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Abstract:
This work focuses on the conceptual development of highly confined, super acidic Brønsted acids and their application in asymmetric catalysis. A new synthetic strategy to access imidodiphosphate-derived Brønsted acids based on a toolbox principle has been developed. This methodology proceeds via consecutive chloride substitutions of hexachlorobisphosphazonium salts, providing rapid access to privileged catalyst motifs, such as imidodiphosphates (IDP), iminoimidodiphosphates (iIDP), and imidodiphosphorimidates (IDPi). Furthermore, this approach enables access to previously elusive catalyst scaffolds with particularly high structural confinement, allowing the highly enantioselective transformation of small and structurally unbiased substrates, as exemplarily demonstrated in the asymmetric sulfoxidation of propyl methyl sulfide. To access extremely acidic catalyst motifs, a new synthesis of arylbis(trifluoromethylsulfonylimino)sulfonamides has been developed, which upon implementation into the imidodiphosphate-framework led to the development of imidodiphosphorbis(iminosulfonylimino)imidates (IDPii). Combinatorial spectroscopic and experimental data reveal higher acidities of the novel IDPii motif than commonly employed super acids, such as trifluoromethanesulfonic acid or bis(trifluoromethanesulfonyl)imide. Most notably, IDPiis allow rendering alcohols into strong electrophilic alkylating reagents under silylium activation, as it has been exemplarily demonstrated in the α-methylation of silyl ketene acetals – transforming methanol into a strong electrophilic methyl surrogate. Although the IDPii catalyst class displays extreme reactivity and overcomes previous limitations regarding substrate activation, no sufficient enantioinduction was observed in the explored asymmetric transformations. Crystallographic analyses of several IDPii catalysts motifs illustrated insufficient BINOL-induced structural confinement. Considered as a solution, a monovalent chiral catalyst scaffold has been conceptually designed, which in combination with the new access of bis(trifluoromethylsulfonylimino)sulfonyl units was believed to give access to a novel extremely Brønsted acidic and highly selective catalyst class. This catalyst design focuses on the tetrahydroindacene motif with two adjacent appendages to confine the active site. Herein, a seminal contribution toward the conceptual design, the synthesis and potential application of this unprecedented catalyst scaffold is disclosed.
