A novel cationic silicon−sulfur Lewis pair with a chiral H 8 -binaphthyl backbone is reported. It catalyzes otherwise sluggish Diels−Alder reactions of cyclohexa-1,3-diene and chalcone derivatives in good yields and decent enantioselectivities (up to 81% ee). The enantioinduction is highest with a [1,1′-biphenyl]-4-yl substituent at the carbonyl carbon atom. This moiety can be later converted into a carboxyl group by Baeyer−Villiger oxidation. The same oxidant also epoxidizes the double bond in the cycloadduct, and the epoxide engages in a lactonization with the carboxylic acid. Synthetically interesting hexahydro-3,6-methanobenzofuran-2(3H)-one skeletons are obtained in one pot.

The stereoselective preparation of diastereomeric dihydrosilepine-derived silicon cations decorated with another binaphthyl unit at the silicon atom is described. A sulfide donor attached to that additional binaphthyl substituent forms an intramolecular Lewis pair with the electron-deficient silicon atom, as verified by 29Si NMR spectroscopy. Both chiral sulfur-stabilized silicon cations act as catalysts in the difficult Diels–Alder reaction of cyclohexa-1,3-diene and chalcone derivatives. Both Lewis acids induce enantioselectivity, but the S,S relative configuration is superior to the S,R configuration. With the former diastereomer, enantiomeric excesses of close to 60% are obtained. These values are the highest achieved to date in this seemingly trivial cycloaddition.

The synthesis and spectroscopic characterization of two novel cationic silicon–sulfur Lewis pairs with a chiral 4,4′-disubstituted binaphthyl silepine backbone are described. Both Lewis acids induce significant enantioselectivity in the model Diels–Alder reaction of cyclohexa-1,3-diene and chalcone but additional substitution of the binaphthyl backbone exerts a minimal effect on enantioinduction compared to previously reported Lewis acids. Another silicon cation with a chiral spirocyclic backbone induces enantioselectivity in the same range but its synthesis is laborious.

The preparation and NMR spectroscopic characterization of a family of axial chiral, sulfur‐stabilized silicon cations is reported. These silicon Lewis acids have been applied as catalysts in difficult enantioselective Diels–Alder reactions of cyclohexa‐1,3‐diene and representative chalcone derivatives. To gain better understanding of the relevant structural elements in these catalysts, their silepine backbone and the aryl thioether group have been systematically modified. These efforts have led to an improved catalyst that induces 53 % ee for chalcone and even 65 % ee for a more hindered derivative. These cyclohexa‐1,3‐diene Diels–Alder reactions are endo selective. However, the corresponding cyclopentadiene Diels–Alder reactions predominantly yield the exo cycloadducts with no enantioinduction.

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