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A Marcus-Theory-Based Approach to Ambident Reactivity

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A Marcus-Theory-Based Approach to Ambident Reactivity Dissertation zur Erlangung des Doktorgrades der Fakultät für Chemie und Pharmazie der Ludwig-Maximilians-Universität München A Marcus-Theory-Based Approach to Ambident Reactivity Robert Martin Breugst aus München 2010 Erklärung Diese Dissertation wurde im Sinne von § 13 Abs. 3 bzw. 4 der Promotionsordnung vom 29. Januar 1998 (in der Fassung der vierten Änderungssatzung vom 26. November 2004) von Herrn Professor Dr. Herbert Mayr betreut. Ehrenwörtliche Versicherung Diese Dissertation wurde selbständig, ohne unerlaubte Hilfe erarbeitet. München, 28.10.2010 _________________________ Dissertation eingereicht am: 28.10.2010 1. Gutachter: Prof. Dr. Herbert Mayr 2. Gutachter: Prof. Dr. Hendrik Zipse Mündliche Prüfung am: 21.12.2010 II Acknowledgements Acknowledgements First of all, I would like to express my cordial gratitude to Professor Dr. Herbert Mayr for the opportunity to compose this thesis in his group. I have cherished all the valuable discussions with him, his endless support, and his inspiring confidence very much and I have always appreciated working under these excellent conditions. From the very first day in his group Professor Mayr made me feel welcome, has always been willing to discuss my ideas, and encouraged me to pursue a scientific career. Furthermore, I would like to thank Professor Dr. Hendrik Zipse for numerous discussions and helpful comments on quantum chemical calculations, and of course also for reviewing my thesis. Additionally, I am very indebted to Professor J. Peter Guthrie, Ph.D., for giving me the opportunity to spend almost 4 months in his group at the University of Western Ontario. During this time, I have learned so many new things and I am grateful for the reams of discussions in his office. The financial support by the Fonds der Chemischen Industrie (scholarship for Ph.D. students) is gratefully acknowledged which did not only fund two years of my time during my Ph.D. but also funded my stay at the University of Western Ontario and my visits to several international conferences. Sincere thanks for the great working atmosphere and many fruitful discussions are given to all my colleagues, especially those from the “Olah” lab: Roland Appel, Dr. Frank Brotzel, Dr. Tanja Kanzian, Dr. Sami Lakhdar, and Christoph Nolte. I would also like to thank Francisco Corral for his collaboration during his undergraduate research course, Nathalie Hampel not only for the synthesis of our reference electrophiles but also for her helping hand for several product studies, and Brigitte Janker for quickly solving any emerging problems. I cannot overemphasize my deepest gratitude to our kind soul Hildegard Lipfert for all her help and support. III Acknowledgements For the fast and efficient proof reading of this thesis, I am very thankful to Johannes Ammer, Roland Appel, Nicola Breugst, Francisco Corral, Waltraud Härtel, Dr. Tanja Kanzian, Hans Laub, Christoph Nolte, and Dr. Nicolas Streidl. I am also grateful to Dr. Armin Ofial for very valuable suggestions and critical comments. Besides the support from my colleagues I am very happy that I had a couple of private “motivational coaches” around who helped me to focus on my work when it was necessary and who also distracted me from it when it was necessary. Therefore, I am grateful to all my friends, especially to Thorsten Allscher who accompanied me for the past nine years of our chemistry studies. My deepest and most sincere gratitude belongs to my whole family, Wolfgang, Irmgard, Nicki, and Waltraud, for their unconditional support. You have always helped me wherever you could to make my life easier and more enjoyable. Thank you very much! IV Parts of this thesis have already been published as follows: Marcus-Analysis of Ambident Reactivity [Marcus-Analyse ambidenter Reaktivität] M. Breugst, H. Zipse, J. P. Guthrie, H. Mayr, Angew. Chem. 2010, 122, 5291-5295; Angew. Chem. Int. Ed. 2010, 49, 5165-5169. Nucleophilic Reactivities of Imide and Amide Anion M. Breugst, T. Tokuyasu, H. Mayr, J. Org. Chem. 2010, 75, 5250-5258. Ambident Reactivities of Pyridone Anions M. Breugst, H. Mayr, J. Am. Chem. Soc. 2010, 132, 15380-15389. A Farewell to the HSAB Principle of Ambident Reactivity H. Mayr, M. Breugst, A. R. Ofial, Angew. Chem. accepted, DOI: 10.1002/anie.201007100. V Table of Contents Table of Contents Chapter 1: Summary 1 Chapter 2: Introduction 13 Chapter 3: Marcus Analysis of Ambident Reactivity 23 Chapter 4: Nucleophilic Reactivities of Imide and Amide Anions 87 Chapter 5: Ambident Reactivities of Pyridone Anions 145 Chapter 6: Ambident Reactivities of the Anions of Nucleobases and Their Subunits 217 Chapter 7: A Farewell to the HSAB Principle of Ambident Reactivity 317 VI Chapter 1: Summary Chapter 1: Summary 1 General Pearson’s principle of hard and soft acids and bases (HSAB) and the related Klopman–Salem concept of charge- and orbital-controlled reactions have been considered to provide a consistent rationalization for ambident reactivity. However, in a series of experimental investigations, it has previously been shown that the reactivities of typical ambident nucleophiles cannot be properly described by these concepts (Scheme 1). This thesis was designed to examine the reactivities of other ambident nucleophiles and to provide a consistent rationalization of ambident reactivity. soft hard soft hard hard soft hard hard soft E E E E E E E E E kinetic thermodynamic kinetic & thermodynamic kinetic kinetic & control control thermodynamic control control thermodynamic control control Scheme 1: Failure of the HSAB principle to correctly predict the regioselectivities of the reactions of hard and soft electrophiles E with some prototypes of ambident nucleophiles. 2 Marcus Analysis of Ambident Reactivity According to the Marcus equation (1), the Gibbs energy of activation can be calculated from 0 ‡ the Gibbs energy of reaction G and the intrinsic barrier G0 , which corresponds to the Gibbs energy of activation (G‡) of an identity reaction (with G0 = 0). ‡ ‡ 0 0 2 ‡ G = G0 + 0.5 G + (G ) / 16 G0 (1) We have extended earlier work by Hoz and co-workers and calculated the intrinsic barriers for the identity methyl transfer reaction [Eq. (2)] at G3(+) and MP2/6-311+G(2d,p) level of theory for different nucleophiles X (e.g., X = F, OMe, NMe2, CH3). Consistent with previous results, we have found that the intrinsic barriers are smaller, when the attacking atom is ‡ ‡ ‡ ‡ further right in the periodic table, i.e., G0 (F) < G0 (OMe) < G0 (NMe2) < G0 (CH3). 1 Chapter 1: Summary The same trend also controls ambident reactivity and it was shown that N-attack of CN–, S- attack of SCN–, and O-attack of enolates are intrinsically favored, in accordance with their relative positions in the periodic table. ‡ Substitution of the calculated intrinsic barriers (G0 ) and the calculated reaction free energies (G0) for the reactions with methyl chloride into the Marcus equation (1) gave the Gibbs energy profiles depicted in Figure 1. Figure 1: Gibbs energy profiles for the methylation of ambident nucleophiles with methyl chloride in the gas phase [G3(+)]. It is shown that the G0 term in the Marcus equation favors C-attack at cyanide, while N- ‡ attack is preferred by the intrinsic barrier (G0 ). As the difference of the intrinsic terms is much smaller than the difference in the reaction free energies (G0), free cyanide ions preferentially react with the carbon atom. In reactions of SCN–, N-attack is preferred by the thermodynamic term G0, while the attack at the sulfur terminus is preferred intrinsically. As the G0-term for S- and N-attack of thiocyanate is rather small, kinetically controlled alkylations of SCN– occur preferentially at the intrinsically preferred sulfur atom. 2 Chapter 1: Summary For enolates, the product stability term (G0) highly favors C-attack over O-attack. However, the significantly higher intrinsic barrier for C-attack compensates this effect and as a result, enolates are either attacked at oxygen or carbon in kinetically controlled reactions, depending on the “freeness” of the enolate anion. 3 Nucleophilic Reactivities of Imide and Amide Anions The reactions of several amide and imide anions with benzhydrylium ions and structurally related quinone methides have been studied kinetically by UV-Vis spectroscopy in DMSO and in acetonitrile solution. 1H- and 13C-NMR spectroscopy revealed that in all cases examined in this work, amides are formed exclusively (N-attack) and no traces of imidates (O-attack) were observed (Scheme 2). Therefore, Kornblum’s interpretation of the ambident reactivity of amide anions – greater SN1 character leads to more O-attack – needs to be revised. Me2N Me2N Me2N O O DMSO DMSO O O + N O N N –KBF4 –KBF4 O BF4 K Me2N Me2N Me2N not observed 84 % Scheme 2: Exemplary reaction of an amide anions with the bis-(4,4’-dimethylamino) benzhydrylium ion in DMSO yielding only the product of N-attack. The second-order rate constants (log k2) for these reactions correlate linearly with the electrophilicity parameters E of the electrophiles according to the correlation equation [Eq. (3)], allowing us to determine the nucleophilicity parameters N and s for these nucleophiles (Figure 2). log k2 = s (N + E) (3) 3 Chapter 1: Summary 8 6 4 2 0 -2 -19 -17 -15 -13 -11 -9 -7 Figure 2: Plots of the rate constants log k2 of the reactions of imide and amide anions with reference electrophiles in DMSO versus their electrophilicity parameters E. The comparison of imide anions with the structurally related carbanions in Figure 3 shows that similar stabilizing effects of imide anions are found for acetyl and ethoxycarbonyl substituents, whereas acetyl groups stabilize carbanions better than ethoxycarbonyl groups. OEt O log k2 O Carbanions 6 O -Nucleophiles O O H 5 O O N Imide Anions O n-Nucleophiles O 4 O N N EtO O O Figure 3: Comparison of the nucleophilic reactivities of structurally related imide anions and + carbanions towards the benzhydrylium ion lil2CH (20 °C; DMSO; + for structure of lil2CH see Figure 2).
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