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Pd/S,O-Ligand Catalysed Regioselective C–H Olefination of Anisole Derivates

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Pd/S,O-Ligand Catalysed Regioselective C–H Olefination of Anisole Derivates Bachelor Thesis Scheikunde Pd/S,O-ligand Catalysed Regioselective C–H Olefination of Anisole Derivates door Rianne van Diest 14 december 2020 Studentnummer 11677635 Onderzoeksinstituut Verantwoordelijk docent Van ’t Hoff Institute for Molecular Sciences Dr. M.A. (Tati) Fernández Ibáñez Onderzoeksgroep Begeleider Synthetic Organic Chemistry Verena (Vivi) Sukowski 1 TABLE OF CONTENTS List of abbreviations ............................................................................................................................. 3 Abstract .................................................................................................................................................. 4 Popular scientific summary ................................................................................................................. 4 1. Introduction ....................................................................................................................................... 5 1.1 C–H activation as attractive strategy for green chemistry ............................................................ 5 1.2 Improving the selectivity of C–H activation with ligands ............................................................ 6 1.3 C–H olefination of anisole derivates ............................................................................................. 7 2. Background information .................................................................................................................. 9 2.1 C–H activation mechanisms .......................................................................................................... 9 2.2 Fujiwara-Moritani reaction and the role of the S,O-ligand ......................................................... 10 2.3 Selectivity of the reaction ........................................................................................................... 12 3. Experimental procedure ................................................................................................................. 13 4. Results and discussion .................................................................................................................... 15 4.1 Substrate scope of Pd/S,O-ligand catalysed C–H olefination of anisole derivates ..................... 15 4.2 Comparison of results with similar literature procedures ........................................................... 20 5. Conclusion ....................................................................................................................................... 21 6. References ........................................................................................................................................ 22 Supporting Information ..................................................................................................................... 23 2 LIST OF ABBREVIATIONS Ac-Leu-OH = N-acetyl-L-leucine AcOH = acetic acid AMLA = ambiphilic metal-ligand activation Ar = aryl BDE = bond dissociation energy BIES = base-assisted internal electrophilic substitution BM = σ-bond metathesis cHex = cyclohexane CMD = concerted metalation-deprotonation DCE = dichloroethane DCM = dichloromethane DMF = N,N-dimethylformamide EA = electrophilic addition EDG = electron donating group EI = electron ionization Eq = equivalent EtOAc = ethyl acetate EWG = electron withdrawing group FG = functional group FI = field ionization FTIR = fourier transform infrared Hep = heptane Hex = hexane HFIP = hexafluoroisopropanol HRMS = high resolution mass spectrometry KIE = kinetic isotope effect M = metal Me = methyl MeOH = methanol NMR = nuclear magnetic resonance OA = oxidative addition OAc = acetate on. = overnight PPh3 = triphenylphosphine RDS = rate-determining step RF = retardation factor Rt. = room temperature S,O- = sulfur,oxygen- tAmOH = tert-amyl alcohol (2-methyl-2-butanol) tBu = tert-butyl TFA = trifluoroacetic acid TFE = trifluoroethanol THF = tetrahydrofuran TLC = thin layer chromatograpy UV = ultraviolet 3 ABSTRACT A general methodology for para-selective C–H olefination of ortho-substituted anisole derivates is described. The reaction proceeds via the Fujiwara-Moritani catalytic cycle and can be performed under mild conditions. It makes use of a S,O-ligand, which improves both reactivity and regioselectivity of the reaction. Various anisole derivates have been screened, with electron-donating as well as electron- withdrawing side groups on the ortho position. The optimal reaction conditions differ for anisole derivates bearing electron donating or withdrawing substituents to ensure high yield and selectivity. This research is a step towards a general methodology for para-selective C–H olefination of all kinds of anisole derivates. Scheme 1: General procedure for regioselective C–H activation with anisole derivates. POPULAR SCIENTIFIC SUMMARY Natural compounds are regularly used, for all kinds of applications. For example, many medicines are isolated from plants. However, this isolation process is not always very sustainable and often costly. Therefore, nature is a great inspiration source for chemists. If we can procedure these medicines in the lab, there is no need for difficult isolation processes from plants. Anisoles are building blocks, that can often be found in medicine. An example is shown in Figure 1. This research focussed on synthesis with these anisoles. These structures contain unreactive carbon-hydrogen bonds, which can be functionalized via a process called C–H activation. In this process, the unreactive carbon-hydrogen bond is replaced by a more Figure 1: Medicine functional bond, like carbon-carbon, or carbon-oxygen. In this way, other (Licochalcone C), with molecules can be attached to anisole, just like putting two Lego blocks the anisole structural together. A problem is the selectivity of the reaction: the new molecule can moiety is shown in red. be attached to every free corner position of the anisole. This research focussed on selective C–H activation of anisole derivates. We developed a general procedure with mild conditions for the C–H activation of these building blocks, which is shown in Scheme 2. C–H activation needs a metal catalyst to improve the reaction. In this research, a palladium catalyst is used. A bio-organic molecule is coordinated to the catalyst, to increase the reactivity of the catalyst and to direct the selectivity towards a certain site of the anisole. This molecule is called a S,O- ligand, because it binds with its sulphur and oxygen atom to the palladium. We used the general method to test various anisole derivates, which provided the desired products in good yields. This efficient and selective functionalization of anisole derivates could provide a useful strategy for the synthesis of medicine and other natural compounds. Scheme 2: General procedure for selective C–H activation with anisole derivates. 4 1. Introduction 1.1 C–H activation as attractive strategy for green chemistry In the last decade, metal-catalysed C-H activation has seen a significant growth as strategy in manipulating the reactivity of C–H bonds.1 C–H activation is applied to replace unreactive C–H bonds by functional groups, i.e. formation of C–C, C–N or C–O bonds. Transition metals like Ru, Rh, Pt, or Pd from which the latter one is mostly used, can insert into C–H bonds and thereby activate the bond.2 A general diagram for this C–H functionalization is shown in Scheme 3. First, the C–H bond is replaced by a C–M bond, which represents the C–H activation step. Next, the metal complex can be functionalized via e.g. transmetallation or oxidative addition. Via a termination step the two organic groups can be connected and thereby resulting in a functionalization of the molecule. A detailed discussion of the mechanism for C–H activation is given in the background information. C–H functionalization can be used to synthesize various organic compounds, like pharmaceutical drugs.3–7 Nature namely produces many complex organic molecules, that are used in all kinds of medicine. In some cases, synthesizing these compounds in the lab turned out to be challenging or even impossible. For example, Licochalcone C (Figure 1) is a medicine that can be extracted from licorice, but the yield of this extraction process is only 15 mg / 2 kg.8 C–H activation could provide an easy, cheap and efficient strategy to synthesize such medicines, so that there is no more need for expensive extraction procedures. In this way, C–H activation can not only help in synthesizing (new) medicines, but also in making more medicines commercially available. Scheme 3: C-H functionalization via metal-catalysed C–H activation (adapted from ref 2).2 Figure 1: Licochalcone C An important advantage for this strategy over traditional synthetic methods, is that there is no need to pre-functionalize the reactant. Instead, the reaction can be performed directly with the C–H bond that is already present in the molecule. This makes C–H activation an efficient and cheap strategy, because no extra synthesis steps have to be used, and as a result, the atom economy is relatively high. Next to that, there is less waste formation. Therefore, C–H activation can also be an attractive strategy for green chemistry. Scheme 4 shows this advantage in a comparison of C–H activation and the traditional methods for C–H functionalization. Scheme 4: C–H functionalization via metal-catalysed C–H activation (a) or via pre-functionalization of the reactant (b).2 In this scheme, FG represents any functional
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