Stable Silenolates and Brook-Type Silenes with Exocyclic Structures

The first silenolates with exocyclic structures [(Me3Si)2Si(Si2Me4)2SiC(R)O]−K+ (2a: R = 1-adamantyl; 2b: mesityl; 2c: o-tolyl) were synthesized by the reaction of the corresponding acylcyclohexasilanes 1a–c with KOtBu. NMR spectroscopy and single-crystal X-ray diffraction analysis suggest that the aryl-substituted silenolates 2b,c exhibit increased character of functionalized silenes as compared to the alkyl-substituted derivative 2a due to the different coordination of the K+ counterion to the SiC(R)O moiety. 2b,c, thus, reacted with ClSiiPr3 to give the exocyclic silenes (Me3Si)2Si(Si2Me4)2Si=C(OSiiPr3)R (3b: R = Mes; 3c: o-Tol), while 2a afforded the Si-silylated acylcyclohexasilane 1d. The thermally remarkably stable compound 3b, which is the first isolated silene with the sp2 silicon atom incorporated into a cyclopolysilane framework, could be fully characterized structurally and spectroscopically.

analyses were carried out on a Hanau Vario Elementar EL apparatus. Photolyses were performed by using a 500 W medium pressure mercury lamp (Original HANAU). Sample solutions were photolyzed under an atmosphere of nitrogen in Pyrex Schlenk tubes immersed in cold water to ensure ambient sample temperature and to prevent irradiation with light of wavelengths < 300 nm. UV absorption spectra were recorded on a Perkin Elmer Lambda 5 spectrometer.

Synthesis of [18]-crown-6 adduct of 2b
The procedure followed was that used for 2a with 300 mg (0.

Synthesis of [18]-crown-6 adduct of 2c
The procedure followed was that used for 2a with 300 mg (

Reaction of 2a with iPr 3 SiCl
A solution of 2a was freshly prepared by stirring a mixture of 46.9 mg (0.447 mmol) of KOtBu and 300 mg (0.447 mmol) of 1a in 5 mL of THF for 1 h at 70°C. Now the resulting red solution was warmed to 0°C and 0.10 g (0.447 mmol) of iPr 3 SiCl were added drop wise. Immediately the solution turned colorless. After aqueous work up with 100 mL of 3 % sulfuric acid the organic layer was separated, dried over Na 2 SO 4 and the solvents were stripped off with a rotary evaporator. Drying in vacuum (0.001 mbar) and crystallization from acetone solution at 30°C afforded 270 mg (80%) of the analytically pure acylcyclohexasilane 1d as white crystals.

Reaction of 2c with iPr 3 SiCl
170 mg (0.876 mmol) of iPr 3 SiCl were added drop wise at 0 °C to a solution of 2c in 5 mL of THF freshly prepared from 500 mg (0.797 mmol) of 1c and 94 mg (0.876 mmol) of KOtBu according to the procedure described above. Immediately the red solution turned yellow. After removal of the volatile components in vacuo the remaining yellow solid was dissolved in heptane, filtered over dry celite and the solvent was stripped off again. The target silene 3c was formed along with several unidentified by-products. Purification of the crude material by crystallization was not successful because it decomposed further even at 70°C.

Reaction of 3b with O 2
A solution of 300 mg (0.410 mmol) of 3b in 6 mL of THF was stirred in contact with air at room temperature for 2 h. After removal of the volatile components on a rotary evaporator the product was chromatographed twice on silica gel, eluting with gradient (heptane, toluene), to give 100 mg (32%) of a semi-solid residue of slightly impure 4b.

Photolysis of 3b
A solution of 300 mg (0.410 mmol) of 4c in 5 mL of benzene was photolyzed with a 500 W mercury lamp at 25 °C for 12 h. At this time 1 H-and 29 Si-NMR analysis showed that the starting material completely had been consumed. After removal of the volatile components on a rotary evaporator 5 mL of pentane were added and the resulting solution was filtered over silica gel.

X-ray Crystallography
For X-ray structure analysis suitable crystals were mounted onto the tip of glass fibres using mineral oil. Data collection was performed on a Bruker Kappa Apex II CCD diffractometer at 100 K using graphite-monochromated Mo K ( = 0.71073Å) radiation. Details of the crystal data and structure refinement are provided in Tables S1 -S4. The SHELX version 6.1 program package was used for the structure solution and refinement. 4 Absorption corrections were applied using the SADABS program. 5 All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were included in the refinement at calculated positions using a riding model as implemented in the SHELXTL program. In the solid state structure of 2a the adamantyl group as well as the crown ether molecule were found disordered over two positions and were accordingly implemented in the structural model. The ratios of occupancy refined to 0.65:0.35 (adamantyl group) and 0.60:0.40 (crown ether), respectively. All nonhydrogen atoms of the disordered parts were refined anisotropically and hydrogen atoms were placed using standard AFIX commands. Crystallographic data (excluding structure factors) have

Computational Methods
All calculations were carried out using the Gaussian09 program package 6 on a computing cluster with blade architecture. For all calculations the mPW1PW91 hybrid functional was used 7 together with the 6-31+G** basis set. After structure optimizations the vibrational frequencies were calculated to ensure minimum structures. The same combination of functional and basis set was used to calculate the excited states via time dependent (TD)-DFT. Compound 3b was modeled by substituting the OSiiPr 3 group by OSiMe 3 .