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UvA-DARE (Digital Academic Repository)

Fluorogenic organocatalytic reactions

Raeisolsadati Oskouei, M.

Publication date 2017 Document Version Other version License Other Link to publication

Citation for published version (APA): Raeisolsadati Oskouei, M. (2017). Fluorogenic organocatalytic reactions.

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Download date:01 Oct 2021 Chapter 1 1

Chapter 1

Introduction

1.1. Organocatalysis Catalyst, the term used to describe a compound that speeds up chemical reactions without being consumed: a world full of questions lies behind this simple term. Nearly all reactions that occur in living cells require catalysts and without them, life would be impossible, but what is the mechanism by which they work? What kind of catalysts are suitable to produce the desired products with high efficiency under mild conditions? How do they direct the reaction to a pathway which requires less activation energy? Many research groups attempt to discover the unknown aspects of the catalyst world. As a result of all this work many different types of catalyst have been developed with abilities to catalyze a large diversity of reactions. Despite the remarkable achievements on the experimental side for many important catalytic reactions in synthetic chemistry, polymer science and pharmaceutical industry, the mechanisms of many catalytic processes still pose questions that need to be answered.1–11 Catalysts are classified generally according to their physical state, their chemical nature, or the nature of the reactions that they catalyze. In this thesis the focus is on organocatalysts. They are defined as small organic molecules, where an inorganic element is not part of the active principle in the catalytic process. Organocatalysts are a class of catalysts with rich structural diversity. They are often chiral, so that they have the potential for enantioselectivity, and can be classified according to their working mechanism:4,8,12,13 - Activation of the reaction partners based on the nucleophilic/electrophilic properties of the catalyst. - Covalent formation of reactive intermediates. - Phase-transfer reactions. The chiral catalyst forms a host-guest complex.

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- Activation of the reagents by hydrogen bonding of the catalyst to the reagents. 1.1.1. Cinchona alkaloid based organocatalysts Considering the nature of the organocatalysts, they are grouped in one of the An important class of organocatalysts are the cinchona alkaloids and their following classes:4,8,12,13 derivatives. The ability of different types of catalysts based on the cinchona - Biomolecules such as proline, phenylalanine, secondary amines in alkaloid skeleton in catalyzing diverse reactions has been amply general, the cinchona alkaloids, certain oligopeptides.14–18 demonstrated.17,29,31–35 They derive their usefulness from the presence of a - Synthetic catalysts derived from biomolecules.18–20 chiral backbone, equipped with multiple functional groups that can be readily - Hydrogen bonding catalysts, including TADDOLS, derivatives of BINOL modified to perform a variety of catalytic tricks. Another factor is that the such as NOBIN, and organocatalysts based on thioureas.21–25 natural precursors are available as pairs of pseudo-enantiomers, allowing the - Ionic liquids such as triazolium salts .26,27 selective synthesis of complementary enantiomers.36,37 In the review published by List and his group in 2005, it is mentioned that most The first asymmetric reaction carried out using a cinchona base was published but not all organocatalysts can be broadly classified as Lewis bases, Lewis by Bredig and Fiske in 1912. They reported that the addition of HCN to acids, Brønsted bases, and Brønsted acids. According to their description, benzaldehyde is accelerated by quinine and quinidine, and the resulting Lewis base catalysts (B:) initiate the catalytic cycle via nucleophilic addition to cyanohydrins are optically active. However, the optical yields achieved were in the substrate (S). The resulting complex undergoes a reaction and then the range of <10% enantiomeric excess (ee).31 About four decades later, releases the product (P) and the catalyst for further turnover. Lewis acid Pracejus used O-acetylquinine as a catalyst (1 mol%) in the addition of catalysts (A) activate nucleophilic substrates (S:) in a similar manner. Brønsted methanol to phenylmethylketene and obtained the product with 74% ee.38 In base and acid catalytic cycles are initiated via a (partial) deprotonation or 1975 Wynberg and Helder reported that they had found that for 11 Michael protonation, respectively.28 reactions, using optically inactive donors and acceptors in the presence of catalytic amounts of optically active quinine, optically active products were obtained. In one case the enantiomeric excess was determined and amounted to 68%.31 The reaction in scheme 2 exemplifies their studies.

Scheme1. Simplified catalytic cycles of Lewis base, Lewis acid, Brønsted base, and Brønsted acid catalysis.28

Interests in metal-free asymmetric catalysis for the production of the chiral Scheme 2. Addition of nitrosulfone to methyl vinyl ketone in the presence of quinine31 building blocks which are used as pharmaceutical intermediates or other chemicals has led researchers to focus on asymmetric or enantioselective Hiemstra and Wynberg reported detailed mechanistic studies of cinchona organocatalysis. By now modern asymmetric catalysis is built on three rather alkaloid catalysts in 1981.39 Scheme 3 shows an example of the reactions that than two pillars, namely biocatalysis, metal catalysis, and organocatalysis. they studied. They showed that the presence of the hydroxyl group in this Thiourea based and cinchona alkaloid based catalysts are two types of catalyst accelerates the reaction. According to their findings the ee of the organocatalyst which have been successfully used to catalyze a diverse set of reaction depends on the nature of the solvent. The reaction at high reactions.8,16,17,29,30 temperatures proceeded faster with slow racemization during the reaction.

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Scheme1. Simplified catalytic cycles of Lewis base, Lewis acid, Brønsted base, and Brønsted acid catalysis.28

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They also studied the influence of the structure of the substrates on the yield and ee of the reaction.39

Scheme 3. An example of the aromatic thiol addition to cyclohexanone catalyzed by 39 cinchona alkaloid. Scheme 5. Organocatalytic addition of aromatic thiols to cyclohexenone and the suggested transition state model for this reaction in the presence of cinchona In general the suggested roles of the functional groups on these catalysts are alkaloid40 presented in scheme 4.16

Scheme 6. (a) Organocatalytic addition of aromatic thiols to cyclohexenone and (b) suggested transition state model for this reaction in the presence of cinchona derived urea organocatalyst. 33

In recent decades considerable progress has been made in developing this class of catalysts. For example, in order to achieve high yields and Scheme 4. The role of functional groups in cinchona alkaloid derivatives16 enantioselectivities, Siva’s group synthesized pentaerythritol tetrabromide-

based chiral quaternary ammonium salts as phase transfer catalysts for In 2016 Houk and Grayson published the result of their DFT calculation studies enantioselective Michael addition reactions between various nitroolefins and on the asymmetric addition of aromatic thiols to cycloalkenones catalyzed by Michael donors (malonates) under mild reaction conditions with very good cinchona alkaloid derivatives (Scheme 5 and 6)33,40. They suggest that the chemical yields (up to 97%) and ee (up to 99%).41 These catalysts have better amine deprotonates the thiol and the resulting protonated amine activates the efficiency in combination with bases such as DIEA and triethylamine. The yields electrophile by Brønsted acid catalysis and the urea group binds the and ee’s of the reactions were higher in polar solvents such as methanol than nucleophilic thiolate by hydrogen bonding. According to their findings the in nonpolar solvents such as toluene. Brønsted acid−hydrogen bonding transition state (TS) model for cinchona

alkaloid catalysis is favored over Wynberg’s widely accepted ion pair−hydrogen

bonding model.33,40

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They also studied the influence of the structure of the substrates on the yield and ee of the reaction.39

Scheme 6. (a) Organocatalytic addition of aromatic thiols to cyclohexenone and (b) suggested transition state model for this reaction in the presence of cinchona derived urea organocatalyst. 33

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Scheme 9. Michael addition of malonate to nitroolefin in the presence of thiourea derivative catalyst43

Besides the extensive research to develop new generations of bifunctional organocatalysts with high reactivity and selectivity, revealing the mechanism of Scheme 7. The structure of synthesized phase transfer organocatalysts41 the action of the catalysts is also in progress, and an interesting research area. Pápai and coworkers reported the result of their studies on 1.1.2. Thiourea based organocatalysts tetrahydropyranylation of alcohol with Schreiner’s catalyst using a The pioneering research on thiourea derivative organocatalysts can be combination of computational and experimental methods in 2016.44 They 42 attributed to the work of Jacobsen’s group in 1998. They developed this type propose that despite the common view that double hydrogen bonding of catalysts for the Strecker reaction (Scheme 8). stablizes the interaction between thiourea and alcohol (HB mechanism), an alternative mechanism in which thiourea acts as a Brønsted acid is more likely. According to their suggested mechanism, thiourea protonates dihydropyran to form an oxacarbenium ion, which reacts with the alcohol (Brønsted acid (BA) mechanism). They have experimentally confirmed the predictions. Reactions with deuterated alcohols yield both syn and anti products, providing further support for the Brønsted acid mechanism.19

Scheme 8. Organocatalytic hydrogen cyanide addition to using thiourea derivative as the catalyst42

synthesis of the bifunctional organocatalyst which is shown in the following 1.2. Fluorescence spectroscopy reaction. They did not mention the mechanism of the catalytic process. Despite the remarkable achievements on the experimental side for many

important organocatalytic reactions, the knowledge about the mechanistic

details still remains limited. For instance, the role of the active sites of the

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Scheme 9. Michael addition of malonate to nitroolefin in the presence of thiourea derivative catalyst43

Scheme 8. Organocatalytic hydrogen cyanide addition to using thiourea derivative as the catalyst42

The authors did not discuss the mechanism of catalysis in this reaction. Takemoto’s group reported highly enantioselective Michael addition of malonates to nitroolefins using metal-free chiral bifunctional organocatalysts Scheme 10. Brønsted acid mechanism versus hydrogen bonding mechanism in tetrahydro pyranylation of alcohol44 (Scheme 9).43 Attempts to optimize the structure of the catalysts resulted in synthesis of the bifunctional organocatalyst which is shown in the following 1.2. Fluorescence spectroscopy reaction. They did not mention the mechanism of the catalytic process. Despite the remarkable achievements on the experimental side for many

important organocatalytic reactions, the knowledge about the mechanistic

details still remains limited. For instance, the role of the active sites of the

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catalysts in the catalytic process as well as the origins of high (or low) The fluorescence quantum yield ϕf and the fluorescence decay time τf are enantioselectivity are not yet clearly established in most cases. primary quantitative measures:

The aim of this research project is to explore the use of fluorescence ϕf = kf / (kf + knr) (1)

spectroscopy to gain information about the interaction between the substrates τf = 1 / (kf + knr) (2)

and the catalysts. This section gives some general background on fluorescence In equations 1 and 2 kf is the rate constant for fluorescence emission.

and its applications. The non-radiative rate constant knr combines the rate constants of inter Goppelsröder used fluorometric analysis for the first time in history in 1867 to system crossing, internal conversion to the ground state, and any other determine Al (III) by the fluorescence of its morin chelate.45 Since 1911, when competing fluorescence quenching process that might occur. In Scheme 11 this Heimstaedt and Lehmann developed the first fluorescence microscopes to is exemplified by photochemical reaction, but one may also think of energy investigate the autofluorescence of bacteria, protozoa, plant and animal transfer processes, in which the excited state energy is transferred to another tissues, and bioorganic substances such as albumin, elastin, and keratin, many chromophore which may emit at longer wavelength (FRET).52,53 important applications based on photoluminescence have been developed, All fluorescence instruments contain three basic items: a source of light, a such as fluorescence microscopy, fluorescent tubes and lamps,46,47 optical sample holder and a detector. The most common detector, also used in the brighteners, plasma screens, forensics, tracers in hydrogeology, fluorescent present work, is a photomultiplier tube. Research grade spectrofluorometers and phosphorescent paints, phosphorescent labels, safety signs, and use single or double diffraction grating monochromators to select the counterfeit detection (security documents, bank notes, art works).48–51 wavelength of the incident light and to spectrally analyse the emitted At room temperature most molecules occupy the lowest vibrational level of fluorescence. The fluorescence is most often measured at a 90° angle relative the ground electronic state, and on absorption of light they are elevated to to the excitation light. For turbid or opaque samples the fluorescence can be produce electronically excited states. Having absorbed energy and reached measured from the front.45 When detecting the emission at a particular one of the higher vibrational levels of an excited state, the molecule rapidly wavelength and scanning the wavelength of excitation, a so-called loses its excess of vibrational energy by collision and falls back to the lowest fluorescence excitation spectrum is obtained. In most cases, this is identical to vibrational level of the excited state (Scheme11). From this state emission of the absorption spectrum of the molecule, because molecules emit from the light in the form of fluorescence can occur, in competition with other relaxed singlet excited state regardless of the excitation wavelength. The processes. excess of energy that is deposited in the molecule by excitation at shorter wavelengths is rapidly dissipated by internal conversion (Kasha’s rule). Among the many different fluorimetric methods54–56 we focused mostly on the use of fluorescence spectroscopy to record the spectra of emitted light. Time-resolved fluorescence was measured using the technique of time- correlated single photon counting. The sample is excited with a series of short laser pulses and the arrival times of detected photons are determined relative to the time of the laser pulse. By accumulating many thousands of photons, a histogram of arrival times can be constructed, from which the decay time can be obtained.52,53 Scheme 11. Jablonski diagram showing the primary excited state processes The interests in exploring the individual behavior of molecules in complex

environments has led to the development of a wide variety of microscopic techniques.57

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The aim of this research project is to explore the use of fluorescence ϕf = kf / (kf + knr) (1) spectroscopy to gain information about the interaction between the substrates τf = 1 / (kf + knr) (2) and the catalysts. This section gives some general background on fluorescence In equations 1 and 2 kf is the rate constant for fluorescence emission. and its applications. The non-radiative rate constant knr combines the rate constants of inter Goppelsröder used fluorometric analysis for the first time in history in 1867 to system crossing, internal conversion to the ground state, and any other determine Al (III) by the fluorescence of its morin chelate.45 Since 1911, when competing fluorescence quenching process that might occur. In Scheme 11 this Heimstaedt and Lehmann developed the first fluorescence microscopes to is exemplified by photochemical reaction, but one may also think of energy investigate the autofluorescence of bacteria, protozoa, plant and animal transfer processes, in which the excited state energy is transferred to another tissues, and bioorganic substances such as albumin, elastin, and keratin, many chromophore which may emit at longer wavelength (FRET).52,53 important applications based on photoluminescence have been developed, All fluorescence instruments contain three basic items: a source of light, a such as fluorescence microscopy, fluorescent tubes and lamps,46,47 optical sample holder and a detector. The most common detector, also used in the brighteners, plasma screens, forensics, tracers in hydrogeology, fluorescent present work, is a photomultiplier tube. Research grade spectrofluorometers and phosphorescent paints, phosphorescent labels, safety signs, and use single or double diffraction grating monochromators to select the counterfeit detection (security documents, bank notes, art works).48–51 wavelength of the incident light and to spectrally analyse the emitted At room temperature most molecules occupy the lowest vibrational level of fluorescence. The fluorescence is most often measured at a 90° angle relative the ground electronic state, and on absorption of light they are elevated to to the excitation light. For turbid or opaque samples the fluorescence can be produce electronically excited states. Having absorbed energy and reached measured from the front.45 When detecting the emission at a particular one of the higher vibrational levels of an excited state, the molecule rapidly wavelength and scanning the wavelength of excitation, a so-called loses its excess of vibrational energy by collision and falls back to the lowest fluorescence excitation spectrum is obtained. In most cases, this is identical to vibrational level of the excited state (Scheme11). From this state emission of the absorption spectrum of the molecule, because molecules emit from the light in the form of fluorescence can occur, in competition with other relaxed singlet excited state regardless of the excitation wavelength. The processes. excess of energy that is deposited in the molecule by excitation at shorter wavelengths is rapidly dissipated by internal conversion (Kasha’s rule). Among the many different fluorimetric methods54–56 we focused mostly on the use of fluorescence spectroscopy to record the spectra of emitted light. Time-resolved fluorescence was measured using the technique of time- correlated single photon counting. The sample is excited with a series of short laser pulses and the arrival times of detected photons are determined relative to the time of the laser pulse. By accumulating many thousands of photons, a histogram of arrival times can be constructed, from which the decay time can be obtained.52,53 Scheme 11. Jablonski diagram showing the primary excited state processes The interests in exploring the individual behavior of molecules in complex

environments has led to the development of a wide variety of microscopic techniques.57

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- Scanning methods such as near-field scanning optical microscopy (NSOM) and them to be distinguished via their absorption or emission spectra or other confocal microscopy luminescence properties. For any process that leads to the generation of - Wide field methods such as total internal reflection and epifluorescence fluorescence the term fluorogenesis can be used. A high contrast can be microscopy. achieved when non-fluorescent reagents react to form fluorescent molecules In this work we used a total internal reflection fluorescence (TIRF) microscope in a fluorogenic reaction.60 In the literature, the term is used more broadly. For for imaging fluorescence at a microscopic level. In this case a dichroic mirror example, adding a fluorescent tag or label to a non-fluorescent reagent is also selects the range of wavelengths of emitted light that can reach the detector. described as a fluorogenic process,60,61 and other forms of fluorescence A schematic representation of the components of a conventional fluorimeter enhancement due to binding62 or aggregation63 are also considered and a total internal reflection (TIRF) microscope are shown in scheme 12. fluorogenic. TIRFM allows for selective excitation of fluorophores near the surface, while Different research groups have used fluorescence labelling for determination molecules further away from the surface (typically >200 nm) are not excited of biomedically important substances with high selectivity and sensitivity.60,64,65 and therefore do not fluoresce. In the liquid layer above the surface non- Beside the progress in fluorescence tagging application, fluorescence bound molecules give rise to a background fluorescence. Because these generating studies play an important role in visualizing the processes in the molecules are mobile, this background is mostly diffuse, while molecules that medical area. For example Liu’s group reported their success to visualize drug bind to the surface long enough give rise to a distinct intensity peak. In this release using a fluorogenic Michael reaction (Scheme 13).66 way, single molecule detection becomes possible.58 (a) (b)

66 Scheme 13. Fluorogenic reaction to visualize the drug delivery process

Scheme 12. The components of (a) fluorescence spectrometer (from Fluorolog-3 Fluorescence imaging techniques have also been used to study the kinetics of Manual); PMT = photomultiplier tube (b) TIRF microscope59 enzyme catalyzed chemical reactions. In 2005, Velonia and coworkers used the scanning confocal microscope (CFM) to investigate the hydrolysis of a non- 1.3. Fluorogenic reactions fluorescent ester to form a highly fluorescent product in the presence of lipase The research described in this thesis is aimed at the quantitative detection of B from Candida Antarctica (CALB) as the catalyst. They were able to observe chemical changes by means of fluorescence spectroscopy. This requires that the real-time catalysis and substrate kinetics of this single-enzyme-catalyzed reactants, products and intermediates have different properties that allow reaction.67 Also reactions in organic chemistry have been studied using

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66 Scheme 13. Fluorogenic reaction to visualize the drug delivery process

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fluorescence imaging methods. In 2011 Herten and coworkers reported the These chromophores have relatively high absorption coefficient, high quantum result of their single-molecule studies on epoxidation of a boron yield, life time in the nanosecond range, narrow absorption and emission dipyrromethene (BODIPY) probe by meta-chloroperoxybenzoic acid (mCPBA) bands and good solubility and (photo)stability in most organic solvents. Many (Scheme 14). As a result of the epoxidation process the yellow fluorescent BODIPY derivatives have proven suitable for single molecule spectroscopy.71–74 BODIPY converts to the green fluorescent product. Making use of this An enormous variety of BODIPYs has already been synthesized, which provides property, they used dual-color detection using a TIRF microscope to visualize a class of compounds containing different functional groups with known the irreversible conversion of the substrate to the product.68 photophysical and spectroscopic properties which can be useful in choosing the desired substrate for the synthetic purpose.70,75 When the dyes are engaged in complexation or reaction any change in the position of the peak of the emitted light or in the intensity of the emitted light will be helpful to follow the process. The best situation will be turning on or off the emission due to the interaction between the reagents. For studying organocatalytic reactions, the BODIPY dyes must be linked to the required functional groups that undergo the reactions that we want to study.

1.5. Outline of the research Scheme 14. Epoxidation of the double bond shortens the conjugated -system. As the In the first part of this project, a number of organocatalytic reactions such as 68 result the product emits at shorter wavelength than the reactant Michael addition,76 Biginelli reaction,77 Henry reaction,78 aldol addition,79 were studied using fluorescence spectroscopy. Then, we focused on the Michael The opportunities of single molecule techniques to obtain a unique insight into reaction and continued our studies on this reaction using TIRF microscopy. the mechanism of chemical reactions were reviewed in 2013 by Cordes and For the Michael addition reaction the substrate must contain an unsaturated Blum.69 Several examples of imaging of catalytic chemical reactions are bond (double bond) next to an electron withdrawing group. We chose BODIPYs discussed in this paper and show the potential of this method on the one hand containing a maleimide group (compound 15)80,81 or a nitro alkene group and the problems and challenges on the other hand. (compound 17)82–84 as the substrates. The reason is that compounds 15 and 17

are weakly fluorescent but the addition products, in which the double bond is 1.4. BODIPYs as the suitable candidates for fluorogenic reactions removed, are highly fluorescent. So, the progress of the reaction can be easily In order to use fluorescence spectroscopy to follow organocatalytic reactions, followed by the use of the fluorescence spectroscopy. These two compounds the first requirement was labeling the studied substrates with a suitable were synthesized following the literature methods. In some cases the method chromophore. We chose BODIPYs (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) was modified to improve the yields of the reactions. We chose the BODIPYs to label the substrates.70 containing aldehyde groups in different parts of the core (compounds 6 and

9),82–84 as the substrates for chromene synthesis (Chapter 4) and also as the substrate for the Biginelli reaction (compound 6, chapter 5). These compounds were also synthesized following literature methods. These BODIPYs are fluorescent themselves but the intensity of the emitted light changes during Scheme 15. The BODIPY core the reaction which provides the possibility to follow the interactions. The

structures of the substrates used are shown in scheme 16.

508735-L-bw-Oskouei 508735-L-bw-Oskouei Processed on: 7-3-2017 Processed on: 7-3-2017 12 Introduction Chapter 1 13 fluorescence imaging methods. In 2011 Herten and coworkers reported the These chromophores have relatively high absorption coefficient, high quantum result of their single-molecule studies on epoxidation of a boron yield, life time in the nanosecond range, narrow absorption and emission dipyrromethene (BODIPY) probe by meta-chloroperoxybenzoic acid (mCPBA) bands and good solubility and (photo)stability in most organic solvents. Many (Scheme 14). As a result of the epoxidation process the yellow fluorescent BODIPY derivatives have proven suitable for single molecule spectroscopy.71–74 BODIPY converts to the green fluorescent product. Making use of this An enormous variety of BODIPYs has already been synthesized, which provides property, they used dual-color detection using a TIRF microscope to visualize a class of compounds containing different functional groups with known the irreversible conversion of the substrate to the product.68 photophysical and spectroscopic properties which can be useful in choosing the desired substrate for the synthetic purpose.70,75 When the dyes are engaged in complexation or reaction any change in the position of the peak of the emitted light or in the intensity of the emitted light will be helpful to follow the process. The best situation will be turning on or off the emission due to the interaction between the reagents. For studying organocatalytic reactions, the BODIPY dyes must be linked to the required functional groups that undergo the reactions that we want to study.

structures of the substrates used are shown in scheme 16.

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Scheme 16. The BODIPYs used for Michael addition reactions and Biginelli reaction

Proline derivatives, thiourea derivatives and cinchona alkaloids and the immobilized version of two of them (scheme 17) were used as the organocatalysts. The results of the studied reactions are presented in five chapters. Chapter 2 contains a description of the experimental techniques, and the synthetic processes to prepare the starting materials, the catalysts and the products. The results of the spectroscopic characterization of the compounds are also presented in this chapter. Chapter 3 is about Michael addition reactions of nucleophiles to the BODIPYs bearing maleimide (15) and nitro alkene groups (17). We studied the reaction between these chromophores and benzyl mercaptan and diethylmalonate in the presence of different catalysts under different conditions. In continuation we chose some of the catalysts and followed these reactions using fluorescence spectroscopy. This method enabled us to get more information about the interaction between the chromophore and the catalyst. We immobilized two catalysts on the surface of Scheme 17. Organocatalysts used in this Thesis the glass and followed the reactions in the case of using benzyl mercaptan as References: the nucleophile. In this chapter, we also present the result of the Michael reaction between benzyl mercaptan and maleimide-BODIPY 15 in the presence (1) Sardon, H.; Pascual, A.; Mecerreyes, D.; Taton, D.; Cramail, H.; Hedrick, of a fluorescent catalyst. Chapter 4 presents the synthesis of fluorescent J. L. Synthesis of Polyurethanes Using Organocatalysis: A Perspective. chromenes. Here, monitoring the reaction with fluorescence showed the Macromolecules 2015, 48, 3153–3165. presence of an intermediate. In chapter 5 we discuss the results of the (2) Atodiresei, I.; Vila, C.; Rueping, M. Asymmetric Organocatalysis in Continuous Flow: Opportunities for Impacting Industrial Catalysis. ACS organocatalytic synthesis of fluorescent derivatives of dihydropyrimidinone Catal. 2015, 5, 1972–1985. using the Biginelli reaction. (3) Chen, D. F.; Han, Z. Y.; Zhou, X. L.; Gong, L. Z. Asymmetric Preliminary results of the single molecule studies of the Michael reaction using Organocatalysis Combined with Metal Catalysis: Concept, Proof of TIRF microscopy are reported in chapter 6. Concept, and beyond. Acc. Chem. Res. 2014, 47, 2365–2377. (4) List, B. Introduction: Organocatalysis. Chem. Rev. 2007, 107, 5413– 5415.

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Scheme 16. The BODIPYs used for Michael addition reactions and Biginelli reaction

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