 4 1.1.2. Disadvantages of electrochemical synthesis…………………………………….. 5 1.1.3. Electrochemical methods………………………………………………………... 5 1.1.3.1. Controlled potential electrolysis (CPE)………………………………. 5 1.1.3.2. Constant current electrolysis (CCE)………………………………….. 6 1.2. Organic thiocyanates………………………………………………………………… 7 1.2.1. Chemical preparation of organic thiocyanates…………………………………... 8 1.2.1.1. Formation of organic thiocyanates by reaction between thiocyanate anions and organic compounds………………………………………………... 8 1.2.1.2. Preparation of organic thiocyanates by using thiocyanogen or related reagents……………………………………………………………………….... 13 1.2.1.3. Reactions of thiocyanogen……………………………………………. 15 1.2.1.4. Common methods for thiocyanation of organic compounds ………… 15 1.2.1.5. Methods of preparing thiocyanogen ……………..…………………... 16 1.2.2. Identification of organic thiocyanates……………………….…………………... 19 1.2.2.1. IR spectroscopy……………………………………………………….. 19 1.2.2.2. 1H- and 13C-NMR……………………………………………………… 19 1.2.2.3. Detection of isothiocyanate and thiocyanate groups by feature reactions……………………………………………………………………….. 20 2. Objectives of the proposed research……………………………………………………… 21 3. Results and discussion……………………………………………………………………... 22 3.1. Choosing “optimal” conditions for good yield and selective electrolysis…………. 22 3.2. Mechanism……………………………………………………………………………. 31 3.2.1. Does the mechanism involve 1e- or 2e- oxidation?............................................... 31 3.2.2. A step-wise heterolytic or concerted thiocyanation mechanism?.......................... 33 3.3. Electrochemical thiocyanation of aromatic compounds…………………………... 37 3.3.1. Controlled potential electrolysis (CPE)…………………………………………. 37 3.3.2. Constant current electrolysis (CCE)…………………………………………….. 46 3.4. Electrochemical thiocyanation of alkenes………………………………………….. 49 4. Conclusions…………………………………………………………………………………. 60 4.1. Electrochemical thiocyanation of aromatic compounds…………………………… 60 4.2. Mechanism of electrochemical thiocyanation of aromatic compounds…………… 61 4.3. Electrochemical thiocyanation of alkenes…………………………………………… 61 4.4. Mechanism of electrochemical thiocyanation of alkenes…………………………… 62 5. Experimental………………………………………………………………………………... 63 5.1. General: instruments, techniques and procedures…………………………………. 63 5.2. Characterization of products………………………………………………………… 65 6. References…………………………………………………………………………………... 74 7. Appendix ................................................................................................................................78 7.1. Basic principles in "organic electrochemistry"…………………………………….. 78 7.1.1. Electrochemical experimental conditions……………………………….………. 78 7.1.1.1. Electrochemical cells…………………………………………………. 78 7.1.1.2. Solvent systems………………………………………….……………. 78 7.1.2. The effect of different parameters on electrochemical reactions………………... 79 7.1.2.1. Type of electrode……………………………………………………... 80 7.1.2.2. Factors affecting the mechanism of electrolysis……………………… 80 7.1.2.3. Effect of solvent………………………………………………………. 81 7.1.2.4. Electrolyte……………………………………………….……………. 82 7.1.2.5. Electrochemical parameters…………………………………………... 83 7.1.2.6. Electricity consumption (charge) passed through electrochemical cell. 84 7.1.2.7. Temperature…………………………………………………………... 85 7.1.2.8. Stereochemistry of substrate………………………………………….. 86 List of Figures Page 1 Divided electrochemical cell………………………………………………………..... 6 2 Undivided electrochemical cell………………………………………………………. 6 3 Electronic structure of thiocyanogen…………………………………………………. 14 4 Dimensional structure and torsion angles of thiocyanogen by theoretical calculations…………………………………………………………………………… 14 5 Cyclic voltammogram of anion SCN- in acetonitrile, scan rate 50 mV/sec.................. 22 6 Cyclic voltammogram of anion SCN- and anisole in acetic acid; scan rate 50 mV/sec………………………………………………………………………………… 24 7 A graphical illustration of data from Table 7 with extrapolation to zero yield……... 29 8 Cyclic voltammogram of anion SCN- with ferrocene in acetic acid……..…………… 32 9 A coulometry experiment with extrapolation to zero current in acetic acid..………… 32 10 Specific adsorption of N,N-dimethylbenzylamine on electrode surface ...................... 87 List of Tables Page 1 Yields of 4-thiocyanatoanisole from CPE …..……………………………………….. 25 2 Effect of electrolyte on the yield of 4-thiocyanatoanisole……………………………. 26 3 Effect of anode material on the yield of 4-thiocyanatoanisole……………………….. 27 4 Effect of thiocyanate salts on the yield of 4-thiocyanatoanisole……………………… 27 5 Effect of NH4SCN concentration on the yield of 4-thiocyanatoanisole……………… 28 6 Effect of NH4SCN/anisole ratio on the yield of 4-thiocyanatoanisole……………….. 28 7 Effect of electricity consumption (NH4SCN, 0.067M)………………………………. 29 8 “Optimal” conditions based on the results described in Tables 1–7…………………. 30 9 Results from constant current electrolysis under the "optimal conditions” (described in Table 8)…………………………………………………………………………….. 31 10 Electrochemical thiocyanation of substituted anisole, toluene, and aniline derivatives…………………………………………………………………………….. 38 11 Electrochemical thiocyanation of other aromatic substrates………………………….. 43 12 Constant current electrochemical thiocyanation of anisole and some representative disubstituted aromatic derivatives……………………………………………………. 47 13 Constant current electrochemical thiocyanation of other disubstituted aromatic derivatives…………………………………………………………………………….. 48 14 Electrochemical thiocyanation of alkenes by CPE electrolysis………………………. 50 List of Schemes Page 1 Electrochemical transformation of functional group……………………………….. 3 2 Charge distribution in thiocyanate anion…………………………………………… 8 3 Substitution reaction of dicarbonyl compounds with thiocyanate anion…………… 10 4 Suggested mechanism for electrochemical thiocyanation in two-phase solution…………………………………………………………………………….... 18 5 Electrochemical thiocyanation of anisol as a model substrate…………………….. 23 6 A step-wise mechanism of thiocyanation of aromatic compounds………………… 33 7 A concerted mechanism of thiocyanation of aromatic compounds……………….... 34 8 Aromatic substrates…………………………………………………………………. 37 9 Products distribution from electrochemical thiocyanation of 2,3-dimethyl-2- 52 butene………………………………………………………………………………. 10 Mechanism of thiocyanation of alkenes to yield an addition product……………… 53 11 Products distribution from electrochemical thiocyanation of 2,3-dimethyl-1- 54 butene……………………………………………………………………………….. 12 Products distribution from electrochemical thiocyanation of cyclohexene………… 55 13 The product from electrochemical thiocyanation of styrene……………………….. 55 14 The product from electrochemical thiocyanation of stilbene……………………..... 56 Abbreviations AN acetonitrile CAN cerium(IV) ammonium nitrate CCE constant current electrolysis CPE controlled potential electrolysis CV cyclic voltammetry DCM dichloromethane DME dropping mercury electrode DMF dimethylformamide DSA dimensionally stable anode F Faraday (1 F = ~96500 Coulombs) GC glassy carbon GLC gas chromatography NBS N-bromosuccinimide NMR nuclear magnetic resonance NTS N-thiocyanatosuccinimide THF tetrahydrofuran Abstract Organic thiocyanates are useful precursors for agrochemicals, dyes, insecticides, and drugs. In organic synthesis, they are also used as a convenient source of ArS- for introducing functional sulfur groups in various organic molecules. Chemical synthesis of aryl thiocyanates can be performed via both electrophilic and radical reactions.
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