Novel olefin isomerizations under Ni(I) catalysis

  • Neuartige Olefinisomerisierungen unter Ni(I)-Katalyse

Güven, Sinem; Schoenebeck, Franziska (Thesis advisor); Albrecht, Markus (Thesis advisor)

Aachen (2019, 2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

This thesis features practical methods in nickel chemistry to install E-olefins through olefin transposition and to access tri-substituted Z-silyl enol ethers via chain walking under mild conditions. Stereoisomerism often gives rise to distinctive physical and chemical characteristics that are prominent for pharmaceuticals, agrochemical, fragrance and flavor industries; therefore, the installation of the carbon-carbon double bonds in a stereoselective manner is crucial in organic chemistry and in its related fields. Also, in organic synthesis, precursors with defined double bond geometries are commonly employed to build up more complex molecular structures. Addressing the importance of the construction of the double bonds stereoselectively, the first chapter of this thesis details E-selective olefin isomerization catalyzed by [Ni(m-Cl)IPr]2 under mild conditions. The contributions detailed in this chapter are to showcase the catalyst efficiency by presenting various synthetic applications, such as one-carbon isomerization of allylic groups and exocycles, and remote functionalization over 2- to 5-carbon bond isomerization of homoallylic units. It is also shown that using catalytic amounts of Lewis acid (TiOiPr)4 circumvents the catalyst deactivation problem with substrates that bear ketone functionality. Furthermore, using (TiOiPr)4 increased the isomerization efficiency of fused cyclic compounds having heteroatom functionalities (O, S). Additionally, the feasibility of one-bond isomerization of a Z- and E/Z alkenes is shown successfully. Under the isomerization conditions, an internal Z- to E- isomerization is also observed. The second part of this thesis showcases a new method developed for the formation of Z-silyl enol ethers from homoallylic ketones with excellent regioselectivities via Ni-catalyzed chain walking, which is in conjugation with the first chapter. A thorough reaction optimization and control experiments showed that reaction conditions require using a metallic reductant, Mn, in the presence of either [Ni(m-Br)IPr]2 or NiBr2(dme)/IPr (each; 10 mol%) along with using iPrBr or nPrBr as a stoichiometric additive. It is found out that Mn activates the precatalyst NiII to generate the [Ni(-Br)IPr]2 species in situ. Single crystal X-ray diffraction was utilized to characterize relevant nickel complexes, showing that Mn0-induced activation of NiII forms NiI species, which further shows the stability of [Ni(-Br)IPr]2 in the presence of Mn. Furthermore, using of [Ni(-Br)IPr]2 as a catalyst in the absence of Mn resulted in one turnover, and the end species of this reaction were successfully crystallized. The X-ray analysis showed the formation of NiII species, highlighting the necessity of a metallic reductant to regenerate the active species. Furthermore, deuterium experiments are applied to understand the effect of alkyl bromide, which shows that alkyl bromide is the hydrogen source since the showcased reaction conditions represent a reductive isomerization. The combination of X-ray analysis, 1H NMR experiments and mechanistic reactions enabled the elucidation of key steps in the formation of Z-selective silyl enol ethers catalyzed by Ni. The broad functional group tolerance of this method was also showcased in the presence of polyaromatics, biphenyls, electron-donating and electron-withdrawing groups, presenting a novel transformation to access trisubstituted Z-silyl enol ethers under base-free reaction medium.

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