Visible-Light-Driven Aerobic Oxidation Reactions Catalyzed by Riboflavin Tetraacetate

201 Pages·2016·37.65 MB·English

Institut für Anorganische Chemie
Visible-Light-Driven Aerobic Oxidation
Reactions Catalyzed by Riboflavin
Tetraacetate

Dissertation

Zur Erlangung des Doktorgrades der Naturwissenschaften
Dr. rer. nat.
an der Fakultät Chemie und Pharmazie der Universität Regensburg

vorgelegt von:
Bernd Mühldorf
aus Burglengenfeld

Regensburg 2016

Der experimentelle Teil der vorliegenden Arbeit wurde in der Zeit zwischen November 2012
und Dezember 2015 unter Anleitung von Prof. Dr. Robert Wolf am Institut für Anorganische
Chemie der Universität Regensburg angefertigt. Meinem Betreuer danke ich herzlich für die
Überlassung dieses spannenden Projektes sowie für die hervorragende Betreuung und
Unterstützung.

Die Arbeit wurde angeleitet von: Prof. Dr. Robert Wolf

Promotionsgesuch eingereicht am: 18.04.2016

Tag der mündlichen Prüfung: 29.04.2016

Promotionsausschuss: Vorsitz Prof. Dr. Richard Buchner
Erstgutachter Prof. Dr. Robert Wolf
Zweitgutachter Prof. Dr. Burkhard König
Dritter Prüfer Prof. Dr. Arno Pfitzner

Prologue
This thesis reports on photocatalytic oxidation reactions catalyzed by the vitamin B derivative
2
riboflavin tetraacetate (RFT). The dissertation seeks to expand the field of RFT-mediated
oxidation reactions, mainly focusing on the challenging visible-light-driven oxygenation of C−H
bonds with air as terminal oxidant. Chapter 1 reviews recent developments in the field of
photocatalytic C−H bond oxygenation. In chapter 2, we show that the photooxygenation of
electron-deficient benzylic substrates is feasible by modifying the reduction potential of RFT
with Lewis acids. Expanding the scope of benzylic substrates accessible with flavin
photocatalysis, chapter 3 delineates how the addition of a non-heme iron complex as co-catalyst
improves the performance of RFT. In chapter 4, the RFT/non-heme iron system was used for the
epoxidation of cycloalkenes. In addition to flavin-mediated C−H bond oxygenations, an aim was
to apply RFT as oxidation catalyst for challenging transformations. Therefore, in chapter 5 we
present two routes for the direct esterification of aldehydes based on flavin photocatalysis.
Chapter 6 describes an enzyme-inspired artificial photosystem for the challenging oxidative
chlorination of arenes using chloride anions as Cl source. Chapter 7 summarizes the results of
this thesis.

Art & Science

Robert Weindl, 2015

Table of Contents
1 Homogeneous Visible-Light-Driven C−H Bond Oxygenation ....................... 1
1.1 Metal Porphyrin and Porphyrinoid Catalysts ......................................................... 1
1.1.1 Iron ................................................................................................................ 1
1.1.2 Manganese .................................................................................................... 9
1.1.3 Ruthenium ................................................................................................... 13
1.1.4 Antimony .................................................................................................... 15
1.1.5 Summary ..................................................................................................... 17
1.2 Tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy) ]2+) and Derivatives ..................... 19
3
1.3 Organic Dyes ....................................................................................................... 23
1.3.1 Eosin Y (EY) ............................................................................................... 23
1.3.2 Methylene Blue (MB+)................................................................................ 25
1.3.3 2,3-Dichloro-5,6-dicyano-p-benzoquinone (DDQ) .................................... 25
1.3.4 3-Cyano-1-methylquinolinium ion (QuCN+) .............................................. 27
1.3.5 Acridinium ions (Acr+-R, R = H, Ph or Mes) ............................................. 28
1.3.6 Antraquinone-2,3-dicarboxylic acid (AQN) ............................................... 35
1.3.7 Riboflavin tetraacetate (RFT) ..................................................................... 36
1.3.8 Summary ..................................................................................................... 38
1.4 Photocatalysis Coupled with Transition Metal Catalysis .................................... 40
1.4.1 Heme and Non-Heme Metal Complexes .................................................... 40
1.4.2 Copper Complexes ...................................................................................... 42
1.4.3 Ruthenium Complexes ................................................................................ 43
1.4.4 Biocatalytic Oxygenation ............................................................................ 44
1.4.5 Summary ..................................................................................................... 46
1.5 References ............................................................................................................ 48
2 Photocatalytic Benzylic C−H Bond Oxidation with a Flavin Scandium
Complex .............................................................................................................. 55
2.1 Introduction .......................................................................................................... 57
2.2 Results and Discussion ........................................................................................ 58
2.3 Conclusion ........................................................................................................... 62
2.4 References ............................................................................................................ 62
2.5 Supporting Information ........................................................................................ 64
3 C−H Photooxygenation of Alkylbenzenes Catalyzed by Riboflavin
Tetraacetate and a Non-Heme Iron Catalyst .................................................. 75
3.1 Introduction .......................................................................................................... 77
3.2 Results and Discussion ........................................................................................ 78
3.3 Conclusion ........................................................................................................... 83
3.5 References ............................................................................................................ 84
3.6 Supporting Information ........................................................................................ 85
4 Aerobic Photooxidation of Cycloalkenes Catalyzed by Riboflavin
Tetraacetate and a Non-Heme Iron Complex ............................................... 101
4.1 Introduction ........................................................................................................ 103
4.2 Results and Discussion ...................................................................................... 105
4.3 Mechanistic Considerations ............................................................................... 108
4.4 Conclusion and Outlook .................................................................................... 112
4.5 Experimental Section ......................................................................................... 113
4.6 References .......................................................................................................... 117
5 Aerobic Photooxidation of Aldehydes to Esters Catalyzed by Riboflavin
Tetraacetate ..................................................................................................... 119
5.1 General Information and Introduction ............................................................... 121
5.2 Flavin-Catalyzed Aerobic Photooxidation of Aldehydes (Part 1): Synthesis of
Methyl Esters ..................................................................................................... 125
5.3 Flavin-Catalyzed Aerobic Photooxidation of Aldehydes (Part 2): Esterification of
Aldehydes with Alkyl Bromides ........................................................................ 133
5.4 Experimental Section ......................................................................................... 140
5.5 Supporting Information ...................................................................................... 142
5.6 Footnotes ............................................................................................................ 151
5.7 References .......................................................................................................... 152
6 Halogenase-Inspired Oxidative Chlorination Using Flavin Photocatalysis 155
6.1 Introduction ........................................................................................................ 157
6.2 Results and Discussion ...................................................................................... 158
6.3 Conclusion ......................................................................................................... 162
6.4 References .......................................................................................................... 163
6.5 Supporting Information ...................................................................................... 165
7 Summary ........................................................................................................... 175
8 Acknowledgement ............................................................................................ 183
9 List of Publications .......................................................................................... 185
10 Curriculum Vitae ............................................................................................. 187
11 Eidesstattliche Erklärung ................................................................................ 189