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Capturing of the Carbonyl Oxide Intermediate in Ozonolysis Using Aldehydes and Ketones

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Capturing of the Carbonyl Oxide Intermediate in Ozonolysis Using Aldehydes and Ketones Degree project C (1KB010) Tove Fällman Uppsala University 2014-03-20 Capturing of the carbonyl oxide intermediate in ozonolysis using aldehydes and ketones Tove Fällman, March 2014 Degree project C (1KB010) in chemistry, 15 credits Supervisors: Anna Lundstedt and Helena Grennberg 1 Degree project C (1KB010) Tove Fällman Uppsala University 2014-03-20 Contents Abbreviations ............................................................................................................................. 3 1. Introduction ............................................................................................................................ 4 1.1 Ozonolysis of alkenes ....................................................................................................... 4 1.3 The mechanism of ozonolysis .......................................................................................... 5 1.4 Capturing of the carbonyl oxide intermediate .................................................................. 5 1.5 This project ....................................................................................................................... 6 2. Results/Discussion ................................................................................................................. 6 2.1 Project plan ....................................................................................................................... 6 2.1.1 Choice of model reaction .......................................................................................... 6 2.1.2 Methods ..................................................................................................................... 7 2.2 Model reaction – The ozonolysis of 1-methyl-1-cyclohexene (1) in the presence of trifluoroacetophenone (2) ..................................................................................................... 10 2.3 Temperature variations in a small scale of ozonolysis of 1 in the presence of 2. .......... 11 2.4 Different carbonyl compounds used to capture the carbonyl oxide of 1. ...................... 11 2.4.1 General results from ozonolysis of 1 in the presence of carbonyl compounds ....... 12 2.4.2 Electron donating versus withdrawing groups ........................................................ 14 2.4.3 Aldehydes versus ketones ....................................................................................... 14 2.4.4 Isolation and analysis of cross-ozonides ................................................................. 14 3. Conclusions and outlook ...................................................................................................... 15 4. Experimental ........................................................................................................................ 15 Determining ozone output .................................................................................................... 16 Standard procedure a: large scale ......................................................................................... 16 Standard procedure b: small scale ........................................................................................ 16 REFERENCES ......................................................................................................................... 19 Acknowledgements .................................................................................................................. 20 2 Degree project C (1KB010) Tove Fällman Uppsala University 2014-03-20 Abbreviations HPLC – High performance liquid chromatography IR– Infrared NMR– Nuclear magnetic resonance PAH – Polycyclic aromatic hydrocarbons TLC – Thin-layer chromatography UV/VIS – Ultra-violet-visual 3 Degree project C (1KB010) Tove Fällman Uppsala University 2014-03-20 1. Introduction 1.1 Ozonolysis of alkenes Organic chemists often stand before the difficulty of synthesizing molecules. For accuracy and precision it is important for them to have lots of tools in the chemical toolbox. Ozonolysis is a classical method to functionalize alkenes, turning them to carbonyl compounds. For simple alkenes this is a commonly used method but for more complex ones the method has its limitations. The selectivity can be very low and the ozone starts attacking what it is not supposed to. There are some very interesting alkenes to functionalize, large molecules with many double bonds and ring formations. A particular group is called polycyclic aromatic hydrocarbons (PAHs, Figure 1.) and is interesting in, for example, the research field of graphene. My supervisor, Anna Lundstedt, is investigating ozonolysis as a way to functionalize PAHs. The current project is a part of the bigger project, looking at ozonolysis of a mono alkene. The conclusions of this project can then hopefully be applied on these complex molecules. Pyrene Phenanthrene Triphenylene Figure 1 - Examples of polycyclic aromatic compounds that can be functionalized using ozonolysis. The product of ozonolysis is called an ozonide. The ozonides themselves are not often used since they are explosive,1 but ozonolysis can give many different products. The outcome is depending on conditions during ozonolysis and workup after ozonolysis. A reductive work up can give a ketone or an aldehyde while an oxidative workup will generate a ketone or a carboxylic acid depending on the substituents of the alkene.2 (Figure 2) reducing conditions Ketone Aldehyde Ozonide oxidizing conditions Ketone Carboxylix acid Figure 2 - Ozonolysis of an alkene yields an ozonide. Different workup procedures followed after ozonolysis will give different products.2 4 Degree project C (1KB010) Tove Fällman Uppsala University 2014-03-20 1.3 The mechanism of ozonolysis The three-step mechanism of ozonolysis was described by Criegee in 1975. In step 1, the reaction between ozone and alkene form a primary ozonide. Ozone works both as a nucleophile and an electrophile, accepting and donating electrons at the same time. And the first step of ozonolysis can be referred to as a cycloaddition reaction. In the second step, the primary ozonide decomposes into a carbonyl oxide and a carbonyl. Before step 3 can happen, the carbonyl must turn 180 °. And in step 3, the carbonyl reacts with the carbonyl oxide in another cycloaddition and forms a monomeric ozonide with a 1,2,4-trioxolane structure. The reaction can also result in an oligomeric ozonide.1, 3 (Figure 3) Ozone . .. Step 1 Step 2 Step 3 .. Alkene Primary ozonide Carbonyl Carbonyl oxide Ozonide Figure 3 There are three steps in the mechanism of ozonolysis. The first includes the reaction with ozone and the formation of a primary ozonide. In step 2, the primary ozonide is broken down to a carbonyl oxide and a carbonyl. The carbonyl oxide and the carbonyl together form an ozonide in step 3.3 In Crigee's paper from 1975, he also describes how the decomposition in step 2 (Figure 3) can happen two different ways if the alkene is unsymmetrical. The ozonide product can then have three different structures (Figure 4).3 and/or Figure 4 - Unsymmetrical alkenes can form two sets of the carbonyl/carbonyl oxide intermediates. Therefore three different ozonides can be formed.3 1.4 Capturing of the carbonyl oxide intermediate If a foreign aldehyde or ketone is added to the reaction-mixture before ozone treatment it is possible to capture the carbonyl oxide intermediate from step 2 in Figure 2. When a foreign carbonyl compound is present during reaction, a cross-ozonide can be obtained. The same can be done with an alcohol but can then yield a hydroperoxy hemiacetal. (Figure 5).3-5 5 Degree project C (1KB010) Tove Fällman Uppsala University 2014-03-20 R''CHO Capturing of the carbonyl ozonide by a foreign carbonyl R'''OH Capturing of the carbonyl oxide by an alcohol Figure 5 The carbonyl oxide can be captured in situ by the presence of another carbonyl, alcohol or water.3 Fluorinated aldehydes and ketones are often used to capture the carbonyl oxide intermediate. It has been reported that the electron deficiency of these fluorinated carbonyl compounds make them extra effective at capturing the carbonyl oxide forming a cross ozonide.6 1.5 This project The aim with this project is to be a part of the investigation of ozonolysis as a synthesis method. It is done by looking at the capturing of the carbonyl oxide intermediate of a model alkene, and varying some variables that could be important to get the use of ozonolysis more diverse. It will be investigated if temperature and scale/concentration is something that influences the reaction outcome. When an optimal temperature is found, and a suitable scale, the carbonyl compound will be exchanged and the properties of it will be investigated. What makes an aldehyde or a ketone a suitable capturer of the carbonyl oxide? The aim is achieved by the use of a model reaction that was reported by Kawamura in 1996.7 The model reaction concerns ozonolysis of 1-methyl-1-cyclohexene (1) with trifluoroacetophenone (2) as additive. Five carbonyl compounds will be used, apart from ketone 2. Fluorination is one property of the carbonyl compound that is analyzed: does it influence the reaction outcome? Can an electron donating group give the opposite effect? Another thing that will be considered is the difference between aldehydes and ketones, could that have an effect on the reaction outcome? 2. Results/Discussion 2.1 Project plan 2.1.1 Choice of model reaction Many things had to be taken into consideration when deciding how to approach the problem. The literature
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