Intramolecular Halo Stabilization of Silyl Cations—Silylated Halonium‐ and Bis‐Halo‐Substituted Siliconium Borates

Abstract The stabilizing neighboring effect of halo substituents on silyl cations was tested for a series of peri‐halo substituted acenaphthyl‐based silyl cations 3. The chloro‐ (3 b), bromo‐ (3 c), and iodo‐ (3 d) stabilized cations were synthesized by the Corey protocol. Structural and NMR spectroscopic investigations for cations 3 b–d supported by the results of density functional calculations, which indicate their halonium ion nature. According to the fluorobenzonitrile (FBN) method, the silyl Lewis acidity decreases along the series of halonium ions 3, the fluoronium ion 3 a being a very strong and the iodonium ion 3 d a moderate Lewis acid. Halonium ions 3 b and 3 c react with starting silanes in a substituent redistribution reaction and form siliconium ions 4 b and 4 c. The structure of siliconium borate 4 c 2[B12Br12] reveals the trigonal bipyramidal coordination environment of the silicon atom with the two bromo substituents in the apical positions.


5,6-diiodoacenaphthene 20
The original procedure [S6] was slightly modified. 5,6-Dibromoacenaphthene (1.0 equiv, 6 g, 19.23 mmol) was dissolved in 400 mL of Diethylether and cooled to -10°C -0 °C. Then a mixture containing n-butyl lithium (1.6 M in n-hexane, 2.4 equiv, 29 mL) and tetramethylethylenediamine (TMEDA) (2.66 equiv, 8 mL) was added dropwise and the reaction mixture was stirred for at least one hour at -10°C -0 °C. After iodine (2.2 equiv, 10.74 g, 42.31 mmol) in 120 mL diethylether was added dropwise over a period of 30 min at -10 °C -0 °C. The dark mixture was stirred for 1 h at the same temperature and was allowed to warm to room temperature overnight. After that 500 mL of aqueous 5 % sodium thiosulfate was added to the mixture with vigorous stirring. The organic phase was first washed with aqueous 5 % sodium thiosulfate, then with water and dried over MgSO4. The solvent was removed under low pressure. The product was purified by column chromatography using n-pentane (Rf = 0.41) as eluent. Yield: 2.30 g (5.67 mmol, 29.5 %). S8 2 Synthesis of the silanes General procedure A: The starting material was dissolved in THF and cooled to -80°C. Then n-butyl lithium (nBuLi) was added dropwise and the reaction mixture was stirred for at least one hour at the same temperature. After that chlorodimethylsilane was added and the mixture was stirred for additional 60 minutes at -80 °C. Then the reaction mixture was allowed to warm to r.t. overnight. After adding NH4Cl solution (10 mL) to the reaction mixture and extraction with Et2O or n-pentane (3 x 20 mL), the organic phases were combined, dried over MgSO4 and the solvent was removed under low pressure. The silanes were purified by column chromatography or crystallization.

chloronium ion 3b and siliconium ion 4b
A solution of trityl borate [Ph3C][B(C6F5)4] (1.0 equiv, 434 µmol, 400 mg) in benzene-d6 was added to a solution of 5-chloro-6-dimethylsilylacenaphthene 1b (1.0 equiv, 434 µmol, 107 mg) in benzene-d6. During the addition, the reaction mixture was cooled with cold water and stirred for 30 min at room temperature. The color of the reaction mixture changed from yellow orange to dark green. Then, the upper, nonpolar phase was removed and the polar phase was analyzed by NMR spectroscopy, which indicated the formation of chloronium borate       (1.0 equiv, 163 µmol, 150 mg) in chlorobenzene-d5 at -40 °C in small portions. The reaction mixture was stirred for 30 min at the same temperature before it was allowed to warm to room temperature and analyzed by NMR spectroscopy, which indicated the formation of chloronium

S30 bromonium ion 3c
In toluene-d8: A solution of trityl borate [Ph3C][B(C6F5)4] (1.0 equiv, 434 µmol, 400 mg) in toluene-d8 was cooled to -10 °C and added to a solution of 5-bromo-6dimethylsilylacenaphthene 1c (1.1 equiv, 477 µmol, 138 mg) in toluene-d8 at -10 °C. Then, the mixture was stirred for 30 min at the same temperature, while the color changed from yellow orange to dark green. After that, the biphasic reaction mixture was allowed to warm to room temperature and the upper, nonpolar phase was removed. The polar phase was washed with 0.3 mL toluene-d8 and bromonium borate 3c[B(C6F5)4] was subsequently analyzed by NMR spectroscopy at T = -10 °C. (1.0 equiv, 401 µmol, 370 mg) in benzene-d6 in small portions. During the addition, the reaction mixture was cooled with cold water. Since benzene-d6 was freezing, the mixture was brought to 12 °C and the remaining silane solution was further added in small portions and stirred vigorously for 30 min at room temperature while warming to room temperature. The color of the reaction mixture changed from yellow orange to brown. Then, the upper, nonpolar phase was removed and the polar phase, containing the bromonium borate 3c[B(C6F5)4], was analyzed by NMR spectroscopy.

S40 bromonium ion 10
A solution of 1-bromo-8-dimethylsilylnaphthalene 2 (1.2 equiv, 481 µmol, 128 mg) in benzene-d6 was added to a solution of trityl borate (1.0 equiv, 401 µmol, 370 mg) in benzene-d6 at r.t. and stirred for 30 min. Then, the upper, nonpolar phase was removed and the polar phase was washed with 0.5 mL benzene-d6. Afterwards, bromonium borate 10[B(C6F5)4] was analyzed by NMR spectroscopy. The sample showed a broad signal for the cation and was measured again several days later, where decomposition was already observed.     Si NMR spectra recorded with the INEPT pulse sequence with parameter sensitive to Si-H groups with large coupling constants were used to identify the by-product with  29 Si = 1.1 (see Figure S52).

S48 iodonium ion 3d
A reaction mixture containing 5-iodo-6-dimethylsilylacenaphthene 1d as main product (with 5-iodoacenaphthene and a small amount of 5-dimethylsilylacenaphthene [S3]) was used for the synthesis of iodonium ion 3d. To ensure a small excess of the silane 1d compared to the trityl borate for complete conversion of the trityl salt, 170 mg of the reaction mixture (containing 1.03 equiv, 414 µmol of the silane 1d) in 1 mL benzene-d6 was added to a solution of trityl borate (401 µmol, 370 mg) in 0.5 mL benzene-d6 at r.t. and stirred for 30 min. The upper, nonpolar phase was removed and iodonium borate 3d[B(C6F5)4] was analyzed by NMR spectroscopy.

General procedure A: A solution of trityl borate [Ph3C][B(C6F5)4] in benzene was added to a
solution of the corresponding silane and 4-fluorobenzonitrile in benzene. During the addition, the reaction mixture was cooled with cold water and the biphasic reaction mixture was stirred for 30 min at room temperature. Subsequently, the phases were separated, the upper, nonpolar phase was removed and the polar phase was washed twice with benzene and then with n-pentane. After removing the solvent under reduced pressure, the residue was dissolved in benzene-d6 and analyzed by NMR spectroscopy.

General procedure B: Trityl borate [Ph3C][B(C6F5)4] and 4-fluorobenzonitrile were dissolved
in benzene-d6. A solution of the corresponding silane in benzene-d6 was added in small portions to the mixture, while it was cooled with cold water, and stirred for 30 min at room temperature. After the NMR spectroscopic analysis of the polar phase, the NMR sample was transferred into a Schlenk tube and washed with n-pentane. The solvent was removed under reduced pressure, the residue was dissolved in benzene-d6 and analyzed by NMR spectroscopy.
General procedure C: After the synthesis of the corresponding halonium ion, as described in
The reaction mixture containing 5-iodo-6-dimethylsilylacenaphthene 1d was used in excess compared to the amount of the trityl borate. The NMR spectroscopic analysis revealed broad signals in 1 H and 29 Si{ 1 H} NMR spectra. Thereafter, the NMR sample was transferred into a Schlenk tube, 4-fluorobenzonitrile (124 µmol, 15 mg) was added, the reaction mixture was stirred for 15 min and analyzed by NMR spectroscopy again.     Photon III C14 CPAD detector). An empirical absorption corrections using equivalent reflections was performed with the program SADABS [S7] or TWINABS [8]. The structure was solved with the program SHELXS [S9] and refined with SHELXL [S10] using the OLEX2 GUI [S11].
These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

Data from X-ray diffraction analysis of silane 1a
Colorless crystals of silane 1a that were suitable for X-ray diffraction analysis were obtained by slow evaporation of n-pentane at -20 °C.

Computational Part
All quantum chemical calculations were carried out using the Gaussian09 package. [S12] The NBO analyses were performed with the version 6.0 which was implemented in the G09 D.01 version including the natural resonance theory (NRT)[S13]. The AIMALL program was used to perform the QTAIM analysis. (T. A. Keith, AIMAll (Version 11.05.16), 2011.) The molecular structure optimizations were performed using the M06-2X functional [S14] along with the def2tzvp basis set for the elements F, Cl, Br, I, Si, C, H and using the corresponding pseudopotential for I [S15], as well as the ultrafinegrid option. Every stationary point was identified by a subsequent frequency calculation as minimum (Number of imaginary frequencies (NIMAG): 0). The SCF energies (E(SCF)) and the computed Gibbs free energies at T = 298.15 K and p = 0.101 MPa (1 atm) in the gas phase (G 298 ) are given in Table S6 for all optimized molecular structures.
For the calculation of the mechanism of the substituent redistribution reaction (Figure 10), the Gibbs free energies, G 298 (benzene) at T = 298.15 K in benzene were used and are given in Table S7. Therefore, the SCF energies (E(SCF)) were calculated at the M06-2X/def2tzvp level of theory with inclusion of solvent effects using the SCIPM model with benzene as solvent. In order to avoid the overestimation of entropy effects in the gas-phase, the pressure of liquid benzene (p = 28.1 MPa (277 atm) was used for the calculation.
Silicon NMR chemical shift calculations were performed using the GIAO method as   The bond dissociation energy for the newly formed siliconhalogen bond is estimated using the isodesmic equations shown in Scheme S4 and the results are given in Table S9. These isodesmic equations are not ideal for two reasons: silanes 1, 2 are destabilized compared to their isomers 16, 18 by the peri-disubstitution. This leads to the prediction of too strong silicon halogen bonds in cations 3 and 10 by the isodesmic equations. In addition, the conformation of the SiMe2 units in cations 17, 19 relative to the naphthalene / acenaphthene backbone allow significant conjugation between the 3p(Si) orbital and the -system of the arene unit, which is not possible in the chalcogenyl stabilized cations 3, 10. This latter imbalance of the isodesmic equations leads to the prediction of too weak Ch -Si bonds. These opposing and in consequence cancelling effects in mind, we suggest that the calculated reaction enthalpies of the isodesmic equations that are shown in Scheme S4 are a good first approximation for the strength of the siliconhalogen linkage in cations 3 and 10.