Intramolecularly Sulfur-Stabilized Silicon Cations with Chiral Binaphthyl Backbones: Synthesis of Three Different Motifs and Their Application in Enantioselective Diels–Alder Reactions

The formation and 29Si
NMR spectroscopic characterization
of silicon cations that are intramolecularly stabilized by a dialkyl
thioether are described. The chemical stability of the silicon–sulfur
Lewis pair and, hence, the viability of the approach, were probed
with a 2-[(alkylthio)­methyl]­phenyl-substituted hydrosilane
as a proxy before three different motifs with chiral binaphthyl backbones
were prepared in multistep sequences. The degree of shielding of the
silicon atom in these cations was found to depend on the substitution
pattern at the silicon atom and the ring size generated by the silicon–sulfur
interaction. These sulfur-stabilized silicon cations are sufficiently
reactive to promote Diels–Alder reactions of cyclohexa-1,3-diene
with various dienophiles; the same set of reactions with cyclopentadiene
is also reported. One of the three chiral Lewis acids induces low,
but promising, enantioselectivity, and 24% ee is the highest value
so far obtained with a cationic tetracoordinate silicon catalyst.