Promoting difficult coupling processes. Synthesis of phosphine-olefin hybrid ligands, reactivity and reductive elimination studies

Resum

The work developed during this doctoral thesis involves the design, synthesis, characterization and evaluation of the catalytic activity of a family of hybrid phosphine-olefin (PEWO) ligands that are able to promote challenging coupling processes. The memory is organized in five main blocks: Introduction, Results and Discussion, Experimental Section, Resumen en español (Spanish Summary) and Annex. First of all, the introduction presents concepts related to the four main topics treated: Ligand design, Reductive elimination promoted by ligands, Negishi reaction mechanism and Reductive elimination in Ni. The results obtained have led so far to the publication of several articles, although there are others that are in the process of being published. In the Results and Discussion block brief comments about the former, as well as full description of the later (drafts) are presented. The experimental characterization of the compounds and detailed explanation of some experiments is gathered in the Experimental Section. In addition, according to the Spanish regulation of Doctoral Thesis, a brief summary of the thesis written in Spanish is included. Finally, the Articles published (with the corresponding Supporting Information) in the original format of the paper are reproduced. These PEWO ligands are tested in Pd-catalyzed cross-coupling processes (and some Ni-catalyzed) showing good results at relatively low temperatures in short reaction times. In addition, the study of the mechanism of the coupling reactions give a broader understanding of the reaction and its competitive routes and leads to optimized reaction conditions. • In Section I, PEWO ligands were tested in a model aryl-alkyl Negishi reaction showing moderate to good selectivity depending on the structure of the phosphine. It was seen that those ligands that were able to coordinate to the metal center in a chelating mode led to better selectivity toward the coupling product. We proposed that olefin coordination cause the acceleration of the reductive elimination rate. • The origin of the lost in selectivity was investigated and presented in Section II. The occurrence of secondary transmetalation processes was found responsible for the presence of Ar–H byproducts. Improved reaction conditions were found and were analyzed in Section III. • Later in Section IV the reductive elimination step was measured and PEWO ligands were among the fastest ligands inducing coupling. 19F NMR spectra confirmed olefin coordination during reductive elimination. • DFT calculations in Section V analyze olefin participation in the reduction of the coupling barrier. It was observed the stabilization of the HOMO MO (Pd dx2-y2) that is being filled when going from PdII to Pd0 with PhPEWO-F. • In Section VI PEWO ligands were found to promote extremely fast coupling processes in Ni, what open new opportunities for difficult catalytic procedures using this first-row transition metal. • Negishi couplings using a great variety of electrophiles and nucleophiles was found very efficient when using the [PdCl2(PhPEWO-F)] catalyst in Section VII. • Finally, PEWO ligands were tested in highly difficult processes. A full stoichiometric, catalytic and DFT study was presented in Section VIII showing the power and limitations of these hybrid ligands. Overall, PEWO ligands showed to be very efficient ancillary ligands for the promotion of difficult coupling processes. Future lines related with its use in Ni catalysis, in challenging cross-coupling processes, and PEWO new designs will be on-going in our group.