An Experimental Acidity Scale for Intramolecularly Stabilized Silyl Lewis Acids

Abstract

A new NMR‐based Lewis acidity scale is suggested and its application is demonstrated for a family of silyl Lewis acids. The reaction of p‐fluorobenzonitrile (FBN) with silyl cations that are internally stabilized by interaction with a remote chalcogenyl or halogen donor yields silylated nitrilium ions with the silicon atom in a trigonal bipyramidal coordination environment. The 19F NMR chemical shifts and the 1J(CF) coupling constants of these nitrilium ions vary in a predictable manner with the donor capability of the stabilizing group. The spectroscopic parameters are suitable probes for scaling the acidity of Lewis acids. These new probes allow for the discrimination between very similar Lewis acids, which is not possible with conventional NMR tests, such as the well‐established Gutmann–Beckett method.

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

“…The Si-Hc oupling constants are in the expected range of 1 J(HSi) = 193 to 203 Hz. [38,39] At rend of the 29 Si NMR resonance movingt owards higherf ield with increasing atomic weight of the remote chalcogen atom is noticeable ( Table 1). The 77 Se and 125 Te NMR chemical shifts of silanes 6, 7 and 10 a,b are in the expected range for diarylchalcogenides.…”

“…The coordination environmento ft he oxygen atom in symmetrically substituted silyloxonium ions 15 and 16 (Scheme 4) is trigonal planar in solution and in the solid state. [38,39] The for- mation of only one isomer of silyloxonium ions 13 a,b suggests also in these unsymmetrical substituted cations ap lanar coordination at the tricoordinated oxygen atom.…”

“…In the first step, the silyl group is introduced to obtain peri-bromos ilyl acenaphthene 5 or naphthalenes 8 and in the second step the chalcogenyl group is installed by lithiation and subsequent treatment with the corresponding dichalcogenide. [38,39] Kinetic resolution using ap rotocol developed by Oestreich et al was successful for the separation of the racemic phenyl substituted silanes 3a, 4a and 9 into their enantiomers. [40] It relies on the dehydrogenative Si-O coupling of silanes and uses 0.5 equivalent tert-butylpyridyl alcohol( R)-11 as an auxiliary and ac opper(I) catalysts ystem (Scheme2).…”

“…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.…”

“…The 77 Se and 125 Te NMR chemical shifts of silanes 6, 7 and 10 a,b are in the expected range for diarylchalcogenides. [38,39,[41][42][43] Remarkably, the selanyl-and tellanyl-substituted derivatives showi na ddition to the Si-H 1 J coupling, ac haracteristicc halcogen-hydrogen through-space coupling ( TS J(ChH)) between the chalcogen atom and the hydrogen atom of the silyl group ( TS J(SeH) = 14-28 Hz and TS J(TeH) = 66 Hz). [38,44,45] Previously,B eckmann and co-workersd isclosed ar elated Si-H•••P interaction in acenaphthene based compounds.…”