Wagner–Meerwein-Type Rearrangements of Germapolysilanes - A Stable Ion Study

The rearrangement of tris(trimethylsilyl)silyltrimethylgermane 1 to give tetrakis(trimethylsilyl)germane 2 was investigated as a typical example for Lewis acid catalyzed Wagner–Meerwein-type rearrangements of polysilanes and polygermasilanes. Direct 29Si NMR spectroscopic evidence is provided for several cationic intermediates during the reaction. The identity of these species was verified by independent synthesis and NMR characterization, and their transformation was followed by NMR spectroscopy.


Tris(trimethylsilyl)silyldimethylmethoxygermane (18)
A solution of 4.50 mmol tris(trimethylsilyl)silylpotassium 4 in 40 mL pentane and a solution of 0.75 g (4.50 mmol) dimethoxydimethylgermane 17 in 10 mL pentane were cooled to 0 °C. The silyl potassium compound was added drop wise to the germane solution. The ice bath was allowed to warm to room temperature overnight. The reaction mixture was then hydrolyzed with 1 M hydrochloric acid. The organic layer was separated and dried over sodium sulfate. The solvent was removed under reduced pressure and the product was purified by Kugelrohr distillation (0.62 g, 36 %). Due to the use of hydrochloric acid about 14% of the corresponding germyl chloride was formed as a by-product, which was detected in the GC chromatograms and NMR spectra. 1 H NMR (499.87 MHz, 305.0 K, C6D6, δ ppm): 0.34 (s, 27H, (CH3)3Si), 0.60 (s, 6H, (CH3)2Ge), 3.54 (s, 3H, CH3OGe). 13    . 29 Si{ 1 H} NMR spectrum of a mixture of tris(trimethylsilyl)silyldimethylmethoxygermane (18) and tris(trimethylsilyl)silylchlorodimethylgermane in C6D6.

S-5
Tris(trimethylsilyl)silyldimethylgermane (14) A solution of 0.62 g (1.62 mmol) tris(trimethylsilyl)silyldimethylmethoxygermane 18 in 30 mL THF and a suspension of 0.062 g (1.62 mmol) LiAlH4 in 50 mL THF were cooled to 0 °C with an ice bath. The solution of silagermane 18 was added to the LiAlH4 suspension and the reaction mixture was stirred for 20 min at 0 °C before it was allowed to warm to room temperature and stirred for another 20 min. The mixture was slowly added to ice cold 2 M sulfuric acid. The phases were separated and the aqueous phase was extracted two times with 50 mL diethyl ether. The combined organic phases were dried over sodium sulfate, filtered and the solvent was removed under reduced pressure. The product was crystallized from ethanol as a waxy, colorless solid (0.38 g, 1.09 mmol, 67 %). 1 H NMR (499.87 MHz, 305.0 K, C6D6, δ ppm): 0.30 (s, 27H, (CH3)3Si), 0.50 (d, 3 JH,H = 4.2 Hz, 6H, (CH3)2Ge), 4.04 (sept, 3 JH,H = 4.2 Hz, 1H, GeH). 13 13 C{ 1 H} NMR spectrum of tris(trimethylsilyl)silyldimethylgermane (14) in C6D6. Figure S3c. 29 Si{ 1 H} NMR spectrum of tris(trimethylsilyl)silyldimethylgermane (14) in C6D6. 10 Solutions of 2.99 mmol tris(trimethylsilyl)germylpotassium 5 in 30 mL DME and of 0.6 mL (excess, 5.52 mmol) chlorodimethylsilane in 30 mL DME were cooled to 0 °C with an ice bath. The germylpotassium compound was slowly added to the chlorosilane solution during 1 h. The ice bath was allowed to warm to room temperature overnight. The reaction mixture was then hydrolyzed with 1 M sulfuric acid. The organic layer was separated and the aqueous phase was extracted with 10 mL diethyl ether. The combined organic phases were dried over sodium sulfate and the filtrate was concentrated to 5 mL under reduced pressure. The product crystallized by adding 2 mL acetonitrile as a colorless, waxy solid (0.847 g, 80.6 %). 1 H NMR (499.87 MHz, 305.0 K, CDCl3, δ ppm) 10

Tris(trimethylsilyl)pentamethyldisilanylgermane (19)
A solution of 1.37 mmol tris(trimethylsilyl)germylpotassium•18-crown-6 5 in 3 mL benzene was added drop wise to a solution of 0.25 g (1.51 mmol) chloropentamethyldisilane 7 in 3 mL benzene. After 5 h the solution mixture was quenched with 1M sulfuric acid and the phases were separated. The aqueous phase was extracted with pentane and the combined organic phases were dried over sodium sulfate, filtered and the sovent was removed under reduced pressure. The product was obtained as colorless crystals by crystallization from

Bis(trimethylsilyl)pentamethyldisilanylgermane (16)
A mixture of 0.21 g (0.49 mmol) germapolysilane 19, 0.062 g (0.51 mmol) KO t Bu and 0.134 g (0.51 mmol) 18-crown-6 ether was dissolved in 2 mL benzene. After the complete formation of the germylpotassium compound 20 was confirmed by NMR spectroscopy, the solution was added to a stirred mixture of 10 mL degassed diethyl ether and 20 mL degassed 2M sulfuric acid cooled with an ice bath. The phases were separated, S-9 the aqueous phase was extracted with degassed diethyl ether and the combined organic phases dried over sodium sulfate. The solvents were removed under reduced pressure and the product was obtained as a colorless oil (0.15 g, 91 %) The germane is sensible to oxygen and should be stored under argon at -20 °C.       General preparation of trialkylsilyl arenium borates (10a-d) 11 Triphenylmethyl tetrakis(pentafluorophenyl)borate was dissolved in 3 mL of the indicated solvent and the silane was added. The formation of two phases could be observed and the biphasic reaction mixture was vigorously stirred for 30 min. The upper, non-polar phase was removed and the lower, polar phase was washed with 2 mL of the used solvent and again the non-polar phase was removed. The polar phase was dried under reduced pressure for 30 min and then dissolved in the respective deuterated solvent.

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General procedure for the rearrangement of tris(trimethylsilyl)silyltrimethylgermane (1) with trialkylsilyl arenium borates (10a-c) A solution of 0.18 g (0.50 mmol) silagermane 1 in 1 mL of the named deuterated solvent was added to a precooled solution of the named trialkylsilyl arenium borate 10a-c. The reaction mixture was stirred for 2 h at the specified temperature and then allowed to warm to room temperature. The polar phase and the non-polar phase were each transferred to separate NMR tubes to be analyzed independently. In the following reactions the NMR spectra of the polar phase showed too may signals to be analyzable but the compounds in the non-polar phase were identified by NMR and GC/MS spectroscopy.

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Silylgermane 18 was slowly added to the borate salt via a Teflon tube and the mixture was stirred at -20 °C for 5 min. The brown polar phase and the light yellow non-polar phase were each transferred to separate NMR tubes at -20 °C and stored at -60 °C for 5 h until the NMR spectra were recorded. At -60°C the polar phase solidifies and no further reaction is expected. The NMR spectra of the polar phase recorded at -20°C contained nearly

Decomposition of silyl toluenium ion 8(C7D8) at higher temperatures
The thermolability of silyl toluenium ion 8(C7D8) is shown by 29 Si{ 1 H} NMR spectroscopy at different temperatures. The amount of decomposition products increases with higher temperatures (Figure S14, b)-f)) and the intensity of 8(C7D8) decreases. In the independent synthesis of 8(C7D8) the same decomposition products already appear at -20 °C (Figure S14, a)).

Computational Details
All quantum chemical calculations were carried out using the Gaussian09 package. 13 In order to be consistent with previously reported data 14 Table S1 and the Diagram of relative ground state energies E and Gibbs free energies at 298.15 K, G 298 , of toluene complexes of isomeric cations 3 -8 is given in Figure S22. The corresponding computed molecular structures are given in the form of their Cartesian coordinates in Table S2. NMR chemical shift computations were performed using the GIAO method as implemented in Gaussian 09 and the M06-L functional along with the 6-311G(2d,p) basis set for molecular structures obtained at the M06-2X/6-311+G(d,p) level of theory. 16 The influence of the highly polar medium on the structure optimizations was modeled using the PCM model 17