Chiral Sulfur-Stabilized Silicon Cations as Lewis Acids for Enantioselective Diels-Alder Reactions

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The intermolecular stabilization of Me3Si+ by Me2S fails due to disproportionation of the resulting sulfonium ion [Me3SiSMe2]+. Hence, little is known about the chemistry of sulfur-stabilized silicon cations. Our laboratory recently showed that intramolecular coordination of soft sulfur donors to tricoordinate silicon cations leads to chemically stable adducts. These "tamed" cations are sufficiently Lewis acidic to catalyze particularly challenging Diels-Alder reactions, e.g., [4+2]-cycloadditions of cyclohexa-1,3-diene and alpha,beta-unsaturated ketones. Rendering these enantioselective is the next big step, and we have already been able to demonstrate that cognate Lewis acids with chiral backbones do indeed induce promising enantioselectivities in selected cases. From the various motifs prepared and tested, a binaphthalene-derived silepine system emerged as a lead structure. The goal of the present project is now to elaborate this candidate into a chiral sulfur-stabilized silicon cation that will catalyze the aforementioned Diels-Alder reactions with high levels of enantioselection. It must be noted here that, to date, there are no catalysts exist to promote these simple looking but exceedingly difficult cycloadditions in asymmetric fashion. It is a fundamental problem in both synthetic chemistry and catalysis with main-group Lewis acids.