Further Structural Modification of Sulfur-Stabilized Silicon Cations with Binaphthyl Backbones

Abstract

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.

Chiral Chalcogenyl‐Substituted Naphthyl‐ and Acenaphthyl‐Silanes and Their Cations

“…This is demonstrated in Figure 4 for a set of 29 Si NMR spectra. The 29 Si NMR chemical shifts of silyl borates 13 and 14 are in the expected region for intramolecularly chalcogenyl‐stabilized silyl cations (δ 29 Si=36–72) [27, 28, 30–33, 38, 39] . As shown previously for related silyl cations, the 29 Si NMR resonance experiences a high‐field shift with increasing size and polarizability of the remote donor atom [38] .…”

“…[25,26] In as eries of investigations, Oestreich and co-workers used different families of intramolecularly stabilized silyl cations to introduce chirality on these silyl Lewis acids. [27][28][29][30][31][32][33] Particularly interesting in the context of our study are silylsulfoniumi ons I-IV ( Figure 1). [28,[30][31][32][33] In thesee xamplest he neighboring group effecto ft he sulfur donor creates new centers of chirality that are controlled by the present stereo element (centered chirality at the silicona tom or axial chirality).…”

“…Later, Ghosez and co‐workers used non‐ionic chiral silyltriflimides derived from substituents from the chiral pool [25, 26] . In a series of investigations, Oestreich and co‐workers used different families of intramolecularly stabilized silyl cations to introduce chirality on these silyl Lewis acids [27–33] . Particularly interesting in the context of our study are silylsulfonium ions I – IV (Figure 1).…”

“…Particularly interesting in the context of our study are silylsulfonium ions I – IV (Figure 1). [28, 30–33] In these examples the neighboring group effect of the sulfur donor creates new centers of chirality that are controlled by the present stereo element (centered chirality at the silicon atom or axial chirality). Consequent structure modifications finally led to the silylsulfonium species IV , which catalyzes challenging Diels–Alder reactions at low temperatures in good yields and decent enantioselectivities (up to 81 % ee ) [32] .…”

Chiral Memory in Silyl-Pyridinium and Quinolinium Cations

“…The work of Siegel, Müller, Oestreich, and others has shown that the interaction between the cationic silylated center and Lewis basic substituents is able to control this reactivity and to modulate the Lewis acidity of silicon . Very recent studies have demonstrated that the addition of basic and chiral ligands around the silyl cation not only stabilizes the latter but also allows chirality transfer with satisfactory enantioselectivity levels in Diels–Alder reactions . The labile nature of the weak LB–Si + interaction also favors the regeneration of the catalyst at the end of the catalytic cycle and could allow the use in asymmetric synthesis of silylium ions bearing a unique silicon-centered chirality, providing the latter is stable during the process.…”