Mechanistic investigations of metal-catalyzed (poly)esterification reactions

The ester bond is an important structural motif in organic molecules, that among others find application as pharmaceuticals, fragrances or coatings. Although the direct coupling of carboxylic acids and alcohols is known for more than a century, it is still the most prominent route to synthesize the ester moiety. This chemical transformation affords the desired ester bond and water as the only by-product, but high reaction temperatures are required. Therefore, the use of a catalyst that can lower the reaction temperature and enable mild reaction conditions is highly desirable. Lewis acidic metals exclusively catalyze the desired esterification reaction and are therefore intensively explored as catalysts. Although a wide variety of different metal salts were found to be effective catalysts, understanding the origin of their catalytic activity is still limited. Mechanistic studies are severely complicated due to in situ transformation of the catalyst, since all reaction components (carboxylic acid, alcohol, ester and water) have the ability to coordinate to the Lewis acidic metal center. In this thesis, we have shed light on the structure of various in situ formed Lewis acidic metal catalysts, potential intermediates and have provided deeper mechanistic understanding of the catalytic cycle. Besides the direct (poly)esterification reaction also the nickel-catalyzed carbon-oxygen bond forming reaction between an organic halide and a carboxylic acid is investigated. Also here, mechanistic understanding of the catalytic cycle is hindered by many side reactions, either by reaction with one of the reaction components, or by other (deactivation) reactions

Spectroscopic Investigation of the Activation of a Chromium-Pyrrolyl Ethene Trimerization Catalyst

1-Hexene is an important α-olefin for polyethylene production and is produced from ethene. Recent developments in the α-olefin industry have led to the successful commercialization of ethene trimerization catalysts. An important industrially applied ethene trimerization system uses a mixture of chromium 2-ethylhexanoate, AlEt3, AlEt2Cl, and 2,5-dimethylpyrrole (DMP). Here, we have studied the activation of this system using catalytic and spectroscopic experiments (XAS, EPR, and UV–vis) under conditions employed in industry. First, chromium 2-ethylhexanoate was prepared and characterized to be [Cr3O­(RCO2)6(H2O)3]­Cl. Next, the activation of chromium 2-ethylhexanoate with AlEt3, AlEt2Cl, and DMP was studied, showing immediate reduction (<5 ms) of the trinuclear Cr­(III) carboxylate and formation of a neutral polynuclear Cr­(II) carboxylate complex. Over time, this Cr­(II) carboxylate complex is partially converted into a chloro-bridged dinuclear Cr­(II) pyrrolyl complex. In cyclohexane, small quantities of an unknown Cr­(I) complex (∼1% after 1 h) are observed, while in toluene, the quantity of Cr­(I) is much higher (∼23% after 1 h). This is due to the formation of cationic bis­(tolyl)­Cr­(I) complexes, which likely leads to the observed inferior performance using toluene as the reaction solvent. Catalytic studies allow us to exclude some of the observed Cr­(I) and Cr­(II) complexes as the active species in this catalytic system. Using this combination of techniques, we have been able to structurally characterize complexes of this selective Cr-catalyzed trimerization system under conditions which are employed in industry

Titanium-catalyzed esterification reactions: beyond Lewis acidity

Esterification is a key reaction and is used in many synthetic and industrial processes, yet the detailed mechanism of operation of often-used (Lewis acid) catalysts is unknown and subject of little research. Here, we report on mechanistic studies of a titanium aminotriphenolate catalyst, using stoichiometric and catalytic reactions combined with kinetic data and density functional theory (DFT) calculations. While often only the Lewis acidity of the Ti-center is taken into account, we found that the amphoteric nature of this catalyst, combining this Lewis acidity with Brønsted basicity of a Ti-bound and in situ formed carboxylate group, is crucial for catalytic activity. Furthermore, hydrogen bonding interactions are essential to pre-organize substrates and to stabilize various intermediates and transition states and thus enhancing the overall catalytic reaction. These findings are not only applicable to this class of catalysts, but could be important for many other esterification catalysts

Mechanistic investigations of metal-catalyzed (poly)esterification reactions

The ester bond is an important structural motif in organic molecules, that among others find application as pharmaceuticals, fragrances or coatings. Although the direct coupling of carboxylic acids and alcohols is known for more than a century, it is still the most prominent route to synthesize the ester moiety. This chemical transformation affords the desired ester bond and water as the only by-product, but high reaction temperatures are required. Therefore, the use of a catalyst that can lower the reaction temperature and enable mild reaction conditions is highly desirable. Lewis acidic metals exclusively catalyze the desired esterification reaction and are therefore intensively explored as catalysts. Although a wide variety of different metal salts were found to be effective catalysts, understanding the origin of their catalytic activity is still limited. Mechanistic studies are severely complicated due to in situ transformation of the catalyst, since all reaction components (carboxylic acid, alcohol, ester and water) have the ability to coordinate to the Lewis acidic metal center. In this thesis, we have shed light on the structure of various in situ formed Lewis acidic metal catalysts, potential intermediates and have provided deeper mechanistic understanding of the catalytic cycle. Besides the direct (poly)esterification reaction also the nickel-catalyzed carbon-oxygen bond forming reaction between an organic halide and a carboxylic acid is investigated. Also here, mechanistic understanding of the catalytic cycle is hindered by many side reactions, either by reaction with one of the reaction components, or by other (deactivation) reactions

Catalytic Synthesis of 1 H-2-Benzoxocins: Cobalt(III)-Carbene Radical Approach to 8-Membered Heterocyclic Enol Ethers

The metallo-radical activation of ortho-allylcarbonyl-aryl N-arylsulfonylhydrazones with the paramagnetic cobalt(II) porphyrin catalyst [CoII(TPP)] (TPP = tetraphenylporphyrin) provides an efficient and powerful method for the synthesis of novel 8-membered heterocyclic enol ethers. The synthetic protocol is versatile and practical and enables the synthesis of a wide range of unique 1H-2-benzoxocins in high yields. The catalytic cyclization reactions proceed with excellent chemoselectivities, have a high functional group tolerance, and provide several opportunities for the synthesis of new bioactive compounds. The reactions are shown to proceed via cobalt(III)-carbene radical intermediates, which are involved in intramolecular hydrogen transfer (HAT) from the allylic position to the carbene radical, followed by a near-barrierless radical rebound step in the coordination sphere of cobalt. The proposed mechanism is supported by experimental observations, density functional theory (DFT) calculations, and spin trapping experiments

Role of the ligand and activator in selective CrtextendashPNP ethene tri- and tetramerization catalysts textendash a spectroscopic study

The reaction of the ethene tetramerization catalyst, ((C6H5)2P)2NiPrCrCl3(THF) (complex 1), and ethene trimerization catalyst, ((o-C6H4OMe)2P)2NMeCrCl3 (complex 2), with alkylaluminum reagents (AlMe3 and MMAO) was investigated using spectroscopic techniques (Cr K-edge XAS, X-band EPR and UV-vis) and catalytic studies. In all cases the majority of chromium was reduced to the divalent oxidation state and only a minor fraction of chromium was reduced further to the monovalent oxidation state. It is demonstrated that MMAO and the ligand (through a pendant ether donor) can facilitate ion pair formation for these divalent Cr complexes, providing insights into the role of the ligand and activator in the activation process. Via the use of dienes, we succeeded in characterizing a monocationic CrII alkene complex, providing evidence that catalysis could proceed via cationic CrII/CrIV intermediates. This is supported by DFT calculations, where it is shown that a mechanism proceeding via dicationic CrII/CrIV intermediates explains the observed product selectivity

Thermal/Blue Light Induced Cross-Linking of Acrylic Coatings with Diazo Compounds

The thermal curing of industrial coatings (e.g., car painting and metal coil coatings) is accompanied by a substantial energy consumption due to the intrinsically high temperatures required during the curing process. Therefore, the development of new photochemical curing processes—preferably using visible light—is in high demand. This work describes new diazo-based cross-linkers that can be used to photocure acrylic coatings using blue light. This work demonstrates that the structure of the tethered diazo compounds influences the cross-linking efficiency, finding that side reactions are suppressed upon engineering greater molecular flexibility. Importantly, this work shows that these diazo compounds can be employed as either thermal or photochemical cross-linkers, exhibiting identical crosslinking performances. The performance of diazo-cross-linked coatings is evaluated to reveal excellent water resistance and demonstrably similar material properties to UV-cured acrylates. These studies pave the way for further usage of diazo-functionalized cross-linkers in the curing of paints and coatings.</p