Resumen de Copper, silver and ruthenium complexes with tridentate ligands. Application as catalysts in aziridination, c-h insertion and hydrogenation reactions
The development of novel types of tridentate ligands has attracted considerable attention in recent years, as a result of their ability to provide stability to metallic complexes. In terms of asymmetric induction, a tridentate ligand might create a more sterically hindered chiral space than a bidentate ligand, reducing disadvantages such as rotation and flexibility in enantiofacial control. Tridentate ligands are widely used in transition metal-catalyzed reactions, leading to efficient processes with great success in a vast number of organic transformations. The final goal of this thesis is the development of novel families of tridentate ligands and their coordination to metal-transitions, exploring their performance in aziridination, C-H insertion and hydrogenation reactions.
Chapter 1 contains a general introduction to this type of ligands and makes reference to some examples in the literature. Chapter 2 sets out the general objectives of this thesis. The research in Chapter 3 describes the synthesis of a substituted pyrazolyl ligand and a chiral trispyrazolylmethane ligand, being the first chiral example of these ligands to date. Other non chiral trispyrazolylmethane ligands were synthesized, with different features in the backbone of the ligand. All ligands synthesized were coordinated to copper (I) in excellent yields, and the corresponding complexes were tested in the aziridination of trans,trans-2,4-hexadien-1-ol, styrene and other olefinic substrates. The chiral trispyrazolylmethane complexes were found to be good catalysts for the aziridination of some olefinic substrates, although in all cases scarce levels of enantioselectivity were obtained.
The research in Chapter 4 describes the synthesis of new electron deficient trisphosphinoborate-based metallic complexes by incorporating electron-withdrawing groups in the backbone of the ligand. In particular, a new trisphosphinoborate ligand containing trifluoromethyl-groups was synthesized and fully characterized. The corresponding thallium, copper and silver complexes were obtained, and the X-ray structures were also elucidated. These trisphosphinoborate-metal complexes were tested in aziridination and nitrene C-H insertion reactions and were found to be excellent catalysts for these transformations. The copper complex catalyzed in excellent yields the aziridination of a wide range of both electron-rich and electron deficient olefins, whereas the silver complex effectively catalyzed the nitrene insertion of C-H bonds in good yields, being another example to the yet scarce number of catalytic systems which verify this reaction. Moreover, the mechanism of copper-catalyzed aziridination was studied, based on the Hammet equation. Collected data confirmed the existence of an electrophilic transition state and the presence of electrophilic radical contributions along the reaction.
The research in Chapter 5 explores the synthesis of new P-stereogenic PNP-type pincer ligands, which were obtained in good yields and with 99% optical purity. These ligands were coordinated to ruthenium, and tested in the asymmetric hydrogenation of a wide range of ketones. For the first time, a P-stereogenic PNP-ruthenium complex catalyzed the reduction of ketones with high conversions and enantiomeric ratios up to 98:2. Moreover, studies towards the elucidation of the active reducing species enabled the development of a novel methodology for the selectively deuteration of the benzylic position of the PNP ligand.
