Unified Approach to Imidodiphosphate-Type Brønsted Acids with Tunable Confinement and Acidity

Oleg Grossmann,Gabriela G Gerosa,Benjamin List,Natascha R Sadlowski,Chandra Kanta De,Sebastian A Schwengers,Joyce A A Grimm

doi:10.1021/jacs.1c07067

Oleg Grossmann, Gabriela G Gerosa + Show 5 more

Open Access

https://doi.org/10.1021/jacs.1c07067

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Abstract

We have designed and realized an efficient and operationally simple single-flask synthesis of imidodiphosphate-based Brønsted acids. The methodology proceeds via consecutive chloride substitutions of hexachlorobisphosphazonium salts, providing rapid access to imidodiphosphates (IDP), iminoimidodiphosphates (iIDP), and imidodiphosphorimidates (IDPi). These privileged acid catalysts feature a broad acidity range (pKa from ∼11 to <2 in MeCN) and a readily tunable confined active site. Our approach enables access to previously elusive catalyst scaffolds with particularly high structural confinement, one of which catalyzes the first highly enantioselective (>95:5 er) sulfoxidation of methyl n-propyl sulfide. Furthermore, the methodology delivers a novel, rationally designed super acidic catalyst motif, imidodiphosphorbis(iminosulfonylimino)imidate (IDPii), the extreme reactivity of which exceeds commonly employed super-Brønsted acids, such as trifluoromethanesulfonic acid. The unique reactivity of one such IDPii catalyst has been demonstrated in the first α-methylation of a silyl ketene acetal with methanol as the electrophilic alkylating reagent.

Highlights

Acid catalysis is arguably the single most general approach to catalysis there is
During the last two decades, organic Brønsted acids have enriched the arsenal of asymmetric catalysis, initially in bifunctional catalysts such as proline or BINOL-derived phosphoric acids (CPA),[2,3] and lately in more purely acidic motifs.[4]
We have generalized the underlying principle of asymmetric Brønsted acid catalysis, in which protons act as the activating principle while chiral, enantiopure anions enable enantiodifferentiation, toward asymmetric counteranion directed catalysis (ACDC), including all types of cationic activation principles.[5]

Summary

■ INTRODUCTION

Acid catalysis is arguably the single most general approach to catalysis there is. It enables the activation of diverse and inherently distinct substrate classes, which, at least in principle, as a necessary and sufficient condition, only require electron density and as such, the potential to catalytically activate the vast majority of all chemical materials exists. It is perhaps not surprising that acidic catalysts have become indispensable tools for chemical synthesis as well as an enabling technology for multimillion-ton-scale productions.[1] During the last two decades, organic Brønsted acids have enriched the arsenal of asymmetric catalysis, initially in bifunctional catalysts such as proline or BINOL-derived phosphoric acids (CPA),[2,3] and lately in more purely acidic motifs.[4] In this context, we have generalized the underlying principle of asymmetric Brønsted acid catalysis, in which protons act as the activating principle while chiral, enantiopure anions enable enantiodifferentiation, toward asymmetric counteranion directed catalysis (ACDC), including all types of cationic activation principles.[5] The high versatility of Brønsted acids inspired the development of ever more acidic catalysts to overcome intrinsic reactivity barriers of weakly basic substrates.[6] the highly selective conversion of small and constitutionally unbiased substrates has long remained challenging due to the rather open active site of most Brønsted acid catalysts and the resulting conformational freedom of protonated reactive intermediates and transition states To overcome these limitations, our group has conceptualized, designed, and established confined acids, the corresponding bases of which possess highly compact anionic active sites. A particular focus is given to previously elusive catalyst scaffolds and toward the development of even more acidic imidodiphosphoryl-based Brønsted acids, which overcomes remaining reactivity barriers and facilitates the development of novel transformations within the ACDC framework.[5,18]

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES