Planar Chiral, Ferrocene‐Stabilized Silicon Cations

Abstract

The preparation of a series of planar chiral, ferrocenyl-substituted hydrosilanes as precursors of ferrocene-stabilized silicon cations is described. These molecules also feature stereogenicity at the silicon atom. The generation and (29)Si NMR spectroscopic characterization of the corresponding silicon cations is reported, and problems arising from interactions of the electron-deficient silicon atom and adjacent C(sp(3))-H bonds or aromatic π donors are discussed. These issues are overcome by tethering another substituent at the silicon atom to the ferrocene backbone. The resulting annulation also imparts conformational rigidity and steric hindrance in such a way that the central chirality at the silicon atom is set with complete diastereocontrol. These chiral Lewis acid catalysts were then tested in difficult Diels-Alder reactions, but no enantioinduction was seen.

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

“…[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).…”

“…All investigated silanes 3, 4, 6, 7, 9 and 10 show the expected 1 HNMR chemical shift for the hydrogen atom which is attached to the silicon atom in the range from d 1 H = 4.95 to 6.29. [38,39] The corresponding 29 Si NMR chemical shifts are between d 29 Si = 1.3 and À21.3. The Si-Hc oupling constants are in the expected range of 1 J(HSi) = 193 to 203 Hz.…”

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

“…Cationic silicon Lewis acids, that is, silylium ion-like species, and chiral variants thereof have been shown to act as competent catalysts for Diels-Alder reactions. [1][2][3] Our laboratory recently introduced binaphthyl-based sulfur-stabilized silicon cations [4][5][6] (S,S)-4 and (S,S)-5 that induced 34% ee and 67% ee, respectively, in the challenging model reaction of cyclohexa-1,3-diene (1) and chalcone (2) (1 + 2 → 3, Scheme 1, top). With H 8 -binaphthyl-based (S,S)-5, 81% ee was reached for more hindered chalcone derivatives.…”

New reactivity at the silicon bridge in sila[1]ferrocenophanes

“…Attenuation of this electrophilicity at silicon has been demonstrated by the installation of a ferrocene (Fc) unit. [56][57][58] Effective comparison can be made between the α-ferrocenyl silyl cation in compound 6 and the isoelectronic borane and carbenium analogues ( Figure 3). 59,60 In all cases, a "dip angle", α* (defined as [180° -(Cp cent -Cp ipso -E)] angle) of the electron-deficient α-substituent towards the iron atom is evident, which increases with the increasing electrophilicity of the substituent from boron to carbon to silicon.…”