Nnulations Morgan Elizabeth Skala
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)ORULGD6WDWH8QLYHUVLW\/LEUDULHV 2020 Accessing Novel Graphene Nanoribbon Architectures through Double peri-Annulations Morgan Elizabeth Skala Follow this and additional works at DigiNole: FSU's Digital Repository. For more information, please contact [email protected] THE FLORIDA STATE UNIVERSITY COLLEGE OF ARTS & SCIENCES ACCESSING NOVEL GRAPHENE NANORIBBION ARCHITECTURES THROUGH DOUBLE PERI-ANNULATIONS By MORGAN E. SKALA A Thesis submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for graduation with Honors in the Major Degree Awarded: Spring, 2020 The members of the Defense Committee approve the thesis of Morgan E. Skala defended on April 14, 2020. ______________________________ Dr. Igor V. Alabugin Thesis Director ______________________________ Dr. Tyler S. Foster Outside Committee Member ______________________________ Dr. Edwin F. Hilinski Committee Member ii ACKNOWLEDGEMENTS I would like to thank Dr. Igor Alabugin for allowing me the opportunity to complete a thesis in his group, for his mentorship and advice, and monetary support. I would like to thank my Honor’s Thesis Committee: Dr. Edwin Hilinski for supporting me through the years and Dr. Foster for his continued support and assistance. I would especially like to thank Edgar Gonzalez- Rodriguez and Miguel Abdo for their dedication and mentorship throughout this project, as well as the other graduate students and post-doctorates in the Alabugin group for their help and encouragement. iii TABLE OF CONTENTS Page ACKNOWLEDGEMENTS............................................................................................................iii TABLE OF CONTENTS................................................................................................................iv ABSTRACT…………………………………………………………………………………….....v CHAPTER I INTRODUCTION………………………………………………………...............1 Background..............................................................................................................1 Thesis Goal..............................................................................................................7 II RESULTS AND DISCUSSION..............................................................................8 Methodology and Results........................................................................................8 Synthetic Expansion to GNRs...............................................................................17 III CONCLUSIONS...................................................................................................19 Further Directions..................................................................................................20 REFERENCES..............................................................................................................................22 SUPPORTING INFORMATION..................................................................................................24 iv ABSTRACT Routes to construct graphene nanoribbons (GNRs) and nanographenes (NGs) from smaller polycyclic aromatic hydrocarbons (PAHs) have long been explored. Foundational work in the Alabugin group has recently culminated in the development of a double peri-annulation protocol where aromatic core precursors containing a propargylic OMe traceless directing group (TDG) are -extended through Sn-radical promoted cyclizations. After cyclization, two Bu3Sn chemical handles are present in the products. These handles can serve for downstream PAH-extension via cross-coupling and cycloaromatization approaches. As proof-of-concept, our group has recently reported the extension of distannylpyrenes via iodination followed by double Suzuki cross- coupling with an aromatic electrophile. By mono cross-coupling of the 1,8-disubstituted-2,7- distannylpyrenes, we hypothesize that extension at one Sn at a time may be possible, allowing to further diversify the PAH library accessible for the construction of GNRs. Mono-Suzuki and Stille couplings were explored under multiple conditions with limited success due to prevalent double cross-couplings in the former and sluggish reactivity in the latter. The broad flexibility of the Bu3Sn handles allows for exploration of other approaches, for example, the borylation of the cyclized product, further cross-coupling with aryl dihalides and cycloaromatization in order to access novel GNR structures. v CHAPTER ONE: INTRODUCTION Background The synthesis of graphene, graphene nanoribbons (GNRs), and nanographenes (NGs) has been a heavily researched area for the past ~15 years. These material’s interesting physical properties have led to widespread applications in optoelectronic devices development.1 General approaches to prepare GNRs and NGs are classified as top-down or bottom-up. Top-down strategies, such as exfoliations of graphite and centrifugation,2a typically do not allow for a good degree of control over the shape and size of the GNR or NG products. Given that the optical and electronic properties that the material will possess relies heavily on its morphology, control over size and shape is of high interest.2 The ability for control makes bottom-up synthesis very advantageous. Furthermore, through bottom-up synthesis, solubility can be improved, and properties can be tuned by design.2a Like puzzle pieces, starting from small PAHs can allow for these scaffolds to be built into almost endless possibilities. An interesting starting point for bottom- up synthesis of NGs and GNRs are alkynes, as they are high energy, carbon-rich functionalities, that can efficiently undergo selective radical cascade reactions.3-4 Most often, alkynes are used in the synthesis of GNRs for annulation at the armchair edge (bay-region) since annulation at the zig-zag edge (L-region) gives odd-alternant hydrocarbons that are non-Kekul structures. We recently show that this problem can be circumvented via double peri-annulationse onto the same core, giving rise to fully aromatic products (Scheme 1).5-6 Expansions at the zig-zag edge are desirable, because they provide access to new types of polyaromatic targets2 With this synthesis, extension of the -system at the zig-zag edge can be accessed, in one or both directions via a library of cross-couplings, to make novel NG and GNR structures. 1 Recently our group reported that radical alkyne peri-annulations at the zig-zag edge may be utilized to extend aromatic cores and give access to olympicenes by using a traceless directing group (TDG) approach.6 TDGs have been also shown to create pentagon-free polyaromatics (Scheme 2).6 A -scission of the directing group at the end of the radical cascade enables the final aromatization.7 Scheme 1. While annulations and ortho-annulations add to the armchair edge of GNRs, peri- annulations occur at the zig-zag edge.5 Scheme 2. Previous work regarding MeO-TDG Sn-radical cyclizations. a) TDG in skipped oligoalkyne radical cascade and fused helicenes. b) Formation of benzanthrones and olympicene with peri-annulations.5 Our work on double peri-annulations onto a naphthalene core gave us access to 5 functionalized pyrenes with Bu3Sn handles. Alternatively, I2 or TFA can be added to the distannylpyrenes to access iodinated or protodestannylated derivatives, respectively. The 2 iodinated structures could be cross-coupled with arylboronic acids via a Suzuki cross-coupling at the 2 and 7 positions, and subsequent oxidative cyclization allowed for the construction of extended polyarenes. (Scheme 3). Preliminary results show that with control over stoichiometry, the 1:1 mixture of 3j and tetrasubstituted product 9 is obtained (as opposed to the desired cross- coupled product). With this result, we hypothesize that mono-cross-couplings should allow for the construction of larger PAHs via variation of the building blocks (i.e. having larger starting materials) and a “mix and match” approach. To circumvent the cross-coupling problem, a change of functional groups has been proposed. The possibility of using the Sn handles as nucleophiles for a Stille mono-cross-coupling prompted us to explore this reaction as an alternative for the failed mono-Suzuki coupling. Scheme 3. Previous work that demonstrates Suzuki cross-couplings and Scholl for extended PAHs.5 Stille cross-coupling is known for its mild conditions and tolerance of a variety of functional groups, which leads to its use in creation of complex molecules.8 Additionally, organostannanes are relatively stable, being rather insensitive to air and moisture.8 While toxicity of tin compounds remains a concern, the toxicity of tri-butyltin derivatives is much lower than that of its ethyl and methyl counterparts.8 With a Stille coupling, the additional step of functionalizing pyrene (i.e. iodination) can be avoided, thus preventing a decrease in overall yield; however, these advantages may not come without disadvantages: The lower polarity and high steric demand of 3 8 SnBu3 can make compounds less nucleophilic in the cross-coupling protocol. This lack of reactivity could be overcome with several potential methods such as the use of bulky phosphines and co-catalysts.8 Mono-Stille cross-couplings have been reported,11-12 as well as sterically hindered Stille cross-couplings.6, 8-10 Success with mono-cross-coupling would give further extension by coupling the monostannylated product with a “central piece” to give longer GNRs (Scheme 4). While little precedent on mono-Stille couplings is found in the literature, a report for a porphyrin scaffold with the Stille reaction occurring