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Alkylation of Csp2 thianthrenium salts and β-Me elimination of an n-propyl chain


Lansbergen,  Beatrice
Research Department Ritter, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Lansbergen, B. (2024). Alkylation of Csp2 thianthrenium salts and β-Me elimination of an n-propyl chain. PhD Thesis, Rheinisch-Westfälische Technische Hochschule, Aachen.

Cite as: https://hdl.handle.net/21.11116/0000-000F-5907-6
Csp2-based thianthrenium salts, such as the aryl and vinyl thianthrenium salts, are useful synthetic functional handles. Csp2-based thianthrenium salts can engage in a wide range of further functionalization reactions, including traditional cross-couplings that convert the thianthrenium group into useful functional groups. Reductive electrophile cross-coupling reactions are advantageous over traditional cross-coupling reactions as they bypass the use of preformed organometallic reagent. Neither aryl nor vinyl thianthrenium were reported to undergo a reductive electrophile cross-coupling reaction. Particularly the reductive alkylation of aryl and vinyl thianthrenium salts is of high interest in both academia and industry since it was discovered that the degree of saturation of a compound can have profound effects on both the physical properties of a molecule as well as the biological activity. Apart from the functionalization of alkyl groups, C–C bond cleavage is another useful strategy in organic synthesis for the functionalization and diversification of alkyl substrates yet remains underdeveloped. The work summarized in this thesis describes novel ways to functionalize alkyl groups via Csp2–Csp3 cross-coupling reactions using Csp2 sulfonium salts as well as a study on a ß-methyl elimination as a method for alkane C–C bond cleavage.
Chapter 1 of this thesis deals with the reductive electrophile cross-coupling between alkyl halides and Csp2 thianthrenium salts. Part 1 in Chapter 1 presents a site-selective two-step C–H alkylation of complex arenes useful for late-stage functionalization, which is achieved via the combination of a site-selective C–H thianthrenation with Pd-catalyzed reductive electrophile cross-coupling. The reductive alkylation reaction can be performed on a range of complex small molecules to access a diverse range of synthetically useful alkylated arenes with high para-selectivity. The robustness of this transformation is further demonstrated by fragment coupling, which has rarely been reported within the context of reductive aryl alkylations. The mechanism of this transformation is consistent with a type of Negishi mechanism where the alkyl zinc reagent is formed in situ. Part 2 reports a novel reductive vinylation of alkyl iodides. The reaction uses a vinyl thianthrenium salt or vinyl bromide, a palladium catalyst, and an alkyl zinc intermediate formed in situ to trap the LnPdII(vinyl) complex formed after oxidative addition before it undergoes undesired homocoupling to form butadiene. Dropwise addition of the vinyl thianthrenium salt and the addition of magnesium bromide was required to better match the fast oxidative addition relative to slow transmetalation. The vinylation reaction has been carried out on complex small molecules to access compounds difficult to access via other methods. The reported transformation also expands on the currently possible reductive Csp2–Csp3 cross-coupling reactions between Csp2–halides and alkyl halides.
Chapter 2 of this thesis describes our attempts to gain understanding of a fundamental reaction: the ß-methyl elimination. Both computational and experimental results are reported. We focused on studying the ß-methyl elimination from an n-propyl chain on scandium d0 complexes with ß-diketiminato (nacnac) ligands. The primary goal of this study was to understand the factors that favor ß-methyl elimination over the often thermodynamically and kinetically more favorable ß-hydride elimination. Though we were unsuccessful at experimentally observing a ß-methyl elimination, key lessons learned are reported, which will hopefully contribute to adding a piece to the puzzle.